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

Abstract

Amorphous poly(propylene oxide), PPO (molecular weights, 425 and ∼106), 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, T1, and spin–spin relaxation times, T2, 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)nNaClO4; n=8–30] have been investigated. In addition, the glass transition temperature for each sample is reported. The recovery of equilibrium magnetization following a π/2 pulse is biexponential as expected for a spin I=3/2 system in the viscous liquid region. Below Tg 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 T2 values are observed over a limited temperature region near the T1 minimum. In the remaining low temperature region only the T2 associated with the central transition could be determined because the large 23Na 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 Tg which is interpreted as an indication of a crossover between two ionic transport mechanisms. Plots of T1 as a function of Tg/T indicate that dynamics for T>Tg are largely determined by the flexibility of the polymer host (the α process) whereas a different process dominates for T>Tg (presumably the β process). A comparison of correlation times derived from NMR relaxation times and ionic conductivities show a marked difference especially in the region of Tg. The conductivity decreases rapidly while the sodium ion motion continues with a correlation time ∼10−6 s through the glass transition.