Conformation of polystyrene chain in ultrathin films obtained by spin coating

This article reports a study of the chain conformation in thin films of amorphous polymer prepared by spin coating. Polystyrene chains of two molecular weights M-w, 230 000 and 660 000, are confined in layers of thickness equal to either 1000 Angstrom, thus greater than their unperturbed radii of gyration R-g, or 100 Angstrom, thus smaller than R-g. The form factor of the chain inside the layer is measured by small-angle neutron scattering. Changes of the scattering are observed as a function of the film thickness in the intermediate scattering vector range. We analyze the data in the frame of the wormlike model. In the 1000 Angstrom films, the chain has the same Gaussian conformation as in the bulk. In the films of 100 Angstrom, the persistence length of the chain is increased (compared to 9.2 Angstrom the value in the bulk) up to 45 Angstrom, for both M-w values. However, the corresponding scattering intensity is higher than expected for such an extended chain.