The local free volume, glass transition, and ionic conductivity in a polymer electrolyte: A positron lifetime study

The size of free-volume holes in neat poly[(ethylene glycol)(23)dimethacrylate] [poly((EG)(23)DMA)] and in the same polymer doped with 0.6 mol/kg LiCF3SO3 have been studied as a function of temperature in the range between 100 and 370 K using positron annihilation lifetime spectroscopy. The results are compared with differential scanning calorimetry and ionic conductivity measurements. In both systems, the hole volume nu (h) shows a typical glass-transition behavior, i.e., a small linear increase with temperature below the glass transition temperature T-g and a steeper increase above T-g. From these measurements T-g was estimated to be 233 K (neat polymer) and 240 K (polymer with salt) and the coefficients of the thermal expansion of the hole volume were determined. The fractional free volume (f=0.080) and the number density of holes (N-h=0.6 nm(-3)) were also estimated. Below T-g the average hole volume of the polymer electrolyte is larger than in the neat polymer. This is consistent with the bulky character of the CF3SO3- anion. Above T-g the salt-doped system shows the lower hole volume of the two systems, probably caused by a reduced segmental mobility as a consequence of the interactions of the Li+ ions with the ethylene oxide units of the polymer. Based on the free-volume theory of Cohen-Turnbull the ionic conductivity sigma is correlated with the mean hole volume nu (h). A linear relation between log(sigmaT (0.5)) and 1/nu (h) was observed to be valid for variations of the conductivity over several orders of magnitudes. From these plots critical hole sizes of gamma nu*=0.65 nm(3) (neat polymer) and 0.87 nm(3) (polymer-salt system) were estimated. The parameters B and T-0 of the Vogel-Tamman-Fulcher equation were also determined, as well as the apparent activation volume DeltaV(app) by pressure-dependent conductivity measurements. The cationic transference number in the polymer-salt system was determined by pulsed field gradient-nuclear magnetic resonance to be t(+)approximate to0.3. (C) 2001 American Institute of Physics.