Computer Simulation Studies Of The Structural And Dynamic Properties Of Confined Polymers Near Solid Surfaces

The configurations and dynamics of polymer chains at and near solid surfaces differ profoundly from those in the bulk. The adsorption and diffusion of confined polymers at surface is always a fundamental and important topic in polymer physics, and has attracted attention for decades already. For example, the hydrophilic polymer-hydrophobic surface system usually appears in the coating industry. Moreover, wetting, surface adhesion, and flow in confined geometries are examples of such systems.In addition, designing a certain pattern to decorate the surface at the nanometer scale is very helpful for controlling the behavior of liquids on laterally heterogeneous surfaces. Related phenomena such as the effect of nanoroughness on wetting and the crossover to capillary filling have an impact on emerging applications, including nanofluidic devices, nanotemplating, and surface rheology. Computer simulations for the processes and mechanism of a single polymer chain adsorbed and diffusing on a surface are very important, as experimental studies on an isolated polymer chain are difficult in most cases.Another examples about the polymers adsorbed on the surface contain the film deposition and porous membrane formation. In modern world polymeric membranes, both porous and compact, are widely used in industry. Examples of industrial applications for the porous membrane are microfiltration, ultrafiltration, reverse osmosis and gas separation. Controlling the morphology of the membrane is of great importance in tailoring them to perform appropriately for specific applications, since it is the size and distribution of pores that largely determines their function. Now there are many techniques to prepare porous polymeric films, in which"immersion precipitation"phase inversion is the most efficient way. Control over membrane structural formation is made challenging in experiment by the complex interplay of thermodynamics and kinetics during the immersion precipitation process. Because there are many properties difficult to control experimentally, modeling and simulations are applied on immersion precipitation process to provide a crucial insight and design guidance.The compact films are generally considered as"protective coatings"which are used to improve material hardness and wear resistance, reduce friction, provide better oxidation resistance and so on. Thus these coatings are employed in a wide range of applications such as aerospace, automotive and surgical/medical tools. This dissertation also investigates the formation and morphology control of the compact polymeric film comprehensively by computer simulations.In this dissertation, we adopt computer simulation to do scientific researches because it can visualize the above-mentioned physical processes directly. It helps us understand and explore the laws inherent in these natural phenomena. According to different research objects, we carry out molecular dynamics (MD) and dissipative particle dynamics (DPD) simulations from the microscopic and mesoscopic angles of view to study the topics mentioned above in detail, respectively. MD simulation method solves the classical equations of motion (Newton equation) according to a force field, in other words, using the suitable potential function to describe the system intramolecular and intermolecular interactions. In this way, a MD simulation generates a trajectory that describes the changes of dynamics variables with time evolution, from which the macroscopic properties (energy, pressure, etc.) can be calculated. Within the DPD method, all the particles interact with each other through three pairwise forces: a conservative force, a dissipative force, and a random force. These forces are very soft, so the integration time step can be larger than that in MD. The time scale in DPD simulation can be at milliseconds. It's also due to the soft repulsion potential, we can unite some molecules or polymer segments into one DPD bead, thus the DPD model can be used to study the systems at micron length scale. The pairwise interactions in the DPD model result in the momentum of the system being conserved. Both above simulation methods not only give apparent images of theoretical models, but also describe the polymer dynamic behavior exhaustively. At present they have become powerful tools in the research fields of Material Science, Chemistry, Physics and Biology.In our study, MD and DPD methods are used to study the adsorption and diffusion behaviors of a single polymer chain on different surfaces, and the formation processes of both porous and compact polymeric membranes on the solid substrates from the microscopic and mesoscopic angles of view, respectively. The main results are as follows:(1) MD simulations are applied to investigate the adsorption and diffusion processes of a single hydrophilic poly(vinyl alcohol) (PVA) chain with different degrees of polymerization (N) on a hydrophobic graphite surface. It is expected that the chain and the surface"dislike"each other because one is hydrophilic and the other is hydrophobic. But surprisingly, a short PVA chain is well adsorbed on the surface to display two-dimensional configuration, accompanied by large changes in the chain configuration. With increasing N, the chain turns gradually from two-dimensional adsorption to possessing certain height in the direction perpendicular to the surface. Moreover, the adsorption energy increases and the diffusion coefficient decreases as N increases. In particular, for N = 20 in equilibrium, the hydroxyls of this short chain are close to the graphite surface in the stable adsorption configuration. In addition, we change the effective dielectric constant to 76.0 to mimic good solvent condition. The chain configurations and the diffusion coefficients both vary in contrast to the foregoing results.(2) MD simulations are applied to study the adsorption of polyethylene (PE) with different N on patterned graphite surfaces that contain nanoscale protrusions. The influence of the nanostructure on the strong attractive interaction inherently in the hydrophobic PE and hydrophobic graphite system is investigated by modifying the top surface area and the height and the shape of the protrusions. The results are analyzed in terms of the chain configuration, the adsorption energy, the global orientational order parameter, and the normalized surface-chain contacting pair number in the first adsorption layer. When the size of the protrusion increases, the adsorption energy, the order parameter, and the normalized surface-chain contacting pair number decrease at a fixed chain length. When the size of the protrusion is fixed, the average adsorption energy per monomer and the order parameter decrease with increasing N because of the stronger intramolecular interactions between the monomers. Changing the protrusion shape in a suitable way will effectively reduce the strong surface-chain interaction.(3) The kinetics process of membrane formation by immersion precipitation is investigated using DPD simulation method. We study the influences of varying the chain length of the polymer composing the membrane, the solvent size in the polymer solution, and the nonsolvent size and amount in the nonsolvent bath on the liquid-liquid demixing process and the membrane morphology in detail. The results are analyzed in terms of the characteristic domain size (R) in the polymeric membrane. R is as a function of simulation time, which first decreases, then increases due to the changes of the number of the interfaces between membrane polymer and nonsolvent. Moreover, the occurrence from spinodal decomposition to late stage domain coarsening is faster for the system with N = 40 than N = 100. When we take the chain-like instead of the one-site solvent, R turns to be larger at the same simulation time for the same system. But when we enlarge the nonsolvent size, it is surprising that the nonsolvent can not diffuse into the polymer solution to exchange for solvent. In addition, the domain size increases with increasing the amount of the nonsolvent, especially in the R coarsening process.(4) DPD simulations are applied to investigate the monolayer and multilayer film formations on different solid substrates by physical deposition. The influences of the polymer concentration and degree of polymerization, the solvent quality, and the interactions between the polymer solution and the solid substrate surface on the film formation dynamics and the mechanism are studied in detail. The results are analyzed in terms of the thickness and the shape of the deposited film, the kinetics of phase separation in the polymer solution, and the contact angle formed between the polymer aggregations and the substrate surface. Moreover, we suggest two strategies, designing a cycle deposition process analogous to"chemical titration"and physically blocking interlayer diffusion by a simple crosslinked network barrier, to deposit the compact monolayer and multilayer films with better quality, respectively.