Phase Transition Of Polymer Chains Studied By Molecular Dynamics Simulations

More than 40 years ago, Stockmayer predicted the collapse transition of polymer chains, which is a fundamental problem in polymer physics. The collapse of polymers is not only useful for designing polymer materials but also can be used to explain some complex biological phenomenons, such as the condensation of DNAs, the assembly of chromatins, and the folding or denaturalization of proteins. Though the collapse of polymers has been studied for many years, the exact nature of it has not been settled yet. In this paper, we mainly investigate the phase behaviors of linear and ring polyethylene chains by molecular dynamics (MD) simulations, and the elastic behaviors of compact polymer chains are also studied by the pruned-enriched-rosenbluth method (PERM).In Chapter 2, the phase behaviors of polyethylene chains are studied by molecular dynamics simulations. First, we investigate the equilibrium properties of polyethylene chains adsorbed on an attractive surface. We find that the adsorption interaction is influential to the collapse of polymer chains. At low temperatures, the chains collapse into globules under weak adsorptions, while they collapse into hairpin structures under strong adsorptions. Second, we study the equilibrium properties of polyethylene chains confined between two adsorption walls. With the confinement-adsorption interactions, some new folding structures are also observed. At some adsorption interactions w and wall seperations D, the chains can also collapse into hairpin structures. At last, a polyethylene chain with one end fixed is considered, since the atomic force microscopy (AFM) tip can produce significant function in polymer crystallization in experiments. We find that when one of the end monomers is fixed the results become very different from the free chains, and the chain can not reach extend coil-like states as the free chains at high temperatures. The collapse of polymer chains is highly changed by fixing one end.In Chapter 3, the phase behaviors of polyethylene ring chains are studied by molecular dynamics simulations. At high temperatures, a fully oblate structure is observed for the ring chains with different chain lengths. For such an oblate structure, a shape factor of...