Synthesis and characterization of self-assembled sulfonated poly(styrene-b-vinylidene fluoride-b-styrene) triblock copolymers for proton conductive membranes

The synthesis of novel ionic triblock copolymers, sulfonated poly(styrene-b-vinylidene fluoride-b-styrene), with varied degrees of sulfonation is described. The synthetic strategy involves a new approach to a chain-end-functionalized fluoropolymer as a macroinitiator followed by atom transfer polymerization of styrene and sulfonation. Characterization of the polymers has been extensively carried out by H-1 and F-19 nuclear magnetic resonance and Fourier-transform infrared spectroscopy, differential scanning calorimetry, and thermogravimetry analysis. Tapping mode atomic force microscopy and transmission electron microscopy were applied to image the microphase separation and ionic aggregates. The dependence of ion-exchange capacity, water uptake, and proton conductivity on the degree of sulfonation has been studied. It has been found that the conductivities of the block copolymers are considerably higher than the random copolymers of polystyrene and sulfonated polystyrene possessing similar ion-exchange capacities. The proton conductivity of the membranes has also been investigated at different temperature and humidity conditions.