Linear sulfonated polyimides containing polyhedral oligomeric silsesquioxane (POSS) in main chain for proton exchange membranes

• Difunctional POSS was introduced in the main chain of linear sulfonated polyimide.
• The POSS aggregates showed good distribution in polymer matrix.
• The membranes revealed high proton conductivity and low methanol permeability.
• The introduction of difunctional POSS enhanced the chemical and dimensional stabilities of membranes.

Linear sulfonated polyimides (SPI's) incorporating a difunctional POSS unit, namely Phenyl8bisaniline-POSS, in the backbone of the main chain at concentration levels of 3, 5 and 10 mol% yield proton exchange membranes. Compared with other POSS hybrid membranes fabricated by direct blending or cross-linking methods, the ultrafine POSS aggregates show more uniform distribution and the obtained SPI-POSS co-polymers exhibit dramatically enhanced comprehensive properties as proton exchange membranes. The results reveal that the POSS in the SPI main chain facilitates the dispersion of POSS as a nano-filler and increases the confined bound water molecules within the formation of continuous proton transformation channels, thereby increasing the proton conductivity in comparison with the plain SPI without POSS. For example, the POSS-SPI membranes with 5 and 10 mol% POSS show higher conductivity in comparison to the POSS free SPI (0.114 and 0.132 S cm−1vs. 0.0998 S cm−1) at 80 °C. Moreover, the obtained POSS-SPI membranes also process enhanced oxidative and hydrolytic stabilities, low swelling ratio combined with high water uptake, adequate thermal stability and mechanical properties, as well as low methanol permeability.

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