Silica-Based Inorganic/Organic Hybrid...
Transcript of Silica-Based Inorganic/Organic Hybrid...
Silica-Based Inorganic/Organic Hybrid
Materials
Von der Fakultt fr Mathematik, Informatik und Naturwissenschaften der RWTH
Aachen University zur Erlangung des akademischen Grades eines Doktors der
Naturwissenschaften genehmigte Dissertation
vorgelegt von
Master of Science
Yongliang Zhao
aus Gansu, V. R. China
Berichter: Universittsprofessor Dr. rer. nat. Martin Mller
Universittsprofessor Dr. rer. nat. Regina Palkovits
Tag der mndlichen Prfung: 25. August 2016
Diese Dissertation ist auf den Internetseiten der Universittsbibliothek online verfgbar.
This research forms part of the research programme of the Dutch Polymer
Institute (DPI), project #747 NanoPA.
Dutch Polymer Institute (DPI), P.O. Box 902, 5600 AX Eindhoven, the
Netherlands
To my family
Contents
Summary .............................................................................................................................I
Zusammenfassung .......................................................................................................... III
List of Abbreviations ........................................................................................................ V
1. Introduction and Outline of the Thesis ...................................................................... 1
1.1 Introduction ...................................................................................................... 2
1.2 Objectives of this thesis ................................................................................... 4
1.3 Outline of the thesis ......................................................................................... 5
1.4 References ........................................................................................................ 6
2. Literature Review ......................................................................................................... 9
2.1 Polymer/inorganic nanocomposites ............................................................... 10
2.1.1 Polymer/silicate nanocomposites ........................................................ 10
2.1.2 Surface modification of silica ............................................................. 11
2.1.3 Polymer nanocomposites containing ultrasmall particles ................... 12
2.1.4 MQ resins ultrasmall core-shell silicate particles ............................ 14
2.1.5 Polyamide 6 ........................................................................................ 16
2.2 Silica-based hybrid core-shell particles ......................................................... 16
2.2.1 Definition and classification of core-shell particles ............................ 16
2.2.2 Organic-silica core-shell particles ....................................................... 18
2.2.3 Design and synthetic strategy ............................................................. 19
2.2.3.1 Polymer core functionalization and silica shell coating .......... 19
2.2.3.2 Layer-by-layer (LBL) process ................................................. 21
2.2.3.3 Pickering (mini)emulsion polymerization ............................... 23
2.2.3.4 Partial extraction of polymer after sol-gel reaction ................. 25
2.2.3.5 Sol-gel microencapsulation of organic liquids......................... 26
2.3 References ...................................................................................................... 30
3. Microencapsulation of Hydrophobic Liquids in Closed All-Silica Colloidosomes(*)
........................................................................................................................................... 39
3.1 Introduction .................................................................................................... 40
3.2 Experimental Section ..................................................................................... 41
3.2.1 Materials ............................................................................................. 41
3.2.2 Preparation of partially hydrophobic silica nanoparticles .................. 42
3.2.3 Formation of all-silica colloidosomes ................................................. 42
3.2.4 Dynamic Light Scattering (DLS) measurements ................................ 42
3.2.5 Fluorescence Microscopy ................................................................... 42
3.2.6 Field-Emission - Scanning Electron Microscopy (FE-SEM) ............. 44
3.2.7 Transmission Electron Microscopy (TEM) ........................................ 44
3.2.8 Thermogravimetric Analysis (TGA) ................................................... 44
3.2.9 Fourier-Transform Infrared (FT-IR) Spectroscopy ............................. 44
3.2.10 1H NMR Spectroscopy ...................................................................... 44
3.3 Results and Discussion .................................................................................. 45
3.4 Conclusions .................................................................................................... 57
3.5 References ...................................................................................................... 58
4. Silica Nanoparticles Catalyze the Formation of Silica Nanocapsules in a
Surfactant-Free Emulsion System(*) .......................................................................... 61
4.1 Introduction .................................................................................................... 62
4.2 Experimental Section ..................................................................................... 63
4.2.1 Materials ............................................................................................. 63
4.2.2 Preparation of methyl-functionalized silica nanoparticles .................. 64
4.2.3 Preparation of silica capsules .............................................................. 64
4.2.4 Characterization methods .................................................................... 64
4.3 Results and Discussion .................................................................................. 67
4.4 Conclusions .................................................................................................... 82
4.5 References ...................................................................................................... 83
5. A Facile One-Step Approach towards Polymer@SiO2 Core-Shell Nanoparticles
via a Surfactant-Free Miniemulsion Polymerization Technique(*) ........................... 87
5.1 Introduction .................................................................................................... 88
5.2 Experimental Section ..................................................................................... 90
5.2.1 Materials ............................................................................................. 90
5.2.2 Preparation of PS@silica composite particles .................................... 92
5.2.3 Interfacial tension (IFT) measurements .............................................. 92
5.2.4 Field-emission scanning electron microscopy (FE-SEM) .................. 92
5.2.5 Transmission electron microscopy (TEM) .......................................... 93
5.2.6 Energy-dispersive X-ray spectroscopy scanning transmission electron
microscopy (EDX-STEM) ........................................................................... 93
5.2.7 Fourier-transform infrared (FT-IR) spectroscopy ............................... 93
5.2.8 Thermogravimetric analysis (TGA) .................................................... 93
5.2.9 Dynamic light scattering (DLS) measurements .................................. 94
5.3 Results and Discussion .................................................................................. 94
5.4 Conclusions .................................................................................................. 107
6. Preparation of Poly (methyl methacrylate)/SiO2 Composite Nanoparticles via a
Surfactant-Free Miniemulsion Polymerization Process ........................................... 111
6.1 Introduction .................................................................................................. 112
6.2 Experimental Section ................................................................................... 115
6.2.1 Materials ........................................................................................... 115
6.2.2 Synthesis of methylsilyl-substituted polyethoxysiloxane ................. 115
6.2.3 Preparation of PMMA/silica composite nanoparticles ..................... 115
6.2.4 NMR measurements .......................................................................... 117
6.2.5 Interfacial tension (IFT) measurements ............................................ 117
6.2.6 Field-emission - scanning electron microscopy (FE-SEM) .............. 117
6.2.7 Transmission electron microscopy (TEM) ........................................ 117
6.2.8 Fourier-transform infrared (FT-IR) spectroscopy ............................. 118
6.2.9 Thermogravimetric analysis (TGA) .................................................. 118
6.2.10 Brunauer-Emmett-Teller (BET) specific surface area measurements
.................................................................................................................... 118
6.3 Results and Discussion ................................................................................ 118
6.4 Conclusions .................................................................................................. 129
6.5 References .................................................................................................... 130
7. Synthesis and Characterization of Ultrasmall Silicate Particles - MQ Resins(*)
......................................................................................................................................... 133
7.1 Introduction .................................................................................................. 134
7.2 Experimental Section ................................................................................... 137
7.2.1 Materials ........................................................................................... 137
7.2.2 Synthesis ........................................................................................... 137
7.2.2.1 Methyl-terminated MQ resins [(Me3SiO1/2)1.0 to 1.6(SiO4/2)1.0] 137
7.2.2.2 Hydride-terminated MQ [(Me3SiO1/2)m(HMe2SiO1/2)n(SiO4/2)1.0]
............................................................................................................ 138
7.2.2.3 Hydrosilylation of hydride-terminated MQ resins ................. 139
7.2.3 Characterization Methods ................................................................. 139
7.2.3.1 NMR measurements ............................................................... 139
7.2.3.2 Gel Permeation Chromatography (GPC) ............................... 139
7.2.3.3 Fourier-Transform Infrared (FT-IR) Spectroscopy ................ 140
7.2.3.4 Dynamic Light Scattering (DLS) measurements ................... 140
7.2.3.5 Atomic Force Microscopy (AFM) ......................................... 140
7.3 Results and Discussion ................................................................................ 140
7.4 Conclusions .................................................................................................. 150
7.5 References .................................................................................................... 151
8. Effect of Ultrasmall MQ Core-Shell Particles on Melt Viscosity of Polyamide 6(*)
......................................................................................................................................... 153
8.1 Introduction .................................................................................................. 154
8.2 Experimental Section ................................................................................... 156
8.2.1 Materials ........................................................................................... 156
8.2.2 Synthesis ........................................................................................... 157
8.2.2.1 Pyrrolidinone-containing silane coupling agent .................... 157
8.2.2.2 Surface modification of SiO2 nanoparticles ........................... 157
8.2.2.3 MQ resins ............................................................................... 158
8.2.2.4 PA6/silica nanocomposites .................................................... 159
8.2.3 Characterization Methods ................................................................. 159
8.2.3.1 Fourier-Transform Infrared (FT-IR) Spectroscopy ................ 159
8.2.3.2 Transmission Electron Microscopy (TEM) ........................... 159
8.2.3.3 Rheological Characterization ................................................. 160
8.2.3.4 Gel Permeation Chromatography (GPC) ............................... 160
8.2.3.5 Dynamic Mechanical Thermal Analysis (DMTA) ................. 160
8.2.3.6 Wide-Angle X-ray Diffraction (WAXD) ............................... 161
8.2.3.7 Tensile Tests ........................................................................... 161
8.2.3.8 Differential Scanning Calorimetry (DSC) ............................. 161
8.2.3.9 Thermogravimetric Analysis (TGA) ...................................... 161
8.3 Results and Discussion ................................................................................ 162
8.4 Conclusions .................................................................................................. 179
8.5 References .................................................................................................... 180
Acknowledgements ....................................................................................................... 183
Curriculum Vitae .......................................................................................................... 185
List of Publications ....................................................................................................... 186
Summary
I
Summary
This dissertation deals with two groups of silica-based composite materials,
namely organic/silica composite particles and polymer nanocomposites containing
ultrasmall silicate particles. For the synthesis of composite particles a silica precursor
polymer - hyperbranched polyethoxysiloxane (PEOS) - is employed in combination
with emulsion technique. PEOS is a highly hydrophobic liquid. Upon hydrolysis it
becomes amphiphilic and interfacial active at the oil/water interface. In an oil-in-water
Pickering emulsion stabilized by long-alkyl substituted silica nanoparticles PEOS acts
as a glue to link the particles together at the interface to form microsized all-silica
colloidosomes enclosing the oil phase. It is further demonstrated that oil-in-water
miniemulsions are formed by dispersing oil/PEOS solutions in water without any
additional particles due to high hydrophobicity and hydrolysis-induced interfacial
activity of PEOS. In such a miniemulsion system methyl-functionalized silica
nanoparticles, which are less interfacial active than the long-alkyl substituted ones, can
catalyze the conversion of PEOS to a mechanically strong silica shell at the oil/water
interface. Thus, the oil phase is encapsulated in monodispersed silica nanocapsules with
almost 100% efficiency. This process depends strongly on pH of the aqueous phase,
which controls the interfacial activity of both PEOS and silica nanoparticles as well as
PEOS hydrolysis and condensation. The catalytic effect of the silica particles is the
result of a delicate interplay between their interfacial activity that allows immersing
catalytically active silanolate groups in the PEOS-containing oil phase and the repulsion
between charged surfaces of these particles and the resulting silica nanocapsules that
splits them apart. By combining PEOS with a hydrophobic monomer, a new type of
surfactant-free miniemulsion polymerization has been developed. Thus, monodisperse
polystyrene@SiO2 nanoparticles are obtained by emulsifying a PEOS/monomer
solution in water and subsequent heating to initiate polymerization. As the
polymerization proceeds, driven by osmotic pressure and incompatibility with
polystyrene PEOS macromolecules migrate continuously towards the oil/water
interface where sol-gel reaction takes place. As soon as the polymerization is completed,
PEOS is fully expelled from the polymer phase and is converted to silica on the
Summary
II
polystyrene surface. This method allows an easy control of silica shell thickness by
varying the PEOS concentration. The particle size, on the other hand, can be regulated
not only by the shearing force, but also by pH of the aqueous medium. In the case of a
less hydrophobic monomer like methyl methacrylate, a more hydrophobic methylsilyl-
substituted PEOS should be employed to obtain stable miniemulsions and subsequently
composite nanoparticles with a narrow particle size distribution. The resulting particles
exhibit a semi-interpenetrating network structure probably because of the compatibility
of the polymer matrix with methylsilyl-substituted PEOS.
