Control of CO2-induced Plasticization in Polyimide Membranes: Coupling of Permeation, Sorption, and Polymer Swelling

摘要

The application of membranes in gas separation and pervaporation requires materials that are resistant to plasticizing feed streams. Thermal annealing and covalent crosslinking have shown to be effective at suppressing plasticization-induced selectivity losses. However, there has not been a comprehensive study of the effect of these treatments on the sorption, diffusion, and swelling processes in glassy polymer membranes. In this study, dilation of polyimide thin films (～120 nm) exposed to high-pressure CO2 (up to 100 atm at 35 °C) was measured by in situ spectroscopic ellipsometry to show the effect of thermal annealing and covalent crosslinking on the film swelling, and to decouple these effects. Analysis of the pressure and time dependence of permeability, sorption , and swelling shows how these parameters are interconnected and how membranes can be stabilized by post-treatments. Both thermal annealing and chemical crosslinking of the polyimide films reduce polymer swelling to prevent large increases in the CO2 diffusion coefficient at high feed pressures. The CO2 permeability and polymer free volume strongly depend on the annealing temperature, and different effects are observed for the crosslinked and uncrosslinked membranes. Separation of natural gas mixtures with the crosslinked polyimides shows that the performance is significantly better and more stable than the cellulose acetate polymers that are used commercially. It is believed that this crosslinking approach can be implemented in an industrial hollow fiber formation process since the reaction is shown to occur in the solid-state, well below the glass transition temperature.