Broadband dielectric spectroscopic studies of molecular motions in a Nafion? membrane vs. annealing time and temperature

摘要

The dynamics of macromolecular motions as coupled to proton migration in annealed Nafion? membranes were explored at temperatures above 100°C using broadband dielectric spectroscopy. Loss permittivity vs. frequency spectra of both β and α relaxations showed increased relaxation times with annealing which was rationalized in terms of water de-sorption and diminished free volume. The α relaxation time increases with increased annealing temperature. A parameter N reflecting connectedness of charge hopping pathways was extracted and indicated that the conductivity network accumulates more charge traps, presumably due to free volume decrease, with increased annealing time. Conductivity increases with increased annealing time at all temperatures despite the decrease in N; this suggests a change in the nature of proton hopping on annealing at high temperatures to be between sulfonic acid groups rather than by hopping across H _2OH ~+OH _2 bonds.