New toughening mechanisms in rubber modified polymers

Rubber toughening usually involves the addition of rubber particles to a ri gid 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 o f electron microscopy techniques. have revealed two new mechanisms of energ y absorption. Direct observations of micromechanical processes in high impa ct polystyrene and copoly(styrene/acrylonitrile) acrylate blends, carried n ut in situ on the stage of a transmission electron microscope have shown th at the rigid. glassy subinclusions found in both 'salami' and hard-soft-har d 'core-shell' rubber particles respond to high tensile stresses by cold dr awing. Fibrillation begins in the rubber phase and then draws fibrils of gl assy 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 t he stage of the transmission electron microscope. hard-soft core-shell part icles consisting of a polyethylene core and an ethylene propylene copolymer rubber shell are able to initiate crazing In the matrix at - 100 C, Hall b elow the glass transition temperature of the ethylene propylene copolymer r ubber. Micrographs illustrating these mechanisms are presented and discusse d. (C) 2001 IoM Communications Ltd.