Nanoscale thin films formed via polyelectrolyte adsorption find important applications as separation membranes, bio- and chemical sensing substrates, and cell-contacting biomaterials. Substrate electric potential is an important control variable, and while studying it influence, the investigators recently made an interesting discovery: the adsorption of certain polycations may become continuous, i.e. asymptotically linear (or nearly linear) in time over hours, upon the application of a modest anodic potential. Continuous adsorption under an applied electric potential, not previously observed, offers exciting technological possibilities while raising fundamental questions: . For what systems, and under what conditions, does continuous adsorption occur? . How do the physico-chemical properties of films formed by continuous adsorption depend on the control variables (voltage, pH, salt, molecular weight)? . What is the mechanism of continuous adsorption?
Preliminary work by the PI has identified systems and conditions where continuous adsorption occurs. In order to understand the dependence of film properties on formation conditions, and thus ultimately to engineer polyelectrolyte films under electric potential control for important applications, the PI will conduct an experimental and computational study toward a mechanistic understanding of continuous polyelectrolyte adsorption under an applied potential. Specifically, the project employs atomic force microscopy (AFM), optical waveguide lightmode spectroscopy (OWLS), X-ray photoelectron spectroscopy (XPS), and molecular simulation to determine the evolution of polymer layer structure, charge, and electrochemical state and to understand these findings in terms of molecular-level events.
Broader Impact: One may envision continuous polyelectrolyte adsorption under an applied electric potential to have a broad impact in several areas of technology including sensing, catalysis, and bioelectronics. Many applications require polyelectrolyte nanofilms of precision thickness and composition, and continuous adsorption easily allows for film growth to any desired level, in a single step and with a single polymer component. In contrast, standard monolayer adsorption is limited to rather thin layers, and layer-by-layer assembly (where thicker layers are possible) requires many steps and at least two components. Certain unique polymer layer characteristics may also be possible. The Pis speculate continuous adsorption to result in a very dense region, with a narrow pore size distribution, near to the substrate, in principle ideal for separations. The single component nature of the films could enable superior ionic conducting membranes, since (unlike layer-by-layer films) all of the polymer's charged sites would be available to coordinate with mobile ions. The educational experiences brought forth by this project will also have a broad impact. Two full-time PhD students will conduct experimental and computational work toward the stated objectives. Undergraduate participation will include at least one student from the STARS program at Yale, which supports minority and female students. Further impact will result from an outreach effort to a New Haven public high school. The Pis propose a laboratory experience, involving polyelectrolyte adsorption experiments, for a local high school student participating in the SCHOLAR program at Yale, a residential experience including classroom and laboratory offerings. Finally, the project proposes to develop a course module on polymer coatings, consisting of a series of lectures and experiments, to be included as part of existing undergraduate and graduate courses in colloids and materials science.
