Payne effect and shear elasticity of silica-filled polymers in concentrated solutions and in molten state

The viscoelastic properties of silica-filled copolymer in concentrated solutions and melts were investigated. A wide variety of rheological observations, such as Payne effect, percolation threshold and elastic effects, of non-porous silica dispersed in a ethylene vinyl acetate copolymer (EVA, 60 wt% of vinyl acetate) were studied in the molten state or diluted solutions in xylene. The concept of the filler network breakdown seems to be adequate in describing the strain-dependence of dynamic mechanical properties. Nevertheless, the degree of nonlinearity was found to be the highest for the highest dilution. It appears for silica concentration lower than the percolation threshold of the filler suggesting another mechanism associated with trapped entanglement. On the other hand, we observe a silica content (Phi(c) = (3.3 +/- 0.1) X 10(-2)) for the onset of a gel-like behaviour in the terminal zone. This critical level, defined as the percolation threshold, appears to be independent of EVA dilution in xylene. Furthermore, this liquid-solid transition can be expressed in the same general relaxation patterns as chemical gelation systems with a self similar behaviour at the rheological gel point. However, the universal properties of percolation are not properly described and elastic effect are better depicted by the cluster-cluster aggregation model. On the other hand, the concentration-dependence of the equilibrium storage modulus of EVA at different dilutions in xylene yields evidence of a screened effect of copolymer chains in the entangled regime (phi greater than or equal to phi(c)) of elastic bending-twisting interaction between filler particles. (C) 2003 Elsevier Science Ltd. All rights reserved.