THERMOREVERSIBLE POLYMER GEL ELECTROLYTES

Thermoreversible polymer gel electrolytes with ionic conductivities in the region of 10(-3) SCM-1 (even at -20-degrees-C) have been prepared from a variety of commercially available polymers and organic solvent s by gel casting from high-temperature solutions at polymer/solvent ra tios down to 10/90. Lithium trifluoromethanesulfonate has been incorpo rated as the ionic species necessary for conduction. A typical gel has polymer/solvent in mass ratio 40/60 and has salt incorporated to give an active O/Li ratio of 12/1. In general, the dynamic modulus (G') of these gels is in excess of 10(5) Pa at low strain, but decreases rapi dly with increasing strain amplitude despite remaining approximately c onstant with strain rate. This drop in modulus, which is attributed to breakdown of the gel network, is completely recoverable. In particula r, one polymer, poly(vinylidene fluoride), was studied in detail. Gels made from this polymer formed self-supporting transparent films. The incorporation of lithium trifluoromethanesulfonate changed the crystal structure and decreased the solvent evaporation rate, at elevated tem peratures, of poly(vinylidene fluoride) gels made with tetraethylene g lycol dimethyl ether. Ionic conductivities of liquid electrolytes (dim ethylformamide with lithium trifluoromethanesulfonate) and correspondi ng gels (dimethylformamide, lithium trifluoromethane-sulfonate and pol y(vinylidene fluoride)) suggest that there is no interaction between s alt and polymer in these gels, although this is still under investigat ion.