Ether-free Sulfonated Polyarylene-Based Proton Exchange Membranes by Superacid-catalyzed Friedel-Crafts Reaction
Proton exchange membrane(PEM)is one of the key components of PEM fuel cells.Sulfonated poly(arylenen ether)-based PEMs have gained much more attention due to easy synthesis and excellent mechanical and thermal properties.However,these PEMs containing ether bonds,which are vulnerable to free radical,show poor oxidative stability.Ether-free polyarylene-based polymer has become the most promising one,giving that no heteroatom is contained on the backbone.Up to now,it has been synthesized only by metal-catalyzed polymerization,Diels-Alder polymerization and superacid-catalyzed polymerization.Superacid-catalyzed polymerization is an easy approach to providing the polymer with pendent alkyl sulfonic acid rather than aryl sulfonic acid.Herein,ether-free polyarylene-based precursors containing trifluoroisopropylidene were successfully synthesized via the trifluoromethanesulfonic acid-catalyzed Friedel-Crafts reaction and then post-sulfonated to obtain ether-free polyarylene-based PEM containing arylene sulfonic acid and trifluoroisopropylidene on the backbone.The effects of the substituents,electron cloud density,and steric hindrance on the molecular weight of the precursor polymers were investigated.The arylene-based monomer with high electron cloud density provided high reaction activity and molecular weight(Mw＞200 kDa),such as fluorene and 9,9-diphenylfluorene,but those after post sulfonation cannot provide flexible membranes due to the rigidity of mainchain.Considering to the balance between electron cloud density and rigidity,a precursor polymer consisting of trifluoroisopropylidene and bibenzyl(PAAF)demonstrated a high molecular weight of 201 kDa and provided flexible membrane after post-sulfonation.Giving that PAAF was not dissolved in concentrated sulfonic acid and easily precipitated from dichloromethane after dropping chlorosulfonic acid,those common post-sulfonation was useless.Chlorosulfonic acid trimethylsilyl ester was used to obtain sulfonated poly(arylene alkylene)-based polymers(sPAAF).sPAAF with various ion exchange capabilities of 1.24-2.40 mmol/g was prepared via control concentration of sulfonated solvent.At 80 ℃ and in a fully hydrated state,its proton conductivity reached up to 102 mS/cm,with a high thermal decomposition temperature of 256 ℃ and good chemical stability,showing a degradation rate between 10％and 20％after Fenton's test.This study aims to lay a theoretical foundation for the development of low-cost,high-performance ether-free sulfonated polyarylene-based PEMs by exploring the relationship between the structure and bulk properties.
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