Predictive modelling of the properties and toughness of polymeric materials, part II: Effect of microstructural properties on the macroscopic response of rubber-modified polymers

The influence of microstructural properties on the macroscopic mechanical behaviour has been studied by finite element predictions of the response of different microstructures of polystyrene (PS) or polycarbonate (PC) containing voids or rubbery particles, subjected to unidirectional extension. The voids represent a low-modulus non-adhering dispersed phase. The rubbery inclusions, which are assumed to be pre-cavitated and perfectly adhering, idealise core-shell particles with a hard rubber shell and a soft non-adhering or pre-cavitated core. The predictions show that the inclusion properties strongly affect the averaged post-yield response of the heterogeneous systems. Especially the post-yield strain softening can be eliminated by the introduction of voids in PC or rubbery particles in PS. Since macroscopic strain softening is believed to be the main cause of catastrophical stress or strain localisations, the softening elimination is believed to be primarily responsible for toughness enhancement of the polystyrene or polycarbonate systems. The results and experiences are extrapolated in order to explain the influence of the absolute length scale of a sub-micron sized morphology on the macroscopic behaviour, especially toughness. Two potential sources of particle-size effects are presented that may result in a stabilised, and thus tougher, macroscopic mechanical response, i.e. the yield stress reduction near a surface or interface because of a locally enhanced mobility of the polymer segments, and the temporary excessive hardening because of a sufficiently small size of the yield zones which results in a reduced effective entanglement distance. The paper concludes with a discussion on the extension of this knowledge to all other, for the moment amorphous, polymers. (C) 2000 Kluwer Academic Publishers