Single chain structure in thin polymer films:: corrections to Flory's and Silberberg's hypotheses

Conformational properties of polymer melts confined between two hard structureless walls are investigated by Monte Carlo simulation of the bond fluctuation model. Parallel and perpendicular components of chain extension, bond-bond correlation function and structure factor are computed and compared with recent theoretical approaches attempting to go beyond Flory's and Silberberg's hypotheses. We demonstrate that for ultrathin films where the-thickness, H, is smaller than the excluded volume screening length (blob size), xi, the chain size parallel to the walls diverges logarithmically, R-2/2N approximate to b(2) + c log(N) with c similar to 1/H. The corresponding bond-bond correlation function decreases like a power law, C(s) = d/s(omega) with s being the curvilinear distance between bonds and omega = 1: Upon increasing the film thickness, H, we find-in contrast to Flory's hypothesis-the bulk exponent omega = 3/2 and, more importantly, a decreasing d(H) that gives direct evidence for an enhanced self-interaction of chain segments reflected at the walls. Systematic deviations from the Kratky plateau as a function of H are found for the single chain form factor parallel to the walls in agreement with the non-monotonic behaviour predicted by theory. This structure in the Kratky plateau might give rise to an erroneous estimation of the chain extension from scattering experiments. For large H the deviations are linear with the wavevector, q, but are very weak. In contrast, for ultrathin films, H < xi, very strong corrections (albeit logarithmic in q) are found suggesting a possible experimental verification of our results.