Ultrasmall silicate particles used in this work are MQ resins having a core-shell
structure with a SiO2-core and an organic shell. The synthesis is based on the acidic co-
hydrolysis and condensation of monofunctional (M, e.g. 1,1,1,3,3,3-
hexamethyldisiloxane) and tetrafunctional (Q, e.g. tetraethoxysilane) organosilicon
compounds in an organic solvent, and the size of MQ resins increases with the decrease
of M to Q ratio. The functionalized MQ resins are prepared by using 1,1,3,3-
tetramethyldisiloxane as a comonomer and subsequent catalytic hydrosilylation with
vinyl-containing compounds, and the degree of modification is defined by the ratio
between 1,1,1,3,3,3-hexamethyldisiloxane and 1,1,3,3-tetramethyldisiloxane. As
confirmed by dynamic light scattering and atomic force microscopy the MQ resin
particles synthesized in this work have a size less than 10 nm and a narrow size
distribution. MQ resins substituted with 1-vinyl-2-pyrollidinone and N,N-
dimethylallylamine are blended with polyamide 6 via a melt extrusion process. It is
shown that 2-pyrrolidinone-functionalized MQ resins are homogenously distributed in
polyamide. By adding only a small amount of such particles (0.5 wt.-%), the viscosity
of the polymer melt is significantly reduced, meanwhile the mechanical properties of
the polymer remain almost unchanged. By comparing the interparticle half-gap and
radius of gyration of the polymer chain it can be concluded that the change of
entanglement density and release of topological constraint caused by MQ resin
nanoparticles account for the viscosity reduction.
Zusammenfassung
III
Zusammenfassung
Die vorliegende Arbeit behandelt zwei Gruppen von Silica-basierten
Kompositmaterialien: SiO2-organische Kompositpartikel und Polymer Nanokomposite
mit ultrakleinen Silikat-Partikeln. Zur Synthese der Kompositpartikel wird ein SiO2-
Precursorpolymer- hochverzweigtes Polyethoxysiloxan (PEOS)- in Verbindung mit
einem Emulsionsverfahren eingesetzt. PEOS ist eine hochgradig hydrophobe
Flssigkeit. Durch die Hydrolyse wird es amphiphil und grenzflchenaktiv an der l-
Wasser Grenzflche. In einer l-in-Wasser- Pickering-Emulsion, die durch mit langen
Alkylketten substituierte SiO2-Nanopartikeln stabilisiert ist, bindet PEOS die Partikel
an der Grenzflche unter Bildung eines SiO2-Kolloidosomes im Mikrobereich, das die
lphase verkapselt. Weiterhin wird gezeigt, dass durch Dispergieren von l-PEOS-
Lsungen in Wasser l-in-Wasser Emulsionen gebildet werden, wobei wegen der
Hydrophobie und der durch Hydrolyse induzierten Grenzflchenaktivitt keine
zustzlichen Partikel zur Stabilisierung bentigt werden. In einer solchen Miniemulsion
knnen methylfunktionalisierte SiO2-Nanopartikel, die weniger grenzflchenaktiv sind
als solche mit langen Alkylresten, die Umwandlung von PEOS zu einer mechanisch
stabilen SiO2-Schale an der l-Wasser Grenzflche katalysieren. Auf diese Weise
gelingt der Einschluss der lphase in monodispersen SiO2-Nanokapseln zu fast 100%.
Der Prozess hngt stark vom pH-Wert der Wasserphase ab, der die
Grenzflchenaktivitt von PEOS und der SiO2-Nanopartikel sowie die Hydrolyse des
PEOS und die Kondensation bestimmt. Der katalytische Effekt der SiO2-Partikel beruht
auf einem Zusammenspiel ihrer Grenzflchenaktivitt, die das Einbringen katalytisch
aktiver Silanolat-Gruppen in die PEOS/lphase ermglicht, sowie der Abstoung
zwischen den geladenen Oberflchen dieser Partikel und den daraus entstehenden SiO2-
Nanokapseln, die diese trennt. Durch Kombination von PEOS und einem hydrophoben
Monomer wurde eine neuartige Tensid-freie Miniemulsionspolymerisation entwickelt.
So erhlt man monodisperse Polystyrol@SiO2 Nanopartikel durch Emulgieren einer
PEOS/Monomer-Lsung in Wasser und anschlieendes Erhitzen zum Starten der
Polymerisation. Im Verlauf der Polymerisation wandern die PEOS-Makromolekle,
getrieben durch den osmotischen Druck und ihre Unvertrglichkeit mit dem Polystyrol,
stetig an die l-Wassergrenzflche, wo die Reaktion stattfindet. Sobald die
Zusammenfassung
IV
Polymerisation abgeschlossen ist, wird PEOS vollstndig aus der Polymerphase
verdrngt und an der Oberflche des Polystyrols zu SiO2 umgesetzt. Dieses Verfahren
erlaubt eine einfache Kontrolle der SiO2-Schalendicke durch Variation der PEOS
Konzentration. Die Partikelgre kann sowohl durch die Scherkrfte als auch ber den
pH-Wert des wssrigen Mediums eingestellt werden. Im Fall weniger hydrophober
Monomerer wie Methylmethacrylat sollte ein strker hydrophobes methylsilyl-
substituiertes PEOS eingesetzt werden, um eine stabile Miniemulsion und in der Folge
Kompositnanopartikel mit enger Grenverteilung zu erreichen. So erhlt man Partikel,
die vermutlich aufgrund der Kompatibilitt von Polymermatrix und dem methylsilyl-
substituierten PEOS ein halbinterpenetrierendes Netzwerk bilden.
Die im Rahmen dieser Arbeit hergestellten ultrakleinen Silikat-Partikel sind MQ-
Harze mit einem SiO2-Kern und einer organischen Schale. Die Synthese basiert auf der
saureren Co-Hydrolyse und Kondensation monofunktionaler (M, z.B. 1,1,1,3,3,3-
Hexamethyldisiloxan) und tetrafunktionaler (Q, z.B. Tetraethoxysilan)
siliziumorganischer Verbindungen in einem organischen Lsungsmittel. Die Gre der
MQ-Harze nimmt mit abnehmendem M/Q-Verhltnis zu. Zur Prparation wird 1,1,3,3-
Tettramethyldisiloxan als Co-Monomer eingesetzt, das nachfolgend einer katalytischen
Hydrosilylierung mit Vinylverbindungen unterworfen wird. Das Ausma der
Modifizierung ist durch Verhltnis von 1,1,1,3,3,3-Hexamethyldisiloxan zu 1,1,3,3-
Tetramethyldisiloxan bestimmt. Mittels dynamischer Lichtstreuung sowie AFM kann
gezeigt werden, dass der Durchmesser der Partikel bei enger Grenverteilung unter 10
nm liegt. MQ-Harze, die mit 1-Vinyl-2-Pyrrolidinon und N,N-Dimethylallylamin
substituiert sind, werden mit Polyamid-6 in einem Schmelzextrusionsprozess vermischt,
wodurch eine homogene Verteilung der mit 2-Pyrollidinon funktionalisierten MQ-
Harze im Polyamid erreicht wird. Bei Zugabe einer geringen Menge dieser Partikel (0.5
Gew.-%) wird die Viskositt der Schmelze deutlich herabgesetzt whrend die
mechanischen Eigenschaften des Polymers nahezu unverndert bleiben. Aus dem
Vergleich des interpartikulren Halbabstands mit dem Trgheitsradius des Polymers
schliet man, das die nderung der Verschlaufungsdichte und der Wegfall der
topologischen Beschrnkung verursacht durch das MQ-Harz fr die Verminderung der
Viskositt verantwortlich sind.
