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Probing slow dynamics in supported thin polymer films.

Zahra Fakhraai1, James A Forrest

  • 1Department of Physics and Guelph-Waterloo Physics Institute, University of Waterloo, 200 University Avenue W., Waterloo, Ontario, Canada N2L 3G1.

Physical Review Letters
|August 11, 2005
PubMed
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Thin polystyrene films show reduced glass transition temperatures (Tg) at slow cooling rates, with dynamics becoming Arrhenius-like and activation energy decreasing with film thickness. This resolves contradictory literature findings.

Area of Science:

  • Materials Science
  • Polymer Physics
  • Physical Chemistry

Background:

  • Glass transition temperature (Tg) in thin polymer films is known to be thickness-dependent.
  • Previous studies have reported conflicting results regarding the influence of cooling rates on Tg in thin films.

Purpose of the Study:

  • To investigate the effect of cooling rate on the glass transition dynamics of thin supported polystyrene films.
  • To elucidate the relationship between cooling rate, relaxation time, and film thickness in determining Tg.
  • To reconcile contradictory findings in the literature concerning thin film Tg behavior.

Main Methods:

  • Variable cooling rate ellipsometry was employed to measure Tg in polystyrene films.
  • Polystyrene films of varying thicknesses, specifically down to 6 nm, were studied.

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  • Cooling rates ranged from approximately 1 K/min to over 90 K/min.

    • Main Results:

    • At slow cooling rates (approx. 1 K/min), Tg was reduced below the bulk value, e.g., 341 K for a 6 nm film.
    • Increasing cooling rates diminished the Tg reduction, with minimal thickness dependence observed above 90 K/min.
    • Film dynamics exhibited Arrhenius behavior with decreasing activation energy as film thickness decreased.

    Conclusions:

    • Cooling rate significantly influences Tg and relaxation dynamics in thin polystyrene films.
    • The observed Arrhenius-like behavior and decreasing activation energy suggest a connection to cooperative motion length scales.
    • This study provides a framework to resolve discrepancies in previous thin film Tg research.