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Programmed morphing of liquid crystal networks
Citation for published version (APA):Haan, de, L. T. (2014). Programmed morphing of liquid crystal networks. Eindhoven: Technische UniversiteitEindhoven. https://doi.org/10.6100/IR781577
DOI:10.6100/IR781577
Document status and date:Published: 13/11/2014
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Programmed morphing of liquid crystal networks
PROEFSCHRIFT
ter verkrijging van de graad van doctor aan de Technische Universiteit Eindhoven, op gezag van de rector magnificus prof.dr.ir. C.J. van Duijn, voor een commissie aangewezen door het College voor Promoties, in het openbaar te verdedigen op
donderdag 13 november 2014 om 16:00 uur
door
Laurens Theobald de Haan
geboren te Apeldoorn
Dit proefschrift is goedgekeurd door de promotoren en de samenstelling van de promotiecommissie is als volgt: voorzitter: prof.dr.ir. J.C. Schouten 1e promotor: prof.dr. A.P.H.J. Schenning 2e promotor: prof.dr. D.J. Broer leden: prof.dr.ing. C.W.M. Bastiaansen (Queen Mary University of London)
prof.dr. A.E. Rowan (Radboud University Nijmegen)
prof.dr. R.P. Sijbesma adviseurs: dr. T.J. White (US Airforce Research Laboratory) dr. C. Sanchez-Somolinos (University of Zaragoza)
A catalogue record is available from the Eindhoven University of Technology
Library ISBN: 978-90-386-3729-7
Copyright © 2014 by Laurens Theobald de Haan
Cover design: Proefschriftmaken.nl || Uitgeverij BOXPress
Printed & Lay Out by: Proefschriftmaken.nl || Uitgeverij BOXPress
The research described in this thesis has been financially supported by the
Netherlands Organization for Scientific Research, Chemical Sciences (NWO-CW),
file number 700.57.444
Table of contents
Chapter 1. Morphing of liquid crystal polymer networks
1.1 Stimuli-responsive materials 1
1.2 Liquid crystal networks 2
1.3 Alignment director variation through the thickness of the polymer film 6
1.3.1 Twisted and splayed alignment profiles: bending 6
1.3.2 Twisted and splayed alignment profiles: coils and helicoids 10
1.4 Director profiling in the xy-plane of the film 15
1.4.1 Continuous circular patterns 15
1.4.2 Discrete patterns 17
1.5 Director patterning in 3 dimensions 19
1.6 Aim and outline of the thesis 20
1.7 References 21
Chapter 2. Programmed deformation of uniaxially aligned actuators with
asymmetry in the molecular trigger
2.1 Introduction 27
2.2 Results and discussion 29
2.2.1 Film preparation, KOH treatment and analysis 29
2.2.2 Deformation behavior of the films 30
2.2.3 Complex folding 33
2.2.4 Curling 34
2.3 Conclusions and outlook 35
2.4 Experimental Section 36
2.5 References 37
Chapter 3. Actuators with a director pattern in the xy-plane of the film
3.1 Introduction 41
3.2 Results and discussion 43
3.2.1 Preparation of photoaligned liquid crystal network films 43
3.2.2 Films based on continuous disclinations 44
3.2.3 Actuation of +1 disclination-based actuator films 46
3.2.4 Disclination-based, discrete patterns 48
3.2.5 Aperture patterns 49
3.3 Conclusions and outlook 51
3.4 Experimental section 52
3.5 References 55
Chapter 4. Folding actuators with 3-dimensional alignment patterns
4.1 Introduction 59
4.2 Results and discussion 60
4.2.1 Alignment pattern design 60
4.2.2 Preparation and optical characterization of the polymer films 61
4.2.3 Actuation of striped twisted nematic films 62
4.2.4 Water-responsive actuators 65
4.2.5 Finite-element analysis 66
4.2.6 Checkerboard pattern 69
4.2.7 Finite element analysis of checkerboard pattern 69
4.2.8 Radimuthal films 70
4.3 Conclusions and outlook 72
4.4 Experimental Section 72
4.5 References 76
Chapter 5. Generating wrinkling patterns in metal/LC-polymer bilayer films
5.1 Introduction 77
5.2 Results and discussion 79
5.2.1 Sample preparation and wrinkle formation 79
5.2.2 Measuring the wrinkle period and amplitude 82
5.2.3 Preparation and examination of various wrinkle patterns 83
5.3 Conclusions and outlook 85
5.4 Experimental section 86
5.5 References 87
Chapter 6. Technology assessment
6.1 Introduction 91
6.2 Soft robotics 91
6.3 Switchable sieves 95
6.4 Smart optics 96
6.5 References 98
List of symbols 101
Summary 103
Samenvatting 105
Curriculum Vitae 107
Publications 109
Acknowledgements 111
This chapter was based on a review: de Haan, L. T.; Schenning, A. P. H. J.; Broer, D. J., Programmed morphing of liquid crystal networks. Polymer, In Press
1
Chapter 1. Morphing of liquid crystal polymer
networks
1.1 Stimuli-responsive materials
In nature, the well-defined and precisely controlled organization of components on
a molecular, supramolecular and mesoscopic length scale is crucial for the complex
morphing processes found in organisms.1 This hierarchical structuring has been a
source of inspiration for materials scientists,2 and has led to the field of functional
supramolecular polymers.3 In order to mimic the systems found in nature, the
materials must have a hierarchical structure that can be tailored to achieve a target
actuation behavior, without the need to assemble separate mechanical components.
One of the focal points in functional supramolecular polymers is stimuli-responsive
polymer materials, which respond to stimuli from the environment by changing
their properties.4 An important class of these materials consists of those which
reversibly change their macroscopic shape upon exposure to a stimulus.5 In
particular, the ability to program a certain morphology change into the material
itself, which can be accessed at any time by applying the stimulus, will lead to
materials that can find applications ranging from moving elements in microfluidic
systems,6 such as shutters and flow controllers, to medical systems7 and robotics,5a, 8
where precisely controlled, complex movements are desired. Complex deformation
has previously been achieved using various techniques, for example by applying
ion-induced plastic strain to metal films9 or application of a dye for localized light
absorption.10 Material based on hydrogels are particularly popular for the
preparation of morphing materials, and complex deformation has been induced
Chapter 1 Morphing of liquid crystal polymer networks
2
using halftone gel lithography on polymer hydrogels,10 and photopatterning of
composite hydrogel sheets.11 However, such materials are limited to wet conditions.
Another promising group of materials that is a candidate for the development of
stimuli-responsive polymer materials is based on nematic liquid crystals which,
unlike hydrogels, can operate in both dry and wet conditions.
1.2 Liquid crystal networks
Liquid crystals (LCs) are a class of materials that possess a phase that has both
anisotropic and fluid-like properties.12 The material consist of rod-like molecules
with orientational alignment that can be described by the alignment director n and
the order parameter S, where S = 3cos θ 1 , with θn being the angular
deviation of the long molecular axis from n (0 < θn < π/2, brackets indicate an
average, see Figure 1.1a).13 Liquid crystals in the nematic phase typically have a
multidomain microstructure, but the alignment of the molecules can be controlled
using various alignment techniques, such as electric fields, magnetic fields, or
command surfaces, which allows excellent control over the molecular alignment
even at macroscopic length scales.
Figure 1.1 a) Schematic representation of the orientational alignment of rod-like molecules in the nematic liquid
crystalline phase. b) Deformation of a liquid crystal polymer network upon decreasing the order parameter S. As
the average θn increases, contraction (λ < 1) occurs parallel to the alignment director (L// λL//), and expansion
occurs perpendicular to it (L┴λ-νL
┴). c) A liquid crystal polymer film depicted as a coordinate system, with the
xy-plane parallel to the surface of the film and the z-axis from z =0 to z = h, where h is the film thickness. The
liquid crystals are shown in a planar alignment, with the alignment director parallel to the xy-plane.
Chapter 1 Morphing of liquid crystal polymer networks
3
When liquid crystals are equipped with polymerizable end groups, and a
crosslinker is present, they can be polymerized into molecularly well-ordered
polymer networks, commonly referred to as liquid crystal networks. UV initiated
radical polymerization is usually the chosen polymerization technique, which can
be performed on liquid crystals containing (meth)acrylate moieties. The polymer
network retains the molecular alignment of the monomers.12, 14 By combining such
reactive mesogens with alignment techniques, excellent control can be obtained
over the molecular arrangement within the material, though the restrictions
imposed by the alignment techniques often require the liquid crystal polymer
networks to be prepared as thin films. Depending on its glass transition
temperature, the polymer network can either behave as an elastomer or a glass at
room temperature, which have different mechanical properties. However, in both
cases the films undergo a shape deformation upon changing the temperature.15 An
increase in temperature lowers S and therefore increases the average θn. This
manifests itself as a contraction λlc (with λlc < 1) of the length of the polymer
network along the nematic director (L//) and an expansion λlc-ν of the length
perpendicular to the director (L┴), where ν (ν > 0) is the Poisson ratio that relates
the responses of L// and L┴ (Figure 1.1b).16
In the presence of molecules that respond to other stimuli, the order parameter of
the polymer network becomes susceptible to other triggers, and the dimensions
may change, for instance, upon contact with water or chemicals or by being
exposed to light of a specific wavelength. The ability to change shape by an
external trigger has made these materials of interest for their use as actuators. The
most popular liquid crystal actuators are those which contain a photoresponsive
molecular trigger, usually based on azobenzene. This building block will undergo a
trans-to-cis conformational change upon UV irradiation, which causes a reduction
of the order parameter and corresponding deformation similar to heating. This
allows for the preparation of light-responsive films, which are attractive since light,
Chapter 1 Morphing of liquid crystal polymer networks
4
when compared to heat or chemical stimuli, is easier to apply locally and without
contact.17
While the linear contraction and expansion behavior of liquid crystal actuators is
interesting,18 their true potential lies in the generation of bending motion to produce
three-dimensional shapes.19 A common strategy to achieve bending is by using a
bilayer consisting of a responsive liquid crystal network and a non-responsive
polymer layer. Alternatively, bending can be achieved by exposing the material to
a stimulus that is inhomogeneous along the z-axis of the film. For example, when a
UV-responsive material containing a UV absorbing dye is exposed to irradiation on
one side, an intensity gradient will develop through the material. This causes a
stronger deformation at one side, and bending takes place.20 However, it would be
more desirable to have access to single-layer, monolithic materials that undergo
complex, 3-dimensional shape deformations without depending on the
inhomogeneity of the stimulus, as this would greatly increase their versatility and
the number of possible applications. To accomplish this, the morphology change
has to be programmed into the molecular structure of the material itself, so that it
can be accessed at any time by application of the stimulus, without receiving any
information about the morphology from the stimulus. As this thesis will show, this
is possible with liquid crystal networks.
In the case of a uniaxially aligned liquid crystal polymer network, n is uniform
throughout the entire matrix (considering ideal circumstances). When a
homogeneous stimulus is used to actuate such a film, contraction and expansion do
not lead to a bending deformation. However, by applying certain alignment
techniques, films having a director variation in one or more directions can be
prepared and more complex deformations can be achieved. This allows the
development of programmable, complex shape deformation. The variation in
director is often referred to as the director profile. To describe the director profile, a
film can be pictured as a coordinate system, in which the z-axis run from z = 0 to
z = h, where h is the thickness of the film, and the xy-plane is positioned parallel to
Chapter 1 Morphing of liquid crystal polymer networks
5
the surface of the film (Figure 1.1c). Paragraphs 1.3 - 1.5 will give an overview of
previously reported director profiles and their corresponding deformations, both
theoretically and experimentally. Initially, profiles that only vary in the z-direction,
but are homogeneous in the xy-plane, are shown. Next, profiles are discussed that
are patterned in the xy-plane but do not vary in the z-direction. Finally, a director
profile that combines both characteristics is presented.
Chapter 1 Morphing of liquid crystal polymer networks
6
1.3 Alignment director variation through the thickness of the polymer film
1.3.1 Twisted and splayed alignment profiles: bending
A straightforward way to obtain bending deformations in liquid crystal networks is
by utilizing the twisted and splayed nematic alignment profiles (Figure 1.2). When
a twisted or splayed alignment is present, there is a 90 degrees change of the
director along the z-axis. This causes the expansion and contraction in the xy-plane
at z = 0 to be different compared to those in the plane at z = h, and this mismatch in
deformation causes bending. Therefore, such films are suitable for the preparation
of actuators that behave as bending cantilevers. The actuators are somewhat similar
in behavior to bilayer systems, except for the fact that instead of a sharp boundary
Figure 1.2 a) Molecular buildup and deformation behavior of a liquid crystal polymer film
with twisted nematic alignment upon decreasing the order parameter S. Initially, anticlastic
deformation leads to a saddle shape, but at higher strains the curvature along one of the
axis is suppressed, and the film bends in only one direction. b) Deformation of a film with
splayed nematic alignment upon decreasing the order parameter S. This film always bends
in only one direction, due to the deformation along one axis being cooperative between the
two faces of the film.
Chapter 1 Morphing of liquid crystal polymer networks
7
between the layers, there is a gradual transition between the two sides of the film.
For twisted films, at z = 0 contraction along the x-axis and expansion along the
y-axis takes place, while the opposite happens at z = h. This leads to an anticlastic
(saddle-shaped) deformation (Figure 1.2a).21 However, such curvature leads to
stretches due to the bending in the x-direction working against the bending in the
y-direction and vice versa. When these stretches become sufficiently large,
suppression of one of the principal curvatures becomes more favorable, which
leads to cantilever-like bending behavior. The strains at which suppression takes
place depend on the sample dimensions, but for the dimensions of a typical
experimental sample, a saddle shape is rarely observed. The direction in which
bending is suppressed is the direction with the lowest curvature, since that
curvature is easier to undo. Therefore, a long ribbon bends along its long axis while
suppressing the bending along the short axis.22 In addition, some out-of-plane
twisting deformation can be expected, due to the chirality of the twisted nematic
alignment,23 and this has also been observed in practice.24 These predictions do not,
however, take into account any outside influences. For example, in clamped
samples often formation of kinks can occur near the clamp, and such effects can
make the deformation of cantilevers with this alignment profile unpredictable and
hard to control.24
A better controlled bending deformation can be achieved in films with splayed
alignment.24 In these films, bending will only be in one direction, and therefore
there is no anticlastic deformation (Figure 1.2b). As the homeotropic side expands
in both the x and y directions, the bending direction of the cantilever is solely
determined by the alignment director on the planar side. It always bends in the
same direction as the alignment director on the planar side, independent of sample
dimensions. As there is no energy cost for anticlastic suppression to achieve
cantilever-like deformation, it is easier for these actuators to bend. The bending is
also less effected by clamping, if the clamp is placed at the edge of the film that
doesn’t bend, which allows better controlled deformations.
Chapter 1 Morphing of liquid crystal polymer networks
8
Twisted and splayed actuators may find applications in devices. For example,
small, inkjet-printed, light responsive actuators with splayed alignment were
prepared that mimic the movements of cilia found in microorganisms.25 A liquid
crystal mixture was inkjet-printed from dichloroethane. The splay alignment was
induced using one rubbed polyimide layer that induced a homeotropic alignment
with an 80 degrees pretilt angle on one side of the film, and a surfactant was added
to the liquid crystal mixture to induce planar alignment at the LC-air interface on
the other side (Figure 1.3a and 1.3b). This resulted in splayed alignment with a
uniform director on the planar side due to the pretilt angle. In a special case the
structures consisted of two domains: One domain contained the azobenzene
derivative A3MA, which undergoes a trans-to-cis isomerization when irradiated at
= 358 nm, while the other domain contained the azobenzene derivative DR1A,
which undergoes this transition at = 490 nm (Figure 1.3b). This allowed
independent actuation of the domains with light of different wavelengths, to induce
Figure 1.3 Light-responsive artificial cilia based on liquid crystal networks with splay
alignment. a) Schematic representation of the alignment, and SEM picture of the cross-section
of the actual film. b) Liquid crystals used to prepare the actuators. c) Schematic representation
of the artificial cilia. The red areas contain DR1A and respond to visible light, while the
yellow areas contain A3MA and respond to UV light. d) Pictures of the printed cilia in action.
Copyright Nature Publishing Group.24
Chapter 1 Morphing of liquid crystal polymer networks
9
asymmetric motion with a different forward and backward stroke, which should be
capable of creating flow in liquids (Figure 1.3c and 1.3d). Such flow generation
could be useful in microfluidic systems.25
By using hydrogen-bridged liquid crystal networks (nOBA), twisted and splayed
actuators were produced that respond to water vapor (Figure 1.4a). After
polymerization, water is not yet capable of penetrating the network, but when the
film is treated with a basic solution (usually KOH) the internal hydrogen bonds are
broken and a hygroscopic salt is formed. When water is absorbed the network
swells, the order parameter is reduced, and the anisotropic deformation results in
stronger expansion perpendicular to the alignment director. In this way, actuators
Figure 1.4 Liquid crystal films based on twisted and splayed alignment that
deform upon water swelling. a) Composition of the reactive liquid crystals used to
make the network. b) Base-treated films that deform based on humidity, at high
(85%) and low (15%) relative humidity. The films were treated with KOH for
various periods of time (0-20 seconds) prior to the experiment. Copyright Institute
of Electrical and Electronics Engineers.25b c) The base-treated liquid crystal film
shows different bending behavior in different solvents, in this case water and
acetone. Copyright Wiley-VCH25a d) The activated film de-swells when the pH is
lowered. Copyright Institute of Electrical and Electronics Engineers.25b
Chapter 1 Morphing of liquid crystal polymer networks
10
that respond to humidity have been prepared (Figure 1.4b). As different solvents
have different swelling capability, the polymer films also respond differently to
solvent polarity upon immersion (Figure 1.4c). The activation with basic solution
will be reversed in an acidic solution, and this means that the actuator will also
respond to solution pH (Figure 1.4d). All these films were pre-bent after
polymerization due to intrinsic stresses, which explains why they were not flat in
the non-swollen state.26
1.3.2 Twisted and splayed alignment profiles: coils and helicoids
Coil-shaped and helicoidal deformation can be achieved by cutting ribbons from a
90 degrees twisted nematic network in such a way that the angle θc between the
alignment director at z = 0 and the long axis of the ribbon is ±45 degrees. Like
twisted nematic actuators that bend, the coiling deformation is caused by a
mismatch in contraction and expansion in the xy-plane at z = 0 and at z = h. The
sign of the value of θc determines the handedness of the resulting coil; when the
angle is positive (clockwise from the long axis), the coil will be right-handed, and
when it is negative (counterclockwise from the long axis), the coil will be left-
handed (Figure 1.5). There is also a small influence of the twist handedness on the
formation of the coils,23 which will be discussed below.
Figure 1.5 Coil formation in ribbons made from twisted nematic films (helicoids
not shown). a) When the angle θc between the long axis of the ribbon and the
nematic director at z = 0 is positive, a right-handed coil is formed. b) When θc is
negative, a left-handed coil is formed.
Chapter 1 Morphing of liquid crystal polymer networks
11
It was shown, both experimentally and through finite-elements simulations, that
cutting ribbons with θc = ±45 degrees can lead to the formation of both helicoids
and coils. In this research, a mixture of a polymerizable, achiral mesogen
(A-6OCB), a nonreactive LC (6OCB), a nonreactive left-handed chiral dopant
(S-811), plus 7% of crosslinker (HDDA), was polymerized between rubbed
polyimide alignment layers with orthogonal alignment directors to create a polymer
network with a 90 degrees left-handed twist (Figure 1.6a). Upon removal of the
nonreactive components, anisotropic shrinkage caused the deformation into a chiral
shape. Ribbons were cut from this film with θc being 45 degrees or -45 degrees,
which were called S- and L-geometry, respectively. The length and thickness of the
Figure 1.6 a) Liquid crystal mixture used to make coil and helicoid ribbons. A-6OCB is a reactive mesogen,
6OCB is a nonreactive mesogen, HDDA is a crosslinker, S-811 is a nonreactive chiral dopant. b) Helicoids
formed by narrow ribbons (thickness 35.2 μm, width 0.23 mm) with a negative θc (L-geometry). c) Helicoids
formed by narrow ribbons (thickness 35.2 μm, width 0.22 mm) with a positive θc (S-geometry). d) Coils
formed by wide ribbons (thickness 35.2 μm, width 0.76 mm) with a negative θc (L-geometry). e) Coils formed
by wide ribbons (thickness 35.2 μm, width 0.83 mm) with a positive θc (S-geometry). f) Plot of the inverse
twist pitch (1/PT) of the helicoids as a function of temperature, T/TNI, where T is current temperature and TNI is
the nematic to isotropic transition point. A left-handed twist pitch is defined as positive and a right handed
twist pitch as negative. The lines indicate theoretical predictions. g) Plot of the inverse pitch (1/PH) and
inverse diameter (1/d) of the coils as a function of temperature. A left-handed twist pitch is defined as positive
and a right handed twist pitch as negative. The open symbols represent the pitch and the filled symbols
represent the diameter. The lines indicate theoretical predictions. Copyright National Academy of Sciences.26a
Chapter 1 Morphing of liquid crystal polymer networks
12
ribbons were kept constant while the width was varied. It was found that narrow
ribbons with a small width/thickness ratio twist around a straight central line to
form helicoids with no unidirectional bending (Figure 1.6b and 6c), while wide
films with a large width/thickness ratio will bend to form curls with strong bending
(Figure 1.6d and 1.6e). The dependence of the shape on the sample dimensions is
caused by the competition between bending energy cost and in-plane elastic energy
cost: Narrow films are easier to be stretched than to be bent, which leads to
helicoid formation, while wide films are easier to be bent than to be stretched, and
form coils. This is similar to the previously shown twisted nematic films, which
either form a saddle shape or a unidirectional bend, depending on whether
suppression of curvature takes place or not. It was found that the shapes uncurl
upon raising the temperature, go through a flat state (at temperature Tflat), and then
curl into the opposite handedness (Figure 1.6b-1.6e). The pitch was plotted as a
function of temperature, and was shown to fit well with the theoretical predictions
of the shape deformations (Figure 1.6f and 1.6g). It was also predicted that at
temperatures near Tflat, the wide ribbons should morph into a helicoid shape, but
this could not be verified in the experiments.27
The deformation of the same ribbons, in which θc was of a different value than 45
or -45 degrees, which was termed X-geometry, was also studied. Only the narrow
ribbons with small width/thickness ratios were considered in this study.
