Transport parameters and solubility coefficients of polymers at their glass transition temperatures

Abstract

Many parameters of polymers exhibit breaks when temperature passes through glass transition. It is also often assumed that fractional free volume (FFV) at the glass transition temperature (Tg) has a standard value (the isofree volume concept). As gas diffusion (D) and permeability (P) coefficients depend on FFV, and mechanism of sorption and permeation is different above and below Tg, a question can be asked if D and P parameters of various gases in polymers have standard values at corresponding Tg, and, if not, how the values of D(Tg) and P(Tg) vary with Tg in different polymers. To examine this problem, two approaches were used: (1) extrapolation to Tg of numerous P and D values measured at ambient temperatures; (2) an analysis of direct data obtained in different polymers at their Tg. In both cases, qualitatively similar results were obtained: the D(Tg) and P(Tg) values increase with growing Tg independently of the nature of gas. Permselectivity Pi(Tg)/Pj(Tg) and selectivity of diffusion Di(Tg)/Dj(Tg) are reduced when Tg increases. The dependence of the solubility coefficients S(Tg) = D(Tg)/P(Tg) is much weaker than those of D(Tg) and P(Tg). This conclusion was confirmed by the results of direct measurements of S in a wide range of temperature including Tg for several gas/polymer systems. An analysis of the results of positron annihilation studies of free volume in polymers led to the conclusion that the observed increases in the D(Tg) and P(Tg) values with Tg are caused mainly by thermal activation of diffusion processes at Tg. © 2000 John Wiley & Sons, Inc. J Appl Polym Sci 76: 1691–1705, 2000