Sorption and diffusion of carbon dioxide in polyimide films

The sorption isotherms of carbon dioxide in a polyimide film sample were measured by a pressure decay method at 30, 40, 50, and 60°C and pressures up to 1.7 MPa. The mean permeability coefficients at the same temperatures were measured by a variable volume method at applied upstream pressures up to 3 MPa. The sorption equilibria at these temperatures were found to be well described in terms of a dual-mode sorption model. The pressure dependences of the mean permeability coefficients at these four levels of temperatures did not satisfactorily follow a dual-mode mobility model, but could be well simulated by a modified mobility model that both the Henry's law (D) and Langmuir (H) populations execute four kinds of diffusive movements such as D → D, D → H, H → D, and H → H. The diffusion coefficients of D → D, D → H, and combined H → H and H → D steps were determined by comparing the experimental permeability data with those predicted via this modified mobility model. The temperature dependences of such diffusion coefficients were consistent with Arrhenius-type equations within the covered temperatures.