Interestingly, it was found that when θc was not ±45 degrees, the formation of
helicoids was not possible and only coils were observed. In other words, only S-
and L- geometry could produce helicoids. Simulations showed that for ribbons with
a small θc (between 3 degrees and 9 degrees), small variations in θc strongly
influenced the pitch, but did not influence the diameter of the coil. It was also
shown that upon gradually increasing θc from 5 degrees to 45 degrees, the shape
gradually changes from a coil into a helicoid (Figure 1.7a). Finally, simulations
showed that at θc = 0, the ribbon still has a non-zero pitch (Figure 1.7b). This is in
agreement with the earlier observation that such ribbons tend to twist out of plane
instead of showing unidirectional bending, and the effect is indeed caused by the
Chapter 1 Morphing of liquid crystal polymer networks
13
chirality of the director twist. However, the effect is very weak and can be
cancelled out by having θc deviate a few degrees from 0. In this case, achiral
bending can be obtained.23
UV-driven coiling actuators have also been prepared by adding azobenzene-based
mesogens to the liquid crystal networks. Ribbons cut at a 45 degrees angle from a
densely crosslinked network with twisted nematic alignment displayed UV-driven
deformation (Figure 1.8a).28 In a different research, curling deformations of UV
responsive, azobenzene-containing films with twisted and splayed alignment were
compared. However, the UV light that was used as the stimulus was polarized, and
the irradiation was carried out on one side of the film, which created an intensity
gradient over the thickness of the film. Therefore, the stimulus was not
homogeneous.29 UV-responsive curling actuators with a right handed domain, a left
handed domain, and a straight domain have been prepared by adjusting the cutting
method, and have been shown to be able to do work by moving the uncurled part
up and down upon switching between UV and visible light (Figure 1.8b).30
Figure 1.7 Simulations of the effect of the angle θc on the shape of narrow ribbons at
T = TNI, where TNI is the nematic to isotropic temperature. a) A series of ribbons with
increasing θc. At low values, a coiling ribbon is observed, which slowly transitions into
a helicoid upon approaching θc = 45 degrees. b) Chirality reversal of the ribbons at
values of θc around zero. At θc = 0, the ribbon still has a chiral shape. The open ring
shape lies around θc = -2.5 degrees. Copyright American Physical Society.22
Chapter 1 Morphing of liquid crystal polymer networks
14
Figure 1.8 a) Light-responsive coiling actuator based on a densely crosslinked
network. Copyright Royal Society of Chemistry.27 b) Coiling actuator with a
left-handed domain, a straight domain, and a right-handed domain. Upon irradiation
with UV or visible light, one of the coiling domains contracts and the other expands.
The liquid crystal mixture used is shown in Figure 6a. Copyright Nature Publishing
Group.29
Chapter 1 Morphing of liquid crystal polymer networks
15
1.4 Director profiling in the xy-plane of the film
1.4.1 Continuous circular patterns
Liquid crystal films that deform into 3-dimensional shapes can also be prepared
using continuous, circular director profiles. These profiles are based on
disclinations, which are the topological defects that occur naturally in planar
aligned nematic liquid crystals without a preferred orientation, and can be observed
as the centers of the brushes in a Schlieren texture when the material is analyzed
using polarized optical microscopy.31 They are described as singularities around
which the alignment director varies continuously. The strength of the disclination
m is defined as the number of times the director rotates when the singularity is
circumnavigated once,32 with the sign depending on whether the rotation is in the
same direction as the circumnavigation (+) or in the opposite direction (-). For
example an azimuthal (concentric circles) or radial (spokes) pattern would be
described as an m = +1 pattern, as the alignment director makes one full rotation
when the disclination is circumnavigated once (Figure 1.9a-c).
There have been numerous theoretical predictions on these systems. Consider, for
instance, a disk-shaped piece of liquid crystal polymer film with an azimuthal
director profile. As the alignment director throughout this film is always parallel to
Figure 1.9 Examples of disclinations, and the
corresponding picture between crossed polarizers. a)
m = +1 azimuthal disclination. b) m = +1 radial
disclination. c) m = +1/2 disclination.
Chapter 1 Morphing of liquid crystal polymer networks
16
the circumference C, this means C = L// = 2πr. Likewise, the radius r is always
perpendicular to the alignment director, so r = L┴. When this film is heated, it is
expected to adopt a new circumference C’ = λL// = λC and a new radius
r’ = L┴ = λ-νr due to anisotropic deformations. Because 0 < λ < 1 and ν > 0, we
know that C’ < 2πr’, which means that the circumference has become too small to
accommodate the radius but is still a circle. This can only be solved by a
deformation out of the plane of the film to form a cone shape. The circumference
of the base of this cone equals C’, and the radius of the base equals r’sinφ, where φ
is the cone opening angle (Figure 1.10). In the case of a disk with radial alignment,
C’ = L┴ = λ-νC and r’ = L// = λr, so C’ > 2πr’ which means that the circumference is
too large for the radius. This can be solved by buckling of the circumference into a
sinusoidal wave, which is positioned on a sphere with radius r’. This will generate
an “anticone” shape (Figure 1.10). At higher strains, the number of buckles can
increase, leading to a more crumpled anticone.16d
Figure 1.10 Shape deformation of films with an
m = +1 disclination pattern upon heating. A film
with an azimuthal pattern will contract along the
radius and expand along the circumference to
form a cone with a base radius r’sinφ, where φ is
the cone opening angle. A film with a radial
pattern will form an anticone shape with a
buckled circumference. Copyright Royal
Society.32
Chapter 1 Morphing of liquid crystal polymer networks
17
1.4.2 Discrete patterns
Discrete patterns can also be used to obtain complex shape deformation in liquid
crystal polymer films. Such patterns consist of multiple domains, with varying
alignment directors between the domains, which are joined by discontinuous
boundaries. To avoid incompatibility between the domains, the deformations on
both sides of the boundary have to match, which can be accomplished by having
the angle between the boundary and the alignment director be the same on both
sides. This is referred to as rank-1 connectedness. Keeping this rule in mind, it is
possible to design a two-dimensional director profile using a number of building
blocks, and predict the change in shape upon actuation. It has been shown that this
kind of blueprinting can be accomplished using a “vocabulary” consisting of six
different types of triangular building blocks.33 The first two types are triangles
which are an angular section from an m = +1 disclination (azimuthal or radial), the
second two types have the alignment director running parallel or perpendicular to
one of the edges, and the third two types have a rank-1 connected border bisecting
the triangle (Figure 1.11a). Upon actuation, the base angle θw of the wedge will
change. When multiple wedges are joined in such a way that the base angles form a
closed circle, the change in θw will create either an angular surplus or deficiency,
and shape deformation in the third dimension will occur (Figure 1.11b). This is
Figure 1.11 a) The six different triangular building blocks from which morphing films with discrete
alignment profiles can be constructed. Copyright SPIE.33b b) Four wedges are joined together to form a
pattern similar to the azimuthal pattern shown earlier. When smoothening is not taken into account,
this film is expected to deform into a pyramidal shape, but smoothening will cause the film to
resemble a cone instead. Copyright American Physical Society.33a
Chapter 1 Morphing of liquid crystal polymer networks
18
similar to the deformation of the films with a circular pattern as described in the
previous paragraph. The sharp boundaries are not expected to induce sharp folds,
as the curvature will be delocalized over the surface to reduce bend energy,
smoothening the surface. However, if more disclinations are present, this
smoothening will cause incompatibilities and will be suppressed to some extent.33
Several blueprint designs have been proposed. For example, two domains with
perpendicular alignment directors, which would normally be incompatible, could
be joined by a zigzag line of alternating points of positive and negative
disclinations. At low strains, such a film could deform by having one domain
crumple to accommodate to the deformation of the other domain. However, at
higher strains it is more likely that the buckling gets divided over the entire surface.
A likely solution is a shape with faceted cylindrical shells joined by an expanding
midsection (Figure 1.12a).33b A different example is a film with a pyramidal pattern
(as in Figure 1.11b), but with a single ±1/2 disclination pair located halfway on one
of the rank-1 connected boundaries. This film is expected to deform into a pyramid
with a spiraling, crumpled moat around it (figure 1.12b).33a
Figure 1.12 Deformation of films with discrete alignment patterns featuring discontinuous
boundaries. a) Two domains with orthogonal alignment directors are joined by a boundary with
disclinations of positive (blue) and negative (red) strength. The actuated shape is expected to
have a crumpled and a flat domain at low strains, and to resemble two faceted cylindrical shells
joined by an expanding midsection at high strains. Copyright SPIE.33b b) A pattern with an
m = +1 disclination at the center and an m = ±1/2 disclination halfway one of the boundaries.
This pattern is expected to result in a pyramid with a spiraling moat. Copyright American
Physical Society.33a
Chapter 1 Morphing of liquid crystal polymer networks
19
All the profiles with patterns in the xy-plane presented here, as well as their
deformations, are based on theoretical predictions. So far, besides the work
presented in this thesis, only one publication has been published that shows the
deformation behavior of liquid crystal polymer films with disclination-based
alignment profiles.34
1.5 Director patterning in 3 dimensions
The previous paragraphs described director patterns that vary either along the
z-direction or in the xy-direction of the film. To make a film which is patterned in
all three directions, these two types of alignment can be combined. Recently,
shape-memory materials with alternating bend and straight domains were prepared
by combining a substrate with a uniaxial alignment layer with a photoaligned
alignment layer with domains with orthogonal alignment directions. This way a
cell with alternating twisted nematic and uniaxial domains was created. Upon
heating, in a ribbon with this director profile the twisted nematic domains showed
bending, while the uniaxial domains remained flat (Figure 1.13).35
Figure 1.13 Ribbons with patterned twisted alignment. A ribbon shape-memory prepared
using a uniaxial planar alignment layer and a patterned alignment layer with orthogonal
domains. The film deforms upon heating by bending of the two outward domains while the
center part remains flat.35
Chapter 1 Morphing of liquid crystal polymer networks
20
1.6 Aim and outline of the thesis
As shown in this chapter, theoretical predictions have revealed that liquid crystal
polymer films with complex alignment profiles can lead to a multitude of exotic
shape deformations upon application of the appropriate stimulus. But despite these
exciting possibilities, these systems have only been sparsely explored in practice.
The only alignment profiles that have been extensively studied are those with a
varying director along the z-direction of the film: the twisted and splayed
alignment.
The aim of this thesis is to explore new ways to prepare liquid crystal polymer
films that reversibly deform into exotic shapes upon exposure to a stimulus, by
programming the molecular organization and orientation in the material. In
chapter 2, a simple method is presented that allows the preparation of a variety of
humidity-responsive actuators with an asymmetry in the molecular trigger by
selectively treating a uniaxially aligned liquid crystal polymer film with a
potassium hydroxide solution to alter the responsiveness to humidity and
selectively create the required molecular trigger. Chapters 3 and 4 discuss the
application of photoalignment to prepare liquid crystal polymer films with complex
alignment profiles in the xy-plane of the film, based on disclinations, which
deformed upon heating. Films were prepared with an azimuthal alignment profile
that deformed into cones, while a radial alignment profile resulted in an anticone
shape. A different alignment pattern, containing slits cut at pre-designed
disclination lines, deformed by opening the slits, resulting in remotely addressable
apertures. Chapter 4 deals with films with alignment director profiles that vary in
all three dimensions. These films possess either a discrete or continuous pattern in
the xy-plane, while also having a 90 degrees twisted nematic alignment. This lead
to films that contracted in a way similar to an accordion, and films that showed
polygonal folding. In chapter 5, it is shown that a patterned liquid crystal polymer
network can be used to steer the direction of surface wrinkles which are formed
when a thin layer of gold is sputter-coated on top of the film, followed by heating.
Chapter 1 Morphing of liquid crystal polymer networks
21
In the last chapter the technology assessment of soft actuators based on liquid
crystal networks is presented, which shows the potential applications of the
materials in soft robotics, switchable sieves, and smart optics, as well as the
challenges that still have to be overcome.
1.7 References
1. Harrington, M. J.; Razghandi, K.; Ditsch, F.; Guiducci, L.; Rueggeberg,
M.; Dunlop, J. W. C.; Fratzl, P.; Neinhuis, C.; Burgert, I., Origami-like unfolding
of hydro-actuated ice plant seed capsules. Nat. Commun. 2011, 2, 337.
2. (a) Forterre, Y.; Skotheim, J. M.; Dumais, J.; Mahadevan, L., How the
Venus flytrap snaps. Nature 2005, 433 (7024), 421-425; (b) Mahadevan, L.,
Polymer science and biology: structure and dynamics at multiple scales. Faraday
Discuss. 2008, 139, 9-19; (c) Meyers, M. A.; Chen, P.-Y.; Lopez, M. I.; Seki, Y.;
Lin, A. Y. M., Biological materials: A materials science approach. J. Mech. Behav.
Biomed. 2011, 4 (5), 626-657; (d) Zhang, S., Fabrication of novel biomaterials
through molecular self-assembly. Nat. Biotechnol. 2003, 21 (10), 1171-1178.
3. Aida, T.; Meijer, E. W.; Stupp, S. I., Functional supramolecular polymers.
Science 2012, 335 (6070), 813-817.
4. (a) Bashari, A.; Nejad, N. H.; Pourjavadi, A., Applications of stimuli
responsive hydrogels: a textile engineering approach. J. Text. I. 2013, 104 (11),
1145-1155; (b) Lee, J. H.; Koh, C. Y.; Singer, J. P.; Jeon, S. J.; Maldovan, M.;
Stein, O.; Thomas, E. L., 25th Anniversary article: Ordered polymer structures for
the engineering of photons and phonons. Adv. Mater. 2014, 26 (4), 532-568; (c)
Liu, K.; Kang, Y. T.; Wang, Z. Q.; Zhang, X., 25th Anniversary article: reversible
and adaptive functional supramolecular materials: "Noncovalent interaction"
matters. Adv. Mater. 2013, 25 (39), 5530-5548; (d) Qing, G. Y.; Li, M. M.; Deng,
L. J.; Lv, Z. Y.; Ding, P.; Sun, T. L., Smart drug release systems based on stimuli-
responsive polymers. Mini-Rev. Med. Chem. 2013, 13 (9), 1369-1380; (e)
Schattling, P.; Jochum, F. D.; Theato, P., Multi-stimuli responsive polymers - the
all-in-one talents. Polym. Chem. 2014, 5 (1), 25-36.
Chapter 1 Morphing of liquid crystal polymer networks
22
5. (a) Bauer, S.; Bauer-Gogonea, S.; Graz, I.; Kaltenbrunner, M.; Keplinger,
C.; Schwodiauer, R., 25th Anniversary article: A soft future: From robots and
sensor skin to energy harvesters. Adv. Mater. 2014, 26 (1), 149-162; (b) Kim, T.;
Zhu, L. Y.; Al-Kaysi, R. O.; Bardeen, C. J., Organic photomechanical materials.
ChemPhysChem 2014, 15 (3), 400-414; (c) Liu, Y. J.; Du, H. Y.; Liu, L. W.; Leng,
J. S., Shape memory polymers and their composites in aerospace applications: a
review. Smart Mater. Struct. 2014, 23 (2), 22.
6. (a) Harrison, C.; Stafford, C. M.; Zhang, W.; Karim, A., Sinusoidal phase
grating created by a tunably buckled surface. Appl. Phys. Lett. 2004, 85 (18),
4016-4018; (b) Ziolkowski, B.; Czugala, M.; Diamond, D., Integrating stimulus
responsive materials and microfluidics: The key to next-generation chemical
sensors. J. Intel. Mat. Syst. Str. 2013, 24 (18), 2221-2238.
7. Woltman, S. J.; Jay, G. D.; Crawford, G. P., Liquid-crystal materials find a
new order in biomedical applications. Nat. Mater. 2007, 6 (12), 929-938.
8. Murata, S.; Kurokawa, H., Self-reconfigurable robots. Robot. Autom. Mag.
2007, 14 (1), 71-78.
9. Chalapat, K.; Chekurov, N.; Jiang, H.; Li, J.; Parviz, B.; Paraoanu, G. S.,
Self-organized origami structures via ion-induced plastic strain. Adv. Mater. 2013,
25 (1), 91-95.
10. Kim, J.; Hanna, J. A.; Byun, M.; Santangelo, C. D.; Hayward, R. C.,
Designing responsive buckled surfaces by halftone gel lithography. Science 2012,
335 (6073), 1201-1205.
11. Thérien-Aubin, H.; Wu, Z. L.; Nie, Z.; Kumacheva, E., Multiple shape
transformations of composite hydrogel sheets. J. Am. Chem. Soc. 2013, 135 (12),
4834-4839.
12. Broer, D. J.; Crawford, G. P.; Zumer, S., Cross-linked Liquid Crystalline
Systems. CRC, Taylor and Francis Group, Boca Raton, FL: 2011.
13. Jakli, A.; Saupe, A., Phase transitions in one- and two-dimensional fluids:
properties of smectic, lamellar and columnar liquid crystals (condensed matter
physics), Taylor & Francis: BocaRaton, Florida, USA: 2006; pp 85-102.
Chapter 1 Morphing of liquid crystal polymer networks
23
14. Broer, D. J.; Boven, J.; Mol, G. N.; Challa, G., In-situ photopolymerization
of oriented liquid-crystalline acrylates, 3. Oriented polymer networks from a
mesogenic diacrylate. Makromolekul. Chem. 1989, 190 (9), 2255-2268.
15. (a) M. Warner; Terentjev, E. M., Liquid crystal elastomers. Oxford
University Press, Oxford: 2003; (b) Ohm, C.; Brehmer, M.; Zentel, R., Liquid
Crystalline Elastomers as Actuators and Sensors. Adv. Mater. 2010, 22 (31), 3366-
3387.
16. (a) Van Oosten, C. L., Responsive liquid crystal networks. Eindhoven
University of Technology, 2009, thesis; (b) Hikmet, R. A. M.; Broer, D. J.,
Dynamic mechanical properties of anisotropic networks formed by liquid
crystalline acrylates. Polymer 1991, 32 (9), 1627-1632; (c) Broer, D. J.; Mol, G.
N., Anisotropic thermal expansion of densely cross-linked oriented polymer
networks. Polym. Eng. Sci. 1991, 31 (9), 625-631; (d) Modes, C. D.; Bhattacharya,
K.; Warner, M., Gaussian curvature from flat elastica sheets. P. Roy. Soc. A-Math.
Phy. 2011, 467 (2128), 1121-1140.
17. (a) Ikeda, T.; Mamiya, J.-i.; Yu, Y., Photomechanics of liquid-crystalline
elastomers and other polymers. Angew. Chem. Int. Edit. 2007, 46 (4), 506-528; (b)
Serak, S.; Tabiryan, N.; Vergara, R.; White, T. J.; Vaia, R. A.; Bunning, T. J.,
Liquid crystalline polymer cantilever oscillators fueled by light. Soft Matter 2010,
6 (4), 779-783; (c) Shankar, M. R.; Smith, M. L.; Tondiglia, V. P.; Lee, K. M.;
McConney, M. E.; Wang, D. H.; Tan, L.-S.; White, T. J., Contactless,
photoinitiated snap-through in azobenzene-functionalized polymers. P. Natl. Acad.
Sci. USA 2013; (d) Yamada, M.; Kondo, M.; Mamiya, J.-i.; Yu, Y.; Kinoshita, M.;
Barrett, C. J.; Ikeda, T., Photomobile polymer materials: towards light-driven
plastic motors. Angew. Chem. Int. Edit. 2008, 47 (27), 4986-4988; (e) Yamada,
M.; Kondo, M.; Miyasato, R.; Naka, Y.; Mamiya, J.-i.; Kinoshita, M.; Shishido, A.;
Yu, Y.; Barrett, C. J.; Ikeda, T., Photomobile polymer materials-various three-
dimensional movements. J. Mater. Chem. 2009, 19 (1), 60-62; (f) Yu, H.; Ikeda, T.,
photocontrollable liquid-crystalline actuators. Adv. Mater. 2011, 23 (19), 2149-
2180; (g) Yu, Y.; Nakano, M.; Ikeda, T., Photomechanics: directed bending of a
polymer film by light. Nature 2003, 425 (6954), 145-145.
Chapter 1 Morphing of liquid crystal polymer networks
24
18. Li, M. H.; Keller, P.; Yang, J.; Albouy, P. A., An artificial muscle with
lamellar structure based on a nematic triblock copolymer. Adv. Mater. 2004, 16
(21), 1922-1925.
19. Sánchez-Ferrer, A.; Fischl, T.; Stubenrauch, M.; Albrecht, A.; Wurmus,
H.; Hoffmann, M.; Finkelmann, H., Liquid-crystalline elastomer microvalve for
microfluidics. Adv. Mater. 2011, 23 (39), 4526-4530.
20. (a) Jin, L.; Jiang, X.; Huo, Y., Light-induced nonhomogeneity and gradient
bending in photochromic liquid crystal elastomers. Sci. China Ser. G 2006, 49 (5),
553-563; (b) Jin, L.; Yan, Y.; Huo, Y. In Light-induced bending and smart control
of photochromic liquid crystal elastomers, 2007; pp 64230W-64230W-8; (c)
Warner, M.; Mahadevan, L., Photoinduced deformations of beams, plates, and
films. Phys. Rev. Lett. 2004, 92 (13), 134302.
21. Warner, M.; Modes, C. D.; Corbett, D., Curvature in nematic elastica
responding to light and heat. P. Roy. Soc. A-Math. Phy. 2010, 466 (2122), 2975-
2989.
22. Warner, M.; Modes, C. D.; Corbett, D., Suppression of curvature in
nematic elastica. P. Roy. Soc. A-Math. Phy. 2010, 466 (2124), 3561-3578.
23. Sawa, Y.; Urayama, K.; Takigawa, T.; Gimenez-Pinto, V.; Mbanga, B. L.;
Ye, F. F.; Selinger, J. V.; Selinger, R. L. B., Shape and chirality transitions in off-
axis twist nematic elastomer ribbons. Phys. Rev. E 2013, 88 (2), 022502.
24. Mol, G. N.; Harris, K. D.; Bastiaansen, C. W. M.; Broer, D. J., Thermo-
mechanical responses of liquid-crystal networks with a splayed molecular
organization. Adv. Funct. Mater. 2005, 15 (7), 1155-1159.
25. van Oosten, C. L.; Bastiaansen, C. W. M.; Broer, D. J., Printed artificial
cilia from liquid-crystal network actuators modularly driven by light. Nat. Mater.
2009, 8 (8), 677-682.
26. (a) Harris, K. D.; Bastiaansen, C. W. M.; Broer, D. J., A glassy bending-
mode polymeric actuator which deforms in response to solvent polarity. Macromol.
Rapid Comm. 2006, 27 (16), 1323-1329; (b) Harris, K. D.; Bastiaansen, C. W. M.;
Broer, D. J., Physical properties of anisotropically swelling hydrogen-bonded
liquid crystal polymer actuators. J. Microelectromech. S. 2007, 16 (2), 480-488; (c)
Chapter 1 Morphing of liquid crystal polymer networks
25
Harris, K. D.; Bastiaansen, C. W. M.; Lub, J.; Broer, D. J., Self-assembled polymer
films for controlled agent-driven motion. Nano Lett. 2005, 5 (9), 1857-1860.
