Temperature evolution of thermoreversible polymer gel electrolytes LiClO4/ethylene carbonate/poly(acrylonitrile)

Thermoreversible polymer gel electrolytes with ionic conductivities of about 10(-3) S cm(-1) at room temperature, were prepared from poly(acrylonitrile) (PAN)-ethylene carbonate (EC)-LiClO4 at different solvent/salt ratios. A temperature dependence was investigated for the electrical and mechanical properties, as well as for spectroscopic changes associate to the solvation mechanism of lithium ions in these gel. The measurements performed on this system concern ionic conductivity, study of self-diffusion coefficient by H-1-NMR, Raman spectroscopy and, finally, an accurate analysis of its mechanical proprieties by rheological tests. The gels present a strong-weak gel transition at about 70 degreesC, independent from salt concentration hence assignable only to the polymeric matrix. However, this transition is completely reversible on cooling, being this a necessary mechanical property for industrial applications. The Raman study has pointed out an increase, on heating, of the Li+ coordination number by EC molecules, as well as a growing number of lithium ions involved in the ionic association (Li+-solvent-ClO4-) and in the Li+-CN interaction. The experimental evidences suggest an evolution of the system toward a less organized, weak gel configuration, when temperature rises above 65 degreesC. (C) 2002 American Institute of Physics.