Dynamic mechanical spectroscopy and conductivity studies of gel electrolytes based on stereocomplexed poly(methyl methacrylate)

Thermally reversible ionically conducting gel electrolytes, comprising blends of atactic and isotactic poly(methyl methacrylate) (PMMA), lithium salt and an organic solvent, have been prepared and characterized. Due to association between isotactic PMMA (i-PMMA) and syndiotactic sequences of atactic PMMA (a-PMMA) strong gels are formed. in solvents which ordinarily do not form gels with high molecular weight a-PMMA, namely, dimethyl carbonate (DMC) and diethyl carbonate (DEC). The plot of the dependence of the elastic moduli on the fraction of i-PMMA passes through a maximum at similar to 1:1 molar ratio between isotactic and syndiotactic fractions of a-PMMA. However, the conductivity of the gels is invariant to the ratio of i-PMMA and a-PMMA. Thus, a gel with a typical polymer content of 15-20 wt%, 1 M lithium salt, and a ratio of i-PMMA to a-PMMA of about 1:3 has an ionic conductivity in the range of 1 x 10(-3)-4 x 10(-3) S cm(-1) and possesses a dynamic elastic modulus one order of magnitude higher than the electrolytes containing only atactic PMMA. This modulus is obtained at values of the frequency between 0.01 and 100 radians, so that even under static conditions the gels do not flow and exhibit reversible elasticity to approximately 100% elongation. Thermal mechanical analysis and calorimetry of these gels show that the physical crosslinks formed by stereocomplexed syndiotactic and isotactic triads melt in the range of 65-85 degrees C depending on the nature of the solvent and lithium salt. The fact that these gel electrolytes are thermoreversible makes them readily processable. After melt casting they form transparent dimensionally stable, self supporting films. (C) 1999 Elsevier Science B.V. All rights reserved.