A diverse range of applications, from organic electronic devices and sensors to coatings and patterning, rely on the properties and performance of polymers under various conditions of confinement, particularly thin film geometries. Generally, interactions between polymer chain segments and interfaces impose enthalpic and entropic penalties on the chain structure, and these effects may be manifested in properties at length scales beyond the size of a chain, up to tens of nanometers (nano-scale). Properties such as the viscosity, chain diffusion coefficients, D, glass transition temperatures, Tg, phase separation temperatures of mixtures and copolymers, as well as morphological instabilities are known to exhibit film thickness dependencies. To date, a comprehensive understanding of such size-dependent phenomena in polymers remains elusive. The foregoing provides the basis for three problems examined in this proposal. (1) One goal is to understand how interfacial forces influence the chain dynamics and to understand the connections between chain dynamics and the film thickness dependent changes exhibited by the Tg in homopolymer and in miscible polymer-polymer systems. The strategy of our proposed experiments is predicated largely on the notion that the glass transition is a manifestation of underlying dynamical features in the system. (2) We also plan to examine how the concentration and spatial organization of nanoparticles affect the morphological stability, chain diffusion and average Tg of polymer-based nanocomposite thin film systems. (3) Finally, we examine the role of interfacial energetics and intermolecular interactions on the late-stage (coarsening) structural evolution in unstable and metastable thin polymer films on substrates. Intellectual merit: In simple liquids, the relation between diffusion and viscosity is established within the context of the Stokes-Einstein equation. With regard to bulk long-chain polymeric melts, the Doi-Edwards theory describes the interrelation between diffusion and various viscoelastic processes exhibited by these materials; the temperature dependence is understood in terms of the WLF (or equivalently the Vogel-Fulcher) equation. In thin films, however, the situation is unclear. Through a series of experiments, guided by predictions based on simulations and theory, connections between the thickness dependent quantities are examined for thin film polymer-polymer mixtures. The second part of this proposal is devoted to developing an understanding of the influence of polymer/nanoparticle interactions on the glass transition and on dynamics (segmental dynamics and morphological instabilities) in thin film polymer-nanoparticle nanocomposites. Questions regarding the effect of interfaces, coupled with size-scale dependencies, are unresolved, though simulations suggest strategies to examine them. Our goal for the third problem, is the development of an understanding of the role of interfacial energetics and intermolecular interactions on late-stage coarsening processes in morphologically unstable and metastable thin , supported, polymer films. Broader Impact: This is an interdisciplinary program cross-cutting different fields, from physical chemistry of surfaces (wetting and self-organization), processing of thin films (morphological stability, viscosity, glass transition) to two dimensional coarsening phenomena (an issue also of interest for the processing thin films for microelectronics and is part of the broader ubiquitous phenomenon of coarsening). While it is true that we now understand a great deal about how to tailor properties of bulk polymers through blending etc. we are at a stage of infancy with regard to thin films. The thickness dependencies of the phase transitions, selfassembly and various morphological changes that result from the actions of intermolecular forces are endemic. An understanding of these issues will help us to develop new rules to design or to tailor properties of thin films for various applications. Clearly, the questions addressed in this proposal have scientific and technological implications. Students participating in this project develop an interdisciplinary background in areas that range from physics, materials and chemistry to engineering.
