Estimation of the Thickness Dependence of the Glass Transition Temperature in Various Thin Polymer Films

Abstract

The glass transition temperature (Tg) in the thin poly(α-methyl styrene) and polysufone films that are coated on the surface of a Si wafer is investigated in this paper. A novel equation was suggested to estimate the thickness dependence of Tg in thin films. Ellipsometry has been used to measure Tg of various thin films as functions of sample thickness, molecular weight, and kind of polymer. One adjustable parameter in the equation, i.e., ξ, was obtained from fitting this function with the experimentally measured Tg. The magnitude of the fitted parameter ξ showed discrepancies between the polymer types but similarity between the molecular weights. This parameter ξ corresponded to the statistical segment length of the polymer. All the measured Tg data of the investigated polymers were superimposed in one curve by normalizing the thickness and the measured Tg with the parameter ξ and bulk Tg, respectively. The origin of Tg depression phenomena in thin films was closely correlated with its segmental length and not with whole chain size.