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 (Tgs) in free-standing polystyrene (PS) films are determined via a multilayer/self-referencing fluorescence method employing a pyrene dye label. Average film Tgs yield a Tg-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 Tg = Tg,bulk – ∼34 K, independent of film thickness and indicative of a strong Tg gradient near a surface. In sufficiently thin films (≤∼56 nm), a 14 nm thick free-surface layer reports Tg that decreases with decreasing film thickness and is equal to the Tg of a 14 nm thick middle layer and the average film Tg. Thus, the strongly perturbed Tg at the two surfaces affects Tg several tens of nanometers into and across the film, resulting in greater Tg reductions than observed in supported films. This study also tests de Gennes' "sliding motion mechanism", devised to explain the MW dependence of the Tg-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 Tg reductions occur only at locations where segments are present from chains forming loops or bridges at a surface. Major Tg reductions (as large as ∼54 K below Tg,bulk) are observed in midlayers of nanoconfined free-standing PS films, disproving a key premise of the mechanism.