Gas permeation in thin films of "high free-volume" glassy perfluoropolymers: part II. CO₂ plasticization and sorption

Carbon dioxide (CO₂) plasticization and sorption effects in both thick and thin films of "high free-volume" glassy perfluoropolymers were studied by monitoring CO₂ permeability and by observing changes in the film thickness and refractive index with ellipsometry measurements. The film thickness, aging time, thermal history and CO₂ exposure protocols have significant effect on the absolute CO₂ permeability and plasticization behavior of both thick and thin films. The extent of CO₂ plasticization increases as film thickness decreases and as the aging time is increased. The as-cast films showed higher plasticization compared to films which were annealed above T_g; however, the CO₂ permeability of both the as-cast and annealed films continuously decreased during the depressurization step unlike other glassy polymers. In general, the various CO₂ exposure protocols revealed lower CO₂ plasticization for perfluoropolymers compared to other reported glassy polymers. The extent of CO₂ sorption obtained from the ellipsometry measurements was found to decrease with the decrease in the excess volume and increase in the aging time for perfluoropolymers; in addition, the structural differences among the various glassy polymers resulting in different polymer-gas interactions also affects the overall sorption characteristics. The lower plasticization in perfluoropolymers compared to Matrimid was also confirmed from the smaller percent increase observed for the experimental diffusion coefficient compared to the theoretically predicted diffusion coefficient from the dual sorption-mobility model. The Langmuir sorption parameter, CH′, and solubility at infinite dilution, S₀, obtained from fitting dual sorption-mobility model to sorption data, showed an excellent linear correlation with (T_g-35) °C. The CO₂ diffusivity and permeability data obtained for thin films of various glassy polymers also showed a strong correlation with free volume. The somewhat unusual behavior of thin films of AF 2400 in comparison to other glassy polymers studied to date is believed to be related to the low cohesive energy density expected of perfluorinated structures and its high free volume resulting from the bulky dioxole comonomer.