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.

Constant-pressure molecular-dynamics simulations have been carried out of a bead−rod model polymer confined between two attractive crystalline substrates. Three different substrate−substrate separations (i.e., film thicknesses) were used and two different polymer−substrate interaction strengths. The density profiles show a monomer layering close to the polymer− substrate interface. A higher density was found in this region compared to the middle bulklike layers of the films. The dependence of the film-averaged density on temperature and thickness was measured for all polymer films. Decreasing the film thickness leads to an increase of this density and of the film-averaged glass-transition temperature. Layer-resolved analysis of the segmental dynamics of the thickest films shows a gradient of the mobility upon approach of the polymer−substrate interface, while the middle-layer dynamics exhibits bulklike behavior. With decreasing film thickness, these gradients become overlapping. All polymer films were deformed uniaxially normal to the substrates beyond their linear viscoelastic regime; their elastic moduli but also their secant moduli at larger strain amplitudes were extracted. In the linear regime, the stiffness was found to increase with decreasing film thickness; this correlates well with the layer-resolved segmental dynamical behavior. A strong drop of the stiffness was observed upon increasing the deformation amplitude; this drop was more pronounced for stiffer films. It is shown that the drop in stiffness can be qualitatively explained by the drop of the relaxation times as well as by the increased heterogeneity of the dynamics in different film layers upon deformation. The thickness dependencies of the structural, dynamical, and mechanical properties become more pronounced with stronger adsorption to the substrates.

Molecular-dynamics simulations have been carried out for a coarse-grained model of a random AB-copolymer confined between two crystalline substrates. The strength of substrate-polymer interactions, and the distance between the two substrates have been varied in a wide range. For thick films the film-averaged segmental mobility decreases for intermediate adsorption strengths, but start to increase for very high substrate-polymer attraction strength. We saw that this non-monotonic behavior is caused by a very strong heterogeneity of the segmental dynamics above the glass-transition temperature: the segmental mobility slows down drastically close to adsorbing substrates, but strongly increases in the middle part of the film. This effect, and its sensitive dependence on film thickness, are explained by finite-size effects in confinement, in combination with glassy boundary layers. It is demonstrated that film-averaged mobility as often measured cannot be understood without resolving local mobility in space and time.