My research interests focus on the rheology of structured polymeric systems, particularly the relationship between material microstructure, chemistry, and macroscopic properties. The use of rheology with other techniques provides a powerful and unique combination to understand and describe the macroscopic behavior of structured systems in terms of their underlying chemistry and/or microstructure. My goals have been to use such an approach to obtain physical insights and new information on systems that are technologically or environmentally relevant. Summaries of some of the group research projects are provided below.
Colloidal silica gels have tremendous potential as novel composite polymer electrolytes because of their mechanical stability, processability and high conductivity. Our effort focuses on determining shear-induced microstructural changes in flocculated fumed silica gels, and relating them to fractal dimensions. We also obtain scaling relationships (analogous to the Cox-Merz rule) between steady and dynamic shear for non-flocculated systems, and correlate colloidal interactions with rheology.
UV cross-linked polymers are increasingly being considered for a wide range of applications because of their environmentally-benign, solvent-free nature and their rapid (on-line) curing speed. We have developed new techniques to continuously monitor the cross-linking behavior of UV curable systems in situ in the rheometer and using FTIR spectroscopy. This enables us to correlate rheology with extent of reaction, and investigate the effects of temperature and monomer functionality on gel point, fractal dimension, reaction order, gelation mechanism, and kinetics of model polymers.
CO2-induced plasticization of polymers offers a novel route to enhance polymerization and processability and develop new systems. First, polymer swelling in the presence of high-pressure CO2 is being examined. Kinetic swelling experiments can be manipulated to provide information about CO2 solubility, diffusion coefficients, and free-volume expansion. Secondly, a high-pressure rheometer is being designed to measure viscosity of polymeric melts with dissolved high-pressure CO2. The ultimate objective is to correlate the swelling and rheological behavior of polymer melts in order to obtain a comprehensive understanding of CO2-induced plasticization in polymer melts.
Hydrophobically modified associative polymers (HASE) and polymer/surfactant complexes are of significant interest because of their potential use in many applications (e.g., coatings, flocculants for waste-water treatment). These materials exhibit a multitude of unusual phenomena with respect to physically entangled systems (sol-gel transition, shear thickening followed by extreme shear thinning, strain hardening), the underlying mechanisms of which remain poorly understood. Using rheology together with microscopy, we are probing the network topology and mode of chain coupling in these systems under different types of deformation, as well as the phase behavior and interaction mechanisms for surfactant/polymer interactions.
Enzymatic modification of water-soluble polymers, such as guar galactomannans, offer a novel and powerful way to develop polymers with tailored architecture and properties. These polymers can be used in application ranging from food additives to oil/gas production. We are studying the kinetics and mechanism of enzymatic hydrolysis using rheology, size exclusion chromatography and mathematical modeling. Of particular interest is the correlation between molecular changes and macroscopic properties as a function of temperature, enzyme concentration and type (side chain vs. main backbone cleavage). We are also interested in developing and characterizing novel blends of enzymatically modified guar with other polysaccharides.
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