Polymer melt droplets adsorbed on a solid wall: A Monte Carlo simulation

Using a coarse-grained bead-spring model of flexible polymers, we study the contact angle of polymer melt droplets sitting at a flat structureless wall, when the strength ε of the adsorption potential $V_{\mathrm{wall}}{\mathrm{(z)=\epsilon /z}}^3$ is varied. For this purpose, droplets containing 2048 or 4096 monomers for chain lengths $\text{N=16}$ and $\text{N=32}$ were carefully equilibrated at temperatures in the range from 74% to 82% of the Theta temperature, and the density profile of these droplets both in the z direction perpendicular to the substrate surface and in the radial direction was obtained. Beyond a critical value of ε we find that the contact angle vanishes, i.e., the droplets spread out and form a flat film. Such flat polymer films are also studied with considerably more polymers (up to 24 576 monomers contained in the simulation box). It is shown that the density profile $\mathrm{\rho (z)}$ is affected by the hard wall (exhibiting the characteristic layering oscillations) up to about $\text{z=5}$ (measuring lengths in units of the length of an effective bond), while at larger z the profile is flat and has the melt density at that temperature, with an interface to the “gas” at about $\text{z=20.}$ Analyzing the capillary wave spectrum of the interfacial fluctuations, the surface tension of the polymer melt is extracted. Via the anisotropy of the local pressure near the wall, the wall excess free energy of the polymer melt is found as well, and the Young equation is tested. Thus methods have been developed that allow a systematic study of polymer–wall interactions and wetting vs dewetting behavior.