[51]. In their work, Merabia et al. [52] modeled the Payne effect by considering that
the stress is supported mainly by the glassy bridges in a direction normal to the
stress. The effect is explained as the local lowering of the glass transition when
stress is applied near the aggregates. Here a plasticizing effect is observed which
induces the formation of glassy bridges and collapses the storage modulus.
Bokobza checked the rheological behavior of elastomers by adding conventional
fillers such as carbon black and silica and found an increase in the modulus. She has
explained this effect as due to the inclusion of rigid filler particles and increase in
polymer–filler interactions and cross-link density [53]. In short the Payne effect can
be related to the destruction of filler network [54], filler deagglomeration [55],
polymer desorption from filler surface [34], and strain softening of polymer shell
surrounding fillers [56].
Cadambi et al. [57] studied the viscoelastic behavior of nanotubes filled hydrogenated nitrilebutadiene rubber by dynamic mechanical thermal analysis and found
that the incorporation of nanotubes enhanced the storage modulus while reducing
the glass transition temperature. Das et al. [58] achieved a good dispersion of
multiwalled carbon nanotubes in a 50:50 blend of styrene–butadiene and butadiene
rubber and observed decrease in intensity of tan δ peak as well as enhanced storage
modulus above the glass transition. Similar results were also found for singlewalled carbon nanotubes filled natural rubber composites [59].
Another dynamic effect common in filled rubber systems is the stress softening
effect or “Mullins effect”. This is observed at high extensions and characterized by
a pronounced decrease in the stress when the filled vulcanizate is stretched for a
second time [56]. This effect is related to the rubber network and filler network as
well, which can be considered as a hysteretic mechanism related to energy dissipation by the material during deformation [55]. Stress softening is observed for
CNTs filled natural rubber [60] and Bhattacharya et al. [61] studied this behavior by
loading–unloading cycles. A strong stress-softening effect is noticed especially at
8.3 wt% of oxidized nanotube loading. Carboxylated multiwalled carbon nanotubes
filled natural rubber also showed significant stress-softening effect [62]. Chen
et al. studied both viscoelastic [63] and stress-softening behavior [64] of EPDM/
zinc dimethacrylate composites. Subramaniam et al. [65] prepared conducting
chloroprene rubber/imidazolium-based ionic liquid modified multi walled carbon
nanotube composites and observed similar trends in viscoelasticity and stresssoftening. Since chloroprene rubber shows relatively high polarity, and therefore,
better affinity to polar fillers, the nonpolar carbon nanotubes are modified. They
have found that ionic liquid modification on nanotubes greatly enhances the
viscoelastic properties of the composites.
3 Conclusion
As the first chapter of the book, the aim of this chapter is to give a brief introduction
about the non linear viscoelastic behavior of rubber and its composites. The term
viscoelasticity and stress softening usually observed in all elastomer systems are
10
D. Ponnamma and S. Thomas
the stress is supported mainly by the glassy bridges in a direction normal to the
stress. The effect is explained as the local lowering of the glass transition when
stress is applied near the aggregates. Here a plasticizing effect is observed which
induces the formation of glassy bridges and collapses the storage modulus.
Bokobza checked the rheological behavior of elastomers by adding conventional
fillers such as carbon black and silica and found an increase in the modulus. She has
explained this effect as due to the inclusion of rigid filler particles and increase in
polymer–filler interactions and cross-link density [53]. In short the Payne effect can
be related to the destruction of filler network [54], filler deagglomeration [55],
polymer desorption from filler surface [34], and strain softening of polymer shell
surrounding fillers [56].
Cadambi et al. [57] studied the viscoelastic behavior of nanotubes filled hydrogenated nitrilebutadiene rubber by dynamic mechanical thermal analysis and found
that the incorporation of nanotubes enhanced the storage modulus while reducing
the glass transition temperature. Das et al. [58] achieved a good dispersion of
multiwalled carbon nanotubes in a 50:50 blend of styrene–butadiene and butadiene
rubber and observed decrease in intensity of tan δ peak as well as enhanced storage
modulus above the glass transition. Similar results were also found for singlewalled carbon nanotubes filled natural rubber composites [59].
Another dynamic effect common in filled rubber systems is the stress softening
effect or “Mullins effect”. This is observed at high extensions and characterized by
a pronounced decrease in the stress when the filled vulcanizate is stretched for a
second time [56]. This effect is related to the rubber network and filler network as
well, which can be considered as a hysteretic mechanism related to energy dissipation by the material during deformation [55]. Stress softening is observed for
CNTs filled natural rubber [60] and Bhattacharya et al. [61] studied this behavior by
loading–unloading cycles. A strong stress-softening effect is noticed especially at
8.3 wt% of oxidized nanotube loading. Carboxylated multiwalled carbon nanotubes
filled natural rubber also showed significant stress-softening effect [62]. Chen
et al. studied both viscoelastic [63] and stress-softening behavior [64] of EPDM/
zinc dimethacrylate composites. Subramaniam et al. [65] prepared conducting
chloroprene rubber/imidazolium-based ionic liquid modified multi walled carbon
nanotube composites and observed similar trends in viscoelasticity and stresssoftening. Since chloroprene rubber shows relatively high polarity, and therefore,
better affinity to polar fillers, the nonpolar carbon nanotubes are modified. They
have found that ionic liquid modification on nanotubes greatly enhances the
viscoelastic properties of the composites.
3 Conclusion
As the first chapter of the book, the aim of this chapter is to give a brief introduction
about the non linear viscoelastic behavior of rubber and its composites. The term
viscoelasticity and stress softening usually observed in all elastomer systems are
10
D. Ponnamma and S. Thomas