Synthesis and properties of sulfonated poly(phosphazene)-graft-poly(styrene-co-N-benzylmaleimide) copolymers via atom transfer radical polymerization for proton exchange membrane

A series of sulfonated poly(phosphazene)-graft-poly(styrene-co-N-benzylmaleimide) (PP-g-PSN) copolymers were prepared via atom transfer radical polymerization (ATRP), followed by regioselective sulfonation which occurred preferentially at the poly(styrene-co-N-benzylmaleimide) sites. The structures of these copolymers were confirmed by Fourier transform infrared (FTIR) spectroscopy, ¹H-NMR, and ³¹P-NMR, respectively. The resulting sulfonated PP-g-PSN membranes showed high water uptakes (WUs), low water swelling ratios (SWs), low methanol permeability coefficients, and proper proton conductivities. In comparison with non-grafting sulfonated poly(bis(phenoxy)phosphazene) (SPBPP) membrane previously reported, the present membranes displayed higher proton conductivity, significantly improved the thermal and oxidative stabilities. Transmission electron microscopy (TEM) observation showed clear phase-separated structures resulting from the difference in polarity between the hydrophobic polyphosphazene backbone and hydrophilic sulfonated poly(styrene-co-N-benzylmaleimide) side chains, indicating effective ionic pathway in these membranes. The results showed that these materials were promising candidate materials for proton exchange membrane (PEM) in direct methanol fuel cell (DMFC) applications.