Ring-opening metathesis polymerization for the preparation of polynorbornene-based proton exchange membranes with high proton conductivity

Abstract

Novel proton exchange membranes based on polynorbornenes with different crosslinker contents were prepared using ring-opening metathesis polymerization (ROMP). The precursor polymers were prepared from norbornene functionalized with benzenesulfonyl chloride (NBSC), norbornene, and dicyclopentadiene (DCPD). DCPD was used as a crosslinker. The polynorbornene-based membranes were obtained by the solution casting method. The sulfonyl chloride groups were transformed into sulfonic acid groups by treating the membrane with potassium hydroxide and hydrochloric acid. NBSC was introduced to replace the sulfonate monomer, which was insoluble in the nonpolar solvents used in ROMP. This approach allowed the preparation of membranes with high ion exchange capacity (IEC) and high proton conductivity. A series of membranes with the same theoretical IEC but with different crosslinker contents were prepared and their properties were measured. The results obtained showed that the proton conductivity, water uptake and swelling ratio of the membranes were all lowered as the crosslink density was enhanced. The proton conductivity of the non-crosslinked membrane at room temperature was 117±4mS/cm. When the ratio of NBSC to DCPD crosslinker was 1:0.406, the proton conductivity, water uptake, swelling ratio and methanol permeability at room temperature were 96±2 mS/cm, 31±2.5%, 10.5±2% and 1.0±0.2×10−6cm2/s, respectively.