Preparation of thermally rearranged poly(benzoxazole-co-imide) membranes containing heteroaromatic moieties for CO2/CH4 separation

Simultaneously achieving excellent mechanical properties and superior gas separation performance remains a great challenge for thermally rearranged polymers when applied as the membrane materials in large scale gas separation applications. Herein, different heteroaromatic non-TR-able codiamines containing benzimidazole/benzoxazole structures were incorporated into ortho-hydroxyl functionalized polyimide backbones, which were subsequently transformed into the poly(benzoxazole-co-polyimide) copolymers by thermal rearrangement (TR). In all cases, the solid-state TR reaction induces the increased preferential intersegmental distances (d-spacing) and fractional free volumes of polymer chains, favorable for the gas permeation. While, the π-π stacking distance exhibits opposite variation tendencies for benzimidazole-based and benzoxazole-based TR-PBOI membranes, which is demonstrated to make a significant effect on the gas selectivity. These TR-PBOI membranes thermally treated at 420 °C for 1 h exhibit higher tensile properties in relative to most of previously reported TR-PBOI membranes with the tensile strength of 97–118 MPa and initial modulus of 2.0–2.4 GPa. Moreover, incorporating heteroaromatic non-TR-able codiamines endows the resultant TR-PBOI membranes with excellent gas separation properties for the CO2/CH4 gas pairs with the CO2 permeability and CO2/CH4 ideal selectivity values exceeding the 1991 upper bound and close to 2008 upper bound. We anticipate this facile method will facilitate the large-scale preparation and application of TR membranes for gas separation.