List of Abbreviations
V
List of Abbreviations
Abbreviation Full Wording
PEOS polyethoxysiloxane
PS polystyrene
PMMA poly(methyl methacrylate)
PA6 polyamide 6
PVP polyvinylpyrrolidone
PEG poly(ethylene glycol)
PDMS polydimethylsiloxane
PP polypropylene
TEOS tetraethyl orthosilicate
CTAB cetyltrimethylammonium bromide
HMDS hexamethyldisiloxane
TMDS tetramethyldisiloxane
AIBA Azobis(isobutyl-amidine hydrochloride)
AIBN Azobisisobutyronitrile
PCMs phase change materials
DLS dynamic light scattering
FE-SEM field-emission scanning electron microscopy
TEM transmission electron microscopy
EDX-STEM energy-dispersive X-ray spectroscopy - scanning
transmission electron microscopy
AFM atomic force microscopy
TGA thermogravimetric analysis
List of Abbreviations
VI
DMTA dynamic mechanical thermal analysis
WAXD wide-angle X-ray diffraction
DSC differential scanning calorimetry
FT-IR fourier-transform infrared
GPC gel permeation chromatography
IFT interfacial tension
NMR nuclear magnetic resonance
BET Brunauer-Emmett-Teller
BJH Barrett-Joyner-Halenda
NL-DFT nonlocal-density functional theory
A@B A is surrounded by B
LBL layer-by-layer
HLB hydrophile-lipophile balance
IEP isoelectric point
o/w oil-in-water
w/o water-in-oil
w/o/w water-in-oil-in-water
o/w/o oil-in-water-in-oil
Rg radius of gyration
volume fraction
m maximum random packing volume fraction
G storage modulus
G loss modulus
Tg glass transition temperature
Chapter 1: Introduction and Outline of the Thesis
1
Chapter 1
Introduction and Outline of the Thesis
Chapter 1: Introduction and Outline of the Thesis
2
1.1 Introduction
Composite technology combining multiple materials differing significantly in
properties to produce a composite material with characteristics different from the
constituents has progressed rapidly since the last decades.1,2 By proper design the
properties of the composites can often go far beyond that achievable with single
materials.3 The combination of inorganic and organic materials is a well-established
and widely used approach with a long history. It is still of significant academic interest
due to almost inexhaustible combinatory possibilities. The inorganic constituents
contribute not only to enhanced physical and chemical properties such as stiffness,
mechanical strength, chemical resistance, thermal stability, optical properties, and
viscoelastic behavior but also to the cost reduction. The organic parts, on the other hand,
render the composite materials flexible, ductile, processable, electrically and thermally
insulated, and possibly functional in specific application areas. However this general
summary does not apply to all events in hybrid materials, because the overall properties
are also strongly determined by the way both constituents combine and by the formed
unique composite structure.4,5
Polymer composites where inorganic fillers are dispersed in a continuous organic
polymer phase are well-established in both theory and practice.6 Especially when one
dimension of the filler falls into the nanoscale (e.g. nanoparticles, nanotubes, layered
silicates, etc.) critical phenomena and unique distinct properties can be anticipated.7 It
is generally accepted that the nanoscale inorganic building blocks may create a
substantial interfacial area among the organic matrix, leading to unique interactions
with matrix and thereby properties that are different from bulk phase even at very low
loadings and are of high benefit for industry. For instance, mechanical strength, thermal
stability, electrical conductivity, and magnetic permeability of the polymers can be
substantially altered when inorganic nanofillers are uniformly dispersed in the polymer
matrices.8-12 Although a lot of effort has been devoted to the development of polymer
nanocomposites, insight in structure-property relations is still largely absent. In most
cases, particles with tens of nanometers are used in preparation of polymer
nanocomposites. However, the situation becomes much more intriguing when the size
Chapter 1: Introduction and Outline of the Thesis
3
of the particles is further diminished below 10 nm, where the interparticle half-gap is
comparable or even smaller than the radius of gyration (Rg) of a polymer coil. This
produces chain confinement and distortion that may promise unusual properties. Indeed,
Mackay et al. showed that the melt viscosity of polymers can be significantly reduced
by introducing ultrasmall nanoparticles with radius of 3-5 nm.13 It is in contradiction to
the prediction of Einstein who claimed that particle addition into fluids including
polymeric ones could lead to an increase in apparent viscosity.14 This phenomenon has
been explained by an increase of the free volume upon adding nanoparticles, as
confirmed by a decrease in the glass transition temperature. It should be noted that the
viscosity reduction was observed only when the interparticle half-gap was smaller than
Rg of the polymer coil.15 There are very few papers in the literature dealing with
nanocomposites containing ultrasmall particles, largely due to the easy agglomeration
and lack of scalable synthesis of such particles.
Within the inorganic-organic hybrid material family, considerable effort is devoted
to the development of colloidal inorganic-organic nanocomposites due to their
promising applications in catalysis, chromatography, coating, biotechnology, etc.4,16,17
Thanks to the huge progress in synthetic techniques in both organic and inorganic
chemistry (e.g. layer-by-layer deposition, in-situ polymerization, in-situ sol-gel
reaction, synthesis of functional inorganic particles, etc.) a wide variety of composite
particles were developed during the last decades.18 Since the properties of such kind of
particles depend not only on the chemical nature of both constitutes, but also on the
designed micro-/nanostructure, the possibility of tailoring the subsequent performance
properties, in principle, is not exhaustive. Composite particles can be designed with
various morphologies, such as raspberry-like, currant bun-like, core-shell, snowman-
like, dumbbell-like, and daisy-shaped structures.19 Among all these configurations,
core-shell particles are the simplest examples that have been extensively studied and
widely applied.20 As typical core-shell particles, organic materials are surrounded by
inorganic materials or vice versa. Coating inorganic particles with organic shells can
protect the inorganic core from chemical corrosion and introduce functional groups to
improve the affinity towards organic environment. On the contrary, if the organic phase
is surrounded by inorganic materials, colloidal stability in aqueous phase and further
Chapter 1: Introduction and Outline of the Thesis
4
surface functionalization can be promised. The preparation strategy for core-shell
composite particles and design of specific structures are of particular interest in
encapsulation technologies, drug delivery and release, catalyst, and so forth.20
Silica as one of the most abundant material on the earth is the most frequently used
inorganic constituent for the preparation of inorganic-organic composite materials.
Being chemically inert, mechanically stable, low toxic, biocompatible, and optically
transparent, silica-based materials are of high demand in various fields of material
science.21 Importantly, silica nanomaterials can be prepared using sol-gel technology
that combines the control over composition and microstructure at the molecular level
with the ability to shape the material to particles of a wide size range, fibers, or thin
films under mild and low-energy conditions.22,23
1.2 Objectives of this thesis
One of the objectives of this thesis is to modify polymer matrices with ultrasmall
silica-based nanoparticels with a size less than 10 nm. The main focus will be laid on
the influence of these particles on the processability of the polymer matrix. For this
purpose, ultrasmall MQ resin particles with a SiO2 (Q) core and an organic shell
consisting of monofunctional components (M) in the form of R3SiO1/2 will be prepared,
and their surface will be substituted with groups that are compatible with the polymer
matrix. The influence of these ultrasmall MQ particles on the rheological behavior of
resulting polymer nanocomposites will be systematically investigated to establish the
structure-property relations by comparing the interparticle half-gap and radius of
gyration of polymer coils. Another goal is to develop a novel and facile approach for
the synthesis of different kinds of silica-based composite particles with the focus on
core-shell structures using a silica precursor polymer - hyperbranched
polyethoxysiloxane (PEOS) - in combination with emulsion technique. PEOS is a
highly hydrophobic liquid synthesized via condensation of tetraethoxysilane with acetic
anhydride.24 It was successfully used as an interfacial glue in water-in-oil Pickering
Chapter 1: Introduction and Outline of the Thesis
5
emulsions to link the stabilizing silica nanoparticles at the interface to obtain all-silica
colloidosomes enclosing an aqueous phase.25,26
1.3 Outline of the thesis
This thesis is structured into 8 chapters.
