https://doi.org/10.1140/epje/i2009-10510-y
Regular Article
Confinement effects on glass transition temperature, transition breadth, and expansivity: Comparison of ellipsometry and fluorescence measurements on polystyrene films
1
Department of Chemical and Biological Engineering, Northwestern University, 60208, Evanston, IL, USA
2
Department of Materials Science and Engineering, Northwestern University, 60208, Evanston, IL, USA
* e-mail: j-torkelson@northwestern.edu
Received:
31
May
2009
Revised:
3
August
2009
Accepted:
21
August
2009
Published online:
26
September
2009
Using ellipsometry, we characterized the nanoconfinement effect on the glass transition temperature (T
gof supported polystyrene (PS) films employing two methods: the intersection of fits to the temperature (Tdependences of rubbery- and glassy-state thicknesses, and the transition mid-point between rubbery- and glassy-state expansivities. The results demonstrate a strong effect of thickness: C. The T -range needed for accurate measurement increases significantly with decreasing thickness, an effect that arises from the broadening of the transition with confinement and a region below T
g where expansivity slowly decreases with decreasing T . As determined from expansivities, the T
g breadth triples in going from bulk films to a 21-nm-thick film; this broadening of the transition may be a more dramatic effect of confinement than the T
g reduction itself. In contrast, there is little effect of confinement on the rubbery- and glassy-state expansivities. Compared with ellipsometry, T
g ’s from fluorescence agree well in bulk films but yield lower values in nanoconfined films: T
g(bulk) - T
g(23 nm) = 15° C via fluorescence. This small difference in the T
g confinement effect reflects differences in how fluorescence and ellipsometry report “average T
g ” with confinement. With decreasing nanoscale thickness, fluorescence may slightly overweight the contribution of the free-surface layer while ellipsometry may evenly weight or underweight its contribution.
PACS: 64.70.P- Glass transitions of specific systems – / 64.70.pj Polymers – / 82.35.Lr Physical properties of polymers – / 68.60.-p Physical properties of thin films, nonelectronic –
© EDP Sciences, SIF, Springer-Verlag Berlin Heidelberg, 2009