Solvent transport in hydrocarbon and perfluorocarbon ionomers

Abstract

Transport behavior of a sulfonated triblock ionomer with a copolymer midblock of styrene and hydrogenated butadiene, Dais analytic (DA) is compared with Nafion™ (DuPont) using water, methanol, dimethyl methylphosphonate (DMMP), and dichlorohexane (DCH). Water solubility in Nafion is lower than that of DA, but the diffusion constant is an order of magnitude higher, indicating essentially unimpeded ionic pathways in Nafion. Differences in the properties or environment of the sulfonic acid groups are evident in the solubility of water and methanol, which are both much lower in the Cs than Ca form of Nafion, but comparable in the salt forms of DA. Water isotherms for DA and several uncrosslinked, sulfonated triblocks can be superimposed on the isotherm for Nafion by vertical scaling factors that represent the saturation concentration for the unrelaxed structure of each triblock ionomer. Water diffusion in both DA and Nafion is limited to ionic pathways and follows a free-volume relation, but the magnitude of the diffusion constant is determined by ionomer structure. High and nearly equal DMMP solubility in acid and salt forms is attributed mainly to solvation of the fluoroether interfacial region of Nafion, whereas, in DA the high solubility is due primarily to acid–base interactions with the sulfonate anion. Cation-modification has little effect on DMMP solubility but results in a dramatic reduction in DMMP diffusion constants. DMMP diffusion in DA and Nafion follow different diffusion pathways: fluoroether interfacial regions in Nafion, but ionic regions in DA.