Per-fluorinated sulfonic acid/PTFE copolymer studied by positron annihilation lifetime and gas permeation techniques

The mechanism of gas permeation in per-fluorinated sulfonic acid/PTFE copolymer Fumapem^® membranes for polymer electrolyte fuel cells has been investigated from the viewpoint of free volume. Three different samples, Fumapem^® F-950, F-1050 and F-14100 membranes with ion exchange capacity (IEC) = 1.05, 0.95 and 0.71 meq/g, respectively were used after drying. Free volume was quantified using the positron annihilation lifetime (PAL) technique and gas permeabilities were measured for O₂ and H₂ as function of temperature. Good linear correlation between the logarithm of the permeabilities at different temperatures and reciprocal free volume indicate that gas permeation in dry Fumapem^® is governed by the free volume. Nevertheless permeabilities are much smaller than the corresponding flexible chain polymer with a similar free volume size due to stiff chains of the perfluoroethylene backbone. (paper)