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Gas Transport Properties of Various Imidized Polyamic Acid Films

The sorption and permeation of CO2 in and through various imidized polyamic acid [PAA] films were investigated. Dual-mode sorption and partial immobilization models were applied to their phenomena and their behavior and structure are discussed. The amount of CO2 sorption increased with imidization in PAA film up to the imide content of ca. 40%, and then decreased. This behavior can be attributed mainly to the variation of the Langmuir sorption capacity term (CH′). In other words, it can be regarded that an unrelaxed volume and/or a microvoid fraction [(Vg−Vl)/Vg] vary with imidization, which are functions of Tg and difference in the expansion coefficient between liquid and glassy states. CH′ was described by the product (Tg−25) and ΔCp, difference of heat capacity between liquid and glassy states used instead of the expansion coefficient. On the other hand, the diffusion coefficient of Henry’s law mode (DD) is enhanced by the decrease of carboxyl group in PAA reducing the diffusion of penetrant with imidization up to ca. 40%, and then is depressed by restricted thermal mobility caused by considerably increased rigidity in the polymer chain with imidization. Variation in the diffusion coefficient of Langmuir mode behaved parallelly to DD.