Block copolymers containing intrinsically proton-conducting blocks tethered with benzimidazole units

ABA triblock copolymers having benzimidazole-tethered end blocks have been prepared and characterized with respect to their intrinsically proton conducting properties. The copolymers were synthesized by using a linear poly( ethylene oxide) (PEO) as a difunctional macroinitiator for the anionic ring-opening polymerization of allyl glycidyl ether. The pendent allyl groups of the end blocks were subsequently linked with 2-(2-benzimidazolyl) ethanethiol via a free radical thiol-ene coupling reaction. Thermogravimetry showed that the final materials were stable up to 210 degrees C under a nitrogen atmosphere. The benzimidazole units of the copolymers were found to crystallize from solution but not from the melt. After heating the materials above the melting point of benzimidazole units, the copolymers displayed single glass transitions temperatures (T-g's). Moreover, the crystallization of the PEO blocks was depressed which may indicate a high degree of interaction between the dissimilar blocks. The proton conductivity was found to be promoted by a high benzimidazole concentration and a high segmental mobility, that is, low Tg's of the copolymers. Increased concentrations of benzimidazole raised the T-g, and the resulting conductivity thus depended on a balance between the charge carrier density and the mobility. Conductivities close to 80 mu S/cm were reached at 160 degrees C by these materials under completely anhydrous conditions.