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Combined X-ray Reflectivity and Infrared Study of the Effect of Hydrogen Bonding of the OH Group on the Relaxation Behavior in Ultrathin Polyvinylphenol Films on SiO2

  • Tulika Sharma
    Tulika Sharma
    Graduate School of Human Development and Environment, Kobe University, 3-11 Tsurukabuto, Nada-Ku, Kobe, Hyogo 657-8501, Japan
    Department of Physics and Astronomy, School of Science and Technology, Kwansei Gakuin University, 1 Gakuen-Uegahara, Sanda 669-1337, Japan
  • Saki Kashihara
    Saki Kashihara
    Department of Physics and Astronomy, School of Science and Technology, Kwansei Gakuin University, 1 Gakuen-Uegahara, Sanda 669-1337, Japan
  • Yuta Yamasaki
    Yuta Yamasaki
    Department of Physics and Astronomy, School of Science and Technology, Kwansei Gakuin University, 1 Gakuen-Uegahara, Sanda 669-1337, Japan
  • Yukihiro Ozaki
    Yukihiro Ozaki
    Molecular Photoscience Research Center, Kobe University, 1-1 Rokkodai, Nada, Kobe, Hyogo 657-8501, Japan
  • Isao Takahashi*
    Isao Takahashi
    Department of Physics and Astronomy, School of Science and Technology, Kwansei Gakuin University, 1 Gakuen-Uegahara, Sanda 669-1337, Japan
    *Email: [email protected]
  • , and 
  • Harumi Sato*
    Harumi Sato
    Graduate School of Human Development and Environment, Kobe University, 3-11 Tsurukabuto, Nada-Ku, Kobe, Hyogo 657-8501, Japan
    Molecular Photoscience Research Center, Kobe University, 1-1 Rokkodai, Nada, Kobe, Hyogo 657-8501, Japan
    *Email: [email protected]
    More by Harumi Sato
Cite this: J. Phys. Chem. B 2023, 127, 35, 7602–7614
Publication Date (Web):August 23, 2023
https://doi.org/10.1021/acs.jpcb.3c01491
Copyright © 2023 American Chemical Society

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    Abstract

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    Utilizing X-ray reflectivity and infrared reflection absorption spectroscopy (IR-RAS), we have investigated the thermal expansion and contraction of ultrathin polyvinylphenol (PVPh) films supported on a silicon (100) substrate capped with an amorphous SiO2 layer. Despite being known to form strong interactions with the SiO2 surface, the thin PVPh films showed a reduction in the glass-transition point Tg, similar to the behavior of polystyrene thin films deposited on SiO2. We explored the relationship between thermal expansivity and film thickness using well-annealed films and found that it decreases with film thickness in the range below twice the radius of gyration of a polymer chain (2Rg) in the glassy state. Thickness expansion in the glassy state and contraction in thickness at temperatures higher than Tg bulk (melt state) showed the presence of two competing relaxation processes. The reported negative thermal expansion in PVPh thin films, which was discovered to be one of the inherent properties, may have been caused by the fast relaxations that take place at the free polymer surface. IR-RAS was utilized to investigate the effect of thickness on hydrogen bonding in PVPh, and it was confirmed that with decreasing thickness, hydrogen bonding becomes weak, and the number of free OH groups increases. Therefore, thinner PVPh samples exhibit lower Tgs as an effect of easier molecular motions.

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    The Supporting Information is available free of charge at https://pubs.acs.org/doi/10.1021/acs.jpcb.3c01491.

    • Annealing protocols adopted for different relaxation measurements of PVPh: (a) annealing condition for Tg measurements; thickness was measured during heating and cooling processes, (b) annealing condition for thickness-dependent relaxation measurements, and (c) annealing condition for the temperature-dependent relaxation measurement for a 65 Å thick film (Figure S1); glass-transition data of PVPh for all of the thicknesses acquired during cooling and heating cycles (Figure S2); temperature-dependent thickness variation in 38 Å thickness PVPh films after annealing each annealing; thickness was measured during heating and cooling processes (Figure S3); before and after annealing AFM images of (a, b) ∼10 Å and (c, d) ∼5.75 Å; AFM images show smooth films with dewetting (Figure S4); and whole-range IR-RAS spectra of different thicknesses (Figure S5) (PDF)

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