Correlations, or lack thereof, between density and dynamics have long been investigated in glasses, with the contrast between the many decade super-Arrhenius change in dynamics on cooling and the correspondingly modest change in density, long being one of the great mysteries of the glass transition. Interfacial perturbations to dynamics in thin films and confined systems have further puzzled and confused this issue with mounting evidence suggesting these factors can be completely uncorrelated near interfaces. Here we examine changes in density with decreasing film thickness for polystyrene (PS), poly(methyl methacrylate) (PMMA), and poly(2-vinylpyridine) (P2VP) thin films based on ellipsometric measurements of the refractive index. For data modeled with the typical homogeneous Cauchy layer optical model, we show how all three polymers display a similarly large increase in refractive index with decreasing film thickness, an observation which has been interpreted by some recent literature studies as indicative of a large (~20-30%) increase in density through the Lorentz-Lorenz relation despite being physically unrealistic. In contrast, we demonstrate that an ellipsometric optical layer model with a gradient in refractive index provides more physically realistic parameters for very thin films. With a depth-dependent refractive index model, thin film data can be well fit to an optical layer model where much of the film’s interior retains a bulk-like refractive index, while only a narrow near free surface layer deviates, exhibiting a strong increase in index. The breadth of this near free surface region exhibiting a sharp gradient in index is observed to grow from 4 nm to 9 nm with decreasing film thickness from 50 nm to 30 nm. Surprisingly we find the near free surface gradient in refractive index to be opposite to expectations based on a simple free volume or density correlation to dynamics. The observed higher refractive index near the free surface could reflect a more optimized molecular packing under high surface mobility, similar to that observed in physical vapor deposited stable molecular glasses. Such an inhomogeneous gradient in film structure would invalidate the use of homogeneous Cauchy layers, as well as the use of the Lorentz-Lorenz relation whose derivation is based on assuming an isotropic distribution of dipoles.

Examination of density and dynamics in thin polymer films

Macromolecular Chemistry: The Second Century

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ACS Spring 2020 National Meeting & Expo

The official archive and repository for all ACS Meetings & Expositions abstracts and select posters or presentations.