Computer simulation of polymer systems in the strongly confined geometry

The conformations and thermodynamic properties of polymers constrained by the presence of surfaces are not understood clearly at the present. The understanding of this system is of practical significance since it is relevant in many technological applications of polymers. A polymer nanofiber presents an attractive system for the study of polymer properties that experience the strongly confinement from the free surface. This work will present the simulation of polymer nanofibers performed by a Monte Carlo algorithm for coarse-grained polymer chains on the Second Nearest Neighbor Diamond (2nd) lattice.; A polyethylene nanofiber is obtained from equilibrated thin film snapshots by applying only one effective periodic boundary condition in the simulation. The presence of attractive long range interaction gives cohesion to the fiber. PE fibers, which contain up to 72 chains of C99 and have the radius ∼5.0 nm, have been produced and equilibrated on the 2nd lattice. In these fibers, the density profiles can be fitted to a hyperbolic tangent function, with end beads being more abundant than the middle beads at the surface. There are orientational preferences at the surface on the scale of individual bonds and whole chains. The center of mass distribution of the chain exhibits oscillatory behavior. Comparison of fibers with different thickness, which contain different numbers of chains, does not indicate significant differences in local and global equilibrium properties for thicknesses in the range 5.6 to 7.6 nm. Surface energies are calculated directly from the on-lattice energetics and presented as a function of the fiber radius.; The topological effect of polymer chains on the nanofiber properties is investigated by a comparison between linear and ring chains of 50 beads on the 2nd lattice. Fibers composed of either linear or cyclic chains exhibit bulk density in the interior region, and the hyperbolic tangent describes the density profile at the surface. The results show slightly differences in the bulk density, interfacial profile and surface energy. Next, the dynamic properties of polyethylene chains in a nanofiber are studied. The mobility increases toward the free surfaces at the scale of individual beads and chains due to the decrease in density. The diffusion of the chain parallel to the surface increases as the nanofiber size decreases. In contrast, the diffusion in the direction normal to the surfaces slows down as the fiber size decreases.; Finally, the fully atomistic model in continuous space of an amorphous polyethylene nanofiber is demonstrated by the reverse-mapping of specific snapshots from the coarse-grained chains. We observe the change in the radial density profile. Compared to the bulk, there is an increase in the potential energy but no significant change in the distribution of torsion angles. The radial distribution function shows similar packing to the bulk.