Metal - polymer interfaces and their effect on the glass transition of thin polystyrene films
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Metal – polymer interfaces and their effect on the glass transition of thin polystyrene films J.S. Sharp School of Physics and Astronomy, University of Nottingham, Nottingham, U.K. NG7 2RD J.A. Forrest Department of Physics and Guelph -Waterloo Physics Institute, University of Waterloo, Waterloo, Ontario, Canada N2L 3G1 ABSTRACT We present a study of polystyrene-metal interfaces and discuss the relationship between the interfacial structure and anomalies in the measured glass transition temperature (Tg) of thin metal capped polystyrene (PS) films. The PS films used in these studies were coated with an evaporated metal layer of either Aluminum (Al) or gold (Au) and the Tg values were measured with ellipsometry. Uncoated PS films were also measured and these samples showed Tg values that were reduced relative to the bulk value for film thicknesses (h) less than 40 nm. Films coated with Au were shown to have measured Tg values that were the same as the bulk value (Tgbulk=370 K) for all the film thicknesses studied (h ≥ 8nm). The Al coated PS films had measured Tg values that were the same as the uncoated PS films. The observed differences are discussed in terms of the differences in the structure of the metal-polymer interfaces produced during thermal evaporation of the metal layers. A novel sample preparation procedure was developed to enable us to use Atomic Force Microscopy (AFM) to directly measure the structure of the buried polymer-metal interfaces. The measurements performed on these systems support the suggestion that the interfacial structure is different for the two metal-polymer interfaces studied and that these differences may be the cause of the anomalies in the measured Tgs of these samples. EFFECT OF INTERFACE ON Tg VALUE The dynamical properties of thin polymer films have generated a significant amount of interest. In particular, a large number of studies have been performed to characterize the film thickness dependence of the measured Tg value of thin PS films [1]. It is well established that the Tg of thin PS films is reduced below the bulk value for films thinner than 40 nm [1]. Significant efforts have been made to develop models and theories in an attempt to try to explain these fascinating observations [1-5]. A detailed study of free standing films provided evidence for a rich phenomenology including evidence for chain confinement effects at very large values of Mw [6,7]. The results of the experiments performed on free standing PS films showed a remarkable correspondence with the measured Tg values of PS films supported on a variety of different substrates. In particular, it has been shown that the measured Tg value of a free standing film of thickness h is the same as that of a supported film of thickness h/2 [2]. This correspondence supports the assertion that the surface to volume ratio of thin film samples has a strong influence on their measured Tg values. The idea that there is a free surface region in polymer films with enhanced dynamics that contributes to the observed
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