Ultrathin Films of Poly(ethylene oxides) on Oxidized Silicon. 1. Spectroscopic Characterization of Film Structure and Crystallization Kinetics
journal contributionposted on 2003-01-31, 00:00 authored by Holger Schönherr, Curtis W. Frank
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.