Chapter 2 provides a literature survey on two kinds of organic-inorganic hybrid
materials, namely nanoparticle-filled bulk composite materials and polymer/silica
hybrid particles. For the bulk nanocomposites the focus is laid on the ultrasmall filler
particles with a size less than 10 nm. In case of hybrid particles organic@SiO2 core-
shell structures are mainly discussed. In Chapter 3, microsized all-silica colloidosomes
enclosing a hydrophobic liquid are prepared by gluing the stabilizing long-alkyl
functionalized silica nanoparticles with PEOS at the interface in an oil-in-water
Pickering emulsion. It is shown that different hydrophobic liquids can be
microencapsulated by this means and the silica shell acts as a good barrier to retard the
release of encapsulated materials. In Chapter 4 we report a new way to prepare
monodisperse silica nanocapsules. PEOS is shown to stabilize solely oil-in-water
miniemulsions. In this system, methylated silica nanoparticles that are less interfacial
active may catalyze the conversion of PEOS into a mechanically stable silica shell at
the oil/water interface instead of being building blocks of the capsule shell. Moreover,
different silica capsular structures ranging from particle-free microcapsules,
colloidosomes and nanocapsules are obtained depending on the pH of the aqueous
phase. In Chapter 5, a new type of surfactant-free miniemulsion polymerization has
been developed by using PEOS as the only stabilizer for a hydrophobic monomer. Thus,
monodisperse polystyrene@SiO2 core-shell nanoparticles are obtained by emulsifying
a PEOS/monomer solution in water and subsequent heating to initiate polymerization.
This method allows an easy control of silica shell thickness by varying the PEOS
concentration. The particle size, on the other hand, can be regulated not only by the
shearing force, but also by pH of the aqueous medium. Chapter 6 extends the
miniemulsion polymerization method to a less hydrophobic monomer, methyl
Chapter 1: Introduction and Outline of the Thesis
6
methacrylate. In this case, a more hydrophobic methylsilyl-substituted PEOS should be
employed to obtain stable miniemulsions and subsequently composite nanoparticles
with narrow particle size distribution. The resulting particles exhibit a semi-
interpenetrating network structure probably because of the compatibility of the polymer
matrix with methylsilyl-substituted PEOS. Chapter 7 describes the synthesis and
characterization of ultrasmall MQ resin particles with different molecular weight (size).
The synthesis is based on acidic co-hydrolysis and condensation of monofunctional (M)
and tetrafunctional (Q) organosilicon components. Further functionalization is carried
out by hydrosilylation of SiH-functionalized MQ resin particles with vinyl-containing
compounds. In Chapter 8, ultrasmall MQ resin particles are blended with polyamide 6
in a melt extrusion process and the resulting nanocomposites are characterized by
different physical techniques. It is shown that 2-pyrrolidinone-functionalized MQ resins
are homogenously distributed in polyamide. By adding only a small amount of such
particles (0.5 wt.-%), the viscosity of the polymer melt is significantly reduced,
meanwhile the mechanical properties of the polymer remain almost unchanged. By
comparing the interparticle half-gap and radius of gyration of the polymer it can be
concluded that the change of entanglement density and release of topological constraint
caused by MQ resin nanoparticles are responsible for the viscosity reduction.
1.4 References
[1] Chung, D. D. L. Composite Materials: Functional Materials for Modern
Technologies; Springer: New York, 2003.
[2] Chung, D. D. L. Composite Materials: Science and Applications, 2nd ed.; Springer:
New York, 2010.
Chapter 1: Introduction and Outline of the Thesis
7
[3] Gay, D. Composite Materials: Design and Applications, 3rd ed.; CRC Press: Boca
Raton, 2015.
[4] Bourgeat-Lami, E.; Lansalot, M. In Hybrid Latex Particles: Preparation With
(Mini)emulsion Polymerization; Springer-Verlag Berlin: Berlin, 2010.
[5] Zou, H.; Wu, S. S.; Shen, J. Chem. Rev. 2008, 108, 3893-3957.
[6] Friedrich, K.; Fakirov, S.; Zhang, Z. Polymer composites: from nano-to-macro-scale;
Springer: New York, 2005.
[7] Koo, J. H. Polymer nanocomposites: processing, characterization, and applications;
McGraw-Hill: London, 2006.
[8] Balazs, A. C.; Emrick, T.; Russell, T. P. Science 2006, 314, 1107-1110.
[9] Caseri, W. In Hybrid Materials: Synthesis, Characterization, and Applications;
Wiley-VCH: Weinheim: Germany, 2004.
[10] Caseri, W. R. Mater. Sci. Technol. 2006, 22, 807-817.
[11] Ajayan, P. M.; Schadler, L. S.; Braun, P. In Nanocomposite Science and Technology;
Wiley-VCH: Weinheim, 2003.
[12] Schaefer, D. W.; Justice, R. S. Macromolecules 2007, 40, 8501-8517.
[13] Mackay, M. E.; Dao, T. T.; Tuteja, A.; Ho, D. L.; Van Horn, B.; Kim, H. C.; Hawker,
C. J. Nat. Mater. 2003, 2, 762-766.
[14] Albert, E. Ann. Phys. 1906, 19, 371-381.
[15] Tuteja, A.; Mackay, M. E.; Hawker, C. J.; Van Horn, B. Macromolecules 2005, 38,
8000-8011.
[16] Chauhan, B. P. Hybrid nanomaterials: synthesis, characterization, and
applications; Wiley: Oxford, 2011.
[17] Kalia, S.; Haldorai, Y. Organic-inorganic hybrid nanomaterials; Springer: London,
2015.
[18] Guozhong, C.; Ying, W. In Nanostructures and Nanomaterials: Synthesis,
Properties, and Applications; Imperial College Press: London, 2004.
Chapter 1: Introduction and Outline of the Thesis
8
[19] Percy, M. J.; Amalvy, J. I.; Barthet, C.; Armes, S. P.; Greaves, S. J.; Watts, J. F.;
Wiese, H. J. Mater. Chem. 2002, 12, 697-702.
[20] Chaudhuri, R. G.; Paria, S. Chem. Rev. 2012, 112, 2373-2433.
[21] Pagliaro, M. Silica-based materials for advanced chemical applications; RSC Pub.:
Cambridge, 2009.
[22] Brinker, C. J.; Scherer, G. W. Sol-gel science: the physics and chemistry of sol-gel
processing; Academic Press: London, 1990.
[23] Wright, J. D.; Sommerdijk, N. A. J. M. Sol-gel materials: chemistry and
applications; Gordon and Breach Science Publishers: Australia, 2001.
[24] Zhu, X. M.; Jaumann, M.; Peter, K.; Mller, M.; Melian, C.; Adams-Buda, A.;
Demco, D. E.; Blmich, B. Macromolecules 2006, 39, 1701-1708.
[25] Wang, H. L.; Zhu, X. M.; Tsarkova, L.; Pich, A.; Mller, M. Acs Nano 2011, 5,
3937-3942.
[26] Zhang, C.; Hu, C. Y.; Zhao, Y. L.; Mller, M.; Yan, K. L.; Zhu, X. M. Langmuir
2013, 29, 15457-15462.
Chapter 2: Literature Review
9
Chapter 2
Literature Review
Chapter 2: Literature Review
10
2.1 Polymer/inorganic nanocomposites
The incorporation of inorganic nanoparticles into polymer matrices to form
nanocomposites is a well-established but still fast growing approach for the
improvement of material properties.1,2 The promise of nanocomposites lies in their
multifunctionality and the possibility of obtaining unique combinations of properties
that are unachievable with traditional materials. In comparison to composites with
micron-sized fillers, nanocomposites exhibit significantly enhanced performance
properties including mechanical strength, stiffness, barrier properties, heat resistance,
flame retardancy, wear resistance and dielectric properties at even low filler volume
fraction.3 The performance properties of the polymer nanocomposites are achieved on
the basis of three major characteristics: a nano-scopically confined polymer matrix, a
nanoscale inorganic constituent, and nanoscale arrangement of these constituents.4 The
list of inorganic nano-fillers includes carbon nanotubes or nanofibers, layered silicates,
layered chalcogenides, graphitic layer, nanoparticles of metals, metal oxides, quantum
dots, and so forth.4,5
2.1.1 Polymer/silicate nanocomposites
Among all these fillers, silica is considered as the most important and widely used
inorganic materials, and polymer/silica nanocomposites have attracted more attention
in both industrial and academic communities, which can be evidenced from the fast
increasing of practical applications and scientific publications.5 Nanoscale colloidal
silica particles and layered silicates are the most widely used reinforcing fillers.
Hectorite (Mx(Mg6-xLix)Si8O20(OH,F)4), saponite (MxMg6(Si8-xAlx)Si8O20(OH)4), and
montmorillonite (Mx(Al4-xMgx)Si8O20(OH)4), where M is an exchangeable monovalent
cation and x is the degree of isomorphous substitution, are among the most commonly
employed smectite-type layered silicates for the preparation of nanocomposites.6
Nanoscale colloidal silica particles of different sizes is commercially available in both
aqueous and organic solvents. There are three principal procedures commonly
employed to disperse nanoparticles as homogeneous as possible in a polymeric matrix:
Chapter 2: Literature Review
11
(a) blending in solution; (b) blending in melt; (c) in-situ processing. Notably, the in-situ
processing can be further divided into two routes: in-situ polymerization in the presence
of silica and in-situ synthesis of silica in polymer matrix.
