Evidence for depth-dependent structural changes in freeze/thaw-cycled dry Nafion® using positron annihilation lifetime spectroscopy (PALS)

The effects of freeze/thaw (F/T) cycling on the structure of dry Nafion® membranes were investigated using positron annihilation lifetime spectroscopy (PALS). The method correlates the lifetime and relative intensity of positrons (anti-electrons) annihilating in the open-volume voids of the membrane with void size and void volume fraction. The results, though still preliminary, show freeze/thaw-induced collapse in the pore volume near the surface of the dry membrane, and also some evidence of slight differences in the two sides of a given membrane. The work demonstrates the utility of the method in studying depth-dependent changes in membrane total free volume along with pore size, structure, configuration and environment as the result of thermal and structural stresses.