Morphology, modulus, and conductivity of a triblock terpolymer/ionic liquid electrolyte membrane

Block polymers are ideally suited for polymer electrolytes exhibiting both high ionic conductivity and superior mechanical robustness because they self-assemble into well-defined nanostructures. Network morphologies exhibiting long-range continuity of both the mechanically robust and conductive domains maximize the macroscopic composite properties but are difficult to achieve in commonly studied diblock copolymer systems. We therefore investigated a polymer electrolyte comprising the triblock terpolymer poly[isoprene-b-(styrene-co- norbornenylethylstyrene)-b-ethylene oxide] (INSO) and the ionic liquid 1-ethyl-3-methylimidazolium bis(trifluoromethylsulfonyl)imide (EMITFSI) because INSO self-assembles into equilibrium network morphologies in which each domain is continuous throughout the sample. Small-angle X-ray scattering and transmission electron microscopy revealed the bulk morphology of INSO to be the O⁷⁰ network prior to cross-linking. The material remained microphase-separated but without long-range order after solvent-casting from dichloromethane, a morphology that was retained after cross-linking and the addition of ionic liquid. Cross-linking had the effect of increasing the elastic modulus by 4 orders of magnitude, from 10⁴ to 10⁸ Pa, and importantly, a high modulus was retained well above the T_g of linear polystyrene. The conductivity was somewhat lower than that expected for a heterogeneous electrolyte, but the results suggest that refinements to the solvent-casting procedure could increase connectivity of the conductive domain and thus macroscopic conductivity.