Simultaneous Thermal Cross-Linking and Decomposition of Side Groups to Mitigate Physical Aging in Poly(oxyindole biphenylylene) Gas Separation Membranes

Physical aging in amorphous polymers causes a decrease in specific volume and thus in the gas transport properties of their membranes. In this work, the effect of simultaneous thermal decomposition of a thermolabile tert-butyl carbonate group, BOC, and cross-linking by a propargyl group (−CH2–CCH) on the gas selectivity–permeability properties of the resulting membranes is studied to learn how membranes with mitigated variations in the gas permeability coefficients with aging time may be produced. The model copolymer is a poly­(oxyindole biphenylylene) that bears BOC and propargyl groups, [(PN-BOC) x -(PN-Pr) y ] n . Systematic studies on the structure/processing/property relationship assessed by TGA, DSC, and permeation measurement using pure gases reveal that a single thermal treatment for 1 h at 240 °C on a neat copolymer membrane, 12–20 μm thickness, is enough to produce chemically robust membranes (insoluble in NMP and DMSO) and that are physically more resistant to aging since the permeability reduction rate approaches zero. The cross-linked membranes possess lower gas permeability coefficients with higher ideal selectivity with respect to the corresponding neat copolymer membranes, i.e., the P(H2) decreases from 60 to 42 Barrers but H2/CH4 selectivity increases by a factor of 2 (21 to 40), and in general the selectivity–permeability properties for the gas pairs H2/CH4, O2/N2, and CO2/CH4 do not present drastic variations with aging time at least from 72 to 2000 h.