Relaxation Dynamics of CO2 Diffusion, Sorption, and Polymer Swelling for Plasticized Polyimide Membranes

Abstract

The CO2 permeability, solubility, and dilation of 6FDA-DAM:DABA 2:1 polyimide membranes exposed to pure CO2 at 40 atm and 35 °C were investigated over long times (up to 20 days). Swelling and sorption measurements were performed on 120 nm films with in situ spectroscopic ellipsometry. The sorption measurements by ellipsometry are compared with sorption into thick films (40−70 μm) by the pressure-decay technique. There is a strong correlation between the CO2 diffusion coefficient and the film swelling. Solid-state covalent cross-linking of the membranes with 1,4-cyclohexanedimethanol and thermal annealing of the polymer films with free acid groups leads to significantly improved ability to maintain a relatively constant CO2 diffusion coefficient over long times. Annealing at temperatures between 100 and 295 °C leads to similar sorption behavior but significantly different diffusion and swelling. Relaxation functions are fit to the permeation, sorption, and swelling data to quantify the relationships between these parameters. There is a correlation between the permeation and swelling relaxation times and the magnitude of the increase in each parameter due to the relaxations with respect to the annealing temperature of the film. However, the relaxation times in the thin film swelling are an order of magnitude faster than in the permeation through thick films. Cross-linking tends to slow down the polymer chain relaxations while decreasing the polymer swelling and stabilizing the CO2 diffusion coefficient under high CO2 feed pressures. Spectroscopic ellipsometry allows for rapid measurement of swelling and sorption of thin films on the length scale of the permselective skin of practical asymmetric membranes (0.1 μm).