Ionic conductivity, glass transition, and local free volume in poly(ethylene oxide) electrolytes: Single and mixed ion conductors

Ethylene oxide based polymer electrolytes which exhibit single-ion (cation Li+ or anion ClO4-) and mixed ion (from the dissociation of LiClO4 salt) conduction were studied by employing positron lifetime annihilation spectroscopy (PALS) and conductivity (sigma) measurements in the temperature range between 170 and 370 K. We present experimental evidence for the validity of (i) the linear expansion of local free volume from PALS, (ii) the Vogel-Tammann-Fulcher (VTF) law for sigma, and (iii) the Cohen-Turnbull equation that relates sigma to the local free volume. These were found to be valid in the temperature range above the end (or freezing) temperature of the glass transition, T-ge approximate to 1.06T(g)(PALS)approximate to1.18T(g)(DSC) [T-g(PALS) and T-g(DSC) are the T-g's from PALS and differential scanning calorimetry (DSC), respectively]. From VTF fits to sigma we obtained a Vogel temperature of T-0 approximate to T-g(DSC) and pseudoactivations energies of B=3.7-5.7 kJ/mol. These parameters disagree with many data published in the literature but are in perfect agreement with the free volume experiments. Moreover, we found T-0=T-g(PALS)-(20-28) K and T-g(DSC)=T-g(PALS)-(25-27) K. Indications for the existence of two relaxation processes near T-g were observed in the free volume expansion curves, which were attributed to the motion of free polymer segments and those interacting with ions. The discrepancy between T-g(DSC) and T-g(PALS) can be attributed to the two-phase microseparation of the polymer electrolytes; DSC responds mainly to the polymer segments in the ion-depleted regions while PALS responds to the polymer segments in the ion-rich regions. From the Cohen-Turnbull plots the critical hole volume required for an elementary jump of an ion was estimated to be gammanu* approximate to 1 nm(3) and was found to be independent of the type of ion. This shows that each type of ionic conductivity is associated with the same segmental mobility. Below T-ge(PALS) the conductivity is larger than expected from the (extrapolated) VTF law, but smaller than displayed in the frozen-in free volume. (C) 2003 American Institute of Physics.