Presenter: Angelica Sanchez
Advisor(s): Saad Khan
Author(s): Angelica M. Sanchez and Saad A. Khan
Graduate Program: Chemical and Biomolecular Engineering

Title: Colloidal gels of fumed silica:  microstructure and temperature effects

Abstract: The interactions of fumed oxides with organic solvents, polymers and biological systems are of great interest as they can be utilized as viscosity modifiers, fillers or adsorbents when mixed in or in contact with such materials. Fumed silica FS, is of particular interest due to the large surface area that its branched structure provides. If these small particles are dispersed in an adequate medium, they form suspensions, flocculated systems or networks; thus an understanding of how to control this microstructure is of paramount importance. Extensive research involving FS dispersions has been conducted; recently, alternative novel applications such as fiber optic cables and composite polymer electrolytes, our major research effort in the past years, have gained especial importance. In this work, various types of FS have been dispersed in different oligoethers and their properties evaluated with the aim to finely tune them for improved performance during end use. Rheology, a reliable and readily available technique, has been employed to characterize the systems and study their microstructure while establishing correlations that can be subsequently employed to tailor the material for the particular application. We examine dispersions of hydrophobic and hydrophilic FS in oligoethers of different molecular weights and end group composition at different temperatures by using dynamic rheology. We observe that hydrophilic FS forms gels in the less polar oligoethers, whereas the hydrophobic one form a network in all the oligoethers employed. Increasing the temperature irreversibly increases the modulus of the system containing hydrophilic FS and the oligoether with the largest end group content. A “concentration” effect due to polymer adsorption and chemical reaction on the particles’ surface seems to explain this anomalous observation. Our results establish a new approach for improving the rheological properties of these materials while establishing the underlying mechanisms dictating such behavior.