Fabrication of high-performance poly(benzoxazole-co-imide) membrane by utilizing synergistic effect of thermal rearrangement and Zn2+ modification

Abstract

Developing membranes that combine excellent gas separation performance with robust resistance to plasticization is highly demanded for membrane separation but remains a challenge. Herein, a series of Zn2+ functionalized poly(benzoxazole-co-imide) membrane was constructed via co-polymerizing (4,4′-hexafluoroisopropylidene) diphthalic anhydride (6FDA) with o-hydroxyl diamine 2,2′-bis(3-amino-4-hydroxyphenyl)-hexafluoropropane (BisAPAF) and 1,3,5-tris(4-aminophenyl)benzene (TAPB) followed by thermal rearrangement (TR) and Zn2+ modification processes. The TR conversion opened the polymer chains with larger d-spacing and higher BET surface area, thus, greatly increased the gas permeability. Meanwhile, the post Zn2+ modification resulted in a huge improved selectivity because of the π-complexation reactions between Zn2+ and CO2 and the formation of a Zn2+ coordination crosslinking network. As a result, due to the synergistic effect of TR and Zn2+ modification, the overall gas transport properties of these newly developed membranes were greatly improved. Furthermore, the dual network structures that arise from the remaining network PI and the Zn2+ coordination crosslinking contribute to the resultant membrane with excellent anti-plasticization capability. A represent membrane termed as PBOI-Zn2+-14h membrane exhibits great CO2/CH4 separation performance, delivering a CO2 permeability of 1048.1 Barrer and a CO2/CH4 selectivity of 46.1 that is exceeded the 2008 Roberson limitation. Furthermore, this membrane possesses remarkable resistance to CO2-induced plasticization, withstanding pressures above 35 Bar. Overall, this study presents an effective way for constructing polymer membranes with both high gas separation performance and strong resistance to plasticization issue.