Role of Polymer Adsorption on Extensional Rheology of Colloid-Polymer Mixtures

About This Grant

This project will study the flow behavior of suspensions consisting of polymers and micron-sized particles during extrusion through a nozzle. When there is no flow, polymer molecules tend to stick to the particles, form bridges between particles, and create a network of interconnected particles. The formation of the network, in turn, affects how the suspension flows through nozzles. This project will investigate the possibility of tuning the extent to which polymers stick to the particles, which will affect the number and shape of the polymer bridges and the flow properties of the suspension. This process represents a simple model of a variety of applications including fabrication of membranes for water purification, batteries for energy storage, and drug formulations to improve delivery. As a result, the project has the potential for significant scientific impact by advancing the science needed to process a class of feedstocks into useful materials. This project will also support the training of graduate students in advanced technical skills needed in a future science and engineering workforce. To communicate the practical importance of this project for society, demonstrations will be developed for the annual Houston Energy Festival for the public and for the STEM Zone Saturday event for K – 12 students and their parents. The objective of this award is to elucidate the effects of polymer bridges on the extensional flow properties of colloid-polymer mixtures. Using a model colloid-polymer mixture with attractive bridging interactions that are tunable through solution pH as well as polymer concentration and molecular weight, the overarching hypothesis that flow-induced conformational changes in the polymers will also affect the extensional flow of colloid-polymer bridging mixtures will be examined. This will be done by (1) linking extensional response of bridging mixtures to polymer macromolecular properties; and (2) determining the effect of normal stresses on extensional flow of bridging mixtures. Optical microscopy experiments will be used to characterize the structure of quiescent mixtures. Data from shear and extensional flow experiments will be used to test models and elucidate the contributions of polymer elongation and particle-induced stresses to the rheological properties. This award will examine the extent to which properties such as the extensional relaxation time and filament lifetime are controlled by flow-driven changes in polymer conformation, which is expected to vary as polymers adsorb to the colloids. The award will also address a longstanding open question about the relationship between normal stresses and extensional thickening, both of which arise from flow-induced polymer deformation. This knowledge is expected to lead to rational design principles to generate complex fluids feedstocks for materials processing routes that involve extensional flow. This award reflects NSF's statutory mission and has been deemed worthy of support through evaluation using the Foundation's intellectual merit and broader impacts review criteria.