Comparing Refractive Index and Density Changes with Decreasing Film Thickness in Thin Supported Films Across Different Polymers

Density or specific volume changes in thin films have been investigated in relation to glass transition and other property changes with decreasing film thickness. Often such density changes are inferred from values of the refractive index through the Lorentz-Lorenz equation. Our group’s previous work on supported polystyrene (PS) films concluded that recently reported large changes in apparent film density are likely erroneous because assumptions in the derivation of the Lorentz-Lorenz relation become invalid for thin films less than ~20 nm. Unexpected non-monotonic changes in refractive index with decreasing thickness were observed for PS films as measured by spectroscopic ellipsometry. Here we compare these PS results to similar measurements of refractive index on poly(2-vinylpyridine) (P2VP) and poly(methyl methacrylate) (PMMA) supported films, polymers typically associated with attractive substrate interactions. Surprisingly, we find P2VP shows identical thickness-dependent refractive index changes to PS, with a more muted behavior for PMMA. We explore whether such trends arise from alignment of monomers near the substrate interface by measuring substrates with different surface chemistries.