TRENDS IN THE TEMPERATURE DEPENDENCY OF SEGMENTAL RELAXATION IN TMPC PS BLENDS

A model to account for the composition dependence and shape of the relaxation spectra of miscible blends in the glass transition zone is applied to dielectric measurements on tetramethyl-Bisphenol A polycarbonate (TMPC) mixed with polystyrene (PS). The shape of the dispersion associated with segmental relaxation is governed primarily by the fluctuations in the degree of cooperativity associated with the distribution in local composition arising from concentration fluctuations. A blend of fixed composition possesses a distribution of local environments; these effect a distribution in the degree of cooperativity of the segmental relaxation of locally rearranging chain units. Pure PS has a lower T(g) than TMPC. Hence, in the blend before cooperative dynamics is considered, PS chains will tend to have segmentally relaxed when segmental relaxation of TMPC is considered. The dynamical constraints imposed by PS segments in a local environment of TMPC are thus mitigated, allowing TMPC segmental relaxation to proceed with a lower degree of intermolecular cooperativity than is the case in less PS-rich environments. The temperature dependence of the frequency-temperature shift factors for the blend are governed by the degree of intermolecular cooperativity of the segmental relaxation. Hence, there exists a direct and verifiable relation between frequency and temperature dependencies of the mixture. The present data on TMPC/PS, though not sufficient to entirely demonstrate such a relationship, are nevertheless consistent with the predicted relation.