Anomalies in the Optical Index of Refraction of Spun Cast Polystyrene Thin Films

Abstract

We used x-ray reflectivity in combination with optical ellipsometry to measure the optical index of refraction, n, in thin spun cast polystyrene films. We have found that n is independent of the molecular weight, but is a sharp function of the film thickness for films less than 100 nm. In all cases the deviation from the bulk, Δ n, is negative and varies linearly with wavelength in the visible region. The magnitude of Δ n, was found to be as large as 0.25 for films 7 nm thick. The bulk index of refraction was recovered in all films after annealing for2habove Tg at 160 °C. X-ray reflectivity measurements of the scattering critical angle show minimal density deviations from the bulk (less than 0.5%) between the annealed and unannealed films. Consequently the large molecular-weight-independent value of Δ n is interpreted as being due to a radially symmetric segmental orientation induced by the spinning process.