27. (a) Sawa, Y.; Ye, F. F.; Urayama, K.; Takigawa, T.; Gimenez-Pinto, V.;
Selinger, R. L. B.; Selinger, J. V., Shape selection of twist-nematic-elastomer
ribbons. P. Natl. Acad. Sci. USA 2011, 108 (16), 6364-6368; (b) Teresi, L.;
Varano, V., Modeling helicoid to spiral-ribbon transitions of twist-nematic
elastomers. Soft Matter 2013, 9 (11), 3081-3088.
28. Harris, K. D.; Cuypers, R.; Scheibe, P.; van Oosten, C. L.; Bastiaansen, C.
W. M.; Lub, J.; Broer, D. J., Large amplitude light-induced motion in high elastic
modulus polymer actuators. J. Mater. Chem. 2005, 15 (47), 5043-5048.
29. Wie, J. J.; Lee, K. M.; Smith, M. L.; Vaia, R. A.; White, T. J., Torsional
mechanical responses in azobenzene functionalized liquid crystalline polymer
networks. Soft Matter 2013, 9 (39), 9303-9310.
30. Iamsaard, S.; Aßhoff, S. J.; Matt, B.; Kudernac, T.; CornelissenJeroen, J.
L. M.; Fletcher, S. P.; Katsonis, N., Conversion of light into macroscopic helical
motion. Nat. Chem. 2014, 6 (3), 229-235
31. Dierking, I., Textures of Liquid Crystals. Wiley-VCH: Weinheim, 2003.
32. Trebin, H. R., The topology of non-uniform media in condensed matter
physics. Adv. Phys. 1982, 31 (3), 195-254.
33. (a) Modes, C. D.; Warner, M., Blueprinting nematic glass: Systematically
constructing and combining active points of curvature for emergent morphology.
Phys. Rev. E 2011, 84 (2), 8; (b) Modes, C. D.; Warner, M., The activated
morphology of grain boundaries in nematic solid sheets. P. Soc. Photo-Opt. Ins.
2012, 8279, 82790Q.
34. McConney, M. E.; Martinez, A.; Tondiglia, V. P.; Lee, K. M.; Langley, D.;
Smalyukh, II; White, T. J., Topography from topology: Photoinduced surface
features generated in liquid crystal polymer networks. Adv. Mater. 2013, 25 (41),
5880-5885.
35. Lee, K. M.; Bunning, T. J.; White, T. J., Autonomous, hands-free shape
memory in glassy, liquid crystalline polymer networks. Adv. Mater. 2012, 24 (21),
2839-2843.
This chapter was based on an article: de Haan, L. T.; Verjans, J. M. N.; Broer, D. J.; Bastiaansen, C. M. W.; Schenning, A. P. H. J., Humidity-responsive liquid crystalline polymer actuators with an asymmetry in the molecular trigger that bend, fold, and curl. J. Am. Chem. Soc. 2014, 136 (30), 10585-10588
27
Chapter 2. Programmed deformation of uniaxially
aligned actuators with asymmetry in the molecular
trigger
2.1 Introduction
In nature, various examples exist of systems that show shape deformation as a
response to changes in humidity. For example, in plants bending deformations in
the opening and closing of pine cones,1 folding and unfolding of ice plant seed
capsules,2 opening of the morning glory flower,3 and helical curling deformation of
Bauhinia Variegate seed pods,4 take place as a result of changes in humidity. Such
responsive bending, folding and curling events in these hierarchically ordered
materials are important for the functioning of natural systems. In materials science
there is an ongoing interest in and demand for fabricating environmentally-
responsive polymer materials that behave similarly to the systems found in nature.5
While light and temperature responsive materials have often been reported, the
fabrication of humidity responsive materials remains less explored. Humidity-
responsive actuators are interesting for many applications, such as power
generators,6 smart textiles,7 and artificial muscles.8 Bending and curling can be
mimicked using bilayers of materials with different swelling behavior, 3-4, 7-9 or by
subjecting only one side of a single-layer film to high humidity.10 However, from a
manufacturing point of view, it would be desirable to have a facile method to
fabricate soft actuators from a single piece of polymer sheet that can be tailored to
achieve bending, folding or curling.6
Chapter 2 Asymmetry in the molecular trigger
28
As shown in the introduction chapter, liquid crystal films which deform as a
response to changes in humidity have previously been prepared by immersion of a
liquid crystal network with hydrogen bonded carboxylic acid moieties in a
potassium hydroxide solution to convert it to a hygroscopic polymer salt.11 Bending
could be achieved in these materials by subjecting a film with planar alignment to
high humidity on only one side, or by subjecting a film with a twisted or splayed
alignment to a uniform humidity change. As such, bending in humidity responsive
LC actuators has been achieved based on either the asymmetry in the molecular
orientation, or by the presence of a gradient in the humidity.
The actuators presented in this chapter are prepared from a single sheet of liquid
crystal film with unidirectional, planar alignment, having no variation in the
alignment director. To induce the complex deformations, the material is locally
treated with a potassium hydroxide solution to create a hygroscopic carboxylic salt
that takes up water from the air and swells in a high humidity environment. In
contrast to previous research, the shape deformation is not induced by asymmetry
in either the stimulus or the alignment director, but by an asymmetry in the
presence of the responsive molecular trigger in the material. This results in soft
actuators that operate in a uniform humidity environment and without requiring a
twisted or splayed alignment director. These soft actuators can be tailored to either
bend, fold or curl in response to changes in humidity, while being prepared from
the same polymer sheet. The ability to create a broad range of very different shapes
using a single material and procedure allows a large variety in the preparation of
actuators. This versatility was demonstrated by preparing actuators that bend
towards or away from a water surface, actuators that show complex
humidity-responsive folding, and curling actuators with controlled handedness that
unwind upon increasing the humidity of the environment.
Chapter 2 Asymmetry in the molecular trigger
29
2.2 Results and discussion
2.2.1 Film preparation, KOH treatment and analysis
Humidity-responsive polymer films were prepared using a liquid crystalline
mixture containing hydrogen-bridged moieties (Figure 2.1a).11 This material was
given a planar, uniaxial alignment using a cell containing rubbed polyimide
alignment layers. After photopolymerization (Figure 2.1b), the film (thickness
18 μm) was retrieved from the cell and cut to the appropriate size. It should be
noted that at this point the films did not yet respond to water. Initially, a water-
responsive actuator with simple bending behavior was prepared by bringing only
one side of the ribbon in contact with a 0.1 M KOH solution, followed by rinsing
with water. The infrared spectrum, measured in reflection before base treatment,
clearly showed the hydrogen bridges as a peak at 1682 cm-1, which corresponds to
the C=O stretching in hydrogen bonded dimers. After the single-side base
treatment and subsequent drying, this peak disappeared and two new peaks
appeared at 1547 and 1395 cm-1, resulting from anti-symmetric and symmetric
COO- stretching, respectively. The spectrum measured at the non-exposed side
remained unchanged.
Figure 2.1 a) Chemical composition of the hydrogen bonded
LC mixture. b) Preparation procedure for making the liquid
crystal films.
Chapter 2 Asymmetry in the molecular trigger
30
The stability of the base-treated liquid crystal polymer networks was investigated
using IR spectroscopy. The films were exposed to various humidity conditions, and
the IR spectra were periodically measured over a period of 10 days. Spectra were
measured in triplo, baseline corrected and normalized. Two films were analyzed,
one which was kept at ambient humidity for the entire experiment, and one which
was kept in a petri dish with water present (the exact humidity was not measured).
Directly after activation, the peak corresponding to the presence of the potassium
salt was clearly present in both films. Over time, the peak in the high humidity film
decreased in intensity, while in the low humidity film it hardly decreased. The
height of the salt peak was plotted as a function of time (Figure 2.2). It was
observed that the signal belonging to the salt peak decreased asymptotically. This
means that the location of the salt moieties in the films is somewhat unstable when
kept in prolonged high humidity conditions, while being more stable in ambient
humidity conditions.
2.2.2 Deformation behavior of the films
During the exposure to the aqueous KOH solution and rinsing with water,
anisotropic swelling of the exposed side caused bending of the film towards the
non-exposed side. When the ribbon was cut with the alignment director
perpendicular to the long axis, bending along the long axis took place
Figure 2.2 Salt peak intensity as a function of time, in a film kept at high humidity and
one kept at low humidity.
Chapter 2 Asymmetry in the molecular trigger
31
(Figure 2.3a). A ribbon in which the alignment was parallel to the long axis showed
bending along the short axis while wet. This behavior is in good agreement with
earlier observations that the swelling should take place predominantly
perpendicular to the alignment director due to the anisotropic nature of the
material,11 and shows that the bending direction of such ribbons is strongly
influenced by the alignment director of the mesogenic units. Unexpectedly, when
the film was dried, bending towards the base-treated side took place. This implies
that the material, upon base exposure and subsequent drying, shrinks in comparison
to the non-exposed side. The shrinkage is anisotropic, and mainly takes place in the
direction perpendicular to the alignment director. Although the mechanism for this
shrinkage is unclear, it may be caused by increasing molecular order, or by a more
dense packing. Alternatively, material might be lost during activation, for instance
through hydrolysis of ester bonds.12 When the film was placed in water again, it
returned to the swollen state, in which it was bending towards the non-exposed
side.
Figure 2.3 Actuation behavior of a one-side base treated planar aligned LC polymer film.
a) Schematic representation of the bending behavior of the ribbon with the alignment
director perpendicular to the long axis, upon activation and when dried. b) Actuation in a
controlled humidity chamber c) Curvature (1/r) of the film as a function of humidity.
Squares indicate data points which were obtained while increasing the humidity, while
circles indicate deceasing humidity. d) Bending of the film when facing the water surface
with the base-treated side (top) or the non-treated side (bottom).
Chapter 2 Asymmetry in the molecular trigger
32
To study the bending behavior in response to environmental humidity, the one-side
base-exposed film with the alignment director perpendicular to the long axis
(Figure 2.3a) was placed in a chamber in which the humidity is homogeneous and
controllable. At high humidity (75%), the sample was straight, while at low
humidity (15%) it was strongly bent towards the base-exposed side (Figure 2.3b)
due to shrinkage caused by the evaporation of water from the base-treated side.
When the bending curvature (1/r) was plotted against the relative humidity
(Figure 2.3c), a linear relationship was observed. The curvature was found to range
from 0 mm-1 at high humidity to 0.6 mm-1 at low humidity. The humidity could not
be increased to a point where bending towards the untreated side took place
(>75 %), such as was observed when the actuator was rinsed with water after the
preparation process. To investigate the response to a humidity gradient, the film
was brought close to a water surface. As expected, bending towards the water
surface took place when the untreated side was facing the water (Figure 2.3d), and
the curvature increased when the distance to the water surface was decreased.
When the film was flipped, it was bending away from the water. This shows that
the direction of the bending is determined by the asymmetry in the responsive
molecular trigger in the polymer film and not by the presence of the humidity
gradient.
Chapter 2 Asymmetry in the molecular trigger
33
2.2.3 Complex folding
Next, complex folding was induced in the polymer films by selective base
treatment of specific areas on the film’s surface (Figure 2.4). An actuator with
three folds in an alternating pattern was prepared. To do so, the outer parts on one
side of the film were first exposed to the base, then the film was rinsed in water and
dried, and a second exposure was carried out on the center part of the film on the
other side (Figure 2.4a). The actuation of this film was tested in the humidity
chamber. The film displayed a folded shape in an environment with low humidity
(15%), while at high humidity (75%) a straight polymer film was again obtained,
which is similar to the actuator with only one base-treated domain. To demonstrate
that sharp, hinge-like folding could also be induced, three narrow parts of the film
were base-treated. A ribbon with three hinges, two on one side of the film and one
on the other side, was prepared (Figure 2.4b) and investigated in the humidity
chamber. At low humidity, three sharply bent hinges were observed and the
actuator was fully contracted. Again, at high humidity (75%) the actuator was
nearly straight.
Figure 2.4 Folded actuators, which are prepared through selective treatment of the film.
a) A ribbon in which the outer parts on one side are exposed to an alkaline solution, and
the center part on the other side (blue regions), actuated in a humidity chamber. b) A
ribbon with three narrow hygroscopic polymer salt bands (blue parts), actuated in the
humidity chamber by increasing the humidity. Arrows indicate the molecular director in
the polymer films.
Chapter 2 Asymmetry in the molecular trigger
34
2.2.4 Curling
Finally, humidity-sensitive curling actuators were prepared. In this experiment, a
ribbon was cut from a planar aligned polymer film in such a way that the nematic
director made a -45 degrees angle with the long axis of the film (Figure 2.5a). After
one-side exposure, the ribbon adopted a right-handed curl shape while wet, with
the base-treated side on the outside. When the ribbon was dried, the handedness of
the curl reversed to left handed, with the base-treated side on the inside. As seen
before, the polymer salt side swells when wet in comparison to the untreated,
non-responsive side, and this swelling predominantly takes place perpendicular to
the molecular director. Since the nematic director makes a -45 degrees angle with
the long axis of the film this will lead to right-handed curling in which the polymer
salt side is on the outside (Figure 2.5a). In the dry state, shrinkage dominates
perpendicular to the molecular director, causing the helix to reverse its handedness.
Figure 2.5 Curling actuators prepared by one-side base exposure of liquid crystal
films. a) Ribbon with the molecular director at a -45 degrees angle with respect
to the long axis of the film, and the resulting curling behavior after activation in
KOH solution, both in the wet and dry state. b) Curling and uncurling of the +45
degrees actuator at high and low humidity, respectively. The behavior of this
actuator is also shown in the humidity chamber with increasing humidity.
Chapter 2 Asymmetry in the molecular trigger
35
When a strip was cut in which the nematic director was at a +45 degrees angle with
the long axis of the film, the resulting curl was left-handed when wet and right-
handed when dry (Figure 2.5b), which shows that the handedness in these humidity
responsive curling actuators is fully directed by the orientation of the molecular
director with respect to the long axis of the film. This ribbon was investigated in a
humidity chamber (Figure 2.5b). At low humidity, the ribbon was tightly curled.
When the humidity was increased, the ribbon started to uncurl. At high humidity it
was straight, similar to the bending and folding actuators. It should be noted this
kind of deformation requires anisotropic strain, and would not be possible with
materials that swell or shrink isotropically.
2.3 Conclusions and outlook
In conclusion, a new, versatile method was developed for the preparation of
polymer actuators based on liquid crystals that can be tailored to bend, fold or curl
as a response to changes in humidity. In this material the deformation is fully
controlled by the asymmetry in the responsive molecular trigger present in the
polymer film. Using the simple procedure of locally exposing a monolithic,
uniaxially aligned hydrogen bonded polymer film to an alkaline solution,
hygroscopic regions are obtained on one side of the film. Anisotropic swelling and
shrinkage was observed on the polymer salt side, which caused a strong bending
deformation of the ribbons. Folding deformation was achieved by patterned base
treatment, and curling deformation could be achieved when the alignment director
of the mesogenic units was put at a -45 degrees or +45 degrees angle with the long
axis of the ribbon, in which the handedness was controlled by the molecular
director.
This approach provides a new method for the preparation of stimuli responsive
actuators from a single polymer sheet. In this novel principle the asymmetry in the
molecular trigger in the anisotropic polymer film play a dominant role, leading to
Chapter 2 Asymmetry in the molecular trigger
36
programmed deformation events in a versatile fashion. This method can in
principle be extended to other materials, such as anisotropically ordered hydrogels.
In the future, even more complex deformation may be achieved using inkjet
printing techniques for the local base treatment, and using photoalignment to
prepare complex three-dimensional director patterns (as is presented in the
following chapters).
2.4 Experimental Section
Preparation of the liquid crystal films: Cleaned glass substrates (3x3 cm) were
spincoated with polyimide (AL 1051, JSR Corp.) at 1000 rpm for 5 seconds, then
5000 rpm for 40 seconds, using a Karl Suss RC8 spincoater. After heating to 180 oC for 1 hour, the plates were rubbed and glued into cells, using 18 micron SP-218
Micropearl plastic spacers (Sekisui Chemical Co.) to secure the cell gap. A mixture
of 3OBA, 5OBA and 6OBA (Merck) was prepared in a 1:1:1 ratio by weight, then
12 w% RM82 (Merck), 0.5-2.0 w% photoinitiator (Irgacure 819, Ciba Specialty
Chemicals), and 200 ppm thermal inhibitor (tert-butyl hydroquinone, Fluka) were
added. The components were dissolved in CH2Cl2, then dried by heating to 50 oC
under air flow, followed by drying in vacuo. The cells were filled at either 95 oC
(nematic) or 105 oC (isotropic), cooled to 80 oC, and cured for 5 min using an
Omnicure S2000 UV source.
Preparation of the actuators: The films were removed from the cells using a razor
blade after immersion in hot water, cut to the right size, and treated with a plasma
asher (Emitech K1050X) for 4 min at 45 Watt to remove remaining traces of
polyimide. Conversion to the KOH salt was carried out by treating the film with an
aqueous 0.1 M KOH solution. When an entire surface was treated, the film was
floated on the surface of the KOH solution for 20 seconds, then rinsed in
demineralized water and dried. When localized activation was performed, a piece
of kitchen sponge was soaked in KOH solution, and the film was placed on top of
Chapter 2 Asymmetry in the molecular trigger
37
it until it visibly started bending, which took a few minutes. The film was then
flushed and dried, and in some cases a second exposure was carried out.
Analysis: ATR FTIR analysis was performed for some of the actuators using a
Varian 670 FTIR spectrometer. Humidity experiments were performed using a
home built humidity chamber, containing a Vaisala HMI41 humidity sensor.
2.5 References
1. Dawson, C.; Vincent, J. F. V.; Rocca, A.-M., How pine cones open. Nature
1997, 390 (6661), 668-668.
2. Harrington, M. J.; Razghandi, K.; Ditsch, F.; Guiducci, L.; Rueggeberg,
M.; Dunlop, J. W. C.; Fratzl, P.; Neinhuis, C.; Burgert, I., Origami-like unfolding
of hydro-actuated ice plant seed capsules. Nat. Commun. 2011, 2, 337.
3. Ma, Y.; Sun, J., Humido- and Thermo-Responsive Free-standing films
mimicking the petals of the morning glory flower. Chem. Mater. 2009, 21 (5), 898-
902.
4. Armon, S.; Efrati, E.; Kupferman, R.; Sharon, E., Geometry and mechanics
in the opening of chiral seed pods. Science 2011, 333 (6050), 1726-1730.
5. (a) Roy, D.; Cambre, J. N.; Sumerlin, B. S., Future perspectives and recent
advances in stimuli-responsive materials. Prog. Polym. Sci. 2010, 35 (1–2),
278-301; (b) Spruell, J. M.; Hawker, C. J., Triggered structural and property
changes in polymeric nanomaterials. Chem. Sci. 2011, 2 (1), 18-26; (c) Stuart, M.
A. C.; Huck, W. T. S.; Genzer, J.; Muller, M.; Ober, C.; Stamm, M.; Sukhorukov,
G. B.; Szleifer, I.; Tsukruk, V. V.; Urban, M.; Winnik, F.; Zauscher, S.; Luzinov,
I.; Minko, S., Emerging applications of stimuli-responsive polymer materials. Nat.
Mater. 2010, 9 (2), 101-113.
6. Ma, M.; Guo, L.; Anderson, D. G.; Langer, R., Bio-inspired polymer
composite actuator and generator driven by water gradients. Science 2013, 339
(6116), 186-189.
Chapter 2 Asymmetry in the molecular trigger
38
7. Dai, M.; Picot, O. T.; Verjans, J. M. N.; de Haan, L. T.; Schenning, A. P.
H. J.; Peijs, T.; Bastiaansen, C. W. M., Humidity-responsive bilayer actuators
based on a liquid-crystalline polymer network. ACS Appl. Mater. Inter. 2013, 5
(11), 4945-4950.
8. (a) Islam, M. R.; Li, X.; Smyth, K.; Serpe, M. J., Polymer-based muscle
expansion and contraction. Angew. Chem. Int. Edit. 2013, 52 (39), 10330-10333;
(b) Ma, Y.; Zhang, Y.; Wu, B.; Sun, W.; Li, Z.; Sun, J., Polyelectrolyte multilayer
films for building energetic walking devices. Angew. Chem. Int. Edit. 2011, 50
(28), 6254-6257.
9. (a) Ionov, L., Biomimetic hydrogel-based actuating systems. Adv. Funct.
Mater. 2013, 23 (36), 4555-4570; (b) Ochoa, M.; Chitnis, G.; Ziaie, B., Laser-
micromachined cellulose acetate adhesive tape as a low-cost smart material. J.
Polym. Sci. Pol. Phys. 2013, 51 (17), 1263-1267; (c) Shen, L.; Fu, J.; Fu, K.;
Picart, C.; Ji, J., Humidity responsive asymmetric free-standing multilayered film.
Langmuir 2010, 26 (22), 16634-16637; (d) Stoychev, G.; Turcaud, S.; Dunlop, J.
W. C.; Ionov, L., Hierarchical multi-step folding of polymer bilayers. Adv. Funct.
Mater. 2013, 23 (18), 2295-2300; (e) Tokarev, I.; Minko, S., Stimuli-responsive
hydrogel thin films. Soft Matter 2009, 5 (3), 511-524; (f) Zou, Y.; Lam, A.;
Brooks, D. E.; Srikantha Phani, A.; Kizhakkedathu, J. N., Bending and stretching
actuation of soft materials through surface-initiated polymerization. Angew. Chem.
Int. Edit. 2011, 50 (22), 5116-5119.
10. (a) Douezan, S.; Wyart, M.; Brochard-Wyart, F.; Cuvelier, D., Curling
instability induced by swelling. Soft Matter 2011, 7 (4), 1506-1511; (b) Geng, Y.;
Almeida, P. L.; Fernandes, S. N.; Cheng, C.; Palffy-Muhoray, P.; Godinho, M. H.,
A cellulose liquid crystal motor: a steam engine of the second kind. Sci. Rep. 2013,
3, 1028.
11. (a) Broer, D. J.; Bastiaansen, C. M. W.; Debije, M. G.; Schenning, A. P. H.
