Influence of molecular weight of SAN on rheological and mechanical properties of ABS-plastics

Rheological and mechanical properties of a crylonitrile-butadiene-styrene polymers (ABS) prepared via bulk polymerization depending on the molecular weight (Mw) of styrene-acrylonitrile copolymer (SAN) have been investigated. The tendencies of attaining the yield stress at steady-state shear flow and approaching to the "plateau" region of storage modulus at low frequencies in oscillatory tests were observed. Both these phenomena are induced by formation of the structural skeleton consisting of polybutacliene (PB) particles arranged in the SAN-matrix. Growth of Mw of SAN leads both to increase of the "plateau" value of storage modulus at low frequencies and the yield stress. This fact can be explained by the influence of Mw of SAN chains grafted onto PB particles on structure formation in ABS melts because of a redistribution of the ratio particle-particle / particle-matrix interactions. The elongational viscosity of ABS melts is a power function of Mw of SAN matrix. The power index of this function increases with the polymer straining that reflects orientation of SAN chains with their length increase. The strain-hardening index of ABS melts increases considerably with increase of SAN matrix Mw. However, it does not depend on presence of PB particles in the material. It means that the value of this index is governed by orientation effect in the SAN matrix. The impact strength of the investigated ABS samples is interrelated with rheological characteristics of ABS melts as well as Mw of SAN. The dependence of impact strength on Mw can be explained by increasing role of orientation effects of SAN chains with Mw increase in the copolymer fibrils connecting the walls of crazes formed at impact action.