Non-linear viscoelastic behaviour and modulus recovery in silica filled polymers

The viscoelastic properties of different silica filled polymers were investigated. A variety of rheological observations was studied in the molten state, such as Payne effect and modulus recovery kinetics, in ethylene vinyl acetate, polystyrene and polypropylene filled with non-porous silica. The concept of the filler network breakdown seems to be adequate in describing the strain-dependence of dynamic mechanical properties of composites. Recovery tests of the complex shear modulus performed by subsequent strain sweep or time sweep experiments show evidence that the initial equilibrium network structure cannot completely restore within several hours. Furthermore, new strain sweep experiment after in situ crystallisation (or glass transition in polystyrene composites) of the sample leads to a total recovery of the linear modulus suggesting that the silica network structure in molten state have the memory of the silica structure in solid state. On the other hand, the complex shear modulus measured at lower strain for cross-linked EVA composites are perfectly recoverable in few seconds as proven by subsequent strain sweeps.