X-ray reflectivity was used to characterize the electron density and rubbery expansion for cross-linked epoxy network films of various thickness adhered to silicon wafers. Reflectivity profiles from all the films were best fit by single layer model. In addition, the electron density of the epoxy films did not depend on film thickness. Epoxy films thicker than 400 angstroms} exhibited similar rubbery expansion. However films thinner than 400 angstroms} displayed significantly smaller rubbery expansion. The distance of this transition from bulk expansion to confined expansion corresponds to 10 cross-links, suggesting that the molecular weight between cross-links is the relevant length scale for polymer networks near an interface. Thickness verses temperature plots from epoxy films < 120 angstroms} thick suggested an increase in the apparent glass transition temperature of the films. Epoxy network films were cast onto silicon wafers with a variety of surface treatments. The thermal expansion behavior of the epoxy films was independent of the surface treatment, which varied both in surface energy, and the strength of bonding interactions with the polymer.
and Wu, W.
Deviations in the Thermal Properties of Ultrathin Polymer Network Films, Macromolecules, [online], https://tsapps.nist.gov/publication/get_pdf.cfm?pub_id=851913
(Accessed March 3, 2024)