Structure–property relationships in SPEEK-PVDF blend membranes: balancing proton conductivity and hydrothermal stability

Sulfonated poly(ether ether ketone) (SPEEK) and poly(vinylidene fluoride) (PVDF) blend membranes were prepared to investigate the balance between hydrothermal stability and proton conductivity for their potential application as proton exchange membrane (PEM). The degree of sulfonation of SPEEK was found to be 66% (IEC=1.99 ± 0.02 meq/g), attained by using a controlled sulfonation procedure. SPEEK was blended with low molecular weight PVDF (71,000 g/mol) in different proportions. The membranes were prepared by solution casting and post-casting temperature’s influence was analysed on the structure morphology and performance of the membrane. Thermogravimetric analysis (TGA) displayed a common three-stage degradation profile, which corresponds to the release of moisture, the breaking down of sulfonic groups and the decomposition of the polymer backbone. Scanning Electron Microscope (SEM) revealed that the miscibility of SPEEK and PVDF is highly dependent on the temperature of solvent evaporation, which affects the formation of hydrophilic and hydrophobic structures. The homogenization of the membranes was evaluated through elemental analysis from Energy Dispersive X-ray Spectroscopy (EDS). In sorption-desorption experiments, it was seen that there are two stages of absorption that are controlled by hydration of sulfonic sites and capillary absorption. It was determined that increased post-casting temperatures result in less absorption and swelling. The proton conductivity and hydrothermal stability showed an inverse relationship with the membranes cast at 60-70 °C. The present study demonstrates that the conditions under which membranes are made affect the performance of the blend membranes.