Tun Abdul Razak Research Centre (TARRC)A RESEARCH & PROMOTION CENTRE OF THE MALAYSIAN RUBBER BOARD
Visualization of polymer-filler interface usingAtomic Force Microscopy
Dr Anna Kepas-Suwara, Materials Scientist
RIEG ATDM Meeting, 22nd March 2019
www.tarrc.co.ukwww.rubberconsultants.com
Polymer-filler interaction
Bound Rubber
➢ Bound rubber plays a crucial role in
rubber reinforcement
➢ Bound rubber is formed by the
interaction between polymer and filler
particles
➢ The formation of bound rubber involve
physical absorption, chemisorption
and mechanical interlocking
There are strain independent contributions to rubber reinforcement :
hydrodynamic effect, polymer network, polymer-filler interaction and strain
dependant filler-filler interaction.
Objectives of these studies:
➢ To visualize and characterize (tan δ mapping) bound rubber in silica and carbon black
filled NR compounds via phase imaging of tapping mode atomic force microscopy (AM-
AFM)
Polymer-filler interactionBound rubber models
Compounding and sample preparation
Ingredients
phr
Compound
1
Compound
2
Compound
3
Compound
5
NR (SMR L) 100 100 100 100
N330 20 50
N234 2 2
Zeosil 1165MP 20 55
Si69 1.6 4.4
Zinc oxide 5 5 5 5
Stearic acid 2 2 2 2
Nonox ZA 1 1 1 1
Sulphur 2.5 2.5 1.2 1.2
TBBS 0.45 0.45 0.9 0.9
1. Mixing and finalizing was
carried out in a Banbury
internal mixer
2. Silane coupling agent has been
used in silica filled compounds
3. Compounds 1 and 2 were
cured at 140oC for 60 minutes
4. Compounds 3 and 5 were
cured at 150oC for 40 minutes
5. To prevent the formation of bloom on the surface, the vulcanizates were extracted in acetone
under cold Soxhlet conditions
6. A smooth surface on the test piece was prepared using a diamond knife in a cryo-
ultramicrotome (RMC PowerTome PC)
Atomic Force MicroscopyAmplitude Modulation mode (AM)
Drive Frequency
Am
plit
ud
eP
has
e
Tip-sample repulsive force
Cantilever in free air
Tip-sample attractive force
➢ The cantilever is oscillated close to its fundamental frequency and it makes a contact with the
surface at the maximum amplitude
➢ The oscillation amplitude is kept constant throughout the scan and it is used as a feedback
parameter to measure the topography of the sample
➢ The tip-sample interaction results in a shift of the phase angle of the oscillating tip; this shift in
the phase angle of the oscillating cantilever can then be related to the tan δ
AFM tips used in the experiments
‘Standard’ tipUltrasharp tip
SSS-NCHR‘Ultrasharp’ tip
Tap300DLC tip‘Standard’ tip
2 nm Radius of curvature <15 nm
<10 Cone Half angle, o 35
330 kHz Resonant frequency 300 kHz
42 N/m Spring constant 40 N/m
Lateral resolution of AFM images
Tip sidewall angleTip radius of curvature
➢ A sharper tip resolves smaller features than a dull tip with a larger radius of curvature
➢ The sidewall angles of the tip determine its ability to probe high-aspect-ratio features
➢ Although the ultrasharp tip offer improved lateral resolution of AFM images their drawback
is that they wear very quickly and therefore significantly increase the cost of the
measurements
Morphology of filled NR compounds Phase images, filler loading: 20phr, ultrasharp tip
N330
Zeosil 1165 MP
low highPhase angle,φ1
fillerbound rubber rubber
➢ Filler particles, bound rubber and filler free rubber are
clearly distinguished from each other
➢ Phase angle shows the lowest value for filler particles
and as the tip travels away from filler the phase angle
increases and reaches its maximum value for the filler
free polymer matrix
➢ The phase angle of the interfacial region, that is located
around the filler particles, has an intermediate value
Morphology of filled NR compounds Phase images, filler loading: 20phr, ultrasharp tip
Compound Bound rubber
thickness, nm
Volume fraction, φFiller Bound
rubber
Rubber Filler
calculatedNR/ 20 phr N330 8.4 0.09 0.07 0.84 0.09NR/ 20 phr Zeosil
1165 MP
16.0 0.08 0.10 0.81 0.08
➢ The carbon black filled NR compound shows much lower thickness of interfacial region (8.4 nm) when
compared to silica filled NR (16 nm)
➢ The calculated volume fraction of filler corresponds well with the volume fraction determined from
the AFM images
➢ Silica filled NR has higher amount of bound rubber when compared with carbon black filled NR
sample
Effect of filler loading on morphology of NR compounds
20 phr N330 50 phr N330
20 phr Zeosil 1165 MP 55 phr Zeosil 1165 MP
➢ Phase images, standard tip
➢ With an increase of filler loading
a decrease in volume fraction of
polymer matrix is apparent and the
bound rubber becomes dominant
➢ In tire tread compounding,
which usually uses high filler
loading (above 50 phr), the bound
rubber has significant influence on
their overall physical and
mechanical properties
Tan δ mapping in AM-AFM mode
cos
sin
tan 0
'
'' A
A
E
E−
==
R. Proksch, D.G. Yablon, Appl. Phys. Lett. 100, 073106 (2012)
Requirement : “Net-repulsive mode”
1999, Garcia et al.
Elastic, ‘storage’
1998, Cleveland et al.
Inelastic, dissipative, ‘loss’
Tan δ maps at 300kHz of carbon black and silica filled NR
filler bound rubber
rubber
➢ The loss tangent of the
interfacial region is lower than
the rubbery phase which
indicates that motion of
polymer chains in close
proximity to filler particles is
restricted
20 phr Zeosil 1165 MP 55 phr Zeosil 1165 MP
20 phr N330 50 phr N330
Tan δ at 300kHz of carbon black and silica filled NR
➢ With increased of filler loading an overall stiffening of the system is observed which is
indicated by the decrease in loss tangent
➢ This effect is the most distinct for the silica filled NR compound where the loss tangent of the
polymer matrix decreases by a factor of 2 as the filler loading increases from 20 phr to 55 phr
Conclusions
➢ Rubber-filler interface in silica (Zeosil 1165 MP) and carbon black (N 330) NR
compounds has been visualized using the AM-AFM technique
➢ Three components, filler, bound rubber and polymer matrix can be distinguished via
a shift in the phase angle of the oscillating cantilever
➢ The thickness of the bound rubber in silica filled NR is about two times that of carbon
black filled NR which may arise from the use of silane coupling agent in silica filled NR
➢ Image analysis performed on AFM images showed that the volume fraction of bound
rubber increases with filler loading
➢ Also, with the increase in volume fraction of filler a decrease in loss tangent is
observed which supports the theory of restricted mobility of rubber in a close proximity
to the filler particles
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Tun Abdul Razak Research Centre (TARRC)A RESEARCH & PROMOTION CENTRE OF THE MALAYSIAN RUBBER BOARD
Thank youAnna Kepas-Suwara, Paul Brown, Stuart Cook and Vincenzo Orlando
www.tarrc.co.ukwww.rubberconsultants.com
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