Solid-state ²H NMR investigation of molecular motion in proton-conducting polyacrylic acid/imidazole composites

The molecular motion of imidazole (Im) in proton-conducting polyacrylic acid (PAA)/Im composite materials (PAA/xIm, where x represents the number of moles of Im per mole of the carboxyl group of PAA) was investigated using solid-state ²H NMR. For the ²H NMR analysis of polymers in which multiple constituents with different kinetic states coexist, we propose the simultaneous analysis of two ²H NMR spectra observed by the quadruple echo method using different echo intervals. The ²H NMR spectra were analyzed by superimposing simulated spectra with different jumping rates of 180° flip and isotropic rotation of Im. For PAA/0.5Im-d₃, Im undergoes 180° flip below 343 K and isotropic rotation above 363 K. For PAA/1.0Im-d₃, Im undergoes 180° flip below 323 K and isotropic rotation above 363 K. In the temperature range of 333–343 K, Im undergoing 180° flip and isotropic rotation coexist in PAA/1.0Im-d₃. The distributions of the jumping rates of 180° flip and isotropic rotation of Im were obtained. The increase in the amount of Im enhances the jumping rate of the isotropic rotation of Im and the proton conductivity of PAA/xIm at high temperatures.