Sulfonated poly(ether sulfone)s for fuel cells by solvent-free polymerization

Poly(ether sulfone)s from 4,4'-difluorodiphenylsulfone (DFDPhS), 4,4'-bistrimethylsiloxy-diphenyisulfone, and 2,5-bis-trimethylsiloxy-biphenyl were obtained by melt polycondensation with high molecular weights (eta(inh) > 0.4 dl/g). The nonsulfonated samples showed a single glass-transition temperature (T-g) in the range from 180 to 230 degrees C depending on the monomer composition in the polymer backbone. The T-g of sulfonated samples could not be detected by differential scanning calorimetry (DSC). Membranes with a theoretical ion-exchange capacity (IEC) ranging from 0 to 2.08 mmol/g should be obtained, assuming only a sulfonation of the pendant phenyl ring. However, the sulfonation with concentrated sulfuric acid always led to the introduction of two sulfonic acid groups into the phenylhydroquinone (PhHQ) moieties in the polymer backbone; one at the desired position at the pendant phenyl ring and one at the phenyl ring in the polymer backbone. Membranes prepared from N-methyl-2-pyrrolidone (NMP) solutions of nonsulfonated and sulfonated samples were transparent and soft to slightly brittle. The water uptake at room temperature increased with increasing IEC from 4 to 50 % (IEC similar to 1.50 mmol/g). On further increase of the IEC, a strong water uptake until dissolution was observed. Methanol diffusion coefficients for samples with an IEC up to 1. 10 mmol/g were one order of magnitude lower than that reported for Nafion (R).