High-performance sulfonated polyimide/polyimide/polyhedral oligosilsesquioxane hybrid membranes for ethanol dehydration applications

A novel miscible polymer blend of copoly(1,5-naphthalene/3,5-benzoic acid-2,2'-bis(3,4-dicarboxyphenyl)) hexafluoropropanedimide 6FDA-NDA/DABA and its sulfonated polyimide has been investigated as a water-selective layer to develop dual-layer hollow fiber membranes for ethanol dehydration via pervaporation. The resultant fibers display an excellent flux of >3.0 kg/m2 h. This is possibly ascribed to a good distribution of sulfonic groups in the polymer matrix, the increased hydrophilicity and enlarged d-space induced by sulfonic groups, which are verified by polarized light microscopy (PLM), contact angle measurement, sorption tests and XRD. Three different post-treatment approaches, namely, thermal treatment, PDMS coating and modification with polyhedral oligosilsesquioxane (POSS) were employed to improve the membrane selectivity. Their effects on polymer chain packing, dense skin layer thickness and ethanol dehydration performance have been investigated with the aid of the advanced PALS technique. Thermal treatment can densify the selective layer structure while PDMS coating provides a thin shielding layer to protect the selective layer from feed-induced swelling. More importantly, POSS modification displays as the most effective way to enhance the membrane selectivity due to the anti-swelling property of POSS inorganic nanoparticles, mobility restriction of polymer chains and narrowed d-space in polymer structure. Compared to various membranes in the literature, the newly developed hollow fibers in the present study have superior fluxes of >2.0 kg/m2 h and good separation factor of >200 for ethanol dehydration. This study may provide useful insights for the exploration of new materials and post-modifications for biofuel separations.