In one approach [74], the level of filler dispersion is expected to play a major role
in determining the filler effects on non-linear responses of nanocomposites while
the other considers the chain stiffening due to reversible trapping of entanglements
to be the primary cause of the observed behavior [78]. For example, it is well known
that rubber-like materials exhibit an appreciable change in their mechanical properties (stress softening) resulting from the first tensile experiment. This phenomenon is well recognized to be caused by the following mechanisms: (1) physical
disentanglement of rubber chains, (2) decrease in the interactions between polymer
molecules and filler surfaces, (3) filler network breakdown and (4) chain scission of
rubber molecules. A number of research papers and reviews have been dedicated to
this behavior termed Payne and Mullins effects [79]. Although different theories
have been proposed, it is essential to understand the filler super structure at different
length scales.
Maier and Goritz [80–82] take into consideration the adsorption/desorption
mechanism by considering the filler particles as multifunctional crosslink with
chains which are either loosely or strongly anchor to the surface. The molecular
interpretation of the Payne effect is then based on a variable network density when
the loosely tied chains are desorbed with the increase of the strain. A compromise is
also suggested, considering that the primary mechanism for the Payne effect
certainly involves the existence of cooperation between the breakdown and reformation of the filler network and the molecular disentanglement of the bound and
free rubber. Another explanation first proposed by Yatsuyanagi et al. [83] considers
the existence of a percolation network through the rigid amorphous layer formed
around the particles. Their interpretation of the Payne effect equally relies on the
competition between desorption and adsorption of this rigid amorphous layer. The
amplitude of the Payne effect for ethylene vinyl acetate copolymer (EVA) at
different silica fraction at 140
C is increased with increase in silica
concentration [84].
The importance of glassy layers in filled polymer has received considerable
attention recently, when it was recognised that a glass transition gradient exists near
the surface and that the dynamics could be either enhanced or slowed down
according to the interaction of the chains with the surface. In a body of work,
Berriot et al. [85–87], Montes and others [88] have clearly shown that, in filled
elastomers, a maximum of reinforcement is obtained when this rigid or slow
dynamics layer forms a continuous path through the filler aggregates. In their
work, Merabia et al. [89] model the Payne effect by considering that the stress is
supported mainly by the cross section of glassy bridges in a direction normal to the
stress. They explain the strain dependence of the elastic modulus by the local
lowering of the glass transition due to the amplification of the stress in the vicinity
of the aggregates. This plasticizing effect induces the yielding of the glassy bridges
and the collapse of the storage modulus.
Darestani Farahani et al. used RPA to study viscoelastic parameters of natural
rubber/reclaimed rubber blends. Viscoelastic behavior of compounds was studied
in strain sweep mode at 100
C, strain range 1–1,200 % and frequency 10 cpm
(cycle per minute). They found that torque (S) increases in higher shear strains,
Non-linear Viscoelastic Behaviour of Rubber-Rubber Blend Composites and. . .
103
in determining the filler effects on non-linear responses of nanocomposites while
the other considers the chain stiffening due to reversible trapping of entanglements
to be the primary cause of the observed behavior [78]. For example, it is well known
that rubber-like materials exhibit an appreciable change in their mechanical properties (stress softening) resulting from the first tensile experiment. This phenomenon is well recognized to be caused by the following mechanisms: (1) physical
disentanglement of rubber chains, (2) decrease in the interactions between polymer
molecules and filler surfaces, (3) filler network breakdown and (4) chain scission of
rubber molecules. A number of research papers and reviews have been dedicated to
this behavior termed Payne and Mullins effects [79]. Although different theories
have been proposed, it is essential to understand the filler super structure at different
length scales.
Maier and Goritz [80–82] take into consideration the adsorption/desorption
mechanism by considering the filler particles as multifunctional crosslink with
chains which are either loosely or strongly anchor to the surface. The molecular
interpretation of the Payne effect is then based on a variable network density when
the loosely tied chains are desorbed with the increase of the strain. A compromise is
also suggested, considering that the primary mechanism for the Payne effect
certainly involves the existence of cooperation between the breakdown and reformation of the filler network and the molecular disentanglement of the bound and
free rubber. Another explanation first proposed by Yatsuyanagi et al. [83] considers
the existence of a percolation network through the rigid amorphous layer formed
around the particles. Their interpretation of the Payne effect equally relies on the
competition between desorption and adsorption of this rigid amorphous layer. The
amplitude of the Payne effect for ethylene vinyl acetate copolymer (EVA) at
different silica fraction at 140
C is increased with increase in silica
concentration [84].
The importance of glassy layers in filled polymer has received considerable
attention recently, when it was recognised that a glass transition gradient exists near
the surface and that the dynamics could be either enhanced or slowed down
according to the interaction of the chains with the surface. In a body of work,
Berriot et al. [85–87], Montes and others [88] have clearly shown that, in filled
elastomers, a maximum of reinforcement is obtained when this rigid or slow
dynamics layer forms a continuous path through the filler aggregates. In their
work, Merabia et al. [89] model the Payne effect by considering that the stress is
supported mainly by the cross section of glassy bridges in a direction normal to the
stress. They explain the strain dependence of the elastic modulus by the local
lowering of the glass transition due to the amplification of the stress in the vicinity
of the aggregates. This plasticizing effect induces the yielding of the glassy bridges
and the collapse of the storage modulus.
Darestani Farahani et al. used RPA to study viscoelastic parameters of natural
rubber/reclaimed rubber blends. Viscoelastic behavior of compounds was studied
in strain sweep mode at 100
C, strain range 1–1,200 % and frequency 10 cpm
(cycle per minute). They found that torque (S) increases in higher shear strains,
Non-linear Viscoelastic Behaviour of Rubber-Rubber Blend Composites and. . .
103