Melt and Glassy Dynamics in Complex Polymer Systems: Miscible Blends and Star-Shaped Polymer Films.

Abstract

Considerable effort has been given towards using polymers as active materials in many types of organic devices such as photovoltaics, active membranes and sensors as well as drug delivery. A number of the applications of polymeric materials involve thin film geometries; therefore the role of interfacial interactions has garnered a lot of interest of late. However, the influence of interfaces on the physical properties of polymer films in not well understood. One such property is physical aging, which leads to a time-dependence of many properties, including: increased brittleness, enhanced modulus and reduced permeability. Therefore, there is a strong technological and scientific importance to understanding the underlying phenomenon behind this process. In this dissertation three classes of polymers are used in order to investigate the role of interfaces on the dynamic properties of polymer thin films: (1) glassy structural relaxation in star-shaped polymers, (2) free surface dynamics of miscible A/B polymer/polymer blends, and (3) polymer nanocomposites. The structural relaxation rate of polystyrene in the bulk and micron thick films is strongly influenced by the chain architecture. Due to the decreased conformational freedom, and entropic constraints of the chains grafted to a central core exhibited by the star-shaped molecules, the stars exhibit up to a 40% reduction in the relaxation rate relative to their linear counterparts. In the case of supported thin films, the thickness dependence of the structural relaxation rate can be described with a universal model that accounts for to the temperature dependence of the structural relaxation rate as well as the distribution in local Tg throughout the film. In a miscible A/B polymer blend and miscible blend nanocomposites, the local surface composition can vastly differ from the bulk. The lower surface tension of the B component will lead to a surface excess. Because of the differences in the composition near the surface, and the reduced friction coefficient in the vicinity of a particle, the polymer chain dynamics near that interface can be vastly different from that of the bulk. All of these findings have implications in the processing and long term implementation of polymer films in many different applications.