Enhancement of anhydrous proton conductive sulfonated poly (ether ether ketone) (SPEEK) membrane based on imidazole molecular mobility and SPEEK thermochromic phenomenon

Abstract

The present work focuses on a systematic study of imidazole derivatives by varying the methylene units in urocanic acid system from mono to heptyl groups as an efficiency proton transfer channel. Methylene units introducing in urocanic acid system can provide various kinds of packing structures, and the chain mobility with developed melting temperature lower than 100 ℃. With this thermal property, alkyl urocanates can form in molten state which are highly molecular mobility under high temperature. Proton conductivity of pure compounds exhibits the significant increase closed to melting temperature as high as 10-4 S cm-1 which performs as good proton conductive species. Sulfonated poly (ether ether ketone), SPEEK, blended with alkyl urocanates are prepared as model membranes. Proton conductivity of blended membranes performs synchronized effect between water and imidazole molecules as high as 10-4 S cm-1 under high temperature compared to neat SPEEK membrane (10-7 S cm-1). It is worth noticing that SPEEK can perform another phenomenon as an reversible thermochormic property. This phenomenon can occur by shifting interaction of hydrogen bond network to π - π stacking of polymer backbone under high temperature. Based on structure of heterocycles, a balance of hydrogen bond network and molecular mobility provide enhancement of proton conductivity and induction of π - π stacking in SPEEK polymer at high temperature generates reversible thermochromic property through sulfonic acid groups and aromatic rings.