Direct blending the silica and polymer solution provides a convenient route to form
polymer/silica hybrid materials after solvent evaporation or fast precipitation of
polymer nanocomposites.7,8 The melt blending method is environmentally benign due
to the absence of organic solvents. Furthermore, it is compatible with current industrial
processes, such as extrusion and injection moulding. Melt blending for nanocomposites
has been developed successfully when the silica particle surface is substituted with
organic groups.9,10 In the in-situ polymerization method, the silica fillers are first
dispersed in the monomer(s) and then the polymerization is triggered.11 A
functionalized silica surface that promises a uniform distribution in organic monomer
is also essential. In a classical in-situ sol-gel process, in-situ formation of silica is based
on co-hydrolysis and then polycondensation of silica precursor molecules under acid or
basic conditions in the solution of polymers.12,13 A reactive extrusion process where the
sol-gel process is carried out in a polymer melt has been developed by DWI, where a
non-volatile silica precursor polymer, hyperbranched polyethoxysiloxane, has been
employed.14,15
2.1.2 Surface modification of silica
Homogenous dispersion of nanoscale silica can lead to an extremely large
interfacial area between polymer matrix and fillers and may promise a dramatic
improvement of macroscopic performance of final materials even at a low volume
fraction.4,5 However, due to the intrinsic chemical difference between organic polymers
and inorganic silica that often cause phase separation, enhancing the interfacial
interaction between two phases of nanocomposites is the most decisive factor that
significantly affects the performance properties of the materials.16 Surface modification
of silica is the most frequently used method to realize a strong interaction and a
subsequent uniform dispersion state. Generally, this modification can be performed in
Chapter 2: Literature Review
12
either a chemical or a physical way.17 Chemical methods mainly involve modification
with either modifiers (e.g. silane coupling agents) or with grafting polymers through
covalent bonding of end-functionalized polymers with the surface (grafting to
method), or in-situ polymerization of monomer initiated from silica surface (grafting
from approach). Modification by physical interaction is much weaker and is realized
by absorbing surfactant molecules (e.g. CTAB, oleic acid) or macromolecules (e.g.
chitosan) on the silica surface through electrostatic interaction18 or hydrogen bonds.19
2.1.3 Polymer nanocomposites containing ultrasmall particles
In most studies on polymer nanocomposites, particles of tens of nanometers are
used. However, the situation becomes much more intriguing when the size of the
particles is further diminished below 10 nm. In this case, molecular dimensions are
approached and the interparticle half-gap (h) is comparable or even smaller than the
radius of gyration (Rg) of a polymer coil (Figure 1 and Table 1). This produces chain
confinement and distortion that can result in depletion driven phase segregation, or in
the case segregation can be avoided, may promise unusual properties.20 From Table 1,
one can expect quite a small h when r is small enough even at a very low .
Figure 1. A typical polymer composite structure, where freely mobile bulk polymer
between nanoparticles and immobilized polymer on the nanoparticle surface are
considered as two main polymer phase.21
Chapter 2: Literature Review
13
Table 1. Interparticle half-gap (h) vs particle radius (r) and volume fraction (). h can
be estimated by the simple relation: = [ ]1 3 1 , where = 0.638
(maximum random packing fraction).22
, %
r, nm
0.5 1 2 5 10 20 40
h, nm
3 12.1 9.0 6.5 4.0 2.6 1.4 0.5
5 20.2 15.0 10.9 6.9 4.3 2.4 0.8
10 40.3 30.0 21.7 13.4 8.5 4.7 1.7
20 80.7 59.9 43.4 26.7 17.1 9.4 3.4
Indeed, Mackay et al. reported on a continuous drop in the viscosity of linear
polystyrene (PS) by blending with intramolecular cross-linked PS nanoparticles with a
radius of 3-5 nm over a broad mass fraction range.22 The viscosity decrease is explained
by the increase of excluded free volume produced around nanoparticles according to
fact that an accompanying drop in glass transition temperature of PS is observed. It was
shown in later publications that the viscosity decrease can only be observed when the
linear PS molecule is entangled and confined by nanoparticles. They attributed the
viscosity decrease to the constraint release caused by addition of nanoparticles,
although they admitted it is a very complicated phenomenon which also involves the
introduction of free volume by the nanoparticles.8,23 This observation is contrary to the
expression proposed by Einstein predicting the increase of the viscosity is solely a
function of the particle volume fraction () and the viscosity of dispersion liquids.24 In
another report, ultrasmall silicate clusters with a radius of 0.35 nm were blended with
linear polydimethylsiloxane (PDMS) and a decrease in viscosity was observed whereas
no viscosity was found by addition of larger silicate particles, again pointing to the
unusual nanoscale effect that nanoparticles may provide.25 They state that the small
clusters appear to solvate the PDMS since the particle size approaches the length
scale of a monomer unit, leading to a decrease in viscosity. Jain et al. reported on a
dramatic viscosity decrease of polypropylene (PP) filled with in-situ prepared silica
Chapter 2: Literature Review
14
nanoparticles with a radius of 7.5-15 nm in a narrow concentration window (up to
approx. 0.5 wt%).26 A reduction in entanglement density caused by adsorption of high
molar mass chains on the surface of silica nanoparticles was eventually thought to
reduce the viscosity. Although the manner of viscosity decrease and mechanism to
elucidate viscosity decrease are not completely consistent in these cases, at least there
is a certain nanoscale effect which cannot be observed with larger nanoparticles.
Very recently, self-healing was demonstrated in a multi-layered nanocomposite
polymer structure, i.e. the nanoparticles dispersed in a polymer matrix migrated to a
crack generated at the interface between the polymer and a glassy layer.27 According to
results of computer simulations, nanoparticles in a polymer fluid can segregate to the
surfaces and into the cracks due to a polymer-induced depletion attraction between
the particles and the surface.20 In this case, only particles comparable in size to the Rg
of were driven from the matrix to the surface in the crack area. Importantly, a
homogeneous dispersion of the nanoparticles in the polymer matrix is a prerequisite for
achieving the above-mentioned properties.
2.1.4 MQ resins ultrasmall core-shell silicate particles
The synthesis of ultrasmall silica particles that are compatible with organic
polymers is a big challenge. On the other hand, MQ resins that are well-known in the
organosilicon industry are three-dimensional silicone polymers composed of a
monofunctional component (M) in the form of R3SiO1/2 and a tetrafunctional unit (Q)
in the form of SiO2, as shown in Table 2. They are widely used in pressure-sensitive
adhesives, silicone rubbers, coatings and so on.28-34 A MQ resin particle can be
considered as an ultrasmall particle with a SiO2 core and an organic alkylsilyl shell. The
particle size ranges from 0.5 to 3 nm and can be simply adjusted by varying the Q/M
ratio. The most frequently used method for MQ synthesis is on the basis of hydrolysis
and condensation of Me3SiCl in the presence of aqueous sodium silicate. Alternatively,
they can be prepared by co-hydrolysis and condensation of tetraethoxysilane (TEOS)
and Me3SiCl or hexamethyldisiloxane with addition of water in an organic solvent.
Chapter 2: Literature Review
15
However, in the first case, the structure control of MQ resins is not easy and gels are
often produced due to a poor control of the rate of silicic acid polymerization.
Furthermore, this method always yields a broad molecular weight distribution of
resulting products.35 In the second method, the reaction takes place under homogeneous
conditions, thus a much better structure control and narrow molecular weight
distribution can be achieved. However, a part of alkoxy groups often remain unreacted
in the products due to incomplete hydrolysis when Me3SiCl is used. This situation
cannot be significantly improved even when more readily hydrolysable alkoxy groups,
for example, tetramethoxysilane, are used as starting material. However, when a
disiloxane (e.g. hexamethyldisiloxane) is used to replace Me3SiCl, MQ particles grow
in a controlled manner and all the alkoxy groups are completely consumed.35
Importantly, by this means 1,1,3,3-tetramethyldisiloxane can be used to synthesize MQ
resins with terminal SiH groups, which can be further modified via a catalytic
hydrosilylation reaction to introduce different groups for improving their compatibility
with polymer matrices.
Table 2. Nomenclature of MDTQ units in organosilicon materials.
Basic monomers Functionality Structure in polymer R/Si Symbol
SiX4 4 Si OOO
O
SiO2 0 Q
R-SiX3 3 Si OCO
O
RSiO3/2 1 T
R2-SiX2 2 Si OCO
C
R2SiO 2 D
R3-SiX 1 Si OCC
C
R3SiO1/2 3 M
Chapter 2: Literature Review
16
2.1.5 Polyamide 6
It is a target of the present thesis to modify polyamide 6 (PA6) by blending with
ultrasmall particles. PAs are one class of the most widely used commodity polymers.
Within the PA family, PA6 is the most widely used one. PAs possess good material
properties, in particular a high impact resistance, stiffness, abrasion resistance,
elongation at break, modulus of elasticity, resiliency, dimensional stability on heating,
barrier properties and puncture resistance. In order to further improve their mechanical
properties, PAs are often mixed with inorganic fillers such as glass fibers, impact
modifiers or nanoparticulate clay minerals. Silica nanoparticles are the most widely
used filler, and PA/silica nanocomposites have been the subject of extensive research
during the past decades. Although plenty of work has been devoted to the development
of PA-based nanocomposites, very limited work on the precessability of PA6 by
addition of small nanoparticles has been reported,36 though this is a very important topic.
