Branched poly(biphenylene‐co‐sulfone)ether ion exchange membranes containing perfluorocyclobutane groups for fuel cell applications

Abstract

A series of branched poly(biphenylene-co-sulfone) ion exchange membranes containing perfluorocyclobutane groups were prepared for fuel cells. Two bifunctional trifluorovinyloxy-terminated monomers (sulfonable 4,4 0 -bis(trifluorovinyloxy)biphenyl and insulfonable 4,4 0 -sulfonyl-bis(trifluorovinyloxy)biphenyl) and a trifunctional trifluorovinyloxy-terminated branching agent (1,1,1-tris(4 0trifluorovinyloxyphenyl)ethane) were synthesized and terpolymerized via thermal [2π + 2π] cyclodimerization to obtain partially fluorinated and branched polymers containing 0-5 mol% of the branching agent. They were then postsulfonated by chlorosulfonic acid at room temperature, cast as membranes, and characterized to evaluate their electrochemical properties for fuel cell applications. As the branching agent content was increased, their polydispersity values highly increased, indicating they became highly branched. It was confirmed that higher branching agent content also increased the ion exchange capacity, water uptake, and proton conductivity of the branched ion exchange membranes containing perfluorocyclobutane groups. This indicates that their electrochemical properties can be easily controlled by the degree of branching.