Distribution of glass transition temperatures in free-standing, nanoconfined polystyrene films: A test of de Gennes' sliding motion mechanism

Effects of nanoscale confinement on the distribution of glass transition temperatures (T_gs) in free-standing polystyrene (PS) films are determined via a multilayer/self-referencing fluorescence method employing a pyrene dye label. Average film T_gs yield a T_g-confinement effect in agreement with the molecular weight (MW) dependence reported by Forrest, Dalnoki-Veress, and Dutcher. Multilayer films, with one pyrene-labeled layer, reveal that a 14 nm thick free-surface layer in sufficiently thick films (≥∼56 nm) exhibits T_g = T_g,bulk - ∼34 K, independent of film thickness and indicative of a strong T_g gradient near a surface. In sufficiently thin films (≥∼56 nm), a 14 nm thick free-surface layer reports T_g that decreases with decreasing film thickness and is equal to the T_g of a 14 nm thick middle layer and the average film T_g. Thus, the strongly perturbed T_g at the two surfaces affects T_g several tens of nanometers into and across the film, resulting in greater T_g reductions than observed in supported films. This study also tests de Gennes' "sliding motion mechanism", devised to explain the MW dependence of the T _g-confinement effect in free-standing films. No midlayer chain in a multilayer film forms loops or bridges reaching a surface. de Gennes' mechanism indicates that T_g reductions occur only at locations where segments are present from chains forming loops or bridges at a surface. Major T _g reductions (as large as ∼54 K below T_g,bulk) are observed in midlayers of nanoconfined free-standing PS films, disproving a key premise of the mechanism.