Sulphonated polysilsesquioxane–polyimide composite membranes: proton exchange membrane properties

This study reports the preparation of a –COOH-containing sulphonated copolymer (SPI-COOH-70) and its composite membranes. The composite membranes (SPI/SS-X) were prepared by using the solution casting route by adding different weight percentages of 3-(trihydroxysilyl) propane-1-sulphonic acid into SPI-COOH-70 solution. The proton exchange membrane properties such as morphology, ion-exchange capacity, water uptake and proton conductivity of the prepared composite membranes were studied as a function of sulphopropylated polysilsesquioxane (SiOPS) filler and temperature. All the SPI/SS-X composite membranes showed high-thermal stability and chemical stability, which are attributed to the presence of polar phosphine oxide and the wholly aromatic nature of the copolymers. The homogeneous distribution of the SiOPS nanoparticles in the polymer matrix observed from the scanning electron microscopy and atomic force microscopy images are attributed to the formation of covalent bonds between the –OH and –COOH groups of SiOPS and SPI-COOH-70, respectively. The composite membranes with 10 wt% SiOPS exhibited much higher proton conductivity (205 mS cm−1), which is much higher than that of the pristine copolymer membrane (114 mS cm−1) under similar experimental conditions. The improvement in proton conductivity is attributed to the presence of pendant sulphonic acid groups in the nanofiller, which provides a better proton transport pathway in the composite membranes.