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09:32
Cooling Rate Dependent Ellipsometry Measurements to Determine the Dynamics of Thin Glassy Films
Published on: January 26, 2016
Reduced glass transition temperatures in thin polymer films: surface effect or artifact?
O Bäumchen1, J D McGraw, J A Forrest
1Department of Physics and Astronomy, McMaster University, Hamilton, Ontario, Canada.
Physical Review Letters
|September 26, 2012
Summary
Free surfaces significantly lower the glass transition temperature (Tg) of thin polystyrene films. Supported films show bulk Tg, proving free interfaces dominate Tg reduction in thin films.
Area of Science:
- Materials Science
- Polymer Physics
- Surface Science
Background:
- The glass transition temperature (Tg) of polymers is a critical property affecting their performance.
- Tg of thin polymer films is known to deviate from bulk values, particularly in freestanding configurations.
- The role of free surfaces versus substrate interactions in this phenomenon requires clarification.
Purpose of the Study:
- To directly investigate the impact of free surfaces on the measured glass transition temperature (Tg) of thin polystyrene films.
- To compare the Tg of freestanding films with supported films of identical material and thickness.
- To determine the primary cause of Tg reduction in thin polymer films.
Main Methods:
- Ellipsometry was employed to measure the temperature-dependent thickness and refractive index of thin polystyrene films (35-114 nm).
- Measurements were conducted on both freestanding films and subsequently on the same films supported on a silicon substrate.
- Changes in the slope of temperature-dependent optical properties were used to determine Tg.
Main Results:
- Freestanding polystyrene films exhibited a reduced glass transition temperature (Tg) compared to the bulk material.
- The same films, when supported on a silicon substrate, displayed Tg values identical to the bulk.
- Tg values for supported films were consistent with previous studies on similar systems.
Conclusions:
- Free interfaces are unequivocally identified as the dominant factor responsible for the reduction in glass transition temperature (Tg) for thin polystyrene films within the studied thickness range.
- The presence of a substrate effectively suppresses the surface effects that lower Tg in freestanding films.
- This study provides clear evidence for the significant influence of surface thermodynamics on polymer film behavior.
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