Probing the distribution of glass transition temperatures and related relaxation behavior in thin and ultrathin films using novel fluorescence approaches

Since the discovery a decade ago that the glass transition temperature (Tg) of ultrathin polymer films may vary by more than 20 K from bulk Tg, substantial research has been focused on understanding how nanoscopic confinement alters Tg. The deviation from bulk Tg as polymer is "nanoconfined" has technological impact in applications such as asymmetric membranes for gas separation, photoresists for the production of microelectronics, and nanocomposites, among others. Although many techniques (e.g. x-ray reflectivity, ellipsometry) have been used to characterize Tg in nanoconfined films, they are limited to measuring average film properties and provide little direct evidence for the influence of interfacial effects.; In this dissertation a new method was developed in which the emission of fluorescence probes is shown to be well suited for identifying Tg in single layer films and in specific locations of films including those present near the free-surface interface, polymer-substrate interface, and in between. Such studies show for the first time that the free-surface Tg of a polystyrene (PS) film is dramatically reduced and that this free-surface effect persists into the film interior over several tens of nanometers where the resulting gradient or distribution in Tgs near the free surface is observed to depend strongly on the degree of nanoconfinement. These results provide the first direct evidence that the origin of the effect of nanoconfinement on Tg is primarily a result of interfacial effects and offer new understanding into how the average Tg of the entire film can result from a distribution in Tgs near interfaces and in between.; The work presented here also stands as the first demonstration that the thickness dependence of Tg in nanoconfined single layer films may be significantly "tuned" via the addition of low molecular weight diluents or by small modifications to the repeat unit structure of PS. These surprising results may be explained by an alteration in the magnitude of the perturbation on Tg at the free surface and/or a reduction in the ability of interfacial effects to propagate into the film interior, which is argued to be related to the level of cooperativity of the polymer.