J., Functional organic materials based on polymerized liquid-crystal monomers:
Supramolecular hydrogen-bonded systems. Angew. Chem. Int. Edit. 2012, 51 (29),
7102-7109; (b) Harris, K. D.; Bastiaansen, C. W. M.; Broer, D. J., Physical
properties of anisotropically swelling hydrogen-bonded liquid crystal polymer
Chapter 2 Asymmetry in the molecular trigger
39
actuators. J. Microelectromech. S. 2007, 16 (2), 480-488; (c) Harris, K. D.;
Bastiaansen, C. W. M.; Lub, J.; Broer, D. J., Self-assembled polymer films for
controlled agent-driven motion. Nano Lett. 2005, 5 (9), 1857-1860.
12. van Kuringen, H. P. C.; Eikelboom, G. M.; Shishmanova, I. K.; Broer, D.
J.; Schenning, A. P. H. J., Responsive Nanoporous Smectic Liquid Crystal Polymer
Networks as Efficient and Selective Adsorbents. Adv. Funct. Mater. 2014, 24 (32),
5045-5051
This chapter was based on: de Haan, L. T.; Sanchez-Somolinos, C.; Bastiaansen, C. M. W.; Schenning, A. P. H. J.; Broer, D. J., Engineering of complex order and the macroscopic deformation of liquid crystal polymer networks. Angew. Chem. Int. Edit. 2012, 51 (50), 12469-12472; Modes, C. D.; Warner, M.; Sanchez-Somolinos, C.; de Haan, L. T.; Broer, D. J., Angular deficits in flat space: remotely controllable apertures in nematic solid sheets. P. Roy. Soc. A.-Math. Phy. 2013, 469 (2153), 20120631
41
Chapter 3. Actuators with a director pattern in the
xy-plane of the film
3.1 Introduction
As described in the previous chapters, the morphing behavior of liquid crystalline
networks has so far been studied mainly in systems with uniaxial alignment, or in
those with a molecular director that varies in only in the z-direction, that is, twisted
nematic and splayed configurations that bend1 or curl2 after applying the stimulus.
In contrast, this chapter describes the preparation of polymer networks which are
patterned in the xy-plane of the film, which respond to temperature increase with
reversible, exotic shape deformations. In contrast to the previously shown
examples, these actuators do not require a gradient in the alignment director,
molecular trigger, or the stimulus in the z-direction to form 3-dimensional shapes.
While theoretically predicted (see Chapter 1),3 the deformation behavior of liquid
crystal networks with disclination alignment profiles had never been
experimentally confirmed.
Initially, circular patterns were prepared based on +1 disclinations: the azimuthal,
radial, and spiral pattern. Networks based on disclinations of +0.5 and +1.5
strength were also prepared. The deformation of the azimuthal and radial films was
studied by heating them up using an IR lamp. Then, discrete patterns based on
disclinations were also prepared, using the “vocabulary” as described in the
introduction chapter, and the deformation of these films was also investigated.
Chapter 3 Director pattern in the xy-plane of the film
42
Finally, a pattern was shown in which slits were cut, which allow in-plane
deformation that leads to opening of the slits. This can be used to prepare sieves or
filters that can be opened and closed through heating.
To create liquid crystal polymer films with a non-uniform director profile in the
xy-plane, an alignment technique that can facilitate the preparation of such
alignments in the reactive mesogens is required. Polyimide layers are favored for
the alignment of liquid crystals, but while advanced techniques for polyimide
buffing are capable of generating non-uniform director profiles,4 the most versatile
technique for the generation of complex director profiles is photoalignment.5 To
prepare photoaligned alignment layers, a photoresponsive material is irradiated
with polarized UV light to cause a permanent or reversible chemical
transformation. The molecules are more likely to absorb the light, and undergo the
transformation, if the transition dipole moment is parallel to the electric vector of
the polarized UV light. This creates anisotropy in the alignment layer, which can
then be transferred to the reactive mesogens that come into contact with it. There
are various photoalignment materials available. A popular class of materials are
based on azobenzene. These materials undergo a reversible trans to cis
rearrangement when they absorb UV light, which causes the molecules to reorient.
As the absorption is more efficient for molecules in which the transition dipole
matches the polarization direction of the light, the alignment of the molecules is
driven towards the direction perpendicular to the polarization direction, and
anisotropy is created. Another class of alignment materials are the linearly
photopolymerizable polymers (LPP’s), which rely on a [2+2] cycloaddition
reaction between the photosensitive side groups of the polymer, which results in
crosslinking of these groups upon exposure to polarized UV light. As the groups
will only crosslink when the transition dipole moment is in the right direction with
respect to the polarization direction of the UV light, anisotropy is created in the
alignment material. The anisotropic layer will align liquid crystals that come in
contact with it, either parallel or perpendicular to the polarization of the UV light
used to prepare it, depending on the material used.6 In the experiments described in
Chapter 3 Director pattern in the xy-plane of the film
43
this thesis, only alignment materials that align parallel to the polarization direction
were used (Staralign 2100 and ROP-108).
3.2 Results and discussion
3.2.1 Preparation of photoaligned liquid crystal network films
For the preparation of the photoalignment layers, a thin film of LPP spincoated
onto glass substrates was used. Two of these substrates were glued together with a
small (~18 μm) gap between them to obtain an alignment cell. When irradiated
with linearly polarized UV light, the alignment material forms an alignment layer,
with the alignment director oriented parallel to the polarization direction of the UV
light. A photomask was used to selectively expose areas of the cell to allow the
creation of various patterns (Figure 3.1a). The cells were filled with a mixture of
polymerizable liquid crystals in the isotropic phase, and after cooling the liquid
Figure 3.1 a) Setup for the preparation of patterned alignment cells. b) Procedure for
making a patterned liquid crystal polymer network from a patterned alignment cell. c)
Liquid crystal mixture for identification of the alignment patterns. d) Liquid crystal mixture
for the preparation of liquid crystal actuators.
Chapter 3 Director pattern in the xy-plane of the film
44
crystalline samples to the nematic phase, photopolymerization was carried out
(Figure 3.1b). Two different nematic liquid crystal mixtures were used. One
mixture consisted of two mesogens, which were combined in a 1:1 ratio to reduce
the clearing point and allow curing at room temperature (Figure 3.1c).7 It contained
a blue colored dichroic dye (Rhodamine 700) that allowed the determination of the
alignment direction of the resulting polymer network when it is placed between
parallel polarizers. The second mixture consisted mostly of mesogens with a single
acrylate group, as the low crosslink density of a film prepared with these
monomers is known to result in a strong shape deformation response upon heating.8
It contained an IR absorbing dye (Lumogen 788 IR) to obtain an actuator which
could be heated by applying IR irradiation (Figure 3.1d). To both mixtures, a
radical initiator (Irgacure 184 or Irgacure 819, respectively) was added to allow
photopolymerization.
3.2.2 Films based on continuous disclinations
Liquid crystal polymer films with director profiles based on m = +1 disclinations
were prepared first: the azimuthal, radial and spiral pattern. The alignment layers
were made by slowly rotating the cells containing the LLP layers while exposing to
linearly polarized UV light through a wedge-shaped photomask (Figure 3.2a).
Different patterns could be prepared by using different polarization directions for
the UV light. When the polarization direction was set to be perpendicular to the
long axis of the wedge, azimuthal alignment layers were obtained. When the
polarization direction of the UV light was set parallel to the wedge, radial
alignment layers were created, while a polarization direction at 45 degrees
produced spiral alignment layers.
Chapter 3 Director pattern in the xy-plane of the film
45
The cells were filled with the mixture containing the dichroic dye (Figure 3.1c),
photopolymerized at room temperature in the nematic phase, and the resulting
films were investigated between polarizers. Between crossed polarizers, the
azimuthal, spiral, and radial films showed no light transmission in the areas where
the mesogenic units are positioned either parallel or perpendicular to the
polarization direction of the incoming polarized light, while the transmission
gradually increased towards the intermediate areas where the long molecular axis is
at a 45 degrees angle (Figure 3.2b). This manifested itself as a dark fan-shaped area
in a bright field. Between parallel polarizers, the areas where the liquid crystals are
aligned parallel to the polarization direction appeared in a deeper shade of blue
compared to those aligned in a different direction (Figure 3.2b). This coloration is
caused by the blue colored dichroic dye, which aligns with the liquid crystals and
shows more absorption when it is aligned parallel to the polarization direction of
the incoming light. Blue colored regions are observed at different areas in the
polymer films, which is compatible with the azimuthal, radial and spiral alignment
director profiles. As expected, the black fan-shaped sections are stationary when
Figure 3.2 a) Photoalignment setup for the preparation of continuous circular alignment
cells. b) Optical characterization of the films with azimuthal, spiral and radial alignment
viewed between crossed polarizers and parallel polarizers. The gray triangles in the
schematic representation of the alignment patterns indicate non-transmissive areas in the
crossed polarizers setup. The blue triangles indicate areas with no transmission between
crossed polarizers, and blue transmission between parallel polarizers. c) Film prepared
using a rotating substrate in combination with the use of circular masks. d) Optical
characterization of films based on disclinations of other strength, aligned using alignment
cells which were prepared using UV light with a rotating polarization direction. The
substrate was rotated counterclockwise.
Chapter 3 Director pattern in the xy-plane of the film
46
the samples are rotated. A polymer film with even higher complexity was prepared.
A structure was made in two steps with an azimuthal alignment on the inner circle
and a radial alignment on the outer circle, which was termed the sun alignment.
The alignment of the resulting film was verified between crossed and parallel
polarizers, and showed the expected properties (Figure 3.2c).
To further explore this method, alignment patterns based on disclinations of +1.5
and +0.5 strength were also prepared. This was done by continuously rotating the
polarization direction of the UV light during the rotation of the cell. Two patterns
were created with the polarization direction rotating at half the speed of the
substrate rotation. In one case the polarization direction and substrate were rotating
in the same direction, while in the other case they were rotating in opposite
directions. The resulting polymer films made using these cells showed the expected
features under crossed and parallel polarizers (Figure 3.2d). As expected, in
contrast to the azimuthal, spiral and radial films, the black fan-shaped sections in
these asymmetric samples were rotating upon rotating the sample between the
polarizers.
3.2.3 Actuation of m = +1 disclination-based actuator films
To show the functionality of the materials, the actuation behavior of freestanding
films upon heating by IR irradiation was investigated. To this end, new films were
prepared using the second liquid crystal mixture (Figure 3.1d). In this case,
photopolymerization was carried out at elevated temperature in the nematic phase
to obtain a polymer film. The actuation behavior of the azimuthal and radial films
upon heating was investigated. A piece of polymer film was cut in a circular shape
from the azimuthal actuator around the center, followed by irradiation with an IR
lamp while holding it in place with a suction gripper at the center. Prior to heating,
the actuator was only slightly bent (Figure 3.3a). Upon switching on the lamp,
deformation was observed into a conical shape within seconds, with the cone apex
Chapter 3 Director pattern in the xy-plane of the film
47
located at the center and pointing downward (Figure 3.3a). When the distance
between the lamp and the actuator was varied, the upward bending angle changed
accordingly (Figure 3.3b), which demonstrates that the degree of deformation
depends on the intensity of IR irradiation. The sample is symmetrical, and the
symmetry is broken to favor the apex-down cone, which is probably caused by the
vacuum gripper pulling on the tip. This was further affirmed by the observation
that when a weaker vacuum was used, the apex-up cone was favored instead, in
which case the symmetry was broken by gravity. When the lamp was switched off,
the film returned to its original shape. The actuation experiment was repeated for
the actuator with a radial alignment pattern. When the IR lamp was switched on,
the film deformed into an anticone shape (Figure 3.3c). The degree of actuation
could again be varied by changing the distance from the IR source.
Figure 3.3 Actuation of films with azimuthal and radial alignments. a)
Actuation behavior of an azimuthal polymer film upon heating with an IR
lamp. b) Relationship between the distance of the sample to the lamp and the
upward bending angle. c) Actuation behavior of a radial polymer film. The
arrows along the radius and along the azimuth indicate the direction of
deformation.
Chapter 3 Director pattern in the xy-plane of the film
48
The behavior of these liquid crystal networks is in agreement with the theoretical
models for mechanical responses of disclinations in nematic crosslinked networks
presented in the introduction chapter.3a In case of an azimuthal alignment pattern, a
reduction of the liquid crystal order upon temperature increase leads to contraction
along the azimuthal direction and an expansion along the radial direction (Figure
3.3a). These strains cannot be accommodated within the sheet plane, causing
deformation of the flat sheet out of the plane into a cone. In the case of a radial
alignment pattern, the strains are in the opposite directions (Figure 3.3c). This
results in an anticone, which is recognizable as a saddle shape.
The actuation behavior of the films with director profiles based on other
disclinations was not investigated. However, in a study performed by a different
research group the deformation of large number of UV-responsive films with
patterns based on disclinations was shown.9
3.2.4 Disclination-based, discrete patterns
Alignment patterns based on the previously described disclinations were also
prepared as discrete patterns. The photoalignment procedure used to generate these
patterns is comparable to the method used for the continuous domains. However,
instead of irradiating a small angular section of a rotating substrate, a static
substrate is irradiated through a photomask in a two-step fashion, changing the
polarization direction in between the two steps. This leads to patterns that have
areas with a single, non-varying alignment director, and are separated by sharp
boundaries. Alignment cells based on these patterns were then filled with the
reactive liquid crystal mixture with a lower crosslinker content (Figure 3.1d), and
after photopolymerization the deformation behavior of the resulting films was
examined. To properly design the discrete patterns, it is important to use the
“vocabulary” for multidomain pattern design as described in the introduction
chapter, containing only rank-1 connected boundaries to avoid incompatibilities.3b, 3c
Chapter 3 Director pattern in the xy-plane of the film
49
Using this procedure, an alignment cell with a four-domain discrete pattern
resembling a m = +1 azimuthal disclination was produced, and a liquid crystal
polymer film was made using this cell as described above. When the film was
heated using an IR lamp, it deformed into the expected conical shape (Figure 3.4a).
The pattern could also be tiled multiple times in a single film, while maintaining
rank-1 connectedness. A film containing roughly thirty of these domains was
prepared and actuated. It strongly deformed into a dented surface, which somewhat
resembled the expected array of pyramids (Figure 3.4b). It is not clear, however,
why the shape is irregular.
3.2.5 Aperture patterns
Discrete alignment patterns based on disclinations can also be utilized to prepare
films with apertures that can be opened and closed when a stimulus is applied.
Such films could lead to an interesting application, where a filter or sieve is
prepared with pores that can be altered in size, for example to unclog them or to
Figure 3.4 a) Discrete m = +1 azimuthal pattern, and
the resulting shape deformation upon IR heating. b)
Tiled azimuthal pattern (partially shown) and the
resulting deformation upon IR heating.
Chapter 3 Director pattern in the xy-plane of the film
50
allow larger particles to pass on command. As previously shown, a film with an
azimuthal +1 disclination will contract along its circumference and expand along
its radius. The new circumference and radius are incompatible, and the only
solution is to form a cone shape. However, when such a film would be cut from the
edge to the center, the strains can instead be relieved by widening the cut. This
solution is favored over formation of the cone shape, as it will stay flat and does
not experience the stretches involved with bending into the third dimension. Using
this principle, a complex director profile was designed to prepare a film with
apertures which could be opened and closed by applying heat. The profile
consisted of two half m = +1 disclinations, with an m = -1 disclination directly in
between. A slit was cut between from the center of one m = +1 disclination to the
center of the other, slicing the m = -1 disclination in half. This allows all
disclinations to release their strains by deforming in the plane of the film, by
opening the angles at the outer edges of the slit and closing the angles at the center
of the slit. The final solution is the slit opening. This profile was then tiled multiple
times in a single film (Figure 3.5a).
Figure 3.5 a) Schematic representation of the alignment
pattern with slits (thick lines) cut between the two positive
disclinations, crossing the negative disclination. The
opening of the slits upon temperature increase is also
shown. b) The actual film, floating on a water bath. A close-
up of one of the apertures is shown before and after heating.
Chapter 3 Director pattern in the xy-plane of the film
51
The film was prepared in the same way as the other films with discrete alignment
profiles. The slits were carefully cut at the correct positions using a razor blade.
The film was then put onto a water bath so it would float, and the water was
gradually heated up to 85 oC using a hot plate. It was observed that the apertures
are indeed opened at this temperature (Figure 3.5b). The response was found to be
reversible upon cooling of the water bath to room temperature. This pattern
resembled a sieve in which the opening and closing of the pores can be controlled
with temperature, although the openings were still large (about 1 cm in length).
3.3 Conclusions and outlook
In conclusion, the results presented in this chapter show that patterned
photo-alignment layers can be used to align polymerizable liquid crystals and
prepare freestanding polymer films with complex order in the xy-plane, based on
disclinations of various strengths. It was demonstrated that m = +1 disclination
patterns can be prepared using a photoalignment layer in combination with a
wedge-shaped mask to irradiate a rotating substrate. Films with patterns based on
m = +1.5 and m = +0.5 disclinations were also prepared. The m = +1 disclination
films with azimuthal and radial patterns reversibly deformed into cone and saddle
shapes, respectively, upon heating with IR irradiation, as was predicted in earlier
studies. It was also shown that disclination-based patterns could be prepared using
discrete, stepwise patterning. These patterns showed deformations similar to their
continuous counterparts, but in addition could be tiled to have multiple
disclinations present in a single film.
The materials shown comprise only a small number of the possible structures that
can be designed and prepared, and it is to be expected that, by using different mask
sizes and patterns, the possible dimensions and shapes can be greatly expanded. As
an example of the applications that could emerge from these systems, a pattern
with slits was prepared, and it was shown that the slits could be opened and closed
Chapter 3 Director pattern in the xy-plane of the film
52
by heating and cooling, respectively. The range of possible applications could be
further expanded by looking at different shapes. Additionally, materials can be
fabricated that respond to different stimuli, and the composition of the liquid
crystal mixtures can be varied in order to change other properties of the material,
leading to new and powerful methods for the fabrication of stimuli-responsive
polymer materials with a complex order having novel optical, mechanical, and
electronic properties.
3.4 Experimental section
All materials were used as received from Merck (RM82 (C6M), RM105, RM23,
RM257 (C3M)), BASF (Lumogen 788, LC756), Ciba Specialty Chemicals
(Irgacure 819, Irgacure 184), Huntsman (Staralign 2100,), Rolic (ROP-108),
Lambdachrome (Rhodamine 700), and Aldrich (t-butyl hydroquinone,
cyclopentanone, dichloromethane). The alignment cells were prepared using 20
micron glass rod spacers and adhesive (Norland NOA63).
The photoalignment material Staralign 2100 or ROP-108 was spincoated onto glass
substrates using a Karl Suss RC8 spincoater. Photoalignment was carried out using
an Ominicure S2000 UV lamp with 320-390 nm filter. A Knight Optical HNBP
sheet polarizer mounted in a Thorlabs PRM1/MZ8 rotation mount was used to
polarize the UV light in various directions, and the substrate was rotated using a
Thorlabs CR1/M-Z7 rotation stage. The rotation of the polarizer and stage was
controlled with Thorlabs TDC001 controllers, which were programmed with APT
software (Thorlabs).
Photopolymerization of the liquid crystal mixtures was carried out using a Philips
PL-S 9W UV lamp for the first mixture (C6M, C3M, Irgacure 184, Rhodamine
700), while an Ominicure S2000 UV lamp with 320-500 nm filter was used for the
second mixture (RM82, RM105, RM23, Irgacure 819, Lumogen 788). For the IR
Chapter 3 Director pattern in the xy-plane of the film
53
actuation experiments a Heraeus Noblelight Shortwave surface irradiator (type
“Duo”, 250 Watt, 57.5 Volt) with gold reflector was used.
Preparation of liquid crystal cells with continuous alignment patterns: A Staralign
2100 solution was spincoated onto two cleaned glass plates, which were heated to
120 ºC for 10 minutes to evaporate the solvent. The LPP substrates were assembled
into cells using glue containing 20 micron diameter glass rod spacers to fix the cell
air gap. In order to generate the desired alignment pattern, the LC cells were
exposed to linearly polarized UV light at 5 mW/cm2. Care was taken to have the
center of the cell coincide with the axis of rotation of the rotating stage. Exposure
was carried out through a photomask whose transmission area was a 9 degrees
angle sector of the circle of rotation, which was placed in such a way that the
intersection of the radii was at the axis of rotation of the sample stage. The sample
stage was rotated counterclockwise at a speed of 0.022 degrees per second. In case
of the patterns without rotational symmetry, the motorized polarizer was rotated at
half this speed in either clockwise or counter clockwise.
Preparation of liquid crystal cells with discrete alignment patterns: ROP-108 was
spincoated onto two cleaned glass plates, which were heated to 115ºC for 10
minutes to evaporate the solvent. The LPP substrates were assembled into cells
using glue containing 18 micron diameter glass rod spacers to fix the cell air gap.
In order to generate the desired alignment pattern, the LC cells were exposed to
linearly polarized UV light through a photomask, then the sample was rotated to
expose the previously unexposed parts, and a second exposure was carried out with
the polarization direction rotated by 90 degrees.
Preparation of densely crosslinked liquid crystal networks: Liquid crystal cells with
continuous alignment patterns were filled at 95 ºC with a mixture of C6M and
C3M in a 20/80 ratio, to which 1%wt of photoinitiator Irgacure 184 and 0.5%wt. of
Rhodamine 700 were added. After filling the cell with the LC mixture, it was
slowly cooled down to room temperature, and the alignment of the LC was
Chapter 3 Director pattern in the xy-plane of the film
54
checked by observing the cell between crossed polarizers. Photopolymerization
was carried out at room temperature by UV irradiation for 30 min.
Preparation of actuator films: RM82, RM105, and RM23 were put into a screw-cap
vial in a 2 : 3 :1 ratio by weight. A small amount of tert-butyl hydroquinone
dissolved in CH2Cl2 was added as a thermal inhibitor. 0.5% w/w Lumogen 788 and
1% w/w Irgacure 819 were added, then more CH2Cl2 was added and the mixture
was stirred at elevated temperature until all components were mixed and the
solvent had evaporated. An alignment cell with an azimuthal or radial pattern was
brought to 80 oC, and the cell gap was filled with the mixture though capillary
action. When the cell was completely filled, it was cooled down slowly to 60 oC.
Then UV polymerization took place by UV irradiation for 5 minutes. Finally, the
cell was opened with a razor blade to retrieve the freestanding film.
Investigation of IR actuation: The films with radial and azimuthal alignment were
actuated by holding them in place with a suction gripper which was mounted on a
height-adjustable clamp, with the IR lamp placed above it. The proximity of the
sample to the lamp was varied by changing the height of the clamp.
Chapter 3 Director pattern in the xy-plane of the film
55
3.5 References
1. (a) Harris, K. D.; Bastiaansen, C. W. M.; Broer, D. J., A glassy bending-
mode polymeric actuator which deforms in response to solvent polarity. Macromol.
