Self-Diffusion and Viscosity of Low Molecular Weight Polystyrene over a Wide Temperature Range

Abstract

The self-diffusion coefficient D and the viscosity η of low molecular weight polystyrene (1.9 × 103 g/mol) have been measured from Tg + 3 °C to Tg + 160 °C. Forward recoil spectrometry (FReS) and pulsed-gradient spin-echo NMR methods were combined to determine D values covering more than 10 orders of magnitude. Over this range, D has the same temperature dependence as T/η, to within 1 decade. The small difference can be explained by the different temperature dependences of segmental and terminal relaxation near Tg. The product of D and the terminal relaxation time is independent of temperature over the entire temperature range to an excellent approximation. In contrast, the analogous quantity for low molecular weight glass-forming liquids can increase by several orders of magnitude as Tg is approached from above. This difference is interpreted in terms of the length scale of the spatially heterogeneous dynamics which develop near Tg.