Ultrathin Films of Poly(ethylene oxides) on Oxidized Silicon. 1. Spectroscopic Characterization of Film Structure and Crystallization Kinetics

We have characterized the structure, molecular orientation, and crystallization kinetics of isothermally crystallized thin (film thickness d < 500 nm) and ultrathin films (d < 100 nm) of poly(ethylene oxides) on oxidized silicon substrates by a combination of microscopic and spectroscopic methods. In situ hot stage atomic force microscopy (AFM) reveals a preferred flat-on orientation of lamellar crystals in films thinner than ca. 300 nm. The mean orientation of the polymer molecules, as measured by transmission and grazing angle reflection FT-IR spectroscopy, fully agrees with the preferred orientation of the PEO helices parallel to the surface-normal direction, as inferred from the AFM data. In addition to a strong film thickness dependence of this preferred chain orientation, the FT-IR data indicate that the degree of crystallinity decreases steadily when the film thickness becomes smaller than ∼200 nm. The local environment of pyrene end-labels in derivatized PEO was characterized by steady-state fluorescence spectroscopy, and the excimer/monomer emission ratio was found to be very sensitive to both film thickness and crystallization temperature. The latter relationship could be described by an Arrhenius equation and yielded an excimer-forming-site energy of 17 ± 2 kJ/mol. Finally, the isothermal crystallization of PEO in ultrathin films was followed spectroscopically in situ. Both fluorescence and FT-IR spectroscopy indicated that the crystallization kinetics are progressively slowed down for decreasing film thickness, presumably due to the increased glass transition temperature of ultrathin PEO films on interactive substrates.