Influence of the silica content in SPEEK-silica membranes prepared from the sol-gel process of polyethoxysiloxane: Morphology and proton mobility

Sulfonated poly(ether ether ketone) (SPEEK)-silica membranes with increasing silica content are presented. The silica is generated in situ via the water free sol-gel process of polyethoxysiloxane (PEOS), a liquid hyperbranched inorganic polymer of low viscosity. The conversion of PEOS into silica during membrane formation induces changes in the hydrophobic-hydrophilic phase separation of the pure ionomer, influencing morphology and proton mobility. The hybrid membranes are studied regarding their morphology (transmission electron microscopy) and their proton mobility at bulk (impedance spectroscopy) and molecular level (1H wide-line NMR, and self-diffusion). Water self-diffusion by stimulated-echo edited spectra shows two components corresponding to fast and slow diffusion. This can be correlated with the water diffusion in the ionic clusters (more hydrophobic) and in the channels (more hydrophilic). The apparent diffusion coefficients depend on the temperature and on the silica content. At 90% RH, the bulk proton conductivity of the composite membranes is higher than the pure ionomer. Starting from 80 °C, the proton conductivity of the SPEEK-silica membranes is independent from the silica content. The samples prepared with low PEOS content (10 and 20 wt.%) are more stable upon successive heating/cooling measuring cycles, showing less dependency on membrane hydration than the pure SPEEK.