Confined polymer chains in a Θ solvent: A model with polymer-solvent interactions

Thermodynamics of polymer chains in the Θ condition confined to a space between two parallel walls was studied by using lattice Monte Carlo simulations. The Θ state was realized by allocating positive interaction to nearest-neighbor pairs of a polymer segment and a solvent molecule that is now explicitly included, rather than giving attractive interaction between polymer segments with no explicit solvent molecules present. The two models can be equivalent when used to specify the Θ state in unconfined solutions, but missing segment-solvent contacts at the wall make the two models different for confined solutions. The effectively attractive wall facilitates entry of polymer chains into narrow slits in the corrected model and lifts the segment density at sites adjacent to the walls. The dependence of the segment density near the wall on the distance from the wall follows a power law different from the one that holds for the conventional model of the Θ state. In particular, when the wall has explicit interaction with the polymer segments, our model makes the profile highly sensitive to the solvent quality. The corrected model explains enhanced adsorption in a poorer solvent reported in experiments.