The evolution of free volume and gas transport properties for the thermal rearrangement of poly(hydroxyamide-co-amide)s membranes

Within a microporous polymer membrane, its high gas separation performance is much dependent on the free volume element architecture. In this study, thermally rearranged poly(benzoxazole-co-amide) (TR-PBOA) copolymer membranes were prepared by in-situ thermal treating poly(o-hydroxyamide-co-amide) (PHAA) precursors, basing on commercially available TR-able and non TR-able diamines with different molar ratio. Free-volume topologies were tailored by controlling the degree of thermal rearrangement and the flexibility of the original chains. Upon thermal conversion, small cavities coalesced into bigger ones, representing hourglass-shaped cavities with larger cavities and small bottlenecks, resulting in the significant increase in permeability. It was found that thermal rearrangement mainly occurred near or above glass transition temperature (Tg) where chain segments obtained enough motion ability, and TR-PBOA membrane prepared at this temperature possessed the maximal selectivity due to effective packing of rigid chains. When thermally treated at temperature much higher than Tg, there was a compromise between thermal conversion and chain annealing. Compared to thermal treatment temperature, the effect of dwelling time on thermal conversion ratio was minor, as the formed rigid structure limited chain motion until enough energy was received at higher temperature. Furthermore, TR-PBOA membranes with appropriate ratio of PBO and PA contents displayed superior mechanical properties and gas transport performance, especially for CO2/CH4 separation (CO2 permeability was about 237 Barrer, CO2/CH4 ideal selectivity was 36.6, plasticization pressure of CO2 was 2.9 MPa) (1 Barrer=10−10 cm3 (STP) cm cm−2 s−1 cmHg−1)