Rapid Comm. 2006, 27 (16), 1323-1329; (b) Harris, K. D.; Bastiaansen, C. W. M.;
Broer, D. J., Physical properties of anisotropically swelling hydrogen-bonded
liquid crystal polymer actuators. J. Microelectromech. S. 2007, 16 (2), 480-488; (c)
Ohm, C.; Brehmer, M.; Zentel, R., Liquid crystalline elastomers as actuators and
sensors. Adv. Mater. 2010, 22 (31), 3366-3387.
2. (a) Iamsaard, S.; Aßhoff, S. J.; Matt, B.; Kudernac, T.; Cornelissen, J. L.
M.; Fletcher, S. P.; Katsonis, N., Conversion of light into macroscopic helical
motion. Nat. Chem. 2014, 6 (3), 229-235; (b) Sawa, Y.; Urayama, K.; Takigawa,
T.; Gimenez-Pinto, V.; Mbanga, B. L.; Ye, F. F.; Selinger, J. V.; Selinger, R. L. B.,
Shape and chirality transitions in off-axis twist nematic elastomer ribbons. Phys.
Rev. E 2013, 88 (2), 022502; (c) Sawa, Y.; Ye, F. F.; Urayama, K.; Takigawa, T.;
Gimenez-Pinto, V.; Selinger, R. L. B.; Selinger, J. V., Shape selection of twist-
nematic-elastomer ribbons. P. Natl. Acad. Sci. USA 2011, 108 (16), 6364-6368; (d)
Wie, J. J.; Lee, K. M.; Smith, M. L.; Vaia, R. A.; White, T. J., Torsional
mechanical responses in azobenzene functionalized liquid crystalline polymer
networks. Soft Matter 2013, 9 (39), 9303-9310.
3. (a) Modes, C. D.; Bhattacharya, K.; Warner, M., Gaussian curvature from
flat elastica sheets. P. Roy. Soc. A-Math Phy. 2011, 467 (2128), 1121-1140; (b)
Modes, C. D.; Warner, M., Blueprinting nematic glass: Systematically constructing
and combining active points of curvature for emergent morphology. Phys. Rev. E
2011, 84 (2), 021711; (c) Modes, C. D.; Warner, M., The activated morphology of
grain boundaries in nematic solid sheets. P. Soc. Photo-Opt. Ins. 2012, 84 (2),
021711.
4. (a) Chen, Y. D.; Fuh, A. Y. G.; Liu, C. K.; Cheng, K. T., Radial liquid
crystal alignment based on circular rubbing of a substrate coated with poly(N-vinyl
carbazole) film. J. Phys. D-Appl. Phys. 2011, 44 (21), 215304; (b) Varghese, S.;
Chapter 3 Director pattern in the xy-plane of the film
56
Narayanankutty, S.; Bastiaansen, C. W. M.; Crawford, G. P.; Broer, D. J.,
Patterned alignment of liquid crystals by µ-rubbing. Adv. Mater. 2004, 16 (18),
1600–1605.
5. (a) Bechtold, I. H.; Oliveira, E. A., Liquid-crystal alignment on anisotropic
homeotropic-planar patterned substrates. Mol. Cryst. Liq. Cryst. 2005, 442 (1),
41-49; (b) Chen, J.; Johnson, D. L.; Bos, P. J.; Sprunt, S.; Lando, J.; Mann, J. A.,
Radially and azimuthally oriented liquid crystal alignment patterns fabricated by
linearly polarized ultraviolet exposure process. Appl. Phys. Lett. 1996, 68 (7),
885-887; (c) Jeng, S.-C.; Hwang, S.-J.; Horng, J.-S.; Lin, K.-R., Electrically
switchable liquid crystal Fresnel lens using UV-modified alignment film. Opt.
Express 2010, 18 (25), 26325-26331; (d) Yaroshchuk, O.; Reznikov, Y.,
Photoalignment of liquid crystals: basics and current trends. J. Mater. Chem. 2012,
22 (2), 286-300.
6. (a) Kim, C.; Trajkovska, A.; Wallace, J. U.; Chen, S. H., New insight into
photoalignment of liquid crystals on coumarin-containing polymer films.
Macromolecules 2006, 39 (11), 3817-3823; (b) Schadt, M.; Schmitt, K.; Kozinkov,
V.; Chigrinov, V., Surface-induced parallel alignment of liquid-crystals by linearly
polymerized photopolymers. Jpn. J. Apl. Phys. 1992, 31 (7), 2155-2164.
7. (a) Broer, D. J.; Crawford, G. P.; Zumer, S., Cross-linked Liquid
Crystalline Systems. CRC, Taylor and Francis Group, Boca Raton, FL: 2011; (b)
Mol, G. N.; Harris, K. D.; Bastiaansen, C. W. M.; Broer, D. J., Thermo-mechanical
responses of liquid-crystal networks with a splayed molecular organization. Adv.
Funct. Mater. 2005, 15 (7), 1155-1159; (c) van Oosten, C. L.; Corbett, D.; Davies,
D.; Warner, M.; Bastiaansen, C. W. M.; Broer, D. J., Bending dynamics and
directionality reversal in liquid crystal network photoactuators. Macromolecules
2008, 41 (22), 8592-8596.
8. Liu, D. Q.; Bastiaansen, C. W. M.; den Toonder, J. M. J.; Broer, D. J.,
Photo-switchable surface topologies in chiral nematic coatings. Angew. Chem. Int.
Edit. 2012, 51 (4), 892-896.
Chapter 3 Director pattern in the xy-plane of the film
57
9. McConney, M. E.; Martinez, A.; Tondiglia, V. P.; Lee, K. M.; Langley, D.;
Smalyukh, II; White, T. J., Topography from topology: photoinduced surface
features generated in liquid crystal polymer networks. Adv. Mater. 2013, 25 (41),
5880-5885.
This chapter was based on: de Haan, L. T.; Gimenez-Pinto, V.; Konya, A.; Nguyen, T. S.; Verjans, J. M. N.; Sanchez-Somolinos, C.; Selinger, J. V.; Selinger, R. L. B.; Broer, D. J.; Schenning, A. P. H. J., Accordion- like Actuators of Multiple 3D Patterned Liquid Crystal Polymer Films. Adv. Funct. Mater 2014, 24 (9), 1251-1258; Modes, C. D.; Warner, M.; Sanchez-Somolinos, C.; de Haan, L. T.; Broer, D. J., Mechanical frustration and spontaneous polygonal folding in active nematic sheets. Phys. Rev. E. 2012, 86 (6), 060701
59
Chapter 4. Folding actuators with 3-dimensional
alignment patterns
4.1 Introduction
As shown in the introduction chapter, most three-dimensionally morphing liquid
crystal polymer films which are reported in literature bend or curl upon application
of a stimulus because they possess a molecular director that varies along the
z-direction: the twisted nematic and splayed configurations.1 Chapter 3 describes
more exotic deformations of liquid crystal polymer films as a result of a
disclination-based director profile, which is patterned in the xy-plane of the film
but uniform through the sample thickness, and thus essentially 2-dimensional. The
next step towards even more complex deformation is to combine these two profiles
to impose a 3-dimensional director profile on the film, with both variation along
the z-direction and in the xy-plane of the polymer film. This can be achieved by
clever design of the alignment cells, which should be similar to the cells as
described in the previous chapter in that they have a patterned alignment profile,
but have orthogonal alignment directions between the two substrates to force the
liquid crystals into a twisted nematic alignment, eventually supported by small
concentrations of chiral material.2
Initially, a striped pattern with a twisted nematic director profile which has an
orthogonal alignment director between domains was prepared, and was shown to
Chapter 4 Folding actuators with 3-dimensional alignment patterns
60
deform into an accordion-like shape upon heating. This pattern was also used in
combination with water-responsive liquid crystalline systems which showed the
same deformation behavior in a high-pH environment.3 A similarly aligned
chessboard pattern was prepared as well, which showed deformation similar to the
film with multiple m = +1 disclinations as presented in chapter 3. Finite-element
analysis reproduced the deformation behavior of the films. Finally, a circular piece
of film with a continuous circular pattern and a twisted nematic alignment was
prepared and actuated, and was shown to fold into a square shape.
4.2 Results and discussion
4.2.1 Alignment pattern design
The first design was an actuator consisting of a ribbon with a nematic director
which was patterned to have four discrete horizontal stripes with a twisted nematic
director of alternating orientation (Figure 4.1a). Stripes 1 and 3 have the director at
z = 0 oriented along the film’s short axis, while at z = h the director was oriented
along the film’s long axis. Stripes 2 and 4 have the director at z = 0 oriented along
the film’s long axis and oriented along the film’s short axis at z = h. The twist in all
stripes is 90 degrees and has uniform handedness.
If the domains would not be joined together, upon decreasing the order parameter,
each domain would initially deform into a saddle shape, followed by repression of
bending in one direction. Each domain is inverted with respect to its neighbors, and
they will bend in opposite directions. As the domains are joined together, this
causes a conflict to arise at the border. Therefore, it is expected that bending in that
direction is repressed, regardless of the dimensions of the domains, and only
bending along the long axis of the film takes place. The final shape is expected to
have four bends, in alternating directions, and will contract along its long axis in a
Chapter 4 Folding actuators with 3-dimensional alignment patterns
61
way similar to an iconic musical instrument, the accordion. This kind of
deformation is potentially interesting, as it achieves a large lateral displacement
between the endpoints of the film with only small strains.
4.2.2 Preparation and optical characterization of the polymer films
The alignment cells were prepared using photoalignment. However, instead of
irradiating a complete cell as described in the previous chapter, the two plates were
irradiated separately, and then an alignment cell was constructed in such a way that
the alignment of the opposing substrates was orthogonal. To make a striped pattern,
a striped photomask was used in a two-step procedure: one exposure was carried
out through the photomask, and then the substrate was rotated 180 degrees to
Figure 4.1 a) Alignment pattern of the accordion-shaped actuator, top view and side view. Arrows
indicate the contraction and expansion directions along the length of the film. Each domain is 5 mm
wide and 7.5 mm long. b) Two-step procedure for the preparation of the alignment layers. The black
arrows indicate the polarization direction of the UV light. Before the second exposure step the
polarization of the light is rotated 90 degrees while the sample is rotated 180 degrees. c) Liquid
crystal mixture used to make a heat responsive actuator (RM82:RM105:RM23 = 2:3:1), including an
IR absorber (Lumogen 788 IR), a radical initiator, and a chiral dopant (RM756). d) Optical analysis
of a four-lane alternating twisted nematic actuator. The film was analyzed between crossed polarizers
(left) and parallel polarizers (middle). To show the difference in orientation between the lanes, it was
also viewed between crossed polarizers with a quarter wave plate on top of the sample in two
different orientations (right).
Chapter 4 Folding actuators with 3-dimensional alignment patterns
62
expose the non-irradiated parts, and the polarization direction was rotated 90
degrees (Figure 4.1b). A cell was made, and the liquid crystal mixture (Figure 4.1c)
was then fed into the cell and cured, using the same procedure and liquid crystal
mixture that was used to prepare the low-crosslink films for the examination of
deformation as described in chapter 3, including the IR absorber.
After retrieving the film from the alignment cell, it was optically analyzed by
placing it between linear polarizers (Figure 4.1d). Twisted nematic liquid crystals
rotate the polarization direction of linearly polarized light by 90 degrees, causing
the film to be bright between crossed polarizers and dark between parallel
polarizers. Furthermore, when the film was placed between crossed polarizers with
a quarter wave plate on top of the sample at a 45 degrees angle, a difference in
brightness between the domains was observed. This is explained by the slow axis
of the wave plate being either parallel or perpendicular to the mid-plane director of
the nematic twist, which adds to the optical retardation or compensates for it,
respectively. The sample was more closely investigated using polarized optical
microscopy, and was found to contain no significant defects aside from the narrow
lines between the patterned domains, which are caused by a slight mismatch of the
alignment layers. These lines are expected not to influence the deformation
behavior of the actuators, as they are positioned in the direction in which bending
will be repressed, and because they represent a negligible fraction of the total
surface area. Surface profilometry and interferometry were used to measure the
final thickness of the film, which was found to range from 12 μm to 19 μm
between multiple samples.
4.2.3 Actuation of striped twisted nematic films
To perform the actuation experiments, a strip of 30 x 5 mm was cut from the
freestanding film with four alternating stripes, which resulted in an aspect ratio of
roughly 1650 : 275 : 1. The film was clamped on its short end and actuated through
Chapter 4 Folding actuators with 3-dimensional alignment patterns
63
heating by exposure to an IR source. It should be noted that the film was already
slightly bent before heating was applied, which was probably caused by stresses
induced during the photopolymerization of the material at elevated temperatures in
the flat state. Upon cooling down to room temperature, the anisotropy of the
system increases, which may have caused the material to deform. Upon heating the
vertically positioned film, the four areas of the film deformed by bending in
opposite directions. A four-bend accordion shape (Figure 4.2a) was clearly
observed, although the deformation of the area closest to the clamp was somewhat
suppressed. The deformation reversed upon switching off the IR source, and the
speed and magnitude of the deformation could be influenced by varying the
intensity of the IR irradiation.
The experiment was repeated for a 12-stripe film of the same dimensions. This film
was both actuated while being held vertically in front of the IR source (Figure
4.2b) and while lying horizontally on a glass surface (Figure 4.2c). Again, the
Figure 4.2 a) Deformations of the actuator (aspect ratio 1650 : 275 :1) upon
irradiation with an IR source for time t in seconds after switching on the light. b)
Deformation of a 12-lane patterned film, while being held vertically in front of the
IR source. c) Deformation of a 12-lane patterned film, while lying horizontally on a
surface. d) Fully contracted “teardrop” state of the twelve-lane polymer actuator. In
the bottom picture, a line drawing of the teardrop folds is overlaid on a section of
the photograph for increased clarity.
Chapter 4 Folding actuators with 3-dimensional alignment patterns
64
accordion shape was observed with the correct number of bends, but the amplitude
was smaller compared to the four-stripe actuator. Interestingly, upon heating the
film on the surface, sharp folds were observed (Figure 4.2c at t = 38 seconds). One
explanation for this is that due to the tips of the folds receiving more direct IR
irradiation, these areas have a higher temperature and bend more sharply. However,
the effect was not observed in the vertical experiments, and might also be caused
by friction between the sample and the glass substrate. In addition to the expected
shape change, a secondary deformation was observed in both experiments
involving the twelve-stripe actuator. In the vertical experiment, a right-handed
twist over the entire length of the film was observed (Figure 4.2b), while in the
horizontal experiment a sideways bending deformation took place. These two
observations are probably different manifestations of the same effect, which might
be related to the twist handedness of the twisted nematic moieties. Finally, it was
found that if the actuator was heated above a certain point it showed a strong
contraction, and the folds obtained a "teardrop" shape, resulting in an overall shape
not unlike a so-called “ruff collar”4 (Figure 4.2d). No further deformation was
observed after this shape was obtained. In this state, the folds would touch and
stick together, so the film did not revert back upon cooling unless the folds were
separated by hand.
Using the same method, a 2-stripe actuator film was prepared. From this film a
strip was cut at 45 degrees angle (25 mm x 5 mm, aspect ratio ~1300 : 260 : 1) to
the long axis of the stripes (Figure 4.3a). When this film was exposed to IR
irradiation, it formed a coiled shaped consisting of a left-handed coil and a right-
Figure 4.3 a) Cutting method for an alternating twisted nematic
coiling actuator. b) Actuation of the alternating twisted nematic
coiling actuator.
Chapter 4 Folding actuators with 3-dimensional alignment patterns
65
handed coil (Figure 4.3b). As with the previous actuators, the coil closest to the
clamping position was slightly suppressed.
4.2.4 Water-responsive actuators
To show that this exotic type of deformation can also be induced by other stimuli,
an actuator was prepared which responds to changes in the pH of an aqueous
environment. To this end, the reactive mesogen mixture with internal hydrogen
bonds presented in chapter 2 (Figure 4.4a) was used to make the liquid crystal
network.3 Upon immersion of such a film in a basic solution, the internal hydrogen
bonds are converted into a carboxylic salt. This causes an anisotropic swelling,
having a larger expansion in the direction perpendicular to the alignment director
Figure 4.4 a) Liquid crystal mixture used to make a pH responsive polymer actuator
(3OBA:5OBA:6OBA = 1:1:1, RM82 12 w%), including the initiator and chiral
dopant. b) Deformation of a pH responsive alternating twisted nematic actuator.
The film is flat at pH 6, and obtains the accordion shape when the pH is increased
to 11. When the pH is then reduced to 3, the flat shape is recovered. c) Normalized
FTIR spectra of the pH responsive film at various stages during the experiment. The
peak at 1682 cm−1 correspond to the C=O stretching in hydrogen bonded dimers,
while the peaks at 1547 and 1395 cm−1 are from antisymmetric and symmetric
COO− stretching, respectively.2
Chapter 4 Folding actuators with 3-dimensional alignment patterns
66
and a smaller expansion parallel to it. A ribbon of freestanding film with four
domains of alternating twisted alignment, prepared using the same alignment cells
as used before (Figure 4.1b), was immersed in demineralized water, with a 40 mg
metal weight attached to the far end to keep the film from floating to the surface or
crumpling up against the clamp. (Figure 4.4b). The pH of the medium was
monitored using a pH meter, and was determined to be 6.1 for demineralized water.
Also, the FTIR spectrum of the material was recorded prior to the experiment to
keep track of the presence of the internal hydrogen bonds (Figure 4.4c). A 0.1 M
KOH solution was added to increase the pH of the environment to 11.4. At this pH
the internal hydrogen bonds of the network were converted to the corresponding
carboxylic potassium salt, which was confirmed in the FTIR spectrum. The
induced hygroscopic behavior in the material caused it to take up water from the
aqueous medium, which in turn caused a decrease in anisotropy, and an accordion
shape was observed (Figure 4.4b). As observed before, the bends close to the
clamped end were somewhat suppressed, but the middle regions were bending as
programmed. After being exposed to the basic solution, the curvature of the
bending domains no longer increased, and the activation process was considered
complete. At this stage, four bends could be identified, which corresponded with
the four alignment domains. This bending is less pronounced when compared to
the heat responsive actuators due to the presence a weight on one end of the film.
When in a next step the pH was decreased to 2.3, the hydrogen bonds were
recovered and the FTIR spectrum reverted back to show the original peaks. As
expected, the film returned to its original flat shape.
4.2.5 Finite-element analysis
To examine the mechanism driving these shape transformations, a finite element
elastodynamics analysis was carried out by the group of Prof. Robin Selinger at
Kent state University. Thin film samples were meshed in three dimensions, rather
than treating them as having infinitesimal thickness, which was commonly done in
Chapter 4 Folding actuators with 3-dimensional alignment patterns
67
earlier simulations to reduce the demands on computing power. The Hamiltonian-
based model used includes the system’s kinetic energy, elastic energy, and
coupling between strain and nematic order. The scalar order parameter at the time
the material is cross-linked is defined by the parameter So. As the temperature
increases, the nematic order parameter S decreases, inducing a contraction of the
sample along the director and an elongation in the two directions perpendicular to
it. It was assumed that the director field is strongly anchored by cross-linking of the
polymer network such that it rotates with the body but does not otherwise reorient
in response to strain. This is the characteristic behavior of a nematic glass, and is a
reasonable approximation for this experimental system below its glass transition
temperature and where there is no external applied mechanical stress.
To represent a change of temperature, the scalar order parameter S was gradually
raised (to represent cooling) or lowered (to represent heating) and was then held
fixed until the sample reached elastic equilibrium. Finite element simulations of
Figure 4.5 Finite element simulation of the shape deformation of a nematic
network with alternating twisted domains when its nematic order decreases on
heating. For stripe width/thickness ratio in the range of 4–25, smooth, sinusoidal
deformation was observed; a) four stripes (ratio = 12.5) and b) eight stripes (ratio
= 6.25). c) Amplitude of resulting undulations increases as a function of stripe
width/thickness ratio; solid line represents the analytical prediction and square
points are simulation data. d) Simulation of a longer sample with stripe
width/thickness ratio = 50 and a larger temperature change (∆S = –0.8) shows
formation of teardrop-shaped accordion folds.
Chapter 4 Folding actuators with 3-dimensional alignment patterns
68
samples with aspect ratio 50 : 10 : 1 and with stripe widths of different sizes are
shown (Figure 4.5a-b), starting with a sample with four stripes of alternating twist
orientation (Figure 4.1a and Figure 4.5a). As temperature increased, formation of
undulations with two crests and two valleys was observed. The handedness of the
director twist induced a slight chiral bend within each stripe; resulting in a small
zigzag deformation along the sample borders. The simulation was repeated for a
sample of the same aspect ratio, divided into 8 stripes (Figure 4.5b). Simulations
were carried out for samples with 2 to 12 stripes, keeping the sample aspect ratio
constant, and it was found that larger stripe width drives larger undulation
amplitude (Figure 4.5c). This is consistent with the experimental finding that a
4-stripe actuator shows larger amplitude compared to a 12-stripe actuator. For a
large enough stripe width to thickness ratio, a transition from a sinusoidal
undulation to the formation of teardrop-shaped accordion folds was observed
(Figure 4.5d). The central axis of the ribbon formed approximately circular arcs of
alternating orientation. Finite element simulation also predicted that the ribbon
flexes across its short axis, with orientation alternating along each circular arc. A
slight out-of-plane twist, similar to that seen in the practical experiments, was also
observed (as seen in Figure 4.2b).
Using a mathematical model, the same group also estimated the amount of work
these actuators can output. For the experimental sample, the dimensions were set to
30 mm × 5 mm × 15 μm and the Young’s modulus was set to 1 GPa (below the
glass transition). The maximum work that can be done by contraction was
estimated to be approximately 3 × 10 −6 J. For comparison, the mass of the sample
is approximately 3 × 10 −6 kg, so Wmax is about three times the work needed to lift
its own mass by its own length.
Chapter 4 Folding actuators with 3-dimensional alignment patterns
69
4.2.6 Checkerboard pattern
To extend the fabrication method to an even more complex deformation, a film was
prepared with two orthogonally superimposed stripe masks to make a checkerboard
pattern. Four irradiation steps were required, while rotating the substrate 90
degrees after each step, to expose the whole substrate area (Figure 4.6a). Again, an
alignment cell was prepared and filled with the mixture of reactive mesogens
(Figure 4.1b), which was cured through UV irradiation, producing a director
pattern with twisted domains (Figure 4.6a). After removal of the film from the cell
(23 x 23 mm, aspect ratio roughly 1810 : 1810 : 1), the actuation experiment was
carried out for the checkerboard film (Figure 4.6b). Upon heating, the film
deformed from a mostly flat shape to form a periodic square pattern of peaks,
depressions, and saddle points. Like the 12-stripe accordion, a deformation over the
entire surface of the film was observed. The corners of the film bent inwards to
give the film an overall dome-shaped structure.
