Crosslinked polybenzimidazole by [2+3] Huisgen cycloaddition for proton exchange membrane

Abstract

An effective method to prepare crosslinked polybenzimidazole (PBI) via [2 + 3] Huisgen cycloadditions has been proposed. Propargyl modified PBI (PBI-PG) has been synthesized firstly. In the presence of diazido crosslinkers, PBI-PG can be thermally crosslinked by [2 + 3] Huisgen cycloadditions. Evidences from 1H NMR and ATR spectra imply a small amount of grafting alkenyl structures on PBI-PG. The grafting alkenyl structures provide another crosslinking mechanism named as alkyne-alkene crosslink. The alkyne-alkene crosslink is highly efficient under heat to form crosslinking networks which display toughness, low tensile strengths, and large swelling ratios in phosphoric acid (PA) solutions. Influences of rigidities and chain lengths of crosslinkers on performances of crosslinked PBIs have been investigated. PBIs crosslinked by rigid crosslinkers behave as brittle materials while that crosslinked by flexible crosslinkers behave as tough materials. Addition of crosslinkers leads to decreased gel fractions but improved tensile strengths, which is attributed to competition results of crosslinking and plasticizing effects. PBI-R2-20, which is crosslinked by a long rigid crosslinker, exhibits the highest tensile strength of 102.33 MPa, which is 66.34 MPa higher than that of PBI-PG. With even lower PA uptakes, PBIs crosslinked by rigid crosslinkers display more thermally stable and higher conductivities than that crosslinked by flexible crosslinkers. After doping in 12 mol/L PA solution, PBI-R2-20, with an acid uptake of 163 wt%, displays the highest tensile strength of 8.1 MPa and the most thermally stable and the highest conductivity which reaches a maximum of 2.30 × 10−2 S cm−1 at 188 °C among all crosslinked membranes.