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Reduced glass transition temperatures in thin polymer films: Surface effect or artifact?

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Bäumchen,  Oliver
Group Dynamics of fluid and biological interfaces, Department of Dynamics of Complex Fluids, Max Planck Institute for Dynamics and Self-Organization, Max Planck Society;

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Citation

Bäumchen, O., McGraw, J. D., Forrest, J. A., & Dalnoki-Veress, K. (2012). Reduced glass transition temperatures in thin polymer films: Surface effect or artifact? Physical Review Letters, 109(5): 055701. doi:10.1103/PhysRevLett.109.055701.


Cite as: http://hdl.handle.net/11858/00-001M-0000-0029-769C-8
Abstract
We have examined the direct effect of manipulating the number of free surfaces on the measured glass transition temperature Tg of thin polystyrene films. Thin films in the range 35  nm<h<114  nm with molecular weights of 592  kg/mol and 1144  kg/mol were studied. Ellipsometry was used to determine the temperature dependence of the thickness and refractive index of freestanding films. By noting the change in slope in each of these quantities, a Tg value can be assigned in quantitative agreement with previously reported results. For thin freestanding films this value is reduced from that of the bulk. The exact same films are then transferred to a Si substrate and the Tg of the resulting supported film was determined. The Tg values of the now supported films are the same as the bulk value and the same as previous reports of similar supported films. These experiments unambiguously show that free interfaces are the dominant cause of the Tg reductions for the film thicknesses studied.