4.2.7 Finite element analysis of checkerboard pattern
To gain an understanding of this actuation pattern, a finite element analysis
simulation of a 3 x 3 checkerboard actuator was performed by the group of Prof.
Robin Selinger, showing shape evolution with increasing temperature (Figure 4.7).
A mesh with aspect ratio of 50 : 50 : 1 was used. The film distorted into an
Figure 4.6 a) Preparation procedure of the checkerboard alignment layers, and top view of the
final pattern (only 4×4 squares shown) b) Deformation upon IR heating of a film with a
checkerboard pattern (aspect ratio 1810 : 1810 : 1).
Chapter 4 Folding actuators with 3-dimensional alignment patterns
70
arrangement of peaks, depressions, and saddle points corresponding with the
square domains. The simulation also shows an overall dome-shape superposed over
the smaller wavelength undulation, in agreement with experimental findings.
4.2.8 Radimuthal films
In a similar way, a heat-responsive network with a continuous pattern in the
xy-plane and a 90 degrees twist along the z-direction was prepared. To do so, an
alignment cell was constructed from two glass substrates, one having a radial
alignment layer and the other one an azimuthal alignment layer. The resulting
polymer film obtained from this cell possessed a liquid crystal configuration with a
gradual transition from azimuthal to radial through the thickness of the film, which
can be described as a circular twisted nematic alignment, and was termed the
“radimuthal” alignment. It is patterned in all three dimensions, as it has both a
circular director variation in the xy-plane and a gradual variation along the
z-direction (Figure 4.8a). It showed the optical properties between parallel and
crossed polarizers that can be expected for twisted nematic systems, namely full
transmission between crossed polarizers and partial transmission between
perpendicular polarizers (Figure 4.8b).
When a radimuthal film was heated, it deformed by folding four of its outside flaps
inward to form a square shape (Figure 4.8c). This behavior can be explained as
follows: when heated, the film experienced forces similar to the twisted nematic
films discussed in the introduction, and tries to deform into a 3-dimensional shape.
Figure 4.7 a) Top view of the director microstructure of a 3 × 3 checkerboard
actuator. b) Finite element simulation of the 3 × 3 checkerboard actuator when
heated to the isotropic state.
Chapter 4 Folding actuators with 3-dimensional alignment patterns
71
However, suppression of curvature by bending in a single direction is not possible
in this system due to geometric constraints. This can be envisioned by dividing the
film into a number of angular sections that behave as twisted nematic actuators and
fold towards the side with radial alignment (Figure 4.8d). However, due to these
domains being joined together, bending can only take place in the outer areas of the
domains. The film therefore deforms into a polygon with curled edges and a flat
center. The number of domains, and as such the number of edges of the polygon, is
determined by a tradeoff between the total amount of bending surface and the
deviation from the preferred bending direction. When the number of domains is
small, more surface area is allowed to bend to release the stress, but as the domains
are larger the bending direction close to the borders of the domains deviates more
strongly from the preferred bending direction (Figure 4.8e). The optimal shape can
be found by minimizing the internal energy of the system (Figure 4.8f), and was
found to have four domains, which corresponds to a square shape.
Figure 4.8 a) Schematic representation of the “radimuthal” alignment profile, including cross sections
at z = 0, z = 0.5h, and z = h. b) The actual film, as seen between crossed polarizers (left) and parallel
polarizers (right). c) Deformation behavior of the film on a hot plate, as well as a schematic drawing of
the deformation. The film folds four flaps inward to form shape resembling a square. d) An angular
section of the film, with the alignment directors and the preferred bending direction. e) Division of a
circular film into four and five sections. The areas that are not joined, and can therefore bend, are
shown in gray. f) Internal energy of the system (U) as a function of the number of sections (mp). The
energy minimizes at mp = 4.
Chapter 4 Folding actuators with 3-dimensional alignment patterns
72
4.3 Conclusions and outlook
A new family of actuators with a 3D-patterned director profile that show exotic
deformations was prepared and analyzed. The results show that exciting new
deformations in liquid crystal networks can be achieved by 3D structuring. This
could eventually lead to new ways to perform self-folding in plastic materials.
Striped actuators showed accordion-like deformations upon exposure to an
appropriate stimulus, which, depending on the chosen chemical composition of the
mixture of reactive mesogens, could either be a temperature or a pH variation. This
kind of out-of-plane actuation achieves a larger total displacement for a given
temperature change than could be accomplished by contracting a monodomain
sample of the same material with uniform nematic director. It was also shown that
these deformations can be predicted with a mathematical simulation based on finite
element analysis. Such actuators are potentially useful in applications where large,
complex displacements are valued above strength and power.
4.4 Experimental Section
Materials: All materials were used as received from Merck (RM82, RM105,
RM23), Philips Research Organics (3-OBA, 6-OBA), Synthon Chemicals
(5-OBA), BASF (Lumogen 788 IR, LC756), Ciba Specialty Chemicals (Irgacure
819), Rolic (ROP-108), and Aldrich (t-butyl hydroquinone, dichloromethane). The
alignment cells were prepared using 18 micron SP-218 Micropearl plastic spacers
(Sekisui Chemical Co.).[25]
Layers of the photoalignment material (ROP-108) were spincoated onto glass
substrates using a Karl Suss RC8 spincoater. Photoalignment was carried out using
an Ominicure S2000 UV lamp. A Newport 10LP-UV precision linear polarizer,
mounted in a Thorlabs PRM1/MZ8 rotation mount, was used to polarize the UV
light in various directions, and the substrate was rotated using a Thorlabs CR1/M-
Z7 rotation stage (irradiation intensity ~13 mW cm-2 in the UVA range). The
Chapter 4 Folding actuators with 3-dimensional alignment patterns
73
rotation of the polarizer and stage was controlled with Thorlabs TDC001
controllers, which were programmed with APT software. Photopolymerization of
the liquid crystal mixtures was carried out using an Omnicure S2000 UV lamp with
320-500 nm filter (~25 mW cm-2). To remove the alignment material from the
surface of the pH-responsive actuators, an Emitech K1050X plasma asher was
used.
The thickness of the actuator films was measured using both a Fogale nanotech
zoomsurf 3D interferometer and a Veeco Dektak 150 surface profilometer. Slide-
on ATR IR spectra of the pH-responsive films were recorded using a Varian 670
FTIR spectrometer. For the IR actuation experiments, a Heraeus Noblelight
Shortwave surface irradiator (type “Duo”, 250 Watt, 57.5 Volt) with gold reflector
was used. During the pH experiments, the pH of the solution was monitored using
an inoLab pH meter.
Preparation of alternating twisted alignment cells: 3x3 cm glass plates were
cleaned with ethanol. ROP-108 was spincoated onto the plates at 1500 RPM for 30
seconds. The plates were heated briefly to 115 oC to evaporate the solvent.
Irradiation of each LPP substrate was carried out through a photomask with
polarized UV light for 50 seconds. When a 4- or 12-stripe pattern was prepared,
two irradiations were carried out on each substrate. After the initial irradiation step,
the substrate was rotated 180 degrees to expose the non-irradiated areas, while the
polarizer was rotated 90 degrees, and a second irradiation was carried out. When a
chessboard pattern was prepared, the substrates were irradiated four times and
rotated 90 degrees between each irradiation step. In all cases, the plates were glued
into cells, using 18 micron plastic spacers to secure the cell gap. Care was taken to
glue the cells in an orthogonal fashion to obtain cells that force the LC material
into a twisted nematic conformation.
Preparation of “radimuthal” alignment cells: These cells were prepared in the same
way as the alternating twisted alignment cells, except that the plates were irradiated
Chapter 4 Folding actuators with 3-dimensional alignment patterns
74
through a wedge-shaped mask on a rotating substrate (as described in the previous
chapter).
Preparation of heat-responsive, patterned, twisted nematic actuators: RM82 (201.1
mg), RM105 (302.2 mg), and RM23 (101.5 mg) were put into a screw-cap vial. A
1.5 mg ml-1 solution of the thermal inhibitor tert-butyl hydroquinone and the chiral
dopant LC756 in CH2Cl2 was added (~ 90 μL), as well as the radical initiator
Irgacure 819 (3.3 mg) and the IR absorber Lumogen 788 IR (0.8 mg). The
components were dissolved in CH2Cl2, which was then evaporated by stirring the
solution at 80 oC. An alignment cell was brought to 80 oC, and the cell gap was
filled with the mixture though capillary action. When the cell was completely
filled, it was cooled down slowly to 60 oC. Then UV polymerization took place in
the nematic phase by UV irradiation for 5 minutes. Finally, the cell was opened to
retrieve the freestanding polymer film. The film thickness was measured using
surface profilometry and interferometry.
Preparation of pH-responsive, alternating twisted nematic actuators: 3-OBA (99.5
mg), 5-OBA (99.8 mg), 6-OBA (99.9 mg), RM82 (42.2 mg), and Irgacure 819 (6.9
mg) were put into a screw-cap vial. Tert-butyl hydroquinone (0.071 mg) and
LC756 (0,35 mg) were added as a solution in CH2Cl2. The components were
dissolved in CH2Cl2, the resulting solution was stirred while heating with a heat
gun, and the solution was vortex-stirred for 5 minutes. The solvent was removed by
heating the solution to 30 oC under argon flow, followed by drying in vacuo for one
hour. An alignment cell was brought to 90 oC, and the cell gap was filled with the
mixture though capillary action. When the cell was completely filled, it was cooled
down slowly to 80 oC. Then UV polymerization took place by UV irradiation for 5
minutes. Finally, the cell was opened to retrieve the freestanding film.
Investigation of IR actuation: Strips were cut from the alternating twisted nematic
films. The following dimensions were used: 4-stripe and 12- stripe actuator:
30 mm x 5 mm, chessboard actuator: 23 mm x 23 mm. The films were actuated
Chapter 4 Folding actuators with 3-dimensional alignment patterns
75
either in the vertical or horizontal position. When actuating in the vertical position,
they were held with a tweezers in front of the IR lamp until actuation took place.
When horizontal actuation was required, the actuator was placed on a glass slide
which was covered with sand to reduce friction. The material was then irradiated
from above.
Investigation of pH actuation: The film was subjected to oxygen plasma ashing for
4 minutes at 45 Watt to remove any remaining alignment material from the surface.
An IR spectrum of the film was measured, and a 40 mg weight was attached to one
end. The film was immersed in demineralized water, which was brought to pH 11.4
by slowly adding a 0.1 M KOH solution, and kept at that level for 17 minutes. The
film was removed from the medium, dried, and an IR spectrum was recorded. The
film was placed back, and the pH was decreased to 2.3 by the slow addition of a
1 M HCl solution. The film was removed from the medium, dried, and an IR
spectrum was recorded.
Finite Element Elastodynamics Simulation: In order to implement finite element
elastodynamic simulations, each thin film sample was discretized into an
unstructured 3-D tetrahedral mesh. Meshes were generated using Salome, an open
source tool downloaded from http://www.salome-platform.org. A mesh of aspect
ratio 50-10-1 with 66,647 tetrahedra and 14,274 nodes was used. In figure 4d a
mesh of aspect ratio 200-10-1 with 270,549 tetrahedra and 52,850 nodes was used.
The sample stripe width/thickness ratio in the experimental studies was
approximately 130. However due to hardware limitations, the largest stripe
width/thickness achievable in the simulations was 50, considering the size of the
required mesh. This ratio was sufficiently high to drive formation of teardrop-
shaped accordion folds. The checker board actuator was modeled using a mesh of
aspect ratio 50-50-1 with 268246 tetrahedra and 52780 nodes.
Chapter 4 Folding actuators with 3-dimensional alignment patterns
76
4.5 References
1. (a) Mol, G. N.; Harris, K. D.; Bastiaansen, C. W. M.; Broer, D. J., Thermo-
mechanical responses of liquid-crystal networks with a splayed molecular
organization. Adv. Funct. Mater. 2005, 15 (7), 1155-1159; (b) Warner, M.; Modes,
C. D.; Corbett, D., Curvature in nematic elastica responding to light and heat. P.
Roy. Soc. A-Math. Phy. 2010, 466 (2122), 2975-2989; (c) Warner, M.; Modes, C.
D.; Corbett, D., Suppression of curvature in nematic elastica. P. Roy. Soc. A-Math.
Phy. 2010, 466 (2124), 3561-3578.
2. Lee, K. M.; Bunning, T. J.; White, T. J., Autonomous, hands-free shape
memory in glassy, liquid crystalline polymer networks. Adv. Mater. 2012, 24 (21),
2839-2843.
3. (a) Broer, D. J.; Bastiaansen, C. M. W.; Debije, M. G.; Schenning, A. P. H.
J., Functional organic materials based on polymerized liquid-crystal monomers:
Supramolecular hydrogen-bonded systems. Angew. Chem. Int. Edit. 2012, 51 (29),
7102-7109; (b) Harris, K. D.; Bastiaansen, C. W. M.; Broer, D. J., Physical
properties of anisotropically swelling hydrogen-bonded liquid crystal polymer
actuators. J. Microelectromech. S. 2007, 16 (2), 480-488; (c) Harris, K. D.;
Bastiaansen, C. W. M.; Lub, J.; Broer, D. J., Self-assembled polymer films for
controlled agent-driven motion. Nano Lett. 2005, 5 (9), 1857-1860.
4. Muller, M. M.; Ben Amar, M.; Guven, J., Conical defects in growing
sheets. Phys. Rev.Lett. 2008, 101 (15), 156104.
77
Chapter 5. Generating wrinkling patterns in
metal/LC-polymer bilayer films
5.1 Introduction
This chapter describes a way in which liquid crystal polymer films with a patterned
director profile in the xy-plane can be used to prepare complex microstructures. In
this case, no use is made of the anisotropic expansion and contraction of the film to
create exotic actuators; instead, the anisotropy of the modulus is used to control the
formation of surface wrinkles in a bilayer system consisting of a liquid crystal
network and a thin layer of gold.
Over the past decade, the elastic instability of thin sheets, such as surface
wrinkling, has gained significant interest from the scientific community.1
Controlled wrinkle formation can provide a quick and easy method for the
spontaneous generation of a variety of microstructured surfaces. Wrinkling can be
induced in bilayer systems consisting of a thin film of a relatively stiff material on
top of a foundation of a softer material as a result of a strain mismatch between the
layers, usually induced by mechanical or heat-driven contraction of the foundation
or expansion of the top layer. It is a well-studied and well-understood process.2
Many applications for wrinkled surfaces have been shown, including preparation of
superhydrophobic3 and antifouling surfaces,4 actuators,5 generation of strain-related
colors,6 alignment of living cells,7 optical focusing,8 fabrication of microlens
arrays,9 fabrication of patterned electrodes,10 stretchable conductors,11 particle
separation,12 and strain-sensitive diffraction gratings.13 Achieving precise control
over the wrinkling properties is vital for the further development of these
applications. While the amplitude and period of the wrinkles can be easily tuned by
changing the thickness of the thin top layer,14 and the preparation of
Chapter 5 Generating wrinkling patterns in metal/LC-polymer bilayer films
78
homogeneously wrinkled surfaces is rather straightforward,2c, 13b, 14a, 15 it has proven to
be far more challenging to generate complex patterns in the wrinkle direction.
There are many examples of research where some control over the wrinkle
direction was obtained, for example by introduction of plateaus and ridges,2a local
photostiffening of the foundation,16 creating areas of reduced adhesion in the
substrate,17 two-step plasma exposure,18 and controlled diffusion of a solvent.19
However, these techniques are restricted to a limited number of patterns that can be
produced, often apply only to small surface areas, and may require extensive and
minute local surface modifications. Arbitrary wrinkling patterns have also been
prepared using either patterned PDMS molds, or path-guided wrinkling by locally
modifying the modulus of the top layer with a laser.19a However, this requires direct
“writing” of the desired pattern instead of allowing the patterns to form
spontaneously, which in a way defeats their purpose as a quick access to a large
variety of microstructured surfaces. This chapter describes a new solution to this
challenge: a simple, contactless method for the spontaneous generation of complex
wrinkle patterns in a bilayer system consisting of a thin layer of gold on an LC
polymer network foundation. The wrinkling is driven by stresses which are induced
during the deposition of the gold layer, which can be released by heating the
system to lower the modulus of the foundation. Wrinkling will take place in the
direction of the lowest foundation modulus, which is perpendicular to the
alignment director. Therefore, the wrinkles will always follow the alignment
director (Figure 5.1a). This allows patterning of the wrinkle direction using
photoalignment.
Chapter 5 Generating wrinkling patterns in metal/LC-polymer bilayer films
79
5.2 Results and discussion
5.2.1 Sample preparation and wrinkle formation
Photoalignment of the polymerizable LC was carried out in the same way as
described in chapters 3 and 4. Alignment cells were prepared by combining two
glass plates coated with a LPP (ROP-108), which was photoaligned by exposure to
polarized ultraviolet light through a variety of photomasks (Figure 5.1b and 5.1c).
The cells were filled with the same mixture of polymerizable liquid crystals that
was previously used to make actuator films (Figure 5.1d). Filling of the cells was
Figure 5.1 a) Mechanism of wrinkle formation. Heating causes the liquid crystal
network to go through the glass transition, which allows release of the stress in the gold
layer through wrinkling, which takes place in the direction with the lowest modulus. b)
Photoalignment setup used for the preparation of discrete patterns. c) Photoalignment
setup used for the preparation of continuous circular patterns. It consists of a UV lamp,
a polarizer, a photomask, and the substrate bearing the photoalignment layer. d)
Polymerizable liquid crystals that were used. e) Preparation procedure to make the
wrinkle patterns out of the patterned alignment cells.
Chapter 5 Generating wrinkling patterns in metal/LC-polymer bilayer films
80
carried out at 80 oC in the isotropic phase, and photopolymerization was carried out
at 60 oC in the nematic phase. The cell was opened by removing one of the glass
plates, and a gold layer was sputter coated on top of the solid LC film (Figure
5.1e). Putting the material on a hotplate at 60 oC then caused wrinkles to appear in
a few minutes, which could be readily detected as a transition from a reflective to a
diffractive surface. The presence of the wrinkles was confirmed through optical
microscopy (Figure 5.2a), and it was clear that the wrinkles always follow the
alignment director of the underlying liquid crystal network. This is in contrast to
previous research using a crosslinked liquid crystal network as the foundation,
where the wrinkles always ran perpendicular to the alignment director when
formed upon heating,20 or parallel to the director when formed upon cooling.14b, 20a, 20b
In these latter cases, the anisotropic contraction of the liquid crystal network, which
takes place parallel to the director upon heating or perpendicular upon cooling, was
responsible for the formation of wrinkles.
To test if anisotropic deformation in the liquid crystal network was required for
wrinkle formation in our samples, an isotropic liquid crystal network was prepared
by polymerization at a temperature above the nematic-to-isotropic transition. When
a gold layer was applied to this network, wrinkling still took place after heating,
but the wrinkles were randomly oriented (Figure 5.2b). These random wrinkles
were similar to the aligned wrinkles in period and amplitude (see also Figure 5.4e),
which indicates that their formation is strictly caused by the changing materials
properties during heating, and the anisotropy only serves to steer the wrinkling in a
specific direction. This means that compressive stress induced in the system during
the sputter coating process21 is the driving force for wrinkling. At room
temperature, this stress δ is below the critical stress δc required to induce wrinkling
of the gold film according to , where Ep and Em are the elastic
moduli of the polymer layer and the metal film, respectively.22 This creates a
metastable state in which wrinkling does not yet take place. Upon heating the
liquid crystal network foundation above the glass transition temperature, the
Chapter 5 Generating wrinkling patterns in metal/LC-polymer bilayer films
81
modulus Ep decreases, lowering the critical stress below the compressive stress and
allowing wrinkling to occur. The modulus of the LC network is anisotropic, with
the lowest modulus perpendicular to the alignment director. This means that δc is
lower in that direction, dictating that the film will buckle perpendicular to the
alignment director, and as a result the wrinkles will always follow the alignment
director (Figure 5.1a). Once grown, these parallel wrinkles rigidify the structure
and prevent the formation of wrinkles along another direction, an effect that is
reminiscent of the high rigidity observed for corrugated cardboards.
Figure 5.2 a) Optical microscopy image of a wrinkled surface. b) Optical microscopy
picture of a wrinkled surface on an isotropic liquid crystal foundation, showing
randomly oriented wrinkles. c) Typical surface profile of the wrinkled gold surface. d)
Zoom-out microscopy picture of the second order, larger wrinkles in which the smaller
wrinkles are nested. e) The same surface profile, viewed over a longer distance to show
the second-order wrinkles. f) AFM image of the wrinkled surface.
Chapter 5 Generating wrinkling patterns in metal/LC-polymer bilayer films
82
5.2.2 Measuring the wrinkle period and amplitude
The wrinkled surfaces were further studied using surface profilometry: a typical
profile, viewed over a 15 μm distance, is shown in Figure 5.2c. Surprisingly, when
viewed over longer distances it became clear that the profiles show a strong
periodic variation in peak height, forming what seems to be a second-order
hierarchically wrinkled structure (Figure 5.2d and 5.2e). Hierarchical wrinkling has
been previously reported in literature, and takes place when the amplitude of the
wrinkles reaches a maximum, causing the wrinkled layer to act as a much thicker
“effective layer” which then wrinkles with a much longer period.12 However, in this
case the smaller wave has varying amplitude, which is not expected for hierarchical
Figure 5.3 a) Plot of the measured average wrinkle period as a function of
sputtered gold layer thickness as estimated from standard sputter coater
deposition rate. The pattern was measured over a 400 μm length, then the
average distance between each peak and valley was calculated. For each
sample 3-4 measurements were carried out. b) Average wrinkle amplitude as a
function of average wrinkle period. The pattern was measured over a 400 μm
length, then the average distance between each peak and valley was
calculated. For each sample 3-4 measurements were carried out. c) Images
taken of samples using the profilometer with (left) 55 nm and (right) 225 nm
sputtered gold layers.
Chapter 5 Generating wrinkling patterns in metal/LC-polymer bilayer films
83
wrinkling. This implies that it is an oscillating amplitude instead. It is not clear why
the amplitude oscillates. AFM was used to confirm the profilometry measurements,
and showed a wrinkled surface with similar characteristics (Figure 5.2f).
