Dielectric Behavior of Glass Transition and Dynamics in Thin Polymer Films

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Glass transition temperature (Tg) and dynamics of thin films of polystyrene (PS) and poly(methyl methacrylate) (PMMA) are investigated using dielectric relaxation spectroscopy and related methods. In case of thin films of polystyrene, the glass transition temperature could successfully be obtained as a function of film thickness and the obtained results are consistent with those observed using other techniques such as ellipsometry. In accordance with the decrease in the glass transition temperature in thin films, the dynamics of the α-process becomes faster with decreasing film thickness and the distribution of the α-relaxation times becomes broader. In case of thin films of poly(methyl methacrylate), the thickness dependence of the α- and β-processes could be observed using the dielectric relaxation spectroscopy. Both the temperatures Tα and Tβ, at which the dielectric losses due to the α- and β-processes show a maximum at a given frequency, decrease with decreasing film thickness. Below a critical thickness, both Tα and Tβ begin to decrease rapidly with decreasing thickness. In order to investigate the positional dependence of the dynamics of the α-processes for polystyrene thin films, we utilize bilayer films, where a thin film of PS labeled with a dye is located on top of the film of unlabeled PS. For such systems, we could extract the contribution only from the upper labeled layer of PS. The glass transition temperature of the upper labeled layer between unlabeled PS layer and aluminum electrode is lower than that of the bulk labeled polystyrene. This result supports that the sample geometry commonly used for dielectric measurements of ultra thin films is the same to that used for other methods as far as the glass transition dynamics are concerned.