Carbon Dioxide-Induced Plasticization of Polyimide Membranes:  Pseudo-Equilibrium Relationships of Diffusion, Sorption, and Swelling

Abstract

The application of membranes in gas separation and pervaporation requires materials that are resistant to plasticizing feed streams. We demonstrate the relationship between CO2 sorption, permeability, and film swelling of a polyimide gas separation membrane and how these properties are affected by systematic changes to the polymer structure induced by thermal annealing and covalent cross-linking. 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 decouple the effects of thermal and chemical treatments on the film swelling. The refractive index of the CO2-swollen polymer is also used to estimate the CO2 sorption for comparison against that measured on thick films (∼50 μm) by the pressure-decay method. Differences in sorption levels in thin and thick films appear to be related to accelerated physical aging of the thin films. Both thermal annealing and covalent cross-linking 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 cross-linked and un-cross-linked membranes. The so-called “plasticization pressure” in permeation experiments (i.e., upturn in the permeation isotherm) appears to correlate with a sorbed CO2 partial molar volume of 29 ± 2 cm3/mol in the polymer. Furthermore, cross-linking of high glass transition polyimides produces a much greater reduction of the CO2-induced dilation than does cross-linking of rubbery polymers such as PDMS for swelling up to 25%.

Gas permeation and physical aging properties of iptycene diamine-based microporous polyimides

“…This is a direct indication of matrix dilation by sorbed CO 2 molecules, which facilities diffusion of the larger CH 4 gas and thereby increases its permeability. This is expected even in relatively rigid-backbone polymers lacking sufficient physical or covalent interactions between chains [31][32][33]. Open symbols: pure-gas; closed symbols: mixed-gas.…”

Effect of the particle size and particle agglomeration on composite membrane performance

“…Wind and coworkers 10,11 crosslinked polyimides containing carboxylic acid side groups with ethylene glycol and aluminum acethylacetonate at high temperatures and suggested covalent crosslinking as a possible mechanism of the reaction. 12 Furthermore, mixed matrix composite films composed of two different materials offer the potential of combining polymer processability with the gasseparation properties of rigid molecular sieve materials.…”

Hyper-crosslinked, hybrid membranes via interfacial polymerization

“…This implies that the cross-linking density is independent of the side at which the reactants are offered during interfacial polymerization reaction, different from the gradients found in conventional interfacial polymerization membranes. 50 However, because different elements display a different propensity towards removal by argon ion etching it is not possible to determine the imide:POSS ratio as function of etching depth with certainty. The differences in the rate of element removal by sputtering are reflected by the gradients in Si/F and C/F ratios as function of sputtering depth ( Figure A5-2, Appendix).…”

“…In addition, metal membranes (palladium) are currently in pilot plant testing phase for gas separation applications. 50,51 Nonetheless, widespread implementation of inorganic membranes is hampered by the lack of stable, cheap, easy-to-process, defect-free inorganic layers. Moreover, the cost price for ceramic supports remains high, while they offer a low specific surface area compared to polymer hollow fibers or spiral-wound modules.…”

“…A relative refractive index above 1 indicates predominant filling of free volume spaces in the hybrid network, and results in an increase of the refractive index of the mixture. This is because the refractive index of the liquid CO 2 (estimated at n ≈ 1.23 [48][49][50][51] ) is much higher than the refractive index of void space (n = 1). Relative refractive indices below 1 indicate predominant dilation of the network as compared to filling of free void spaces in the network.…”