Proton conductive aromatic block copolymers from a new bistriazole monomer

Abstract

We have designed and synthesized a new bistriazole compound, 3,3′-(1,3-phenylene)bis[4-phenyl-5-(4-fluorophenyl)-4H-1,2,4-triazole], as a comonomer for a series of sulfonated block poly(arylene ether) copolymers. The bistriazole was successfully synthesized from isophthalic dihydrazide and 4-fluorobenzoyl chloride via bisoxadiazole. The bistriazole monomer was polymerized with 4,4′-biphenol or 4,4′-dihydroxydiphenyl ether to obtain hydroxyl-terminated hydrophobic oligomers, which were copolymerized with sulfonated oligomers to obtain the title block copolymers. The block copolymers were high-molecular-weight (M_w = 91–500 kDa) and provided tough and bendable membranes by solution casting. Because of the sequenced block copolymer structures, the membranes exhibited hydrophilic/hydrophobic phase-separated morphology as confirmed by scanning transmission electron microscopic (STEM) images. The membranes showed proton conductivity under humidified conditions; the highest proton conductivity was 6 × 10⁻² S cm⁻¹ at 95% relative humidity (RH) and 80 °C. The membranes were mechanically stable with high storage moduli (ca. 10⁹ Pa) and loss moduli (ca. 10⁸ Pa). These mechanical properties were independent on the ion exchange capacity (IEC) of the membranes. The triazole groups were effective in improving the mechanical and oxidative stability of the sulfonated poly(arylene ether) block copolymer membranes.