Obtained ionic conductivities >10^-3 S/cm, two order of magnitude higher than typical polymer electrolytes. NMR and electrochemistry studies indicate little interaction between the self-supporting fumed silica network and the highly mobile cations and anions. As such, the transport of ions is determined by the lithium salt and PEG combination and is relatively independent of the filler.
Found a direct correlation between dynamic rheology and colloidal interaction parameters for systems containing alkyl-tethered silica dispersed in (salt free) polar media. We have been able to derive, for the first time, a scaling relationship between the elastic modulus and the solubility parameter mismatch between the tethered chain and solvent. This level of comprehensive understanding for these complex systems should enable ways to a priori predict mechanical properties.
Found new evidence for solvation forces dictated by hydrogen bonding in dispersions of hydrophilic fumed silica in a variety of (salt free) polar organic media. In strongly hydrogen-bonding liquids, a solvation layer is envisioned to form on the silica surface through hydrogen-bonding between liquid molecules and surface silanol groups (Si-OH). This stabilizes the silica particles. In contrast, in the case of liquids with limited hydrogen-bonding ability, silanols on adjacent silica particles are thought to interact directly by hydrogen bonding. This leads to particle flocculation and ultimately to gelation.
Investigated the interaction of fumed silica filler, PEG, and lithium salts and the resulting rheological characteristics. The rheology of this system is primarily a function of filler volume fraction following established power-law relationships for the elastic modulus, G' ~ fn and yield stress, ty~ fp. Addition of salt slightly increases gel properties (G' and ty). Although salt concentration is important, the salt type (which is critical to ion transport) is unimportant. Addition of salt to the solvating medium appears to increase the polarity of the medium leading to an increase in gel modulus reminiscent of the relationship developed for salt-free systems.
Investigated wall slip in colloidal gels. When geometries with smooth surfaces are used, wall slip occurs at high stress/strain amplitudes as a result of formation of a particle-lean layer. Formation of slip layer can be controlled by modifying the surface energy of the wall. A surface with a hydrophobic PDMS layer has improved association with the hydrophobic fumed silica particles resulting in a decrease in the observed wall slip (see figure below).
Polymer electrolytes are becoming increasingly important because of their potential use in rechargeable solid-state lithium batteries. However, the successful use of these materials requires them to be mechanically strong, yet be processible and have high conductivities. Our novel approach of making composite polymer electrolytes using fumed silica nano-fillers (see figure below) in low molecular weight polyethylene glycol (PEG) (and its derivatives) is capable of simultaneously satisfying all of these criteria. Key to the success of our method are: (i) the unique rheological behavior of fumed silica; i.e. it can form strong network structures that are also processible, and, (ii) the use of low molecular weight PEG which allows for high conductivity.
