Linear stress relaxation behavior of amorphous ethylene-styrene interpolymers

Amorphous ethylene-styrene interpolymers (ESIs) provide an excellent model system for testing contemporary concepts of molecular dynamics. Moreover, t he results can be extrapolated to the chain reptation dynamics of amorphous polyethylene at ambient temperature which cannot be assessed by direct exp erimental techniques. Previous studies of creep and nonlinear stress relaxa tion were extended to test theoretical relaxation functions for monodispers e polymers and proposed combining rules against the linear stress relaxatio n behavior of ESIs in the plateau and terminal regions. Master curves were constructed by time-temperature superposition of data at temperatures from T-g to T-g + 30. The temperature dependence of the shift factor was indepen dent of molecular weight and styrene content and was well described by the WLF equation. The master curves were satisfactorily fit by the empirical KW W equation. Relaxation master curves were modeled with theoretical relaxati on functions for monodisperse polymers, appropriate combining rules, and kn own molecular weight distributions. The Doi-Edwards single reptation model did not give satisfactory results. However, the des Cloizeaux double reptat ion 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 enta nglement 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-t ail styrene chain insertions.