Dynamics of sodium ions in NaClO4 complexed in poly(propylene-oxide):: A 23Na nuclear magnetic resonance study

Amorphous poly(propylene oxide), PPO (molecular weights, 425 and similar to 10(6)), complexed with NaClO4 salt has ionic conductivities as high as 10(-5) S/cm at room temperature. In an attempt to directly study the dynamics of the Na+ ions, (I=3/2) nuclear magnetic resonance (NMR) spin-lattice relaxation times, T-1, and spin-spin relaxation times, T-2, at a resonance frequency of 77.0 MHz have been measured over the temperature range from 150 to 390 K. A range of salt compositions [(PPO)(n)NaClO4; n=8-30] have been investigated. In addition, the glass transition temperature for each sample is reported. The recovery of equilibrium magnetization following a pi/2 pulse is biexponential as expected for a spin I=3/2 system in the viscous liquid region. Below T-g a better fit to the magnetization recovery curve can be obtained if Kohlrausch-Williams-Watts (KWW) functions are used in place of the exponential functions. At low temperature the molecular motion has slowed to the point where a common spin temperature is not achieved by ionic diffusion. Two T-2 values are observed over a limited temperature region near the T1 minimum. In the remaining low temperature region only the T-2 associated with the central transition could be determined because the large Na-23 quadrupole coupling constant made detection of the satellite transitions impossible. There is no evidence from the NMR relaxation time data for two Na+ populations. The experimental spin-lattice relaxation times can be analyzed to determine the correlation time for the ionic motion as a function of reciprocal temperature. The correlation time data display a distinct change in behavior at T-g which is interpreted as an indication of a crossover between two ionic transport mechanisms. Plots of T-1 as a function of T-g/T indicate that dynamics for T>T-g are largely determined by the flexibility of the polymer host (the alpha process) whereas a different process dominates for T>T-g (presumably the beta process). A comparison of correlation times derived from NMR relaxation times and ionic conductivities show a marked difference especially in the region of T-g. The conductivity decreases rapidly while the sodium ion motion continues with a correlation time similar to 10(-6) s through the glass transition. (C) 1998 American Institute of Physics.