Cross-linked sulfonated poly(ether imide)/silica organic–inorganic hybrid materials: proton exchange membrane properties

Abstract

A series of sulfonated poly(ether imide)–silica hybrid membranes (SPI/S-X) were prepared from fluorine-containing ter-copolyimide and ∼15 nm colloidal silica particles. The soluble poly(ether imide) with carboxylic acid pendant groups was synthesized by one pot high temperature polycondensation reaction using a sulfonated diamine, 4,4′-diaminostilbene-2,2′-disulfonic acid (DSDSA), fluorinated quadri diamine (QA), 3,5-diaminobenzoic acid (DABA) and 1,4,5,8-naphthalenetetracarboxylic dianhydride (NTDA). Such pendant carboxylic acid groups undergo condensation reaction with colloidal silica to provide organic–inorganic bonding that effectively improves the compatibility of nanocomposite membranes. The structure of the synthesized polymer was analysed by FT-IR and ¹H NMR spectroscopy. The esterification reaction was carried out between colloidal silica particles (with different loading) and the carboxylic acid containing polymer chains by thermal treatment in the presence of p-toluenesulfonic acid (PTSA), which catalysed the esterification reaction. The prepared hybrid membranes exhibited excellent thermal and oxidative stability. The mechanical properties of the membranes were enhanced with a certain wt% of silica loading whereas the swelling ratio and oxygen permeability of the membranes were reduced by cross-linking. The microstructure of the resulting hybrid membranes were extensively investigated by SEM and TEM. Good compatibility and uniform distribution of the silica nanoparticles in the hybrid membrane was observed by SEM. An excellent nano-phase separated structure was observed under TEM which indicated the formation of well-dispersed hydrophilic domains throughout the membrane. The hybrid membranes showed significantly high proton conductivity in water medium which is in the range of commercially available Nafion® membranes under similar experimental conditions.