Humidity control and water management in polymer electrolyte membranes for fuel cells are still of high importance to improve the fuel cells' efficiencies. In this study, poly (vinyl alcohol) (PVA)was crosslinked using 15 wt% sulfosuccinic acid (SSA) by a solution casting method and additionally thermally crosslinked at 100 • C. Positron annihilation lifetime spectroscopy was used to study the mean free volume size and the distribution at different humidity. A slight decrease in the free volume

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... was found up to a relative humidity of 30% whereas it increases strongly for a relative humidity of more than 30%. The volume of the voids duplicates from 0.036 to 0.078 nm 3 by changing the relative humidity from 30 to 80%. Thermogravimetric analyzer was used to determine the thermal stability of the membrane. From thermogravimetric analyzer data, it was found that the PVA with 15 wt% SSA membrane are chemically stable up to 200 • C.