Thermal degradation of PA chains often occurs during melt processing under
particularly high temperature (above 250 oC), thus resulting in a loss in performance
properties. It is notable that the research group of Voit has performed a tremendous
amount of work on modifying the melt viscosity of PAs by blending with
hyperbranched polymers. The reduction of the polymer melt viscosity is explained by
the plasticization effect of the hyperbranched polymers.37-39
2.2 Silica-based hybrid core-shell particles
2.2.1 Definition and classification of core-shell particles
Core-shell particles are probably the most common colloidal composites. As the
name implies, core-shell composite particles, which are composed of two or more
materials, can be generally defined as particles comprising a core (inner material) and
a shell (outer layer material).40 To date, different geometries of core-shell particles have
been synthesized and summarized in several reviews, as schematically shown in Figure
Chapter 2: Literature Review
17
2.40 Concentric core-shell particles by far are most commonly synthesized and applied,
where a spherical core is fully surrounded or coated by a continuous shell of a different
material (Figure 2a). Recently, core-shell particles with different shapes (e.g. hexagon-
like, shuttle-like structure) are also of high research interest due to their novel
performance properties. Generally, they are synthesized by templating against a
nonspherical core (Figure 2b). When one single core is replaced with a cluster of several
small particles, further deposition of shell materials leads to a typical core-shell particle
with multiple cores (Figure 2c). One spherical core can also be closely encapsulated via
alternative coating of different materials, thereby forming a concentric multilayer shell
around the core (matryoshka-like particles, Figure 2d). After the nearest layer around
core particles is removed either by calcination or chemical etching, hollow particles
with movable core particles are synthesized (Figure 2e). In fact, plenty of core-shell
particles have been reported in the literatures and some are already available in
industrial applications. As a result, the classification of all core-shell particles in one
fixed criterion, such as industrial application, unique property, or assembly method,
seems difficult but essential on a certain occasion. Classification by the combination of
constituent materials (inorganic or organic), so far, is broadly adopted. Thus the core-
shell particles can be mainly divided into four groups: (i) inorganic-inorganic; (ii)
organic-organic; (iii) inorganic-organic; (iv) organic-inorganic.
Figure 2. Different shaped core-shell particles: (a) spherical core-shell particles, (b)
hexagonal core-shell particles, (c) multiple cores coated by single shell materials, (d)
matryoshka material, and (e) movable core within hollow shell material.
Chapter 2: Literature Review
18
2.2.2 Organic-silica core-shell particles
For the sake of topic concerned in this thesis, organic-inorganic core-shell particles,
in particular organic-silica core-shell particles will be discussed in detail. In this class
of core-shell particles, the core can be hard polymers, organic liquids, and also can be
organic phase change materials. A large variety of homopolymers, such as PS,41-46
poly(ethylene glycol),47 polyurethane,48 poly(vinyl benzyl chloride),49 poly (vinyl
pyrrolidone),50 dextrose,51 and some of copolymers like poly(acrylonitrile-butadiene-
styrene),52 poly(styrene-acrylic acid),53 poly(styrene-methyl methacrylate),54 poly(N-
isopropylacrylamide-co-acrylic acid),55 have already been used as cores to synthesize
core-shell particles. Organic liquids, to some extent can be regarded as soft templates
(or soft cores) for coating shell materials and then be encapsulated to form core-shell
particles.56-63 Phase change materials (PCMs) are a special group of chemicals (e.g. wax,
fatty acids, and fatty acid esters) that are liquid droplets for shell coating above phase
transition temperature and then keep the shape after cooling.64-69 The inorganic shell
can also be made of different materials, such as metals,42,49,70 metal oxides,41,47,51,54,71
metal chalcogenides,53 and silica.41,72-86 The inorganic shell, especially a metal oxide
coating on an organic core can greatly increase overall mechanical strength of particles,
resistance to oxidation, thermal and colloidal stability, and abrasion resistance.87-91
Among these inorganic materials, silica is a promising shell material owing to some
basic advantages over other inorganic (metals or metal oxide) or organic coatings.
Coating an organic core with a silica layer to form a core-shell particle is an important
technique to impart hydrophilicity, biocompatibility and modifiability that are intrinsic
properties of silica to the organic material.92-94 Furthermore, it also reduces the bulk
conductivity and increases the colloidal stability in aqueous phase. Due to its chemical
inertness, silica shell can protect the core from environmental chemical reactions that
may induce deterioration of core materials. The unique biocompatibility and optical
transparency make silica be widely applied in biotechnology and optical coatings.
Moreover, the silica surface is easily functionalized with other molecules, implying
enormous potential in various application areas.95,96
Chapter 2: Literature Review
19
2.2.3 Design and synthetic strategy
The formation of organic-silica core-shell particles has been described in a large
number of patents and scientific publications, however this list is not exhaustive. In
general, the core-shell particles can be synthesized by either a two-step process
including first core preparation and subsequent shell synthesis or a one-step reaction
where the core and shell are formed simultaneously. The two-step synthesis technique
can be summarized into two categories regarding the availability of core particles: (i)
the core particles are synthesized independently and then introduced to a new system
with appropriate surface treatment for further silica coating; (ii) the core particles are
prepared in-situ optionally with help of surface-active materials, and then followed by
a shell material coating. The one-step process means that the core is formed in the
presence of preformed shell and vice versa. For example, the core monomer is
polymerized in the presence of silica nanoparticles, namely Pickering polymerization.
The nanoparticles are thus immobilized on the surface of the polymer core, forming a
unique silica shell with permeation properties. Conversely, the continuous silica shell
can be formed around a liquid core by a one-step sol-gel reaction of silica precursor,
which is a typical technique in sol-gel microencapsulation. Upon different synthetic
routes, the morphology of the silica shell can either be a continuous film that closely
surrounds the organic core, or be discrete silica particles that are immobilized on the
surface of the inner core (so-called colloidosome97-99), or possess a composite structure
that contains both silica film and silica nanoparticle.100-102 Correspondingly, the organic
core encapsulated can be hard polymer particles, liquid droplets, or the combination of
both.
2.2.3.1 Polymer core functionalization and silica shell coating
Thanks to the huge progress in polymerization techniques, polymer particles,
especially PS particles are commercially available and widely used as core templates
for synthesis of core-shell materials.76 However, the intrinsically hydrophobic property
of PS makes the dispersion in aqueous phase and surface coating by silica difficult due
Chapter 2: Literature Review
20
to a lack of effective interaction. So functionalization of polymer surface plays a vital
role in determining the subsequent silica coating and final overall morphology of core-
shell particles. Normally, the surface functionalization of polymer core can be divided
into post-treatment and prior-treatment approaches. The post-treatment method is based
on introduction of functional groups or active molecules onto preformed polymer
surface via chemical or physical routes. These surface-active groups or molecules will
structurally direct the further silica coating, forming dense, porous or highly ordered
shell structure. Han et al. introduced sulfonate groups onto the surface of PS by means
of sulfonating reaction (Figure 3).83 Kim et al. developed a plasma technique to
hydroxylate the surface of PS surface in order to increase the compatibility between
polymer core and silica shell (Figure 3).80 In both methods, it is required to transfer
these functionalized polymer particles into a mixture of alcohol and ammonia solution
for subsequent shell formation on polymer surface according to the well-established
Stber method.103
Figure 3. Schematic illustration of polymer surface treatment via sulfonating reaction
and plasma and further silica coating to prepare a core-shell particle.
The prior-treatment method, which incorporates functional groups by co-
polymerization of styrene with other functional monomers, from synthetic view of point,
is more general and flexible due to the large range of available functional groups. For
example, Bourgeat-Lami et al. and Wu et al. introduced silanol groups to the PS surface
by co-polymerization of methacryloxypropyltrimethoxysilane with styrene, allowing
Chapter 2: Literature Review
21
further silica precipitation by condensation of the surface silanol groups with TEOS
(Figure 4).45,104-106 4-Vinylpyridine is another functional monomer used by Zou et al. to
prepare PS latex particles bearing basic groups for further silica coating (Figure 4).107
In most cases surfactant molecules, like polyvinylpyrrolidone (PVP),77,81,108-110
cetyltrimethylammonium bromide (CTAB),72,86 or poly-L-lysine46 are essential in
stabilizing emulsion droplets during the polymerization step and directing silica coating
on polymer core in a controllable, homogeneous way.
Figure 4. Schematic representation of the copolymerization step and sol-gel reaction
of TEOS involved in preparation of polymer-silica core-shell particles.
2.2.3.2 Layer-by-layer (LBL) process
In addition to chemical bonds, noncovalent interactions between polymer core and
silica shell can also be utilized to prepare core-shell type particles. Layer-by-layer
method for the preparation of polymer@SiO2 core-shell particles is based on the
principle of alternate depositing of oppositely charged polyelectrolytes and silica onto
a surface of polymer particle through electrostatic self-assembly. It is worthwhile to
note that the adsorption of particles onto a solid substrate in LBL manner was firstly
proposed by Iler in the middle 1960s.111 Then, Decher et al. put forward this LBL
method by combination of polycations and polyanions in the early 1990s.112,113 In both
Chapter 2: Literature Review
22
cases, macroscopically flat, charged surfaces were employed as substrates. Caruso et al.
have made a huge progress in applying LBL method to coat colloidal particles, thus
allowing to form composite core-shell particles.41,71,92,114,115 Negatively charged
polystyrene latex with a diameter of 640 nm were used as templates for sequential
electrostatic deposition of a linear cationic polymer poly(diallyldimethylammonium
chloride) (PDADMAC) and 25 nm silica nanoparticles with negative charge (Figure
5).71,116 The thickness of the composite multilayers could be adjusted by controlling the
number of deposition cycles. After removal of PS core particles via calcination or
solvent dissolution, submicron-sized hollow particles with wall thickness ranging from
tens to hundreds of nanometers were successfully produced. The LBL process also
makes the alternating deposition of oppositely charged polyelectrolytes onto core
particles through electrostatic interaction possible, thus forming a composite organic
shell around the core materials.92
Figure 5. Illustration of procedures for preparing hybrid core-shell particles and hollow
particles by LBL method.71
Besides, The LBL self-assembly technique is also an effective way to tune the
surface property of core particles by adsorbing polymers. Chiu et al. showed the ability
to grow silica directly on negatively charged surface of PS particles or latex
nanocapsules (Figure 6).117 The surface is first functionalized with alternately deposited,
oppositely charged polyelectrolytes, namely cationic PDADMAC and anionic
poly(styrenesulfonate). After the surface is finally capped with PDADMAC, the sol-gel
Chapter 2: Literature Review
23
reaction of TEOS takes place to form a silica layer. Upon increasing cycling times, the
thickness of the shell, which has a sandwiched architecture, can be well controlled and
the shell exhibits a low permeability and high mechanical strength after the core is
removed.
