Effect of Alkanediol Additives on the Properties of Polyphenylene-Based Proton Exchange Membranes

Novel sulfonated and carboxylated polyphenylene (SPP-QP-BA) containing additives were synthesized as proton exchange membranes. As the additives, linear and flexible alkanediols with various chain lengths (Cn: n = 4 for butyl, n = 8 for octyl, and n = 12 for dodecyl) were investigated. SPP-QP-BA(Cn) composite membranes possessed nanoscale phase-separated morphologies. As the alkyl chain length increased, the ionic domain size, water uptake, and proton conductivity slightly increased. Although the effect of the chain length on these properties was rather minor, the elongation property was affected much by the alkyl chain length and significantly increased with decreasing the alkyl chain length. The SPP-QP-BA(C4) membrane having the shortest alkyl chain length displayed the best mechanical property (yield stress: 43 MPa; maximum strain: 99%), which was much higher than that (yield stress: 39 MPa; maximum strain: 68%) of our previous SPP-QP. In addition, SPP-QP-BA(Cn) membranes showed excellent chemical stability similar to the SPP-QP.