Mechanical Properties and Morphology of Copolymer Modified Polymer Blends

SYNOPSIS Most polymers produced by industry have one or a few outstanding properties. Therefore, it was natural to combine different polymers into alloys, composites, or polyblends. It can be expected that a binary heterogeneous blend should have a modulus that is a not‐too‐complicated function of the moduli and of the volume fractions of the two polymers. Sometimes synergistic effects can be reached, leading to special features, as for instance, an unexpected high impact strength. In high‐impact polystrene and in other rubber‐modified toughened glassy polymers the adhesion between the rubber particles and the glassy matrix necessary for good properties is provided for by the presence of a graft copolymer formed during polymerization. The amounts and structures of such graft copolymers and of the partly cross‐linked elastomer particles are difficult to determine. Therefore, research was carried out on blends of polystrene (PS) and low‐density polyethylene (PE) or ethylene‐propylene copolymer (EP) to which well‐defined block and graft polymers PS‐b‐PE, PS‐g‐PE or PS‐g‐EP were added in the melt. Particle dimensions of the dispersed phase were strongly influenced by the viscosity of the molten blend and by the concentration of copolymer. Young's modulus of the PS blends as a function of the volume fractions of added copolymer and of homopolymers could be used as a tool for estimation of the number of phases and of the morphology, as investigated by optical and electronoptical means. The mechanisms of fracture under impact—crazing and shearing—were well correlated with the morphology observed.