Detailed atomistic Monte Carlo simulation of grafted polymer melts. I. Thermodynamic and conformational properties

Abstract

The thermodynamic and conformational properties of polymer melts grafted on a solid substrate as obtained from detailed, atomistic Monte Carlo simulations with the end-bridging algorithm are presented. The interface between a basal graphite plane (as well as a non-interacting hard surface) and a bulk polyethylene (PE) melt, a few or all chains of which are grafted on the plane, has been studied. Three different PE melts, of mean molecular length ${\mathrm{C}}_{78},$ ${\mathrm{C}}_{156},$ and ${\mathrm{C}}_{250},$ have been investigated, at grafting densities ranging from 0.54 to 2.62 nm⁻². For melts composed of grafted and free chains, it is observed that, at moderate to high surface densities (σ⩾1 nm⁻²), the region close to the substrate is fully occupied by segments belonging to grafted chains, which are forced by their chemical grafting to have their first segment on the interface. As the grafting density increases, free chains are progressively expelled from the surface region, in agreement with scaling arguments and the predictions of lattice-based self-consistent mean-field (SCF) theory. For melts grafted on a graphite plane, it is also seen that the local melt density in the region closest to the interface is systematically higher than in the bulk, exhibiting distinct local maxima due to polymer adsorption. Results for the chain conformation tensor demonstrate that chains are significantly stretched in the direction perpendicular to the surface, even for moderate surface densities. For the ${\mathrm{C}}_{250}$ (PE) melt at a grafting density σ=1.31 nm⁻², for example, the average chain dimension perpendicular to the interface is 1.9 times larger than its equilibrium value in the bulk. The profile of the chain end density is seen to exhibit universal behavior in agreement with the predictions of the SCF theory. Additional results concerning the mean chain conformational path and the structure of the interfacial area for both systems studied (fully grafted and mixtures of grafted and free chain systems) are also presented.