Molecular Dynamics Simulation Studies On The Structure And Dynamic Evolution Of Polymers At Surface Or Interface

Research on the structure and dynamic evolution of polymers at surface or interface is in a state of active growth and exciting development. The most fundamental and important topic is the adsorption and diffusion of confined polymers at surface. The configurations and dynamics of polymer chains at and near solid surfaces differ profoundly from that in the bulk. If polymer chains with well-defined length adsorbed onto solid substrate, well-ordered assembled monolayer could be prepared. Conjugated molecules are the most common candidates and they can be used in active electronic and optoelectronic organic materials. Many techniques could prepare thin uniform polymer films. However, dewetting will take place if a partially wetting film is placed on a solid substrate. Therefore it is of much importance to study the stability of the adsorbed polymer thin film. Another promising building block to self-assemble into highly ordered structures is rod-coil block copolymers. The local packing of rigid rod gives rise to additional orientational ordering within the self-assembled structures. It is interesting to balance the competition between maximizing the rod-rod contacts and the entropy contributed by the coils, and hence to prepare electronic and optoelectronic materials in nanoscale.Molecular dynamics (MD) simulation method is adopted to probe the laws inherent in the above-mentioned physical processes. MD simulation method solves the classical equation of motion (Newton equation) according to a force field to describe the system intra- and inter-molecular interactions. A trajectory which contains the dynamic quantities varying with time will be generated and therefore the macroscopic properties (energy, pressure, etc.) can be calculated. Besides traditional explicit MD simulation method, coarse-grained molecular dynamics (CGMD) method which ignores the chemical detail of atoms and therefore can provide physical quality in large time and space scale is also adopt. In this thesis, we investigated not only the adsorption of single-chain polystyrene on the graphite surface and the self-assembly of permethyldecasilane on silicon surface using atomistic MD method, but also the dewetting behavior of liquid film on partial wetting solid surface and the self-assembly of tethered nanorods using CGMD method. The main results are as follows:(1) A molecular dynamics simulation was used to investigate the adsorption of single-chain polystyrene on the graphite surface. The results show that the structures of the adsorbed PS are usually elongate in shape rather than circular from the top view; there is an exponent relationship between the component of the mean square radius of gyration parallel to the surface and the number of monomers, and the exponent obtained is about 1.04; the main driving force is the van der Waals force between chain segment and surface.(2) Atomistic molecular dynamics simulations have been used to investigate the adsorption of permethyldecasilane (MS10) on silicon (001) surface. The condition under which the self-assembled monolayer forms is examined. The properties of the well-ordered structures, including the packing patterns, the equilibrium distances between two neighbor chains and the tilt angles are calculated to characterize the structure of the self-assembled monolayer. The results are comparable with those in experiment.(3) Coarse-grained molecular dynamics simulations have been carried out to investigate the dewetting behavior of liquid film on partial wetting solid surface. Spontaneous dewetting is initiated by removing a band of strip from both ends of the liquid film which has achieved equilibrium under 3D periodic condition. The solid-liquid interaction and temperature are varied to show their influence on the dewetting dynamics during dewetting as well as the shape evolution of the liquid film. Consistent with the results obtained in previous experiment and simulations, the liquid film recedes at a constant speed initially with different couplings and temperatures. Furthermore, smaller coupling parameters or higher temperatures tend to accelerate recession speed of the liquid film and shorten the constant-speed duration. Moreover, obvious rims are not observed always and both coupling parameter and temperature could influence the appearance of the obvious rims.(4) Coarse-grained molecular dynamics simulations have been used to investigate the self-assembly of tethered nanorods with relatively high aspect ratio. The tether number and location are varied to show their influence on the self-assembly of the nanorods. We find that laterally-tethered nanorods self-assemble into structures with flat interfaces; these structures include stepped ribbons, stepped lamellae and lamellae with rods packing into bilayer sheets. The stepped lamellar phase is observed for the first time in this study. End-tethered nanorods are prone to self-assemble into structures with curved interfaces, and the assembled structures observed here include spherical micelles and nematically aligned cylinders. The cylinder phase exists at high number densities, instead of the lamellar phase typically found for end-tethered nanorods with relatively lower aspect ratio.