Localized chain mobility and gas transport properties of thermoplastic aromatic polymers

The gas transport properties of some polyarylates, polysulfones, 3,3′,4,4′-biphenyltetracarboxylicdianhydride (BPDA)- and 4,4′-(hexafluoroisopropylidene)diphthalic anhydride (6FDA)-based polyimides were correlated to their respective dynamic–mechanical γ relaxation temperatures, Tγ. The CO2 and O2 permeability coefficients (P) at 35 °C increased with a decrease in Tγ, i.e. an increase in the localized chain mobility. A linear relationship between ln P and Tγ was observed, based on which a model for prediction of gas permeability was proposed. Within the ln P versus Tγ plot, best-fit curves for each polymer class resulted in a collection of parallel straight lines. For the polyarylates and polyimides, permeability correlations with Tγ were better or comparable to those obtained by correlations with fractional free volume, Vf. In fact, polyimide permeabilities exhibited no correlation at all with Vf. With the exception of polysulfones, no significant correlations were observed between Tγ and Vf. Thus, the correlation between P and Tγ is not a simple consequence of correlations between P versus 1/Vf and Tγ versus Vf.