Effect of casting solvent on the morphology and performance of sulfonated polyethersulfone membranes

Sulfonated polyethersulfone (SPES) membranes were prepared using dimethylformamide (DMF), dimethylacetamide (DMAc) and N-methyl-2-pyrrolidone (NMP) as casting solvents, referred as SPES–DMF, SPES–DMAc and SPES–NMP membranes, respectively. Comprehensive and comparative studies have shown that casting solvents have significant effects on the morphology and performance of SPES membranes. The 1H NMR spectra revealed the formation of strong hydrogen bonding between residual DMF and SPES, accompanying the thermal decomposition of DMF during the membrane preparation. The X-ray diffraction indicated a more perturbed randomness of amorphous inside of the SPES–DMF membranes than other two membranes. Atomic force microscope demonstrated different surface morphologies of three solvent cast SPES membranes, which appeared to be governed by solvent volatility combined with solvent–polymer interactions. A more co-continuous hydrophilic cluster-like structure was observed on the surface of SPES–DMAc membranes. The SPES–DMF membranes exhibited relatively low ionic exchange capacity, water uptake and large contact angle compared to the SPES–DMAc and SPES–NMP membranes. The SPES–DMAc membranes achieved the highest conductivity values at room temperature among the three membranes. These resulting differences were tentatively correlated with the different degrees of interaction between the SPES and solvents. Moreover, the conductivity differences seemed to be correlated to the morphological differences.