Nonlinear Growth of Polyelectrolyte Multilayers: Chain Dynamics and Film Structure
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PART I: NON-TECHNICAL SUMMARYThis project focuses on controlling the structure of functional polymer coatings. Such coatings could be used, e.g., for biomedical or optical applications. Examples might be coatings that can deliver drugs locally in a multi-stage, time-resolved protocol, or structured polymer films that can serve as precursors for better anti-reflective optical coatings. To support these advanced applications, (a) the coatings should be able to be easily deposited on a variety of surfaces via an environmentally friendly process, and (b) their structure should be easily controlled by the deposition procedure and conditions. The layer-by-layer (LbL) technique is a unique method which, through a deposition process entirely in water can create conformal coatings of controlled thickness on virtually any surface. The challenge, however, is to control the mixing of polymer molecules within the coated film to preserve film layering. This project will explore the effect of various molecular and deposition parameters on film layering, with the goal of achieving structured films which can be deposited through fewer steps. Advanced instrumental techniques will be used to take snapshots of how multilayered structures, water content, and film mechanical properties can be manipulated.Importantly, this project will create a fertile training ground for the participating graduate, undergraduate and high-school students. The PI is currently the academic advisor of the "Women in Materials Science" (WIMS) organization, which is strongly involved in many outreach activities, ranging from on-campus tours and demonstrations for Girl Scouts and high-school students to visits to local schools with the goal of encouraging and engaging female and minority 5-7th graders to pursue careers in science and engineering.PART II: TECHNICAL SUMMARYThe ability to control dynamics and the structure of layer-by-layer (LbL) polyelectrolyte films lies at the heart of advanced biomedical and optical applications of polyelectrolyte multilayer (PEM) films. Of specific interest are films with defined and controlled internal stratification. Yet nonlinearly growing LbL films (NL-LbLs) which deposit at larger thicknesses and therefore are highly desirable for many applications, suffer from molecular intermixing. This proposal aims to (a) uncover the mechanism of polymer chain diffusion within NL-PEMs during film growth, (b) establish correlations between molecular interactions, dynamics, and order for a broad range of NL-PEM films and (c) use this knowledge to control structure of NL-PEMs and develop strategies for constructing stratified and gradient LbL films. This project will involve synthesizing well-defined polybase-polyacid (PB-PA) pairs associated through ionic pairing or hydrogen-bonding. Interchain dynamics will be studied during PEM assembly, as well as at a post-assembly step. Neutron reflectometry (NR) will be used to track diffusion of polymer chains across the film thickness, in situ ellipsometry to determine film water content, and nanoindentation to explore film mechanical properties during film construction. At the post-assembly step, NR will be applied to resolve internal stratification, and fluorescence recovery after pattern photobleaching (FRAP) to study lateral mobility of assembled chains. This knowledge will then be used to rationally construct films with programmable density, water content, charge balance, and permeability. It will also enable rationally designed diffusional barriers and gradients of water content within NL-PEM films, in order to achieve films useful as matrices for conformal antireflective coatings or for sequential drug delivery via improved control of permeability of functional molecules.