Temperature dependence of local segmental motion in polystyrene and its variation with molecular weight

Dielectric measurements are reported for the alpha-relaxation in polystyrene (PS) of varying molecular weights. Although the segmental relaxation dispersion was essentially invariant to M-w, the T-g-normalized temperature dependence (fragility) increases systematically with molecular weight. The latter result corroborates dynamic mechanical and light scattering studies, but is at odds with the reported (T-T-g) superpositioning of the shift factors for the recoverable creep compliance of PS. The failure of the dielectric relaxations time to superimpose when expressed as a function of T-T-g is consistent with the analysis of equation of state data for PS. We find that volume is not the dominant control variable, and in fact, temperature exerts a stronger influence on the relaxation times. (C) 2003 American Institute of Physics.