Surfactant Mobilization of Adsorbed Polymer and its Effect on the Severity of Co-Adsorption Hysteresis

Carnegie Mellon University, Pittsburgh PA

Investigators

Abstract

Project Abstract Tilton, r., Washburn,N; Carnegie-Mellon Univ.; CTS-0625135 Intellectual Merit. Two observations motivate the proposed research: i) that polymers often reside in persistent hysteretic states at the solid/liquid interface whereby the extent of adsorption and the polymer conformation are sensitive to the adsorption history and ii) that the signs of hysteresis are sometimes increased, but sometimes decreased, when polymers adsorb in the presence of surfactants that bind to the polymer but are repelled from the solid surface. The primary objective is to determine the mechanism by which surfactant binding controls the dynamics of adsorbed polymers and thus their ability to sample different conformations and their susceptibility to hysteretic co-adsorption. The state-of-the-art in polymer/surfactant co-adsorption research is that although the literature contains studies of an increasing number of experimental systems, no systematically designed experimental study has been performed that tests any hypothesis for the origin and control of hysteretic co-adsorption, and no theoretical model has been developed to explain the phenomenon. The project will provide the systematically designed experimental benchmarks that are needed for the development of appropriate theory. While the emphasis of the proposal is mainly on the experimental studies, kinetic Monte Carlo simulations will be developed and compared to trends in the data. The particular focus of the kinetic MC simulations is to mimic experimental hysteresis measurements by recording the dynamics of adsorbed layer response to a step-change in solution composition. The hypothesis underlying the proposed research is that bound surfactants solubilize and mobilize polymer "train" segments, thereby decreasing hysteresis. In order to access and mobilize train segments, surfactants must surmount the repulsive energy barrier from the solid surface. The following measurements will be made in model systems with systematically varying polymer/surfactant binding energies and surfactant/surface repulsive energies: i) extent of adsorption from polymer surfactant mixtures via different adsorption pathways, ii) surfactant influence on the extent and kinetics of exchange between adsorbed polymers and polymers entering from the bulk solution, and iii) surfactant influence on adsorbed polymer surface diffusion coefficients. Broader Impact. The research project will broadly benefit industrial complex fluid formulation practices. Mixed self-assembly and co-adsorption of polymers and surfactants exert powerful controls over the macroscopic properties of complex fluids. These are found in the manufacture and application of materials such as pharmaceuticals, ceramics, paints, inks, and other advanced coatings. Polymer /surfactant mixtures exert strongly coupled forces that fine tune material properties, but with the unfortunate consequence that these materials are particularly difficult to formulate. Formulation difficulties are exacerbated by the prevalence of hysteresis effects, where the final material properties are sensitive to the order of component addition and mixing conditions. By determining the origins of hysteretic co-adsorption, this project will enable new complex fluid formulation methods that will lead to improved products and processes over a spectrum of industries. The project includes university, industrial, and K-12 educational activities. The PI will develop a laboratory module on Ellipsometric Measurement of Surfactant Adsorption" for a new Advanced Colloid and Surface Characterization university course. A concise lecture on theory, practice, and interpretation of ellipsometry will be offered together with the lab module as part of the industrial short course Essential Instrumental Methods for Colloid, Interface, and Complex Fluid Characterization. The team will collaborate with Carnegie Mellon's Engineering Your Future program that brings female high school students onto campus for a day of hands-on activities illustrating engineering in everyday life.