Linear Stress Relaxation Behavior of Amorphous Ethylene−Styrene Interpolymers

Abstract

Amorphous ethylene−styrene interpolymers (ESIs) provide an excellent model system for testing contemporary concepts of molecular dynamics. Moreover, the results can be extrapolated to the chain reptation dynamics of amorphous polyethylene at ambient temperature which cannot be assessed by direct experimental techniques. Previous studies of creep and nonlinear stress relaxation were extended to test theoretical relaxation functions for monodisperse polymers and proposed combining rules against the linear stress relaxation behavior of ESIs in the plateau and terminal regions. Master curves were constructed by time−temperature superposition of data at temperatures from Tg to Tg + 30. The temperature dependence of the shift factor was independent of molecular weight and styrene content and was well described by the WLF equation. The master curves were satisfactorily fit by the empirical KWW equation. Relaxation master curves were modeled with theoretical relaxation functions for monodisperse polymers, appropriate combining rules, and known molecular weight distributions. The Doi−Edwards single reptation model did not give satisfactory results. However, the des Cloizeaux double reptation approach successfully described the relaxation master curves. Excellent agreement was found between the resulting plateau moduli and those from a previous study of creep in the glass transition region. The calculated entanglement molecular weight (1250−2290 g mol-1) was much closer to that of polyethylene (480−860 g mol-1) than to that of polystyrene (13 500 g mol-1) due to the unique chain microstructure of these ESIs with no head-to-tail styrene chain insertions.