Gas sorption, diffusion, and permeation in thermally rearranged poly(benzoxazole-co-imide) membranes

•TR process allows great increase in diffusivity and minor increase in solubility.•Increase in gas permeability of TR copolymer is due to enhanced free volume.•Polymer rigidity hinders close chain packing.•Hindered packing results in high gas solubility by large Langmuir sorption capacity.

Thermally rearranged polymer membranes exhibit extraordinary gas permeability based on a rigid polymer structure with a high free volume element. In this study, TR poly(benzoxazole-co-imide) membranes from 4,4′-hexafluoroisopropylidene diphthalic anhydride (6FDA), 3,3′-dihydroxyl-4-4′-diamino-biphenyl (HAB), 2,2′-bis(3-amino-4-hydroxyphenyl)-hexafluoropropane (bisAPAF), and 2,4,6-trimethyl-m-phenylenediamine (DAM) were prepared to improve their gas separation properties. Copolymer membranes of polyimides and TR polybenzoxazoles may be desirable to generate efficient gas transport properties as well as to process polymers into fiber or film forms. Gas permeability, diffusivity, and solubility of the precursor polyimide and TR poly(benzoxazole-co-imide) membranes were investigated to characterize gas transport properties for small gas molecules including H2, O2, N2, CH4, and CO2. Thermal rearrangement process resulted in an increase in polymer free volume, which improved the diffusion and sorption coefficients.

Graphical abstractSolubility coefficients of (a) AD-polymers, and (b) HD-polymers as a function of critical temperature of small gas molecules measured at 1 atm, 35 °C.Figure optionsDownload full-size imageDownload high-quality image (183 K)Download as PowerPoint slide