SORPTION AND DIFFUSION OF GASES IN A POLYIMIDE

Sorption and diffusion of gases (CO2, N2, and He) in a polyimide (PI2080) film were measured by using an apparatus which gives the sorption rate curves from the initial state to the equilibrium state. Nonlinear isotherms observed for CO2 sorption were interpreted successfully in terms of the dual-mode model for sorption in glassy polymers. Linear isotherms observed for N2 and He seemed to obey Henry's law. Two diffusion coefficients (D(I) and D(E)) were obtained using the short-time method and the long-time method for a Fickian diffusion model, together with the equilibrium solubility (C(e)) from each experiment. The initial sorption rate curves agreed with the calculated curves using D(I), however near sorption equilibrium the curves are in accord with the calculated curves using D(E). These observations suggest that some relaxation process is superimposed on the diffusion process. The non-Fickian transport data were correlated successfully with a model that combines time-dependent diffusion and the Fickian model.