On the glass transition in ultrathin polymer films of different molecular architecture

Optical waveguide spectroscopy has been employed to monitor the temperature dependence of the refractive index, kappa = delta n/delta T, and of the thermal (linear) expansivity, beta = (delta d/delta T).d(-1), for thin poly(methyl methacrylate) (PMMA) films of thicknesses d > 0.2 mu m independently from each other. A break in the linearized slope of n (and d) was identified as the film glass transition temperature, T-g. For thinner samples only surface plasmon resonances could be analyzed, but the kinetic mode, i.e. reflectivity-versus-temperature scans, still gave reliable T-g values. The PMMA films of different thicknesses, ranging from d approximate to 3 nm to d approximate to 800 nm, were prepared by three different techniques: by spin-casting from solution, by the grafting-from approach and by the Langmuir/Blodgett/Kuhn technique. We found that all these films prepared on hydrophobic substrates show the expected decrease of T-g for ultrathin samples (d < 100 nm). However, this behavior is independent of the strongly varying intramolecular architecture and organization of the macromolecular chains in the various films.