A molecular model for tensile shear banding and the brittle‐ductile transition in poly(vinyl chloride)

Abstract

AbstractRoetling's adaptation of the Ree‐Eyring model to represent tensile shear banding of polymers in terms of two relaxation processes and Mansfield's model to describe alpha and beta relaxation processes in terms of polymer segment mobility are combined with the particulate structure of poly(vinyl chloride) (PVC) to give a more detailed connection between the macroscopic yielding process and the microscopic mobility of polymer segments. Mansfield's model for relaxation processes is cast in terms of intra‐ and inter‐molecular interactions that serve to hinder orientational motions of polymer segments. The viscoelastic beta process in PVC is interpreted in terms of this model to be a hindered rotation of a segment about its main chain axis. About half of the activation energy of the beta process can be estimated from the barriers hindering rotation in simple alkyl halides. According to tensile yield studies it is the beta process that governs the tensile yield behavior of PVC at high loading rates or low temperatures, i.e. the brittle to ductile transition. Viscoelastic measurements were made on milled compression molded powder blends of PVC of differing K‐values (55–69), over a frequency range of 0.1 to 500 Rad/sec. and a temperature range of −140°C to 20°C. The dependence of the complex compliance on frequency and temperature was represented in terms of a five parameter model proposed by Havriliak and Negami to represent the dielectric dispersions of polymers. The five parameters and their dependence on temperature were estimated using the multi‐response statistical techniques developed by Havriliak and Watts. The parameters are interpreted in terms of Mansfield segmental jumping model.