Water absorption and proton conductivity of sulfonated acrylamide copolymers

Copolymers of 2-acrylamido-2-methyl-1-propanesulfonic acid (AMPS) and N-tert-butylacrylamide (tBAA) have been evaluated as proton conducting materials. The water absorption of copolymers with varying equivalent weights (EW), i.e. g polymer/mol sulfonic acid groups, was studied at different relative humidities (rh) by gravimetry and calorimetry. Melt endotherms for water were detected by DSC in copolymers containing ≥45 mol% AMPS when equilibrated at 93% rh, and ≥36 mol% AMPS when equilibrated at 98% rh. A study of the kinetics of the water absorption at 98% rh showed that equilibration was faster for the copolymers than for the homopolymer PAMPS. The proton conductivity (σ) of the samples was investigated as a function of temperature, water content, and EW. ac Impedance measurements between −20 and 70 °C showed a value of σ as high as 0.2 S/cm at 70 °C for a copolymer containing 54 mol% AMPS equilibrated at 98% rh. It was concluded that both the EW and the amount of water in the polymers controlled the conductivity at all temperatures. For example, increasing amounts of water in the polymers generally gave increasing σ above the melting point of the water, but decreasing σ below the same temperature.