Polyelectrolytes are large water-soluble molecules that contain electric charges. When water solutions of positively and negatively charged polyelectrolytes are mixed together, complexes are often formed that have either liquid-like or solid-like properties. The ability to tailor these properties has led to their use in a variety of applications, ranging from personal care to industrial waste processing and water treatment. This project is aimed at understanding how the relevant properties of these materials originate from the detailed structure of the components from which they are formed. This understanding will be generated by developing a series of well-characterized model materials systems, and studying their mechanical properties with several experimental techniques. In addition, new processing methods will be developed that enable polyelectrolyte complexes to easily be coated onto different material surfaces. The characterization methods include the use of high frequency sound waves to probe the material response. This technique is widely applicable to a variety of coatings with both protective and aesthetic functions. The project is relevant to membranes for water filtration and includes education and research training of students, broadening participation, and outreach activities.
Polyelectrolyte complexes formed by the interaction of oppositely charged macromolecules are an important class of soft, polymeric materials. These materials are of interest largely because of their mechanical and transport properties. The mechanical properties can span the full spectrum of behaviors from low-viscosity liquids to tough viscoelastic materials to brittle solids, in a manner that can be reversibly controlled through changes in the salt concentration or pH. The primary aim of this project is to understand the factors that control this behavior using well-characterized model systems. A secondary aim is to use this information to develop surface modifications to enhance the performance of membranes used for water purification. The focus of the project is on polyelectrolyte complexes in thin film form, both because of the utility of these materials as surface modifiers, and because the thin film geometry is particularly convenient for the proposed investigations. There are three aspects of these investigations, beginning with new deposition mechanisms based on the electrochemical control of the pH at the surface of interest. The second set of experiments is aimed at mapping out the phase behavior of these materials, including the relationship between equilibrium water content of a film and the salt concentration of the aqueous medium with which it is in contact. The third element of the proposed program is the most extensive, and involves mechanical characterization of the polyelectrolyte complex films. Acoustic methods will be used to characterize the linear viscoelastic properties of these materials on a time-scale of about 60 nanoseconds, approaching the timescale that is accessible by molecular dynamics simulations, bridging the gap between experiment and computational modeling. In addition, the nonlinear properties of these materials will be investigated using creep and fracture experiments designed specifically for investigations of thin films in the hydrated state.