thermal stability, and low flammability. However, their high price poses strong
limitations on their large-scale use, and other restrictions are set by the difficult
control of the agglomeration of the individual nanotubes into low-modulus bundles
that control the mechanical stability and strength of the SWNT polymer
nanocomposites. Elastomeric applications generally require and make use of the
large deformation extensibility and resilience of the elastomer. Upon incorporation
of stiff fillers into elastomers, it is generally desired to enhance the stiffness (i.e.,
enhance the initial stiffness and retain this stiffness enhancement for overall large
strain deformation behavior) while also retaining the important attributes of large
strain resilient behavior and large strain-to break.
The outstanding properties of carbon nanotubes have generated scientific and
technical interests in the development of nanotube-reinforced polymer composites.
Das et al., investigated a novel mixing approach for achieving a good dispersion of
MWCNTs in a rubber blend. In this approach the CNTs were incorporated into a
50:50 blend of solution-styrene–butadiene rubber and butadiene rubber. First, the
CNTs were predispersed in ethanol and then this CNT–alcohol suspension was
mixed with the rubber blend at elevated temperature. The rubber nanocomposites
prepared by such method exhibit significantly enhanced physical properties already
at very low nanotube concentrations. Dynamic mechanical analysis indicates that
the incorporation of CNTs affects the glass transition behaviour by reducing the
height of the tan δ peak considerably. Above the glass transition temperature the
storage modulus has been increased after incorporation of a small amount of CNTs.
Finally, the ‘Payne effect’, an indication of filler–filler interactions, was observed at
very low concentrations of CNT in the rubber matrix [98].
The storage modulus of unfilled rubbers, E
0 , depends on frequency and temperature and is independent of the deformation amplitude. In contrast, E
0 for the filled
rubber shows a significant dependency on the dynamic deformation, here the value
considerably decreases with an increasing strain amplitude. This non-linear behaviour of filled rubbers is known as ‘Payne effect’ [74, 99] and has been explained by
the existence of a filler network in the rubber matrix above the percolation threshold. With increasing strain amplitude the filler network is breaking down which
results in lowering of the E
0 value. Figure 17 supplies an evidence of the existence
of a carbon nanotube filler network in an S-SBR–BR blend prepared by the wet
mixing method. For a pure rubber (not shown here) and at small CNT loading (up to
2 phr) no ‘Payne effect’ is observed. However, with the increase of the MWCNT
content a gradual decrease in E
0 is observed in strain sweeps. So, even with 3 phr of
MWCNT the tubes obviously form a continuous filler network in the rubber matrix.
The OH-functionalised sample with 5 phr of CNT shows a significant lower E
0
value compared to the unfunctionalised one, with additional silane modification the
value increases. This shows the opposite behaviour as known for silica filled
samples, where silanisation reduces the Payne Effect to a certain amount. The
concentration of hydroxyl groups, present on the surface of the nanotubes, is
obviously not comparable to silica and does not allow the formation of hydrogen
bonds between two adjacent modified tubes, hence the silanisation is not reducing
the filler–filler interaction.
112
A.B. Nair et al.
limitations on their large-scale use, and other restrictions are set by the difficult
control of the agglomeration of the individual nanotubes into low-modulus bundles
that control the mechanical stability and strength of the SWNT polymer
nanocomposites. Elastomeric applications generally require and make use of the
large deformation extensibility and resilience of the elastomer. Upon incorporation
of stiff fillers into elastomers, it is generally desired to enhance the stiffness (i.e.,
enhance the initial stiffness and retain this stiffness enhancement for overall large
strain deformation behavior) while also retaining the important attributes of large
strain resilient behavior and large strain-to break.
The outstanding properties of carbon nanotubes have generated scientific and
technical interests in the development of nanotube-reinforced polymer composites.
Das et al., investigated a novel mixing approach for achieving a good dispersion of
MWCNTs in a rubber blend. In this approach the CNTs were incorporated into a
50:50 blend of solution-styrene–butadiene rubber and butadiene rubber. First, the
CNTs were predispersed in ethanol and then this CNT–alcohol suspension was
mixed with the rubber blend at elevated temperature. The rubber nanocomposites
prepared by such method exhibit significantly enhanced physical properties already
at very low nanotube concentrations. Dynamic mechanical analysis indicates that
the incorporation of CNTs affects the glass transition behaviour by reducing the
height of the tan δ peak considerably. Above the glass transition temperature the
storage modulus has been increased after incorporation of a small amount of CNTs.
Finally, the ‘Payne effect’, an indication of filler–filler interactions, was observed at
very low concentrations of CNT in the rubber matrix [98].
The storage modulus of unfilled rubbers, E
0 , depends on frequency and temperature and is independent of the deformation amplitude. In contrast, E
0 for the filled
rubber shows a significant dependency on the dynamic deformation, here the value
considerably decreases with an increasing strain amplitude. This non-linear behaviour of filled rubbers is known as ‘Payne effect’ [74, 99] and has been explained by
the existence of a filler network in the rubber matrix above the percolation threshold. With increasing strain amplitude the filler network is breaking down which
results in lowering of the E
0 value. Figure 17 supplies an evidence of the existence
of a carbon nanotube filler network in an S-SBR–BR blend prepared by the wet
mixing method. For a pure rubber (not shown here) and at small CNT loading (up to
2 phr) no ‘Payne effect’ is observed. However, with the increase of the MWCNT
content a gradual decrease in E
0 is observed in strain sweeps. So, even with 3 phr of
MWCNT the tubes obviously form a continuous filler network in the rubber matrix.
The OH-functionalised sample with 5 phr of CNT shows a significant lower E
0
value compared to the unfunctionalised one, with additional silane modification the
value increases. This shows the opposite behaviour as known for silica filled
samples, where silanisation reduces the Payne Effect to a certain amount. The
concentration of hydroxyl groups, present on the surface of the nanotubes, is
obviously not comparable to silica and does not allow the formation of hydrogen
bonds between two adjacent modified tubes, hence the silanisation is not reducing
the filler–filler interaction.
112
A.B. Nair et al.