New toughening mechanisms in rubber modified polymers

Rubber toughening usually involves the addition of rubber particles to a rigid polymer in order to promote energy absorption through the initiation of local yielding, which takes the form of multiple crazing and/or extensive shear yielding. In addition to these classic mechanisms, recent studies of deformation mechanisms in various rubber modified polymers, using a range of electron microscopy techniques, have revealed two new mechanisms of energy absorption. Direct observations of micromechanical processes in high impact polystyrene and copoly(styrene/acrylonitrile)-acrylate blends, carried out in situ on the stage of a transmission electron microscope have shown that the rigid, glassy subinclusions found in both ʹsalamiʹ and hard-soft-hard ʹcore-shellʹ rubber particles respond to high tensile stresses by cold drawing. Fibrillation begins in the rubber phase and then draws fibrils of glassy polymer from the subinclusions, causing initially spherical inclusions to become flattened discs before finally disintegrating. In addition, when thin sections of rubber toughened polypropylene are stretched in situ on the stage of the transmission electron microscope, hard-soft core-shell particles consisting of a polyethylene core and an ethylene/propylene copolymer rubber shell are able to initiate crazing in the matrix at - 100°C, well below the glass transition temperature of the ethylene/propylene copolymer rubber. Micrographs illustrating these mechanisms are presented and discussed.