relaxation measurements, however these techniques are rather time consuming. The
dynamic mechanical measurements provide a complementary approach where
stress or strain parameters and its dependence on viscoelasticity of polymers can
be appropriately checked.
Fillers of various dimensions are added to polymers to alter its processability,
properties and uses. Such micro and nano composites obtained may have tremendous possibilities in industries and information on their viscoelasticity is very
necessary as far as their processing and applicability are concerned. The dynamic
properties of filled elastomers have been a subject of active research since they
affect the performance of tyres such as skid, traction, and rolling resistance.
Elastomer nanocomposites are most important materials characterized by excellent
elasticity and flexibility, and are widely used in various applications such as cables,
tyres, tubing, dielectric materials and sensors [1–5]. The non linear features
observed in filled elastomers upon a simple shear are as follows. The dynamic
storage and loss moduli of the composites are only dependent on the dynamic
strains and not on the static strain. In the same way the stress strain curves also do
not depend on static strain. Moreover the initial modulus under constant strain rate
is highly rate dependent whereas the terminal modulus is independent of strain rate.
This initial to terminal modulus ratio in the stress-strain curves is the same as the
ratio of the dynamic storage moduli obtained at low and high strains.
The major mechanism behind the reinforcement in elastomer composites and
their nonlinear behavior is based on the filler-matrix interactions and not on the
filler cluster formation/agglomeration or percolation. The interfacial interactions
cause in temporary (labile) bond formation between the polymer chains and the
filler surface and this results in trapped entanglements. Such molecular entanglements affect the matrix polymer chain motions both near and far fields and greatly
enhance the non-Gaussian (Langevin) chain behavior influencing the storage and
loss moduli in various extents. When a strain (stress) is applied on the composite,
these trapped entanglements get released and this leads to the reduction in the
dynamic moduli. The reinforcement of elastomers by nanofillers and the nonlinear
viscoelastic properties of the nanocomposites are very much similar to the phenomenon of Payne effect observed in filled elastomers. This suggests a common
mechanism of network formation and breakage arising from the trapped entanglements of macromolecular chains of the elastomer matrices in presence of fillers.
The investigation of the nonlinear dynamic mechanical properties of filled
elastomers has long background since A. R. Payne explored this behavior in carbon
black filled elastomers [6]. He observed a decrease in dynamic storage modulus in
filled elastomers with increasing strain amplitude. However the mechanisms for
reinforcement and nonlinearity remain controversial. As already mentioned filler
agglomeration and network formation cause high levels of reinforcement in the
nanocomposite systems and the deagglomeration and network breakdown result in
the nonlinearity with strain [7, 8]. All these mechanisms depend highly on the
nature of filler and the mixing method used for composite preparation. The concentration of filler also influences the composite viscoelasticity as many reports
concentrate on the non linear viscoelastic properties at high filler volume fractions.
44
K.K. Sadasivuni and Y. Grohens
dynamic mechanical measurements provide a complementary approach where
stress or strain parameters and its dependence on viscoelasticity of polymers can
be appropriately checked.
Fillers of various dimensions are added to polymers to alter its processability,
properties and uses. Such micro and nano composites obtained may have tremendous possibilities in industries and information on their viscoelasticity is very
necessary as far as their processing and applicability are concerned. The dynamic
properties of filled elastomers have been a subject of active research since they
affect the performance of tyres such as skid, traction, and rolling resistance.
Elastomer nanocomposites are most important materials characterized by excellent
elasticity and flexibility, and are widely used in various applications such as cables,
tyres, tubing, dielectric materials and sensors [1–5]. The non linear features
observed in filled elastomers upon a simple shear are as follows. The dynamic
storage and loss moduli of the composites are only dependent on the dynamic
strains and not on the static strain. In the same way the stress strain curves also do
not depend on static strain. Moreover the initial modulus under constant strain rate
is highly rate dependent whereas the terminal modulus is independent of strain rate.
This initial to terminal modulus ratio in the stress-strain curves is the same as the
ratio of the dynamic storage moduli obtained at low and high strains.
The major mechanism behind the reinforcement in elastomer composites and
their nonlinear behavior is based on the filler-matrix interactions and not on the
filler cluster formation/agglomeration or percolation. The interfacial interactions
cause in temporary (labile) bond formation between the polymer chains and the
filler surface and this results in trapped entanglements. Such molecular entanglements affect the matrix polymer chain motions both near and far fields and greatly
enhance the non-Gaussian (Langevin) chain behavior influencing the storage and
loss moduli in various extents. When a strain (stress) is applied on the composite,
these trapped entanglements get released and this leads to the reduction in the
dynamic moduli. The reinforcement of elastomers by nanofillers and the nonlinear
viscoelastic properties of the nanocomposites are very much similar to the phenomenon of Payne effect observed in filled elastomers. This suggests a common
mechanism of network formation and breakage arising from the trapped entanglements of macromolecular chains of the elastomer matrices in presence of fillers.
The investigation of the nonlinear dynamic mechanical properties of filled
elastomers has long background since A. R. Payne explored this behavior in carbon
black filled elastomers [6]. He observed a decrease in dynamic storage modulus in
filled elastomers with increasing strain amplitude. However the mechanisms for
reinforcement and nonlinearity remain controversial. As already mentioned filler
agglomeration and network formation cause high levels of reinforcement in the
nanocomposite systems and the deagglomeration and network breakdown result in
the nonlinearity with strain [7, 8]. All these mechanisms depend highly on the
nature of filler and the mixing method used for composite preparation. The concentration of filler also influences the composite viscoelasticity as many reports
concentrate on the non linear viscoelastic properties at high filler volume fractions.
44
K.K. Sadasivuni and Y. Grohens