Eur. Phys. J. E (2007) 24: 55-60
https://doi.org/10.1140/epje/i2007-10210-8

Regular Article

Dynamic anisotropy and heterogeneity of polystyrene thin films as studied by inelastic neutron scattering

¹ Institute for Chemical Research, Kyoto University, 611-0011, Uji, Kyoto-fu, Japan
² Chiba Institute of Technology, 275-0023, Narashino, Chiba-ken, Japan
³ Rutherford Appleton Laboratory, ISIS facility, OX11 0QX, Chilton, Didcot Oxon, UK
⁴ Department of Materials, University of Oxford, Parks Road, OX1 3PH, Oxford, UK

^* e-mail: kanaya@scl.kyoto-u.ac.jp

Received: 1 May 2007
Accepted: 20 July 2007
Published online: 3 September 2007

Abstract

We studied the dynamic anisotropy and heterogeniety of polystyrene thin films in glassy state with inelastic neutorn scattering. Adjusting the scattering vector to the directions parallel and perpendicular to the film surface, we observed the elastic scattering intensities as a function of temperature. It was found for the 200Å film that the elastic intesity decreased with increasing temperature more rapidly in the perpendicular direction than in the pararell direction, showing the higher mobility in the perpendicular direction. However, such dynamical anisotropy was not observed in the 1000Å film. The decrease in the mobility was observed with the film thickness in both the directions. These results were explained in terms of an interface hard layer. We also evelauated the dynamical heterogeniety from the non-Gaussian parameter A₀, which increased with decreasing the film thickness, showing the increase in the dynamical heterogeneity. Assuming a simple bi-layer model consisting of the interface hard layer and the bulk-like layer, we analyzed the thickness dependence of the non-Gaussian parameter A₀ and the mean square displacement 〈u ²〉 to find that the hard layer has a thickness of ∼ 130 Å and a mean square displacement of ∼ 0.018 Å^2 at 230K.