Mechanical behavior of dual-rubber-modified SAN

The use of either an SBS block copolymer or an emulsion-made methyl-methacrylate-grafted rubber alone failed to give any significant increase in the toughness of brittle poly(styrene-co-acrylonitrile) (SAN), even at concentrations up to 50%. However, a combination of the two rubber modifiers produced a strong synergistic toughening. All specimens were prepared by injection molding; however, annealing to relax orientation did not significantly alter this synergistic effect. The use of mechanical dilatometry showed that post-yield deformation of blends containing both modifiers involved some crazing just after yielding but subsequent deformation was mostly due to shear yielding. Addition of SBS alone to SAN resulted in a mixed crazing/shear yielding mode of post-yield deformation with the relative proportions of the two mechanisms being constant up to failure. In contrast, the small emulsion graft particles alone gave rise to a predominant shear yielding deformation. A qualitative mechanism for the synergism is discussed.