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Updated: Sep 22, 2025
09:32
Cooling Rate Dependent Ellipsometry Measurements to Determine the Dynamics of Thin Glassy Films
Published on: January 26, 2016
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When Does a Glass Transition Temperature Not Signify a Glass Transition?
J A Forrest1,2, K Dalnoki-Veress3,4
1Department of Physics and Astronomy, University of Waterloo, Waterloo, Ontario, Canada, N2L 3G1.
ACS Macro Letters
|May 20, 2022
Summary
A new model reveals that the measured glass transition temperature (Tg) in thin films reflects only surface dynamics, not the bulk material. This suggests dilatometric Tg measurements may not accurately represent the true glass transition in thin film systems.
Area of Science:
- Materials Science
- Physical Chemistry
- Polymer Science
Background:
- The glass transition temperature (Tg) is a critical parameter for polymers and amorphous solids.
- Thin film measurements of Tg often deviate from bulk values due to surface effects.
- Dilatometric measurements detect Tg via changes in film thickness with temperature.
Purpose of the Study:
- To develop a simple model linking enhanced surface mobility to dilatometric Tg in thin films.
- To investigate the validity of dilatometric Tg as a measure of bulk dynamics in thin films.
- To understand the influence of surface dynamics on measured Tg values.
Main Methods:
- Development of a simple physical model for thin film dynamics.
- Incorporation of enhanced surface mobility and a characteristic length scale.
- Comparison of model predictions with existing experimental data for thin film Tg.
Main Results:
- The model demonstrates that dilatometric Tg in thin films primarily reports the dynamics of a near-surface layer.
- The measured dilatometric Tg is shown to be an inaccurate reporter of bulk material dynamics.
- Model calculations show strong agreement with a wide range of experimental thin film Tg data.
Conclusions:
- Dilatometric Tg measurements in thin films are heavily influenced by surface effects.
- The "kink" observed in dilatometric measurements does not represent a true bulk glass transition.
- The model provides a framework for understanding surface-influenced dynamics in thin films.
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