Soluble sulfonated polybenzothiazoles containing naphthalene for use as proton exchange membranes

• Sulfonated polybenzothiazoles with naphthalene units were synthesized.
• Naphthalene and 6FA units in sulfonated polybenzothiazoles improved solubility.
• Sulfonated polybenzothiazoles had excellent thermal, and mechanical stabilities.
• The sPBT-6FA65 membrane showed high proton conductivity.

Two series of sulfonated polybenzothiazoles containing naphthalene, derived from either 2,2-bis(4-carboxyphenyl) hexafluoropropane (6FA) or 2,6-naphthalene dicarboxylic acid (NA) were synthesized by polycondensation with non-sulfonated monomer 2,5-diamino-1,4-benzenedithiol dihydrochloride (DABDT) and sulfonated monomer 4,8-disulfonyl-2,6-naphthalene dicarboxylic acid (DSNA). The sPBT-6FA series polymers containing DSNA were soluble in polar aprotic solvents such as dimethyl sulfoxide or 1-methyl-2-pyrrolidinone, whereas sPBT-NA65 with NA was not soluble in common polar aprotic solvents. Therefore, the incorporation of both the naphthalene and flexible hexafluoroisopropylidene units enhanced the solubility of the sPBT polymer series. The polymers were evaluated as proton exchange membranes and exhibited excellent dimensional stability, high thermal and oxidative stabilities, good mechanical properties, and high proton conductivities. The proton conductivities of sPBT-6FA65 were 0.25 S cm−1 at 80 °C in water and 0.018 S cm−1 at 120 °C under 35% RH.

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