Anomalies in the optical index of refraction of spun cast polystyrene thin films

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, Δn, is negative and varies linearly with wavelength in the visible region. The magnitude of Δn, 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 for 2 h above T_g 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 Δn is interpreted as being due to a radially symmetric segmental orientation induced by the spinning process.