Thermally Rearranged Polybenzimidazole Membranes for Gas Separation

Abstract

Glassy polymers like PTMSP and PIM were reported to have regular microcavities in the polymer matrix recently. In our previous research, we reported extraordinarily permeable polymer membranes derived from ortho-functional polyimide via thermal rearrangement reaction. Polybenzoxazole and polybenzothiazole were obtained from polyimide containing hydroxyl and thionyl group respectively, however, polypyrrolone was derived from amine-containing polyimide (PAI) instead of polybenzimidazole due to the competition between secondary and tertiary amines in its thermal rearrangement. In this study, we primarily focused on an interesting route for the preparation of polybenzimidazole (PBI) membrane from polypyrrolone(PPy) with microcavities for gas separation. Hexafluoroisopropylidene diphthalic anhydride(6FDA) and diaminobenzidine(DBZ) monomers were polymerized to obtain PAI, which was thermally rearranged for the formation of PPy. This rearrangement contributed in the growth of fractional free volume (FFV) from 12.4% to 21% for PAI and PPy, respectively. The membranes were then treated in alkaline solution. The FFV was reduced to 13% by this alkaline treatment. Finally, the heat treatment at 400-450 oC induced the decarboxylation leading to the thermal rearrangement of PPy, giving rise to PBI membranes with microcavities These membranes had 26.7% FFV, primarily due to the regeneration of FFV by latter thermal rearrangement at 450oC. Thermal stability and microscopic characteristics of the polymer were investigated by several analytical methods such as TGA, X-ray diffraction, nitrogen sorption isotherms etc. Gas permeation properties of the TR PBI film were characterized for He, H2, CO2, O2, N2, and CH4 gas molecules. TR PBI membrane showed improved gas permeation characteristics over revisited upper bound comparing with TR PPy membranes, particularly for the smaller gas molecules like helium and hydrogen.