Oxidative crosslinking of copolyimides at sub-Tg temperatures to enhance resistance against CO2-induced plasticization

Abstract

Polymer crosslinking via thermal oxidation is a well-established technique that enhances the gas selectivity of a polymer membrane, but usually at the expense of lower gas permeability. As thermal oxidation is typically performed at temperatures higher than the glass transition temperature (Tg) of polymers, large energy footprints are incurred while pore structures in asymmetric membranes – the preferred physical configuration of polymer membranes are collapsed. To overcome these drawbacks, here we report a strategy to simultaneously enhance both gas permeability and selectivity via thermal oxidation at sub-Tg temperatures. This was achieved with a copolymer consisting a component that enhanced Tg, and another component with functional groups that decomposed and induced crosslinking at lower temperatures. We used a series of complementary characterization techniques to pinpoint the actual mechanism underpinning our sub-Tg thermal oxidation approach. Crosslinking slightly reduced the d-space between polymer chains but increased polymer fractional free volume and most importantly, inhibited CO2-induced plasticization. The pure CO2 permeability of the crosslinked polymer reached 88.5 Barrer with a CO2/CH4 ideal selectivity of 38.8, while no plasticization behavior was detected at a CO2 pressure up to 30 atm at 35 °C. When separating CO2:CH4 (1:1) mixed gas at total pressures between 10 and 60 atm, the CO2 permeability and CO2/CH4 selectivity of this crosslinked polymer were reduced to 55 Barrer and 29 due to competitive permeation.