Structural Evolution and Glass Transition Behavior of Adsorbed Polymer Nanolayers as Modulated by Interfacial Interactions

Abstract

Incorporation of a second phase into a polymer system can be used to enhance local structure and properties, enabling useful technologies with applications in healthcare, energy, the environment, and myriad commodity products. In systems such as polymer nanocomposites and supported thin films, the large specific interfacial area can elicit significant deviations from bulk behavior of the overall polymer system. However, the ability to precisely anticipate and control polymer properties through incorporation of interfaces hinges upon a comprehensive understanding of the complex phenomena therein.One such phenomenon is polymer adsorption, wherein segments within a polymer chain physically adhere to an adjacent substrate. Polymer melts typically undergo adsorption in response to heating above the glass transition temperature Tg, corresponding to common processing conditions for polymer systems. Once regarded trivially as “dead,” adsorbed layers are now recognized as capable of undergoing glass transitions and relaxations. However, the unknown details of their structure, properties, and evolution are an obstacle to control over glassy properties near interfaces. This dissertation details progress in elucidation of the structure-property-processing relationships within adsorbed polymer nanolayers. Building upon previous work investigating planar adsorbed layers of simple polymer systems, this work leverages a combination of new techniques for sample preparation and direct characterization of adsorbed layer structure and adsorbed layer Tg, which enables new insights into the influence of chemical and geometrical factors in governing interactions at an adsorbed interface. To ascertain the evolution of adsorbed layer structure and Tg at the polymer matrix-nanoparticle interface in polymer nanocomposites, we developed a model approach for isolating adsorbed layers from a polymer nanocomposite via Guiselin’s experiment. Adsorbed layer structure and evolution were characterized by direct visualization via transmission electron microscopy (TEM) under cryogenic conditions, and the evolution of Tg was characterized via fluorescence spectroscopy. Analogous in-situ experiments wherein the nanoparticles with adsorbed layers were redispersed within a polymer matrix enabled a characterization of the factors influencing Tg characterization in polymer nanocomposites. Both adsorbed layer structure and Tg were found to co-evolve and to be annealing-time dependent. Follow-up work leveraged our model approach to ascertain the role of nanoparticle size and curvature on adsorbed layer structure. TEM imaging revealed that the structure of adsorbed layers was dependent upon nanoparticle size, with larger nanoparticles favoring a greater degree of chain flattening. Finally, we investigated the role of interaction strength in the evolution of adsorbed layer Tg. Analogous polystyrene and poly(methyl methacrylate) planar adsorbed layers on silica—corresponding to weakly- and strongly-interacting pairings, respectively—were characterized via fluorescence spectroscopy and ellipsometry. Tg was found to co-evolve with structure in both polymers. Perhaps surprisingly, the adsorbed layer Tg was reduced from that of the bulk in both weakly- and strongly-interacting pairings, however the magnitude of deviation from bulk Tg varied with interaction strength. The findings presented herein are offered as a springboard for future work elucidating fundamental structure-property-processing relationships at interfaces featuring adsorption, which inform the design of useful materials.