Crosslinked polybenzimidazole via a Diels–Alder reaction for proton conducting membranes

Abstract

The crosslinking of polybenzimidazole (PBI) is a potential strategy to improve the mechanical properties and dimensional stability of acid-doped membranes, as well as to retain additives in the membranes. An effective method to prepare crosslinked PBI with a well-defined structure via a Diels–Alder reaction between vinylbenzyl functionalized PBI (PBI–VB) and α,α′-difurfuryloxy-p-xylene (DFX) is proposed. The chemical structure of PBI–VB is confirmed by FTIR and ¹H NMR. The model reaction of styrene and DFX is employed to clarify the crosslinking reaction of PBI and DFX. During the crosslinking process, three kinds of chemical reaction may happen. The first is a Diels–Alder reaction of DFX with the vinyl groups of PBI–VB. The second is the self-polymerization of vinyl groups. The third is the grafting of difuran groups via a Diels–Alder reaction. The first two reactions contribute the most to the crosslinking of the PBI membrane. With the addition of DFX, there is competition between these two kinds of crosslinking reactions. When the feed ratio of DFX is below 20%, the tensile strength of the crosslinked membranes increases with increasing content of DFX. The crosslinking of the membrane is mainly a results of Diels–Alder reactions. When the feed ratio of DFX exceeds 20%, the tensile strength decreases slightly. Besides the crosslinking via Diels–Alder reactions, the crosslinking of the membrane is also contributed by the self-polymerization of vinyl groups and the grafting of difuran groups. The crosslinked PBI membrane exhibits improved mechanical strength, higher physical and chemical stability, as well as higher phosphoric acid (PA) retention ability. After doping with PA, the crosslinked membrane exhibits good proton conductivity over a temperature range of 60 to 180 °C.