Glass transition temperatures of isotactic poly(methymethacrylate) thin films and individual chains probed by multi wavelength ellipsometry

International audience; Ellipsometry is used as a technique to determine glass temperature transitions of isotactic Poly(methyl methacrylate) (iPMMA) ultra-thin films and isolated chains deposited on a silicon wafer surface. The importance of systematically probing the samples at different wavelengths is pointed out in this work since for a similar thin film, slope changes in delta are more or less pronounced according to the wavelength used. The technique performed on iPMMA thin films and isolated globular chains, deposited on Piranha treated surfaces, reveals the presence of multiple transitions, whereas films of comparable thicknesses deposited on HF treated silicon wafers show only one transition. The results confirm the assumptions of a layered structure yielding specific () which depends on depth location, which is in good agreement with recent theories of glass transitions in confined polymers, and emphasize the influence of polymer-substrate interactions and chain disentanglement.