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Updated: May 6, 2026
Cooling Rate Dependent Ellipsometry Measurements to Determine the Dynamics of Thin Glassy Films
Published on: January 27, 2016
Mixed equilibrium/nonequilibrium effects govern surface mobility in polymer glasses
Jianquan Xu1, Asieh Ghanekarade2, Li Li1
1School of Chemistry and Chemical Engineering, Key Laboratory of Surface & Interface Science of Polymer Materials of Zhejiang Province, Zhejiang Sci-Tech University, Hangzhou 310018, China.
The polymer glass transition temperature (Tg) is lower at the surface. Non-equilibrium conditions in simulations created a linear Tg-gradient, matching experimental findings and revealing insights into polymer surface behavior.
Area of Science:
- Polymer Science
- Materials Science
- Surface Science
Background:
- The glass transition temperature (Tg) of polymers is known to be lower near free surfaces.
- Discrepancies exist between experimental and simulated Tg-gradients due to differing methodologies for determining Tg.
Purpose of the Study:
- To reconcile the differing depth dependence of Tg-gradients observed experimentally and in simulations.
- To investigate the impact of non-equilibrium effects on the surface Tg of polymer glasses.
Main Methods:
- Utilized angle-resolved X-ray photoelectron spectroscopy, sum-frequency generation vibrational spectroscopy, contact angle measurements, and molecular dynamics simulations.
- Compared equilibrium simulations with non-equilibrium simulations by reducing thermal annealing time.
- Studied the dependence of Tg on heating/cooling rates for polymer films of varying thickness.
Main Results:
- Experimental Tg-gradients showed a linear variation with depth, while equilibrium simulations exhibited a double exponential dependence.
- Non-equilibrium simulations, with reduced annealing time, produced a linear Tg-gradient consistent with experimental data.
- Simulations also showed reduced relaxation times and decreased mass density under non-equilibrium conditions.
Conclusions:
- Non-equilibrium effects significantly influence the determination of Tg-gradients near polymer surfaces.
- The linear Tg-gradient observed experimentally is attributed to the kinetically arrested, non-equilibrium nature of the glassy state during measurement.
- Simulations must account for non-equilibrium conditions to accurately replicate experimental observations of surface Tg.
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