Figure 6. Schematic illustration of strategy for coating polyelectrolyte and silica onto
PS particles.
2.2.3.3 Pickering (mini)emulsion polymerization
Instead of organic surfactant molecules, solid particles or 2D layers have also been
reported to stabilize emulsion droplets, which are often referred to Pickering
emulsions.118,119 It is generally accepted that the stability of Pickering emulsions is due
to the coherent particle layer around the droplets that acts as a steric barrier against
coalescence.120,121 It has also been reported that repulsive colloidal particles, which give
dilute planar monolayers on the droplet surface, can serve likewise as efficient emulsion
stabilizers, and the stabilization is explained by particle bridges.122,123 At the very
beginning of this century, the emulsion polymerization in the presence of solid particles
has been investigated by many research groups to prepare core-shell particles.100,124-134
In such processes, solids are thought to act as stabilizer for emulsion droplets or
polymer nuclei during polymerization and as building blocks of the resulting shell.
Notably, the particulate shell composed of closely packed particles is quite different
from the continuous and homogeneous shell obtained from sol-gel reaction of TEOS.
Bon et al. proposed a concept of Pickering miniemulsion polymerization where
nanoparticles or nanoclays were used to stabilize the initial emulsion droplets of
monomer. After polymerization these particles became irreversibly attached onto the
surface, thus forming composite particles with a core-shell structure (Figure 7).127
Chapter 2: Literature Review
24
Figure 7. Synthesis of laponite RD armored polystyrene latex particles via Pickering
miniemulsion polymerization.
Armes et al. have contributed a lot to the emulsion polymerization of monomer(s)
in the presence of silica nanoparticles to prepare polymer/silica nanocomposite core-
shell particles (Figure 8).100 The strategy is based on an initial electrostatic adsorption
of cationic initiator azobis(isobutyl-amidine hydrochloride) (AIBA) onto the surface of
silica nanoparticles and a subsequent surface initiation of polymerization. As the
nascent particles grow in size, the silica coverage on polymer surface is reduced and
free anionic silica nanoparticles that remain in water are eventually adsorbed onto the
polymer surface, leading to typical core-shell particles. Though it is argued that
polymerization in this case is unlike the traditional Pickering emulsion polymerization
because the initial emulsion droplets have a larger size than that of the final
nanocomposite particles, the silica nanoparticles indeed participate in the stabilization
process during polymerization and in building up the shell. Other research groups, such
as Bourgeat-Lami et al., Wu et al. and Wang et al. also extend this topic by synthesizing
novel core-shell particles in their own way.125,126,128,133,134 Despite the diversity and
distinction in experimental manipulation and mechanism clarification, polymerization
in the presence of solid particles or 2D layers is a facile, general and effective way to
synthesize special core-shell particles.
Chapter 2: Literature Review
25
Figure 8. Schematic representation of polymer@silica nanocomposite particles by
emulsion (co)polymerization in the presence of a glycerol-functionalized ultrafine silica
sol.
2.2.3.4 Partial extraction of polymer after sol-gel reaction
In the early 2000s the group of Avnir developed a very unique and novel way to
prepare core-shell particles, which is based on performing sol-gel reaction under basic
alcohol medium within surfactant-stabilized droplets containing PS (not latex particles)
in TEOS.84 As the sol-gel reaction proceeds fast at the interface, the hydrophobic TEOS
molecules are converted to a hydrophilic rigid shell that entraps the host PS without
permitting them to accumulate into a large PS phase, thereby forming bicontinuous
PS/silica composite particles (Figure 9a). When placing these composite particles in
methylene chloride, the entrapped PS molecules can be extracted from the particle
interior. After the polymer is removed by partial extraction, a core-shell structure is
formed, in which the shell consists of porous silica and the core comprises a PS/silica
composite (Figure 9b). The shell thickness can be controlled simply by the duration of
the extraction process. If PS is completely removed by calcination or solvent extraction,
mesoporous rather than hollow silica particles are obtained. This synthetic procedure
involves a one-step sol-gel reaction, and promises an efficient way to prepare
monodisperse submicron-sized composite particles.
Chapter 2: Literature Review
26
Figure 9. TEM images of (a) composite sol-gel particles of PS and silica; (b) SiO2-
shell/(PS+SiO2)-core particles achieved by partial extraction of hydrophobic phase.84
2.2.3.5 Sol-gel microencapsulation of organic liquids
The synthetic strategies discussed above mainly focus on forming silica shell on
polymer templates which are prepared either before or during the formation of core-
shell particles. In some cases, the hard polymer cores also sustain the silica shell with
enhanced mechanical integrity or immobilize the shell particles together on the polymer
surface. Apart from hard polymer cores, soft liquids can also be used as core templates
for preparation of core-shell particles. It is well described in several reviews that
capsules with liquid cores have been widely used in industry ranging from food and
cosmetics to pharmacy and medicine, implementing protection and controlled release
of active substances.40,59,60,135 Organic polymers are the most widely used shell
materials and silica is another promising alternative. Sol-gel microencapsulation of
organic liquids is considered as the most efficient, simple and economic approach to
synthesize silica-shell/liquid core particles. In brief, the emulsion droplets, which can
be oil-in-water (o/w) or water-in-oil (w/o) type, provide a templating interface for the
hydrolysis and condensation of silica precursors. Surfactants (e.g. cationic, anionic, and
nonionic surfactants) are essential not only for stabilizing emulsion droplets but also
for directing further silica precipitation.
The type of emulsion used as templates, in the final analysis, is determined by the
property of target molecules to be encapsulated. In w/o emulsions, droplets of an
Chapter 2: Literature Review
27
aqueous phase, which may contain hydrophilic active substances, are dispersed into a
hydrophobic oil with aid of surfactants. The encapsulation starts with the hydrolysis
and condensation of the silica precursor around the hydrophilic droplet, which in turn
plays a role of template. Once the precursor is fully converted into silica, hydrophilic
materials are encapsulated within the silica shell (Figure 10).136 Hence, by changing the
emulsion parameters, such as hydrophile-lipophile balance (HLB) value of surfactant,
water/oil ratio, aqueous pH, amount of silica precursor, one can delicately control the
overall particle size and the shell thickness as well as the core size. TEOS, one of the
mostly used silica precursors, can be initially located in the oil phase or water droplets.
With help of ethanol, TEOS dissolves in water and forms sol droplets in the oil phase.
Since the nonpolar alkyl chains of TEOS partly hydrolyzed prefer the nonpolar paraffin
oil, the hydrolysis and later condensation take place at the water/oil interface, leading
to a thickness growth of silica shell from oil/water interface toward the core of water
droplets.137
Figure 10. Procedure for direct microencapsulation of aqueous phase within silica shell
through a sol-gel reaction.
In principle, the microencapsulation of hydrophobic substances within a silica
shell is not possible in the case of w/o emulsion except the use of w/o/w or o/w/o double
emulsions, avoiding fast migration of lipophilic materials in the oil phase.60 The most
facile way is to directly encapsulate the hydrophobic compounds in an o/w emulsion
combined with a directed interfacial sol-gel reaction. Hydrophobic substances, such as
flavors, proteins, polymers and other chemicals are first dissolved in appropriate oils
and the homogeneous mixture is then dispersed into an aqueous phase with the aid of
Chapter 2: Literature Review
28
surfactants and/or high shear force in some cases, yielding stable o/w type emulsions.
The silica precursor initially located in the dispersed hydrophobic phase continuously
diffuses from the droplet interior to the water/oil interface until complete consumption.
The resulting particles have a silica shell and a hydrophobic liquid core, which can also
be controllably liberated through the porous shell under external stimulus to produce
silica hollow particles. As another representative of organic core materials, phase
change materials which have a characteristic transition temperature can be coated with
silica in a similar o/w way mentioned above. Remarkably, they serve as soft liquid cores
which can be easily shaped and emulsified during preparation, and as hard solid cores
which in turn sustain the silica shell without collapse after cooling.
In both o/w and w/o cases, corresponding surfactants are essential to stabilize the
emulsion droplets and in some cases to direct the formation of unique shell structure.
In recent years, it has been reported that amphiphilic silica precursor can self-assemble
into different structures or morphologies without any surfactants, and can be eventually
converted into silica after hydrolysis and condensation. The amphiphilicity of silica
precursor reported so far is generally produced from the fact that hydrophobic
alkoxysilane can be hydrolyzed into hydrophilic silanol groups under catalysis of acid
or base.138,139 For example, well-defined siloxane oligomers with three alkoxysilane
functionalities and a chemically bonded alkyl chains have been synthesized by
Shimojima et al.139 Various hybrid mesostructures, namely lamellar, 2D hexagonal, and
2D monoclinic structures, are formed from these compounds without any other
structure-directing agents (Figure 11).
Figure 11. Schematic presentation of self-assembly of the silica precursor into three
types of hybrid mesostructures.139
Chapter 2: Literature Review
29
Another novel way to render the silica precursor amphiphilic is to introduce
hydrophilic groups (e.g. ethylene glycols). However, the monomeric silanes that
partially substituted with ethylene glycols cannot effectively self-assemble into
sophisticated structures probably due to their high solubility and fast hydrolysis in water.
