The reference point for liquid relaxation processes. I. Melt viscosity of polystyrene

Abstract

AbstractRecent theories have suggested or implied that below the experimental glass temperature, Tg, there exists a temperature, T0, which governs the temperature dependence of relaxation processes in a liquid. In the “free‐volume” approach, T0 should be associated with an “occupied” volume, v0. In the present paper, published melt viscosity—temperature data for a series of polystyrene fractions (M = 1675–134,000) are shown to conform to a “modified” Arrhenius equation: η = A exp {B/(T ‐ T0)}. From this and from a previous free‐volume treatment of the same data, the reference point (T0, v0) is derived. Either or both of these approaches indicate that this reference point is situated on the liquid specific volume—temperature curve, extrapolated linearly into the glass region. In the series of polystyrene fractions for which Tg increases from 40 to 100°C., the corresponding T0 values increase from 5 to 50°C. Further relationships between the parameters in the modified Arrhenius, the Doolittle free‐volume, and the Williams‐Landel‐Ferry (WLF) equations, are expressed.