A MOLECULAR-MODEL FOR THE STRENGTH OF THE DIELECTRIC BETA-RELAXATION IN METHYL ACRYLATE AND VINYL-ACETATE POLYMERS

As a test of the supposition that flexible side-group motion is responsible for subglass relaxation in some polymers, a molecular model for the strength of the dielectric beta subglass relaxation in poly(methyl acrylate) (PMA) and poly(vinyl acetate) (PVAc) homopolymers and their copolymers with ethylene (MA/E, VA/E) has been developed. The model is based on a rotational isomeric state (RIS) treatment for the overall relaxation, alpha glass-rubber relaxation + beta process. The RIS model gives good agreement with the experimental total relaxation strengths for the homo- and copolymers in both systems. Then the RIS model is specialized for consideration of the beta-process strengths only. A good account of the experimental behavior was obtained throughout the subglass temperature range for the beta processes in both PMA and PVAc homopolymers, including the marked difference in strengths in the two systems and their strong temperature dependence. The strength of the beta processes in the dilute copolymers is complicated by the possibility of additional contribution by main-chain subglass motions ("gamma process") in the polyethylene segments. In MA/E copolymers the modeled process, which includes side-group motions only, accounts for about half of the measured strength. In VA/E experimentally the process is weak and has not been accurately quantified. The modeling results are consistent with the experimental behavior.