Interfacial effects and the glass transition in ultrathin films of poly(tert-butyl methacrylate)

Glass transition studies were performed on ultrathin spin cast films of poly(tert-butyl methacrylate) (PtBMA) with thickness values in the range 6-415 nm. Ellipsometry was used to measure the dilatometric properties of uncapped PtBMA films as well as films that had been capped with 5 nm thick evaporated Aluminum (M) layers. Al-capped films were also prepared using a recently developed 2(h/2) sample preparation procedure that is designed to completely remove the effects of the free polymer surface. These studies revealed that the glass transition temperature (T-g) in uncapped PtBMA films is reduced from the bulk value (383 K) by similar to 28 K for films as thin as 6 nm. Similar T-g depressions were also reported for Al-capped films that were prepared by thermally evaporating Al directly on to the PtBMA films. However, films that were prepared using the 2(h/2) sample preparation procedure displayed bulk like T(g)s for films as thin as 6 nm. These experiments illustrate that evaporating metal coatings on polymer films does not necessarily remove the effects of the free surface, but that if care is taken to ensure that the properties of the free surface are removed, the thickness dependence of the T-g in thin polymer films can be suppressed. These combined experiments provide strong evidence that free polymer surfaces are responsible for the T-g reductions that are routinely observed in ultrathin polymer films.