Sorption, diffusion, and permeability of humid gases and aging of thermally rearranged (TR) polymer membranes from a novel ortho-hydroxypolyimide

Abstract

We studied in this work the properties of a new membrane (TR-PBO) obtained by solid state thermal rearrangement at 450 degrees C of a recently developed polyimide precursor, (mHAB-6FDA), which was synthesized by reaction of (3,3-diamino-4,4-dihydroxybiphenyl, mHAB) with 2,2-bis(3,4-dicarboxyphenyl) hexafluoropropane dianhydride (6FDA). The mHAB monomer is an isomer of the commercial 3,3'-dihydroxybenzidine (pHAB), used to form the more popular polyimide precursor pHAB-6FDA. TR-PBO membranes obtained from mHAB-6FDA showed excellent CO2 permeability (720 Barrer) and good CO2/CH4 ideal selectivity of 23. We found out that the thermal rearrangement enhances the solubility and diffusion coefficients of CO2 at 10 bar by factors as high as 1.3 and 5, respectively. Larger enhancements, however, were observed in the case of CH4, causing the diffusivity selectivity to decrease by a factor of 2.6 and the solubility selectivity to decrease by a factor of similar to 1.5 upon rearrangement. The pure gas solubility was modeled with the Dual Sorption Mode model and the NELF model. The two models were then used to predict the mixed gas behavior in terms of solubility-selectivity, highlighting the effects of competition that are consistent with those observed in other glassy polymers. We also performed moisture sorption tests and gas permeability measurements in the presence of humidity. It was observed that the thermal rearrangement increases the membrane hydrophobicity and, consistently, the CO2 and CH4 permeability of mTR-PBO membranes is much more stable in the presence of humidity than that of the precursor polyimide membranes. Finally, the effect of aging on the membrane performance was analyzed. A 30% decrease in the CO2 permeability of TR polymer membranes (around 50 mu m thick) was observed after 6 months, while the selectivity increased by 20%. These results indicate that, even after 6 months, the performance of the TR polymer membrane was outstanding and close to Robeson's upper bound.