Single chain structure in thin polymer films: corrections to Flory’s and Silberberg’shypotheses

Abstract

Conformational properties of polymer melts confined between two hard structurelesswalls are investigated by Monte Carlo simulation of the bond fluctuationmodel. Parallel and perpendicular components of chain extension, bond–bondcorrelation function and structure factor are computed and compared withrecent theoretical approaches attempting to go beyond Flory’s and Silberberg’shypotheses. We demonstrate that for ultrathin films where the thickness,H, is smaller than the excluded volume screening length (blob size),ξ, the chain size parallel to the walls diverges logarithmically,R2/2N≈b2+clog(N) with c∼1/H. The corresponding bond–bond correlation function decreases like a power law,C(s) = d/sω with s being the curvilinear distance between bonds andω = 1. Upon increasingthe film thickness, H, we find—in contrast to Flory’s hypothesis—the bulk exponentω = 3/2 and, more importantly,a decreasing d(H) that gives direct evidence for an enhanced self-interaction of chain segments reflectedat the walls. Systematic deviations from the Kratky plateau as a function ofH are found for the single chain form factor parallel to the walls in agreement with thenon-monotonic behaviour predicted by theory. This structure in the Kratky plateau might giverise to an erroneous estimation of the chain extension from scattering experiments. For largeH the deviations are linear with the wavevector,q, but are very weak. In contrast, for ultrathin films,H<ξ, very strong corrections (albeit logarithmic inq) are found suggesting a possible experimental verification of our results.