Slowing Down versus Acceleration in the Dynamics of Confined Polymer Films

We have performed molecular dynamics (MD) simulations on coarse-grained polymer films which are confined between two attractive crystalline Lennard-Jones substrates with three different substrate?substrate separations. Two different polymer?substrate interactions strengths have been studied. Detailed analysis of the structural properties of each film showed a layering of the monomers close to the polymer?substrate interface and a preferential orientation of the bonds parallel to the substrate surface; both depend on substrate attraction strength and temperature, but not on film thickness. The rotational and translational segmental dynamics were analyzed for each film thickness in different film layers, for a wide range of temperatures and for both substrate attraction strengths. For all simulated films, the segmental dynamics was found to be faster than that in the bulk. For relatively thick films and energetically neutral polymer?substrate interaction, a dramatic slowing down of the polymer mobility was found close to the polymer?substrate interface, when compared with the middle of the film, thus providing stiffness enhancement due to the presence of the attractive substrates. With decreasing substrate?substrate separation these gradients in stiffness became overlapping. However, this did not lead to an overall stiffness enhancement in the film, as expected; instead, a large shift toward lower overall mobility was found for the thinner films, which was attributed to finite-size scaling effects.