To avoid this problem, a highly hydrophobic silica precursor polymer, hyperbranched
polyethoxysiloxane (PEOS), is partly substituted with poly(ethylene glycol) (PEG) and
then used for guiding the formation of nanostructured silica materials via self-
templating in water (Figure 12).140 Mesoporous particles, hollow nanocapsules, and
ultrasmall solid particles are prepared simply by controlling substitution degree of
PEOS with PEG. This self-assembly technique of functionalized silica precursor
polymers without any surfactants can certainly become a new approach towards silica-
shell/organic-core hybrid materials.
Figure 12. Schematic illustration of self-assembly of different PEG-PEOS molecules
in water and their conversion to silica particles of different morphologies.140
Chapter 2: Literature Review
30
2.3 References
[1] Ajayan, P. M.; Schadler, L. S.; Braun, P. In Nanocomposite Science and Technology;
Wiley-VCH: Weinheim, 2003.
[2] Koo, J. H.; The McGraw-Hill Companies, Inc.: New York, 2006.
[3] Brechet, Y.; Cavaille, J. Y. Y.; Chabert, E.; Chazeau, L.; Dendievel, R.; Flandin, L.;
Gauthier, C. Adv. Eng. Mater. 2001, 3, 571-577.
[4] Vaia, R. A.; Giannelis, E. P. Mrs Bull. 2001, 26, 394-401.
[5] Zou, H.; Wu, S. S.; Shen, J. Chem. Rev. 2008, 108, 3893-3957.
[6] Ray, S. S.; Okamoto, M. Prog. Polym. Sci. 2003, 28, 1539-1641.
[7] Vollenberg, P. H. T.; Heikens, D. Polymer 1989, 30, 1656-1662.
[8] Tuteja, A.; Duxbury, P. M.; Mackay, M. E. Macromolecules 2007, 40, 9427-9434.
[9] Ruan, W. H.; Zhang, M. Q.; Rong, M. Z.; Friedrich, K. J. Mater. Sci. 2004, 39, 3475-
3478.
[10] Garcia, M.; van Vliet, G.; ten Cate, M. G. J.; Chavez, F.; Norder, B.; Kooi, B.; van
Zyl, W. E.; Verweij, H.; Blank, D. H. A. Polym. Adv. Technol. 2004, 15, 164-172.
[11] Yang, F.; Ou, Y. C.; Yu, Z. Z. J. Appl. Polym. Sci. 1998, 69, 355-361.
[12] Jang, J.; Park, H. J. Appl. Polym. Sci. 2002, 83, 1817-1823.
[13] Landry, C. J. T.; Coltrain, B. K.; Landry, M. R.; Fitzgerald, J. J.; Long, V. K.
Macromolecules 1993, 26, 3702-3712.
[14] Dou, Q. Z.; Zhu, X. M.; Peter, K.; Demco, D. E.; Moller, M.; Melian, C. J. Sol-Gel
Sci. Technol. 2008, 48, 51-60.
[15] Zhu, X. M.; Melian, C.; Dou, Q. Z.; Peter, K.; Demco, D. E.; Mller, M.; Anokhin,
D. V.; Le Meins, J. M.; Ivanov, D. A. Macromolecules 2010, 43, 6067-6074.
[16] Schadler, L. S.; Kumar, S. K.; Benicewicz, B. C.; Lewis, S. L.; Harton, S. E. Mrs
Bull. 2007, 32, 335-340.
[17] Rong, M. Z.; Zhang, M. Q.; Ruan, W. H. Mater. Sci. Technol. 2006, 22, 787-796.
Chapter 2: Literature Review
31
[18] Wu, T. M.; Chu, M. S. J. Appl. Polym. Sci. 2005, 98, 2058-2063.
[19] Yang, X. M.; Dai, T. Y.; Lu, Y. Polymer 2006, 47, 441-447.
[20] Lee, J. Y.; Buxton, G. A.; Balazs, A. C. J. Chem. Phys. 2004, 121, 5531-5540.
[21] Sun, C. X. Controlling the Rheology of Polymer/Silica Nanocomposites; PhD
Thesis, Eindhoven University of Technology, 2010.
[22] Mackay, M. E.; Dao, T. T.; Tuteja, A.; Ho, D. L.; Van Horn, B.; Kim, H. C.; Hawker,
C. J. Nat. Mater. 2003, 2, 762-766.
[23] Tuteja, A.; Mackay, M. E.; Hawker, C. J.; Van Horn, B. Macromolecules 2005, 38,
8000-8011.
[24] Einstein, A. Ann. Phys. (Leipz.) 1906, 19, 371-381.
[25] Roberts, C.; Cosgrove, T.; Schmidt, R. G.; Gordon, G. V. Macromolecules 2001,
34, 538-543.
[26] Jain, S.; Goossens, J. G. P.; Peters, G. W. M.; van Duin, M.; Lemstra, P. J. Soft
Matter 2008, 4, 1848-1854.
[27] Gupta, S.; Zhang, Q. L.; Emrick, T.; Balazs, A. C.; Russell, T. P. Nat. Mater. 2006,
5, 229-233.
[28] Brady, S. A. U.S. Pat. 3,627,851 1971.
[29] Flannigan, W. T. U.S. Pat. 3,772,247 1973.
[30] Bessmer, S. J.; Lampe, W. R. U.S. Pat. 3,888,815 1975.
[31] Smith, R. A. U.S. Pat. 4,490,500 1984.
[32] Modic, F. J. U.S. Pat. 4,500,584 1985.
[33] Mbah, G. C. U.S. Pat. 4,882,398 1989.
[34] Lin, S. B. Int. J. Adhes. Adhes. 1994, 14, 185-191.
[35] Shirahata, A. U.S. Pat. 4,707,531 1987.
[36] Jain, S.; Grtz, H.-H.; Gabriel, C.; Desbois, P. 2008; Vol. WO 2008/084013.
[37] Huber, T.; Potschke, P.; Pompe, G.; Hassler, R.; Voit, B.; Grutke, S.; Gruber, F.
Chapter 2: Literature Review
32
Macromol. Mater. Eng. 2000, 280, 33-40.
[38] Monticelli, O.; Oliva, D.; Russo, S.; Clausnitzer, C.; Potschke, P.; Voit, B.
Macromol. Mater. Eng. 2003, 288, 318-325.
[39] Fan, Z. R.; Jaehnichen, K.; Desbois, P.; Haeussler, L.; Vogel, R.; Voit, B. J. Polym.
Sci., Part A: Polym. Chem. 2009, 47, 3558-3572.
[40] Chaudhuri, R. G.; Paria, S. Chem. Rev. 2012, 112, 2373-2433.
[41] Caruso, R. A.; Susha, A.; Caruso, F. Chem. Mater. 2001, 13, 400-409.
[42] Kawahashi, N.; Shiho, H. J. Mater. Chem. 2000, 10, 2294-2297.
[43] Shiho, H.; Kawahashi, N. J. Colloid Interface Sci. 2000, 226, 91-97.
[44] Shiho, H.; Kawahashi, N. Colloid Polym. Sci. 2000, 278, 270-274.
[45] Tissot, I.; Reymond, J. P.; Lefebvre, F.; Bourgeat-Lami, E. Chem. Mater. 2002, 14,
1325-1331.
[46] Yang, J.; Lind, J. U.; Trogler, W. C. Chem. Mater. 2008, 20, 2875-2877.
[47] Sung, Y. M.; Lee, J. K.; Chae, W. S. Cryst. Growth Des. 2006, 6, 805-808.
[48] Phadtare, S.; Kumar, A.; Vinod, V. P.; Dash, C.; Palaskar, D. V.; Rao, M.; Shukla,
P. G.; Sivaram, S.; Sastry, M. Chem. Mater. 2003, 15, 1944-1949.
[49] Cheng, Z.; Zhang, L.; Zhu, X.; Kang, E. T.; Neoh, K. G. J. Polym. Sci., Part A:
Polym. Chem. 2008, 46, 2119-2131.
[50] Wang, D. B.; Song, C. X.; Lin, Y. S.; Hu, Z. S. Mater. Lett. 2006, 60, 77-80.
[51] Nadagouda, M. N.; Varma, R. S. Smart Mater. Struct. 2006, 15, 1260-1265.
[52] Kim, I. J.; Kwon, O. S.; Park, J. B.; Joo, H. Curr. Appl. Phys. 2006, 6, 43-47.
[53] Song, C. X.; Gu, G. H.; Lin, Y. S.; Wang, H.; Guo, Y.; Fu, X.; Hu, Z. S. Mater. Res.
Bull. 2003, 38, 917-924.
[54] Song, C.; Yu, W.; Zhao, B.; Zhang, H.; Tang, C.; Sun, K.; Wu, X.; Dong, L.; Chen,
Y. Catal. Commun. 2009, 10, 650-654.
[55] Yang, J. X.; Hu, D. D.; Fang, Y.; Bai, C. L.; Wang, H. Y. Chem. Mater. 2006, 18,
4902-4907.
Chapter 2: Literature Review
33
[56] Barbe, C.; Bartlett, J.; Kong, L. G.; Finnie, K.; Lin, H. Q.; Larkin, M.; Calleja, S.;
Bush, A.; Calleja, G. Adv. Mater. 2004, 16, 1959-1966.
[57] Fidalgo, A.; Ciriminna, R.; Ilharco, L. M.; Sciortino, M.; Pagliaro, M.
ChemPlusChem 2012, 77, 536-540.
[58] Veith, S. R.; Perren, M.; Pratsinis, S. E. J. Colloid Interface Sci. 2005, 283, 495