(303f) Solubility Parameters, Water Activity Coefficients and Proton Mobility of Sulfonated Poly (styrene-isobutylene-styrene), Sulfonated Poly (ether ether ketone), and Sulfonated Poly (2-ethoxyethyl methacrylate) Membranes

Maritza Pérez-Pérez and David Suleiman

Chemical Engineering Department

University of Puerto Rico

Mayagüez, PR 00681-9000

Solubility parameter,s water activity coefficients and proton mobility were studied for various sulfonated block copolymer membranes used as proton exchanged membranes (PEM) for direct methanol fuel cell (DMFC) applications. Equilibrium swelling and absorption properties of Sulfonated Poly (styrene-isobutylene-styrene) (S-SIBS), Sulfonated Poly (ether ether ketone) (SPEEK), and Sulfonated Poly (2-ethoxyethyl methacrylate) (SPEEM) were measured using 14 different solvents. The differences in the microstructures were studied using Small Angle X-Ray Scattering (SAXS) and Atomic Force Microscopy (AFM). Two solubility parameters were obtained, one corresponding to the polymer backbone (20.31 J^1/2 cm^-3/2) and the other corresponding to the solubility parameter of the sulfonic acid domains (36.1 J^1/2 cm^-3/2). The effective water activity coefficients and proton mobility in these ionomers were determined varying the temperature and relative humidity in the polymers. The ionic concentration dependence of proton mobility is attributed to the differences in the microstructure of the polymer, because the different environment can influence the sulfonic, ether, and ester groups inside of the hydrated nano-channels. The membranes possessed nano-channels between 1 nm and 25 nm and water content values between 10 and 31. Because of that, at high sulfonation levels the polymer present higher tortuosity and in some cases higher proton mobility than the state-of-the-art Nafion.