Tunability of Glass Transition Temperatures in Nanoscopically Confined Polymer Systems: Impacts of Length Scale, Surfaces and Interfaces.

摘要

Over the past seventeen years, many studies have investigated the effect of nanoconfinement on the glass transition temperature (T g) of polymers, especially in polystyrene (PS). There is general agreement among these studies that Tg decreases with decreasing thickness for supported PS films. This behavior originates from an enhanced mobility layer at the free surface relative to bulk polymer. Many related studies have been inspired by the fact that a range of future technologies will require the use of polymeric materials that are confined at characteristic lengths of ∼100 nm or less.;This dissertation is focused on obtaining a better fundamental understanding of how surfaces, interfaces, and nanoconfinement alter T g and related relaxation behavior. Novel self-referencing fluorescence techniques were devised to study confinement effects on T g and physical aging behavior in thin and ultrathin polymer films, especially free-standing films. Such studies showed that there is a greater Tg reduction with confinement in PS free-standing films compared to substrate-supported films due to the free-surface effect which propagates into the film interior from both film surfaces. This concept was experimentally proven by measuring local T gs at specific locations within the free-standing films.;The work also demonstrated new, large-magnitude nanoconfinement effects which may yield material properties with tunable Tg dynamics that cannot be obtained in conventional materials. It was shown that the Tg of ultrathin PS films can increase with decreasing film thickness when the films are placed atop immiscible polymer layers with higher Tgs. This is the first study to document increases in PS Tg with nanoconfinement. This study indicates that the T g dynamics in nanoconfined polymers can be coupled to the dynamics of neighboring immiscible polymers over many tens of nanometer length scales.;Lastly, a strategy to control confinement effects was further developed. In particular, the Tg-confinement effect in dry poly(vinyl acetate) films was suppressed by the presence of very low levels of sorbed water. This suppression is hypothesized to arise from a connection between the size scale of cooperative dynamics associated with Tg, which decreases with sorbed water, and the size scale of the nanoconfinement effect.