A series of films with varying gold layer thicknesses was prepared by varying the
sputter time during the application of the gold. It was observed that the wrinkle
amplitude and period increased with thicker gold layers (Figure 5.3a and 5.3b),
which was expected due to the relations ~ , where λw is the wrinkle
period and h is the thickness of the metal film, and ~ , where A is the
wrinkle amplitude. From these relations, it is expected that the wrinkle period
increases linearly with gold layer thickness, and the wrinkle amplitude increases
linearly with increasing period.22 However, at higher values the error in the
measurements increased substantially, so it was difficult to verify this. It was also
observed that when thicker gold layers were used, the surface became more
irregular (Figure 5.3c), which could explain the high error in the measurements.
5.2.3 Preparation and examination of various wrinkle patterns
Alignment cells with various alignment patterns (Figure 5.4a) were used to prepare
a variety of different alignment profiles within the liquid crystal networks. Discrete
patterns, such as lines (patterns I and II), squares (pattern III), and text (pattern VII)
were prepared using photomasks (Figure 5.1b). Continuous circular patterns, such
as azimuthal (pattern IV), radial (pattern V), and an even higher complexity pattern
(pattern VI) were prepared through photoalignment on a rotating substrate
(Figure 5.1c). The samples diffract light perpendicular to the alignment direction of
the wrinkles, which allows visual characterization of the patterns and results in
striking optical effects. The patterns were also more closely analyzed with
microscopy, which showed the relatively smooth and localized boundary transition
Chapter 5 Generating wrinkling patterns in metal/LC-polymer bilayer films
84
between two domains (Figure 5.4b and 5.4c). Of special interest is pattern VI
(Figure 5.4d), which would be especially difficult to prepare using previously
reported wrinkling techniques, without the use of pre-patterned molds or laser
writing on the gold surface.
To determine if the domains sizes of the different patterns had any effect on the
period and/or amplitude of the wrinkles, an in-depth analysis of patterns I, II, and
III generated in the 55 nm gold layer was performed, and the average wrinkle
height and period were determined for all three alignments (Figure 5.4e). For the
two samples with rectangular domains (pattern I and II), the results were grouped
based on whether the wrinkle direction in the measured domain was parallel or
Figure 5.4 a) Overview of the different patterns, both schematically and visually. In pattern VII, the
alignment is vertical in the text and horizontal around it. b) Microscopy image of the domain border
in pattern I. c) AFM image of the domain border in pattern II. d) Microscopy image of the center of
pattern VI. The four “lobes” in the pattern are clearly visible. e) Table showing the average wrinkle
period and height of patterns I-VI, as measured with profilometry on a 400 μm trajectory at multiple
locations. For patterns with rectangular domains (I and II), the data is divided between measurements
parallel and perpendicular to the long axis of the domain. The amplitude of the wrinkles on the
isotropic network is also shown, although the period could not be accurately measured.
Chapter 5 Generating wrinkling patterns in metal/LC-polymer bilayer films
85
perpendicular to the long axis of the domain. It appears that wrinkles in the
domains perpendicular to the long axis are quite similar in period and height
compared to domains parallel to the long axis. A series of samples were measured
for patterns displaying 1, 2, 4, 12, and 36 domains per side (with 1, 4, 16, 144, and
1296 individual ‘squares’ produced on LC layers of between 7 and 30 μm thick:
the variations were not large and there was no obvious correlation between LC
layer patterning and the period and amplitude of the generated wrinkles. Thus, at
least at the distance scales of these experiments, there is little impact of the
proximity of the domain boundaries on either the height or period of the wrinkling.
This allows reproducible wrinkling over a range of pattern sizes and forms.
Using the same technique, the films with curved patterns (Figure 5.4a, patterns
IV-VI) were also examined. From these pictures, it became clear that these
wrinkles also follow the curved alignment of the liquid crystal network.
Profilometry measurements were performed on pattern VI (Figure 5.4e) at
locations far from the central region, as the strong curvature of the wrinkles near
the center distorted the results. The wrinkle amplitude and depth were comparable
to patterns I and II. This implies that continuous circular patterns behave similarly
to discrete patterns. Thus, the photopatterning technique allows generation of a
myriad of different wrinkling alignments without adversely affecting the
periodicity or height of the wrinkling pattern.
5.3 Conclusions and outlook
In conclusion, a simple, contactless method to generate well-defined, controlled,
and complex wrinkling patterns has been demonstrated, which involves
sputtercoating a thin layer of gold on photoaligned liquid crystal networks,
followed by heating. The wrinkling always takes place perpendicular to the
alignment director of the LC network, due to the lower modulus in that direction.
Interestingly, a periodic oscillation in the amplitude of the wrinkles was observed.
Chapter 5 Generating wrinkling patterns in metal/LC-polymer bilayer films
86
There appears to be little or no dependence of wrinkle period and height on the
specific pattern, suggesting that the proximity of pattern boundaries has little
impact on the wrinkle formation. The method is versatile, straightforward, and does
not require a complex setup. The process requires no physical contact with the
substrate or fabrication of expensive masters or masks, and could be applied over
large surfaces, if desired. The preparation of complex diffraction gratings has been
shown using this technique, but it could be easily extended to any of the
applications mentioned earlier, such as alignment of cells and patterned electronics.
5.4 Experimental section
A photoalignment material (ROP-108, Rolic) was spin coated on clean 30 x 30
mm2 glass substrates at 1500 RPM for 30 seconds, followed by brief heating to 115 oC to remove the solvent. The substrates were glued into cells using 18 μm spacers.
The completed cells were then subjected to irradiation with polarized UV light via
mask exposure in various ways to create the patterns (Figure 5.1b and 5.1c). The
cells were filled at 80 oC with a mixture of reactive liquid crystals (RM82, RM105
and RM23 in a 2 : 3 : 1 ratio, Merck) containing a radical initiator (Irgacure 819,
Ciba) to allow photopolymerization, and an inhibitor (tert-butyl hydroquinone,
Fluka) to prevent thermal polymerization. The filled cell was allowed to cool to 60 oC, after which the mixture was cured for 5 minutes using UV irradiation. After
cooling to room temperature one plate of the cell was removed, and the polymer
film was sputter coated with gold at room temperature in an Emitek K575X sputter
coater operated at 65 W, where the sputtering duration determined the thickness of
the gold layer. The gold coated LC films were heated on a hot stage at 60 oC until
wrinkling became visible due to the optical effects caused by the generation of
diffractive structures. The entire procedure for producing the films is depicted in
Figure 5.1. The surface of one of the films was observed through an optical
microscope (Leica DM-6000M)), and a profilometer (Veeco Dektak 150, Bruker)
was used to characterize the surface of all the samples.
Chapter 5 Generating wrinkling patterns in metal/LC-polymer bilayer films
87
5.5 References
1. (a) Chen, C. M.; Yang, S., Wrinkling instabilities in polymer films and
their applications. Polym. Int. 2012, 61 (7), 1041-1047; (b) Chung, J. Y.; Nolte, A.
J.; Stafford, C. M., Surface wrinkling: A versatile platform for measuring thin-film
properties. Adv. Mater. 2011, 23 (3), 349-368; (c) Genzer, J.; Groenewold, J., Soft
matter with hard skin: From skin wrinkles to templating and material
characterization. Soft Matter 2006, 2 (4), 310-323; (d) Grinthal, A.; Aizenberg, J.,
Adaptive all the way down: Building responsive materials from hierarchies of
chemomechanical feedback. Chem. Soc. Rev. 2013, 42 (17), 7072-7085; (e) Li, B.;
Cao, Y.-P.; Feng, X.-Q.; Gao, H., Mechanics of morphological instabilities and
surface wrinkling in soft materials: a review. Soft Matter 2012, 8 (21), 5728-5745;
(f) Singamaneni, S.; Tsukruk, V. V., Buckling instabilities in periodic composite
polymeric materials. Soft Matter 2010, 6 (22), 5681-5692.
2. (a) Bowden, N.; Brittain, S.; Evans, A. G.; Hutchinson, J. W.; Whitesides,
G. M., Spontaneous formation of ordered structures in thin films of metals
supported on an elastomeric polymer. Nature 1998, 393 (6681), 146-149; (b) Brau,
F.; Vandeparre, H.; Sabbah, A.; Poulard, C.; Boudaoud, A.; Damman, P., Multiple-
length-scale elastic instability mimics parametric resonance of nonlinear
oscillators. Nat. Phys. 2011, 7 (1), 56-60; (c) Cerda, E.; Mahadevan, L., Geometry
and physics of wrinkling. Phys. Rev. Lett. 2003, 90 (7), 074302; (d) Diamant, H.;
Witten, T. A., Compression induced folding of a sheet: An integrable system. Phys.
Rev. Lett. 2011, 107 (16), 164302; (e) Huang, J.; Juszkiewicz, M.; de Jeu, W. H.;
Cerda, E.; Emrick, T.; Menon, N.; Russell, T. P., Capillary wrinkling of floating
thin polymer films. Science 2007, 317 (5838), 650-653; (f) King, H.; Schroll, R.
D.; Davidovitch, B.; Menon, N., Elastic sheet on a liquid drop reveals wrinkling
and crumpling as distinct symmetry-breaking instabilities. P. Natl. Acad. Sci. USA.
2012, 109 (25), 9716-9720; (g) Pocivavsek, L.; Dellsy, R.; Kern, A.; Johnson, S.;
Lin, B.; Lee, K. Y. C.; Cerda, E., Stress and fold localization in thin elastic
membranes. Science 2008, 320 (5878), 912-916.
Chapter 5 Generating wrinkling patterns in metal/LC-polymer bilayer films
88
3. (a) Kim, S.-J.; Park, H.-J.; Lee, J.-C.; Park, S.; Ireland, P.; Park, S.-H., A
simple method to generate hierarchical nanoscale structures on microwrinkles for
hydrophobic applications. Mater. Lett. 2013, 105, 50-53; (b) Kim, Y. H.; Lee, Y.
M.; Lee, J. Y.; Ko, M. J.; Yoo, P. J., Hierarchical nanoflake surface driven by
spontaneous wrinkling of polyelectrolyte/metal complexed films. ACS Nano 2012,
6 (2), 1082-1093; (c) Li, Y.; Dai, S.; John, J.; Carter, K. R., Superhydrophobic
surfaces from hierarchically structured wrinkled polymers. ACS Appl. Mater. Inter.
2013, 5 (21), 11066-11073.
4. Efimenko, K.; Finlay, J.; Callow, M. E.; Callow, J. A.; Genzer, J.,
Development and testing of hierarchically wrinkled coatings for marine
antifouling. ACS Appl. Mater. Inter. 2009, 1 (5), 1031-1040.
5. Watanabe, M.; Shirai, H.; Hirai, T., Wrinkled polypyrrole electrode for
electroactive polymer actuators. J. Appl. Phys. 2002, 92 (8), 4631-4637.
6. Xie, T.; Xiao, X.; Li, J.; Wang, R., Encoding localized strain history
through wrinkle based structural colors. Adv. Mater. 2010, 22 (39), 4390-4394.
7. (a) Greco, F.; Fujie, T.; Ricotti, L.; Taccola, S.; Mazzolai, B.; Mattoli, V.,
Microwrinkled conducting polymer interface for anisotropic multicellular
alignment. ACS Appl. Mater. Inter. 2012, 5 (3), 573-584; (b) Yang, P.; Baker, R.
M.; Henderson, J. H.; Mather, P. T., In vitro wrinkle formation via shape memory
dynamically aligns adherent cells. Soft Matter 2013, 9 (18), 4705-4714.
8. Li, R.; Yi, H.; Hu, X.; Chen, L.; Shi, G.; Wang, W.; Yang, T., Generation
of diffraction-free optical beams using wrinkled membranes. Sci. Rep. 2013, 3,
2775.
9. Chan, E. P.; Crosby, A. J., Fabricating microlens arrays by surface
wrinkling. Adv. Mater. 2006, 18 (24), 3238–3242.
10. Wu, H.; Menon, M.; Gates, E.; Balasubramanian, A.; Bettinger, C. J.,
Reconfigurable topography for rapid solution processing of transparent conductors.
Adv. Mater. 2014, 26 (5), 706-711.
11. Lacour, S. P.; Wagner, S.; Huang, Z.; Suo, Z., Stretchable gold conductors
on elastomeric substrates. Appl. Phys. Lett. 2003, 82 (15), 2404-2406.
Chapter 5 Generating wrinkling patterns in metal/LC-polymer bilayer films
89
12. Efimenko, K.; Rackaitis, M.; Manias, E.; Vaziri, A.; Mahadevan, L.;
Genzer, J., Nested self-similar wrinkling patterns in skins. Nat. Mater. 2005, 4 (4),
293-297.
13. (a) Bowden, N.; Huck, W. T. S.; Paul, K. E.; Whitesides, G. M., The
controlled formation of ordered, sinusoidal structures by plasma oxidation of an
elastomeric polymer. Appl. Phys. Lett. 1999, 75 (17), 2557-2559; (b) Harrison, C.;
Stafford, C. M.; Zhang, W.; Karim, A., Sinusoidal phase grating created by a
tunably buckled surface. Appl. Phys. Lett. 2004, 85 (18), 4016-4018.
14. (a) Chen, Z. B.; Kim, Y. Y.; Krishnaswamy, S., Anisotropic wrinkle
formation on shape memory polymer substrates. J. Appl. Phys. 2012, 112 (12),
124319; (b) Kang, S. H.; Na, J.-H.; Moon, S. N.; Lee, W. I.; Yoo, P. J.; Lee, S.-D.,
Self-organized anisotropic wrinkling of molecularly aligned liquid crystalline
polymer. Langmuir 2012, 28 (7), 3576-3582; (c) Serrano, J. R.; Xu, Q. Q.; Cahill,
D. G., Stress-induced wrinkling of sputtered SiO2 films on
polymethylmethacrylate. J. Vac. Sci. Technol. A 2006, 24 (2), 324-327.
15. (a) Ahn, S. H.; Guo, L. J., Spontaneous formation of periodic
nanostructures by localized dynamic wrinkling. Nano Lett. 2010, 10 (10),
4228-4234; (b) Chen, Y. C.; Crosby, A. J., Wrinkling of inhomogeneously strained
thin polymer films. Soft Matter 2013, 9 (1), 43-47; (c) Okayasu, T.; Zhang, H. L.;
Bucknall, D. G.; Briggs, G. A. D., Spontaneous formation of ordered lateral
patterns in polymer thin-film structures. Adv. Funct. Mater. 2004, 14 (11),
1081-1088; (d) Watanabe, M.; Mizukami, K., Well-ordered wrinkling patterns on
chemically oxidized poly(dimethylsiloxane) surfaces. Macromolecules 2012, 45
(17), 7128-7134.
16. Huck, W. T. S.; Bowden, N.; Onck, P.; Pardoen, T.; Hutchinson, J. W.;
Whitesides, G. M., Ordering of spontaneously formed buckles on planar surfaces.
Langmuir 2000, 16 (7), 3497-3501.
17. Vandeparre, H.; Léopoldès, J.; Poulard, C.; Desprez, S.; Derue, G.; Gay,
C.; Damman, P., Slippery or sticky boundary conditions: Control of wrinkling in
metal-capped thin polymer films by selective adhesion to substrates. Phys. Rev.
Lett. 2007, 99 (18), 188302.
Chapter 5 Generating wrinkling patterns in metal/LC-polymer bilayer films
90
18. Ding, W. L.; Yang, Y.; Zhao, Y.; Jiang, S. C.; Cao, Y. P.; Lu, C. H., Well-
defined orthogonal surface wrinkles directed by the wrinkled boundary. Soft
Matter 2013, 9 (14), 3720-3726.
19. (a) Guo, C. F.; Nayyar, V.; Zhang, Z.; Chen, Y.; Miao, J.; Huang, R.; Liu,
Q., Path-guided wrinkling of nanoscale metal films. Adv. Mater. 2012, 24 (22),
3010-3014; (b) Vandeparre, H.; Desbief, S.; Lazzaroni, R.; Gay, C.; Damman, P.,
Confined wrinkling: Impact on pattern morphology and periodicity. Soft Matter
2011, 7 (15), 6878-6882.
20. (a) Agrawal, A.; Luchette, P.; Palffy-Muhoray, P.; Biswal, S. L.; Chapman,
W. G.; Verduzco, R., Surface wrinkling in liquid crystal elastomers. Soft Matter
2012, 8 (27), 7138-7142; (b) Agrawal, A.; Yun, T. H.; Pesek, S. L.; Chapman, W.
G.; Verduzco, R., Shape-responsive liquid crystal elastomer bilayers. Soft Matter
2014, 10 (9), 1411-1415; (c) Greco, F.; Domenici, V.; Romiti, S.; Assaf, T.;
Zupancic, B.; Milavec, J.; Zalar, B.; Mazzolai, B.; Mattoli, V., Reversible
heat-induced microwrinkling of PEDOT:PSS nanofilm Surface over a
monodomain liquid crystal elastomer. Mol. Cryst. Liq. Cryst. 2013, 572 (1), 40-49.
21. Thornton, J. A.; Hoffman, D. W., Stress-related effects in thin-films. Thin
Solid Films 1989, 171 (1), 5-31.
22. D., L. L.; M., L. E., Theory of Elasticity. Vol. 7. Pergamon Press: 1970.
91
Chapter 6. Technology assessment
6.1 Introduction
In this thesis a range of liquid crystal polymer actuators are described, which have
been fabricated by controlling the supramolecular organization in the materials in
three dimensions in order to open up potential applications in a variety of fields. It
has been shown that both through chemical patterning and photoalignment, many
new, exotic shapes can be created in these materials. Their versatility makes these
materials suitable for the development of advanced devices, as the deformations
can be tailored to achieve specific needs and requirements. Such small-scale, well
controlled actuation could find a variety of applications, for example in
microfluidics, soft robotics and medical devices.
However, during the course of this research it has also become clear that, for such
practical applications, further research is required. This research should proceed in
two directions: improving the mechanical properties of the developed materials to
do work, and towards functional shapes, such as switchable optics and
remote-addressable sieves. In addition, it would be useful to obtain actuators that
respond to other stimuli, such as UV-irradiation and electric fields, which are
attractive stimuli from an application point of view.
6.2 Soft robotics
During the last few decades, soft robotics have gained increased interest. Soft
robotics differentiates itself from hard robotics in that the actuation is distributed
over the whole actuator instead of being located at the joints.1 Such devices can be
used to handle fragile objects or to move across difficult terrain. A good example
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consists of elastomers with pneumatic networks, which have been used to make
walking robots that can cross obstacles, grippers2 and tentacles3 capable of accurate
manipulation, and elastomeric origami.4 However, there are still challenges to
overcome in this field, such as miniaturization, in which liquid crystal actuators
could play an important part, as they can be fabricated on microscale.5
For many applications actuators should have a large end-to-end displacements,
while many of the reported materials only expand and contract by a few percent.6
This can be solved by using the striped director profiles with alternating twisted
nematic alignment, which will deform in an accordion-like manner. This allows the
actuators to reach a significantly larger end-to-end displacement compared to
uniaxial actuators of the same material, as strains of a few percent can cause the
actuator to contract to only a fraction of its original length. This way, the
displacement can be significantly amplified, which makes the actuators more
useful than their uniaxial counterparts in applications that require such large
displacements. For example, the actuator could be used to move a shutter that
blocks a channel, which can be moved out of the way by applying a stimulus,
enabling smart flow control. The actuators could also be used in medical devices to
perform operations where a small device, which fits into the body, has to achieve a
large displacement.
For such applications a certain mechanical strength is required, and actuators
should be capable of performing work. Of course, increasing the displacement will
also lead to a dramatic reduction in the amount of weight that the actuator can
displace. Some experiments were carried out to estimate how much work the
accordion actuator with 12 stripes can perform (see Chapter 4). A setup was
designed in which the actuator pulled a weight up a ramp, of which the incline
could be adjusted to change the effective weight pulled (Figure 6.1a). The
temperature of the ramp could gradually be increased until the film had reached its
maximum contraction. The maximum work the film could perform was determined
by performing a series of experiments with inclines of increasing steepness. Note
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that the friction between the sample and the surface was ignored. Two films with a
12 stripe twisted nematic alignment profile where tested: One consisting of the LC
mixture that was used for heat-responsive actuators in previous chapters, and a
100% crosslinked network prepared from a mixture of only diacrylates. From this
experiment it was found that the less densely crosslinked film could perform
1.41 x 10-6 J of maximum work (Figure 1b), while the fully crosslinked film could
perform 2.40 x 10-6 J of maximum work (Figure 1c). Although the amount of work
that can be performed is limited, these results show that a higher crosslink density
leads to an increase in maximum work performance.
Besides increasing the crosslink density of the networks, there are several ways in
which the chemistry of the materials could be improved to increase the ability of
the actuators to do work. For example, it is likely that a network of main-chain
liquid crystal polymers would generate a larger force, and therefore be capable of
Figure 6.1 Experiments to estimate the ability of accordion-shaped actuators to
do work. a) The experimental setup. The actuator is clamped on one side while
positioned on a ramp that can be heated, and a weight is attached to the other
side. b) The chemical composition of the less densely crosslinked soft actuator,
and the resulting measurements. c) The chemical composition of the fully
crosslinked soft actuator, and the resulting measurements.
Chapter 6 Technology assessment
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displacing more weight, leading to an increase in maximum work output. Such a
network can be prepared using reactive liquid crystals with thiol and vinyl
functional groups instead of acrylates. Such groups can undergo a step-growth
polymerization when free radical species are introduced, forming head-to-tail thiol
ethers that result in a main-chain liquid crystal network. Another possibility to
improve the mechanical properties of the actuators is by combining the liquid
crystal polymer network with a different, non-LC species. Carbon nanotubes have
been successfully blended with elastomers to reinforce the polymer, leading to
enhanced mechanical properties.7 Unfortunately, mixing carbon nanotubes with
other molecules is difficult due to the strong tendency of the nanotubes to
aggregate. Another option would be to prepare an interpenetrating network (IPN)
between an LC polymer and a non-LC polymer. This can be done by adding a
non-reactive molecule to the reactive LC mixture, which can be removed from the
matrix after polymerization by evaporation or dissolution. Then the network can be
swollen with a reactive species, which is polymerized to form the second network.
In addition to changing the chemistry of the actuators, another possibility would be
to look into a different alignment profile. The twisted nematic alignment is not the
ideal alignment profile for this type of actuator, as the anticlastic suppression of the
separate domains to bend in only one direction works against the lateral
displacement, and the chirality of the twisted nematic alignment leads to
out-of-plane twisting of the actuator. These effects are not present when a splayed
alignment is used, making this the alignment of choice for any bending actuator.
However, an alternating striped alignment profile is significantly more difficult to
prepare using photoalignment, as it requires substrates that alternate between
planar and homeotropic alignment. This has previously been achieved in
coumarin-based LPP alignment layers by only irradiating the parts of the substrate
where planar alignment was desired, followed by a treatment of the substrate with a
thiol species bearing a long alkyl tail, in presence of a radical initiator.8 The thiol
reacted with the double bond present in the non-irradiated areas, inducing a
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homeotropic alignment. In such way it would be possible to fabricate an alternating
splayed patterned actuator.
