Tunable Free Volume Structure on the Gas Separation Performance of Thermally Rearranged Poly(benzoxazole-co-imide) Membranes Studied by Positron Annihilation

In this work, two series of poly(benzoxazole-co-imide) membranes were synthesized through thermal rearrangement (TR) reaction at 450 °C. Different kinds and proportions of the nonrearrangeable (non-TR-able) diamines were incorporated into the rearrangeable (TR-able) precursors to modify the free volume structure of the resulting membranes. Results of positron annihilation measurements exhibit that the increase of TR-able content or number of methyl groups leads to an increase of both free volume size and fractional free volume, while the incorporation of non-TR-able diamines obviously improves the mechanical properties of the membranes. The pure gas permeability shows a perfect linear relationship with the fractional free volume of the samples, confirming the decisive effect of the free volume structure on gas transport. Meanwhile, the gas selectivity is also found to depend on the fractional free volume. In addition, the high fractional free volume obtained by TR reaction leads to exceptionally high H₂/CH₄ separation performance, with majority of the samples exceeding the 2008 Robeson upper bound. The plasticization resistivity of the membranes is also enhanced by the TR reaction. Our study exhibits that the free volume structure of the poly(benzoxazole-co-imide) membranes can be tailored for appropriate gas separation performance. Furthermore, our results show that positron annihilation technique is an effective tool for exploring the microstructural information on gas separation membranes.