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

aksuwara@tarrc.co.uk

www.tarrc.co.ukwww.rubberconsultants.com