Highly phosphonated poly(N-phenylacrylamide) for proton exchange membranes

A novel highly phosphonated poly(N-phenylacrylamide) (PDPAA) with an ion-exchange capacity (IEC) of 6.72 mequiv/g was synthesized by the radical polymerization of N-[2,4-bis(diethoxyphosphinoyl)phenyl]acrylamide (DEPAA), followed by the hydrolysis with trimethylsilyl bromide. Then, the crosslinked PDPAA membrane was successfully prepared by the electrophilic substitution reaction between the aromatic rings of PDPAA and the carbocation formed from hexamethoxymethylmelamine (CYMEL) as a crosslinker in the presence of methanesulfonic acid. The crosslinked PDPAA membrane had high oxidative stability against Fenton's reagent at room temperature. The proton conductivity of the crosslinked PDPAA membrane was 8.8 × 10⁻² S/cm at 95% relative humidity (RH) and 80 °C, which was comparable to Nafion 112. Under low RH, the crosslinked PDPAA membrane showed the proton conductivity of 1.9 × 10⁻³ and 4.7 × 10⁻⁵ S/cm at 50 and 30% RH, respectively. The proton conductivity of the crosslinked PDPAA membrane lied in the highest class among the reported phosphonated polymers, and, consequently, the very high local concentration of the acids of PDPAA (IEC = 6.72 mequiv/g) achieved high and effective proton conduction under high RH.