Composite polymer electrolytes based on poly(ethylene glycol) and hydrophobic fumed silica: Dynamic rheology and microstructure

Dynamic rheological techniques are used to probe the microstructures present in fumed silica-based composite polymer electrolytes. These electrolytes are obtained by dispersing hydrophobic fumed silica in a poly(ethylene glycol)-lithium salt solution and display high conductivities (sigma(298K) > 10(-3) S/cm), mechanical stability, and easy processability. The materials behave as soft solids (gels) under ambient conditions due to the presence of a three-dimensional network of silica entities. Network formation occurs as a result of van der Waals (dispersion) forces between the nonpolar surface layers on silica units. Factors which affect the van der Waals interaction, and hence the gel network density, include the nature of the PEG end group, the presence of ionic species, and the size of the hydrophobic surface group on the silica. The composites also exhibit shear-thinning behavior due to the shear-induced disruption of network bonds, and this behavior can be advantageously utilized in electrolyte processing.