Dynamic Processes at the Nanoscale in Polymer Systems

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A diverse range of applications, from organic electronic devices and sensors to coatingsand patterning, rely on the properties and performance of polymers under various conditions ofconfinement, particularly thin film geometries. Generally, interactions between polymer chainsegments and interfaces impose enthalpic and entropic penalties on the chain structure, and theseeffects may be manifested in properties at length scales beyond the size of a chain, up to tens ofnanometers (nano-scale). Properties such as the viscosity, chain diffusion coefficients, D,glass transition temperatures, Tg, phase separation temperatures of mixtures and copolymers, aswell as morphological instabilities are known to exhibit film thickness dependencies. To date, acomprehensive understanding of such size-dependent phenomena in polymers remains elusive. The foregoing provides the basis for three problems examined in this proposal. (1) Onegoal is to understand how interfacial forces influence the chain dynamics and to understand theconnections between chain dynamics and the film thickness dependent changes exhibited by theTg in homopolymer and in miscible polymer-polymer systems. The strategy of our proposedexperiments is predicated largely on the notion that the glass transition is a manifestation ofunderlying dynamical features in the system. (2) We also plan to examine how the concentrationand spatial organization of nanoparticles affect the morphological stability, chain diffusion andaverage Tg of polymer-based nanocomposite thin film systems. (3) Finally, we examine the roleof interfacial energetics and intermolecular interactions on the late-stage (coarsening) structuralevolution in unstable and metastable thin polymer films on substrates. Intellectual merit: In simple liquids, the relation between diffusion and viscosity isestablished within the context of the Stokes-Einstein equation. With regard to bulk long-chainpolymeric melts, the Doi-Edwards theory describes the interrelation between diffusion andvarious viscoelastic processes exhibited by these materials; the temperature dependence isunderstood 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 basedon simulations and theory, connections between the thickness dependent quantities are examinedfor thin film polymer-polymer mixtures. The second part of this proposal is devoted todeveloping an understanding of the influence of polymer/nanoparticle interactions on the glasstransition and on dynamics (segmental dynamics and morphological instabilities) in thin filmpolymer-nanoparticle nanocomposites. Questions regarding the effect of interfaces, coupled withsize-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 interfacialenergetics and intermolecular interactions on late-stage coarsening processes in morphologicallyunstable and metastable thin , supported, polymer films. Broader Impact: This is an interdisciplinary program cross-cutting different fields, fromphysical 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 thebroader ubiquitous phenomenon of coarsening). While it is true that we now understand a greatdeal about how to tailor properties of bulk polymers through blending etc. we are at a stage ofinfancy with regard to thin films. The thickness dependencies of the phase transitions, selfassemblyand various morphological changes that result from the actions of intermolecular forcesare endemic. An understanding of these issues will help us to develop new rules to design or totailor properties of thin films for various applications. Clearly, the questions addressed in thisproposal have scientific and technological implications. Students participating in this projectdevelop an interdisciplinary background in areas that range from physics, materials and chemistryto engineering.