6.3 Switchable sieves
A sieve with remotely addressable openings, which are opened and closed on
demand, would be a useful device as it would enable adjustable size selectivity and
unclogging. Discrete, disclination-based alignment profiles can be used to prepare
a film with slits that can be opened and closed through heating and cooling,
respectively. However, the pores that were shown in Chapter 3 are still too large
for this application to work, and miniaturization of both the domains of the
alignment profile and the apertures themselves is required. For the preparation of
small sized sieves, photoalignment on a small scale is required. Direct-writing of
the alignment pattern onto the substrate bearing the photoalignment material, using
a focused UV laser beam, can be used to prepare such smaller features. To do so,
an LPP substrate is placed onto an xy translation stage, and the polarization
direction of the beam is regulated using a polarization modulator. The movement
of the substrate stage and the polarization of the beam can be programmed to
generate arbitrary patterns, with micro-scale accuracy.9 The biggest difficulty in the
preparation of the addressable apertures is the cutting of the slits. In the
experiments presented earlier, the slits were cut using a razor blade. However, on
the length scales at which the direct-writing procedure operates, this is no longer an
option. A solution to this problem is to use laser cutting to apply the slits.
Using a direct laser writing setup, an aperture pattern, as presented in Chapter 3,
was prepared on an LPP-bearing substrate, in which the slits were 1 mm wide. A
liquid crystal mixture was spincoated from solution on top of the substrate and
cured. Then the slits were cut in the film using the direct laser writing setup at
maximum focus and energy. The resulting film had the correct pattern, but the slits
were often present in a slightly offset position. As the film was thin, and weakened
Chapter 6 Technology assessment
96
due to the presence of the slits, it could only be delaminated by framing it with
pieces of tape to prevent it from collapsing. The heat response of the slits was
investigated using optical microscopy in combination with a heating stage
(Figure 6.2). It seemed that the slits indeed opened at elevated temperatures. It
must be noted, however, that the framing of the film also caused buckling when
heated, which might cause the slits to appear open even while they were not. This
could be solved by preparing thicker films, which have less tendency to buckle and
do not required a frame to be handled.
6.4 Smart optics
In optics, shape is one of the most important parameters determining how an object
interacts with light. Therefore, adjustable, complex shape deformation could find
applications in optical devices. For example, the actuators with an azimuthal
alignment profile shift from a flat sheet to a cone, which could be used as a
switchable lens. In case of IR heating, the cone can be smoothened to better
resemble a lens by exposing the sheet only at the outer radius, which leads to an
inside-out temperature gradient. This has been tested and found to work. Another
example would be an accordion-shaped actuator with a thin cholesteric layer on top
of it. A cholesteric layer reflects light based on the angle of incidence, which
means that the reflective properties change when the film deforms from the flat
shape to the contracted shape. Finally, the patterned wrinkle surfaces have been
shown to possess diffractive properties. These systems presented here are non-
reversible, but reversible wrinkling in LC-based systems have been shown in the
literature. The procedure of photoalignment could be applied to such materials,
which would allow the preparation of gratings with complex, switchable
diffraction.
For this application, it would be useful to look into actuators that respond to
different triggers then the ones presented in this work, as the presence of high
Chapter 6 Technology assessment
97
temperatures or humidity might not always be desired in electronic devices. A
stimulus that is often used is UV light. To make the films responsive to UV
irradiation, azobenzene-based liquid crystals can be added to the polymerizable
mixture.10 The actuators could also be made to respond to electricity. This can
potentially been done using dielectric molecules with smectic C* alignment.11
Additionally, the addition of carbon nanotubes can give the polymer network
dielectric properties.12
In contrast to the previously mentioned applications in soft robotics, in optics the
ability to do work is of little importance. However, there is a different problem that
has to be addressed: the anchoring of the film. For most applications, the film has
to be attached inside a device. This will hamper the deformation of the film,
leading to a different shape or no deformation at all. For example, a film with an
azimuthal alignment profile could be glued to a metal or plastic ring to place it into
a device. When heated, it cannot contract along its circumference, so the cone
shape will not be observed, or will be repressed. This problem could be solved by
attaching the film to a substrate which can deform with it, or by attaching the film
Figure 6.2 Polarized optical microscopy images of
an aperture film with 1 mm slits, shown at varying
temperatures.
Chapter 6 Technology assessment
98
while already deformed and then allow it to relax into a different shape. However,
in both cases the film might respond by crumpling instead of deformation into a
smooth shape. Other solutions could be to attach the film to springs, or to have it
float on a liquid. Of course, in the latter case the surface tension of the liquid might
have a significant effect on the final shape.
6.5 References
1. Bauer, S.; Bauer-Gogonea, S.; Graz, I.; Kaltenbrunner, M.; Keplinger, C.;
Schwödiauer, R., 25th Anniversary article: A soft future: From robots and sensor
skin to energy harvesters. Adv. Mater. 2014, 26 (1), 149-162.
2. Ilievski, F.; Mazzeo, A. D.; Shepherd, R. F.; Chen, X.; Whitesides, G. M.,
Soft robotics for chemists. Angew. Chem. Int. Edit. 2011, 50 (8), 1890-1895.
3. Martinez, R. V.; Branch, J. L.; Fish, C. R.; Jin, L.; Shepherd, R. F.; Nunes,
R. M. D.; Suo, Z.; Whitesides, G. M., Robotic tentacles with three-dimensional
mobility based on flexible elastomers. Adv. Mater. 2013, 25 (2), 205-212.
4. Martinez, R. V.; Fish, C. R.; Chen, X.; Whitesides, G. M., Elastomeric
origami: Programmable paper-elastomer composites as pneumatic actuators. Adv.
Funct. Mater. 2012, 22 (7), 1376-1384.
5. (a) van Oosten, C. L.; Bastiaansen, C. W. M.; Broer, D. J., Printed artificial
cilia from liquid-crystal network actuators modularly driven by light. Nat. Mater.
2009, 8 (8), 677-682; (b) van Oosten, C. L.; Harris, K. D.; Bastiaansen, C. W. M.;
Broer, D. J., Glassy photomechanical liquid-crystal network actuators for
microscale devices. Eur. Phys. J. E 2007, 23 (3), 329-336.
6. Broer, D. J.; Crawford, G. P.; Zumer, S., Cross-linked Liquid Crystalline
Systems. CRC, Taylor and Francis Group, Boca Raton, FL: 2011.
7. Bokobza, L., Multiwall carbon nanotube elastomeric composites: A
review. Polymer 2007, 48 (17), 4907-4920.
8. Wilderbeek, H. T. A.; Teunissen, J. P.; Bastiaansen, C. W. M.; Broer, D. J.,
Patterned alignment of liquid crystals on selectively thiol-functionalized
photo-orientation layers. Adv. Mater. 2003, 15 (12), 985-988.
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9. Miskiewicz, M. N.; Escuti, M. J., Direct-writing of complex liquid crystal
patterns. Opt. Express 2014, 22 (10), 12691-12706.
10. Ikeda, T.; Mamiya, J.-i.; Yu, Y., Photomechanics of liquid-crystalline
elastomers and other polymers. Angew. Chem. Int. Ed. 2007, 46 (4), 506-528.
11. Jakli, A.; Toth-Katona, T.; Scharf, T.; Schadt, M.; Saupe, A.,
Piezoelectricity of a ferroelectric liquid crystal with a glass transition. Phys. Rev. E
2002, 66 (1), 011701.
12. Courty, S.; Mine, J.; Tajbakhsh, A. R.; Terentjev, E. M., Nematic
elastomers with aligned carbon nanotubes: New electromechanical actuators.
Europhys. Lett. 2003, 64 (5), 654-660.
101
List of symbols
λ wavelength of light
λlc contration of a liquid crystal network along the director
λw wrinkle period
δ stress
δc critical stress required for wrinkling
θn angular deviation of liquid crystal molecules from the alignment director
θc angle between the alignment director at z = 0 and the long axis of a rbbon
θw base angle of a wedge-shaped building block
ν Poisson ratio
φ cone opening angle
A wrinkling amplitude
C circumference
d coil diameter
Ep elastic modulus polymer foundation
Em elastic modulus metal layer
h film thickness
L// length parallel to the alignment director
L┴ length perpendicular to the alignment director
m disclination strength
mp number of inward folding flaps of a heated radimuthal film
n alignment director
PT helicoid pitch
PH coil pitch
r radius
S order parameter
S0 initial order parameter
T temperature
Tflat temperature at which a coiling actuator is flat
TNI temperature of nematic to isotropic phase transition
t time
U internal energy of a radimuthal film
x y z coordinate system axis
103
Summary
Programmed morphing of liquid crystal networks
This thesis describes the development of polymer films that show programmed
shape deformations upon exposure to a stimulus, such as heat, light, or humidity.
The stimuli-responsive materials are based on liquid crystal polymer networks,
which can be prepared by photopolymerization of reactive liquid crystals. Using
photoalignment, alignment layers that force the molecules into complex alignment
director patterns can be prepared, and this allows the programming of shape
changes in the material. Exotic shape deformations were achieved in materials with
alignment director variations of the molecules in one, two, and three spatial
dimensions.
The liquid crystal actuators with the least complex alignment pattern are those
which have uniaxial planar alignment, which means the molecules are aligned
parallel to the surface of the film in one direction. Such networks contract and
expand when a stimulus is applied, but can be made to bend by making one side of
the film more responsive to the stimulus compared to the other side. Selective
treatment of hydrogen-bonded liquid crystal networks with a basic solution was
used to create areas on the film which shrink when dry and swell when wet, which
resulted in patterned actuators that respond to changes in humidity by forming
exotic shapes.
Films with a complex director profile in the plane of the film were also prepared.
Using a circular azimuthal pattern, a film could be prepared that, upon heating,
contracts along the circumference and expands along the radius, which leads to a
cone-shaped deformation. Likewise, a film with a radial pattern experiences the
opposite forces and deforms into an anti-cone, which manifests as a saddle shape.
By including an IR-absorbing dye, the films could be heated with IR irradiation to
104
allow contactless activation of the shape change. Using a similar pattern, a film
with slits that open and close upon heating and cooling was prepared.
A film with director variation in all three dimensions was prepared using
photoalignment of a film with twisted nematic alignment, which has a continuous
director variation through the thickness of the film. This technique was used to
create films with stripe patterns that deform into accordion-like shapes upon
heating. Finally, it was shown that the wrinkling behavior of a bilayer system of
liquid crystal polymer and gold could be controlled by photoalignment, as the
wrinkles run parallel to the alignment director of the liquid crystals.
In conclusion, a number of new, exotic shape deformations of liquid crystal
networks were programmed and demonstrated. These results can lead to
applications in various fields, such as microfluidics, microrobotics, and medical
systems.
105
Samenvatting
Programmeerbare vervorming van vloeibaar kristallijne netwerken
Dit proefschrift beschrijft de ontwikkeling van polymeerfilms die een
ingeprogrammeerde vormverandering ondergaan wanneer ze worden blootgesteld
aan een stimulus zoals warmte, licht of vocht. Deze stimulus-responsieve
materialen zijn gebaseerd op vloeibaar kristallijne polymeernetwerken, welke
kunnen worden vervaardigd door het polymeriseren van reactieve vloeibare
kristallen. Door gebruik te maken van foto-uitlijning kunnen uitlijnlagen worden
gemaakt die de moleculen dwingen om complexe patronen te volgen, en dit maakt
het mogelijk om de vervorming in het materiaal zelf te programmeren. Dit heeft
geleid to exotische vormsveranderingen in materialen met variaties in de uitlijning
in een, twee en drie ruimtelijke dimensies.
De vloeibaar kristallijne actuatoren met de minste complexiteit hebben een
uniaxiale planaire uitlijning, wat betekent dat de moleculen parallel aan het
oppervlak van de film zijn uitgelijnd, en allemaal in dezelfde richting liggen. Dit
netwerk wordt korter in een richting en langer in de andere richting wanneer een
stimulus wordt toegepast, maar kan alleen buigen als een kant sterker op de
stimulus reageert dan de andere kant. Door middel van selectieve behandeling van
waterstof-gebrugde vloebaar kristallijne netwerken met een basische oplossing
zijn delen van de film gevoelig gemaakt voor water, zodat ze krimpen in een droge
omgeving en zwellen in een vochtige omgeving. Op deze manier zijn
gepatroneerde actuatoren verkregen die reageren op de luchtvochtigheid door
exotische vormsveranderingen te ondergaan.
Verder zijn er films met een complex uitlijnpatroon in het vlak van de film
vervaardigd. Door gebruik te maken van een azimuthaal patroon kon een
cirkelvormige film worden gemaakt die op verwarming reageert door langs de
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omtrek samen te trekken en langs de radius uit te zetten, wat uiteindelijk leidt tot
een kegelvormige vormsverandering. Een film met een radieel patroon vervormt op
de omgekeerde manier, wat leidt tot een vorm die wordt beschreven als een
anti-kegel, welke eruitziet als een zadelvorm. Door een IR-absorberende kleurstof
toe te voegen konden de films worden verhit door middel van IR straling, zodat de
vormsverandering kon worden geactiveerd zonder contact te maken met de film.
Door gebruik te maken van een soortgelijk patroon kon ook een film worden
gemaakt met openingen die open en dicht gaan door te verhitten en af te koelen.
Een film met een variatie in de uitlijning in alle drie de ruimtelijke dimensies kon
worden vervaardigd door het foto-uitlijnen van een film met een gedraaide
nematische uitlijning die een doorlopende variatie door de dikte van de film heeft.
Door middel van deze techniek kon een film gemaakt worden met een
strepenpatroon, die bij verhitten deformeert op een manier die wel iets weg heeft
van de beweging van een accordeon. Als laatste is gedemonstreerd dat het
rimpelgedrag van een bilaag-systeem, bestaande uit een vloeibaar kristallijn
polymeernetwerk en een laagje goud, kon worden gestuurd door middel van foto-
uitlijning. Dit was mogelijk doordat de rimpels de uitlijnrichting van de vloeibare
kristallen volgen.
Uiteindelijk kan worden geconcludeerd dat een aantal nieuwe, exotische
vormsverandingen in vloeibaar kristallijne netwerken zijn geprogrammeerd. Deze
zijn gedemonstreerd door een stimulus toe te passen. De resultaten kunnen leiden
tot toepassingen in verschillende onderzoeksgebieden, zoals lab-on-a-chip,
microrobotica en gezondheidszorg.
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Curriculum Vitae
Laurens Theobald de Haan was born on the 30th of May, 1986 in Apeldoorn, the
Netherlands. After completing his secondary education at the Udens College in
Uden, he studied Molecular Life Sciences at the Radboud University in Nijmegen.
During his first master’s project, he worked on the preparation of polymer
networks with azide side groups using RAFT polymerization to obtain coatings
that could be functionalized using click chemistry. His second master’s project
involved the synthesis of ionic liquid crystals and the study of their use as a
reaction medium for enzymes. After obtaining his master’s degree, he started his
PhD project on complex liquid crystal polymer actuators under the supervision of
prof. Albert Schenning and prof. Dick Broer. The results of this project are
presented in this thesis.
Laurens Theobald de Haan werd geboren op 30 mei 1986 in Apeldoorn, Nederland.
Na afronding van de middelbare school aan het Udens College in Uden begon hij
zijn studie Moleculaire Levenswetenschappen aan de Radboud Universiteit in
Nijmegen. Tijdens zijn eerste masterproject werkte hij aan het vervaardigen van
polymeernetwerken met azide zijgroepen door middel van RAFT polymerisatie,
die vervolgens gefunctionaliseerd konden worden middels clickchemie. Zijn
tweede masterproject bestond uit de synthese van ionische vloeibare kristallen en
het bestuderen van de geschiktheid van deze materialen als een reactiemedium
voor enzymen. Na het verkrijgen van zijn masterdiploma begon hij aan zijn
promotieproject, met als onderwerp complexe actuatoren gebaseerd op
gepolymerizeerde vloeibare kristallen, onder begeleiding van prof. Albert
Schenning en prof. Dick Broer. Het resultaat van dit project is beschreven in dit
proefschrift.
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Publications
Publications related to this thesis
de Haan, L. T.; Leclère, P.; Damman, P.; Schenning, A. P. H. J.; Debije M. G., On
demand wrinkling patterns in thin metal films generated from self-assembling
liquid crystals. Adv. Funct Mater., Submitted.
de Haan, L. T.; Schenning, A. P. H. J.; Broer, D. J., Programmed morphing of
liquid crystal networks. Polymer, In Press.
de Haan, L. T.; Verjans, J. M. N.; Broer, D. J.; Bastiaansen, C. M. W.; Schenning,
A. P. H. J., Humidity-responsive liquid crystalline polymer actuators with an
asymmetry in the molecular trigger that bend, fold, and curl. J. Am. Chem. Soc.
2014, 136 (30), 10585-10588.
de Haan, L. T.; Gimenez-Pinto, V.; Konya, A.; Nguyen, T. S.; Verjans, J. M. N.;
Sanchez-Somolinos, C.; Selinger, J. V.; Selinger, R. L. B.; Broer, D. J.;
Schenning, A. P. H. J., Accordion- like actuators of multiple 3D patterned liquid
crystal polymer films. Adv. Funct. Mater 2014, 24 (9), 1251-1258.
Dai, M.; Pico, O. T.; Verjans , J. M. N.; de Haan, L. T.; Schenning, A. P. H. J.;
Peijs, T.; Bastiaansen, C. W. M., Humidity-responsive bilayer actuators based on a
liquid-crystalline polymer network. ACS Appl. Mater. Inter. 2013, 5 (11),
4945-4950.
Modes, C. D.; Warner, M.; Sanchez-Somolinos, C.; de Haan, L. T.; Broer, D. J.,
Angular deficits in flat space: remotely controllable apertures in nematic solid
sheets. P. Roy. Soc. A.-Math. Phy. 2013, 469 (2153), 20120631.
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Modes, C. D.; Warner, M.; Sanchez-Somolinos, C.; de Haan, L. T.; Broer, D. J.,
Mechanical frustration and spontaneous polygonal folding in active nematic sheets.
Phys. Rev. E. 2012, 86 (6), 060701.
de Haan, L. T.; Sanchez-Somolinos, C.; Bastiaansen, C. M. W.; Schenning, A. P.
H. J.; Broer, D. J., Engineering of complex order and the macroscopic deformation
of liquid crystal polymer networks. Angew. Chem. Int. Edit. 2012, 51 (50),
12469-12472.
Other publications
Fernandez, A. A.; de Haan, L. T.; Kouwer, P. H. J., Towards room-temperature
ionic liquid crystals. J. Mater. Chem. A. 2013, 1 (2), 354-357.
Canalle, L. A.; van Berkel, S. S.; de Haan, L. T.; van Hest, J. C. M., Copper-free
clickable coatings. Adv. Funct. Mater. 2009, 19 (21), 3464-3470.
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Acknowledgements
During my time as a PhD candidate, I had the pleasure of working together with
many great people. Here, I would like to thank everyone who contributed to the
completion of this thesis by providing help or support, for engaging in stimulating
discussions or for providing useful advice.
I would like to thank my promoters, Albert Schenning and Dick Broer, for giving
me the opportunity to work in their amazing group. Albert, I want to thank you for
your daily support, your enthusiasm and positive outlook, and for allowing me to
choose my own directions within my research project. I wish you the best of luck
as the new professor of the SFD group. Dick, I could always count on your
extensive knowledge of liquid crystal polymers, which has been of great help,
especially when both me and Albert were still new to this field.
I would like to thank the members the reading committee, who took the time to
read and comment on the thesis. Cees Bastiaansen, Alan Rowan, Rint Sijbesma,
Carlos Sanchez-Somolinos, and Timothy White, thank you for your interest in my
thesis, and your corrections.
During this project I have had a number of collaborations with research groups
abroad. From Kent State University, I would like to thank Vianney Gimenez-Pinto,
Andrew Konya, Than-Son Nguyen, Jonathan Selinger, and Robin Selinger for their
work on finite-element analysis of my patterned twisted nematic films, and the
great paper we wrote together. From North Carolina State University I would like
to thank Michael Escuti and Matthew Miskiewicz for inviting me over to their lab
to do some experiments. It was a great experience to visit the United States, and I
think we accomplished a lot in the lab. I have also worked closely with Carl Modes
and Mark Warner from the University of Cambridge, who showed in their papers
the theoretical predictions of the experiments described in chapter 3. I would like to
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thank you for the interesting and inspiring discussion we had, for your advice, and
for allowing me to contribute to two of your publications. From the University of
Mons I would like to thank Pascal Damman and Phillipe Leclère for their help with
the gold wrinkle patterns. You helped us understand a topic that nobody in our
group had much knowledge of, and we couldn’t have written our paper without
this. I especially want to thank Carlos Sanchez-Somolinos from the University of
Zaragoza. Carlos, thank you for your work on this project, both before and after I
started my PhD program, so I was able to quickly have some great results. Also,
thank you for having me visit you three times, and for allowing me to take the
photoalignment setup with me to Eindhoven. You have been a great collaborator,
and friend.
I also had the pleasure of supervising a number of students who worked on my
project. Jullien Verjans, Jelle Lendemijer and Stefan van Uden, thank you for your
hard work and for the many things you have taught me about supervising students.
I would also like to thank Dylan Davies, who worked as a student on my project
before I stared, and did some great preliminary experiments.
During my time at SFD I have worked with a number of amazing colleagues.
Michael Debije, thank you for all the great discussions we had, about work and
other things, and for inviting me to your board game sessions. Amol, Altug,
Antonio, Anne Hélène, Baran, Berry, Casper, Claudia, Danqing, Debarshi, Derya,
Dirk Jan, Felix, Hilal, Hitesh, Huub, Indu, Irén, Ivelina, Jelle R., Jelle S., Jeroen,
Joost, Jurica, Jurgen, Koen, Kamlesh, Lihua, Luc, Marjolijn, Michiel, Mian,
Monali, My, Natalia, Nicole, Paul, Pauline, Peter, Pim, Pit, Robert, Shufen, Sander,
Stephanie, Tamara, Tom, Wanyu, Xiao, Xiaoran, Yang, Youseli, Zeno, thank you
for the great times at work and outside of it.
I would like to thank my family and friends for their love and support. I thank my
father, Theo de Haan, and my brother, Martin de Haan, for always being there for
me.
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Finally, I want to thank my mother, Emmy de Haan van Randwijk. I cannot
describe how much you have done for me, my brother, and my father. You were
always there when I needed someone to talk to, you always showed interest in
everything I did, and you will always be the strongest person I have ever known.
I miss you.
Rest in peace.