Polymer electrolyte membrane based on polyacrylate with phosphonic acid via long alkyl side chains

A series of novel phosphonated polymers with phosphonic acid groups via different lengths of flexible pendant side chains was synthesized. The radical polymerization of the corresponding diethyl esters of acrylate monomers, followed by hydrolysis with trimethylsilyl bromide produced the expected polyacrylates. Among the three polymers, the cross-linked poly[6-(acryloyloxy)hexylphosphonic acid] (PAHPA) membrane prepared by the recombination of polymer radicals with benzoyl peroxide as a radical initiator showed excellent proton conductivity comparable to that of the Nafion 117 membrane in the range of 30–80% relative humidity (RH) at 80 °C, regardless of the significant low water uptake behavior. Furthermore, well-defined phase separation between the hydrophobic and hydrophilic domains was clearly observed by scanning transmission electron microscopy (STEM) and synchrotron X-ray scattering measurements. To the best of our knowledge, the cross-linked PAHPA membrane shows the best proton conductivity performance among phosphonated polymers already reported.