Simulation Study On Chain Entanglement And Dynamics In Polymer Thin Films

Polymer thin films have received an increasing amount of interests in polymer physics due to their widespread technological applications including protective coatings, lithographic devices, organic photovoltaics, data storage devices, micro reactors, reverse osmosis membranes, etc. Studies based on some important and fascinating systems such as adsorbed polymer films and ultra-thin polymer layers in lubricated contacts with solid surfaces can help improve the properties of adhesion, wetting, lubrication and wear in the interfacial regions, which is of great significance in tribology and material processing.Polymer films on the nanometer scale exhibit significant changes in their physical properties when compared with the bulk. The important reason is that confinement effects as well as the surface and interfacial effects may lead to distorted conformation and altered entanglement and dynamics of polymer chains, which may affect the viscoelasticity of polymer films. However, mechanisms of variations in the entanglement and dynamics of chains in polymer thin films have not yet been clearly understood. In experiments, microscopic changes of chain entanglement and dynamics are mainly deduced from the observation of rheological properties of polymer films due to the restrictions imposed by the characterization methods. The accuracy of those conjectures remains elusive. Besides, theories related to polymer films are far from perfect, and a universal theory that can describe complex polymer film systems of multi-factors is still missing. In simulations, although many efforts have been conducted with the goal of understanding chain dynamics in polymer films, few studies have taken into account the variation in the entanglement of long chains and its influence on chain dynamics due to the limitation of computational capabilities and the lack of analytical methods.Monte Carlo simulation method has the advantage of high computational efficiency. Moreover, information on chain entanglement can be directly obtained using the primitive path analysis method. Therefore, in this dissertation, we use Monte Carlo simulation combined with the primitive path analysis method to investigate the effects of confinement, interfacial interaction, and polydispersity on chain entanglement and dynamics in polymer melts confined by smooth walls, and reveal the entanglement effects on chain dynamics in complex polymer films. The main results are summarized as follows:1. Chain entanglement and dynamics in confined polymer films: In this section, chain entanglement and dynamics in polymer melts confined by neutral walls are investigated using Monte Carlo simulation combined with the primitive path analysis method. The effects of chain length and film thickness on the entanglement and dynamics of confined chains are explored. Simulation results reveal a depletion layer of chains near the walls where chains are compressed in the direction perpendicular to the walls and strongly stretched in the parallel direction. The primitive path segments tend to lie down in the plane of the film and chains become disentangled in films thinner than double radius of gyration. The competition between the disentanglement effect and the confinement imposed by the neighboring particles results in two variation stages of chain dynamics with the increase of film thickness. In thinner films, chains become disentangled while the confinement imposed by the neighboring particles is significantly enhanced. As chain length increases, the dominant factor of chain dynamics in thinner films turns from the confinement effect to the disentanglement effect, leading to a crossover of chain dynamics from rapid slowing down to significant speeding up. In thicker films, the degree of chain entanglement hardly changes, thus the confinement of the neighboring particles dominates the variation of chain dynamics.2. Effect of interfacial interaction on chain entanglement and dynamics in confined polymer films: In this section, chain entanglement and dynamics in polymer melts which have a variable strength of interaction with the confining walls are investigated using Monte Carlo simulation combined with the primitive path analysis method. The wall-polymer interaction systematically varies from attraction to repulsion. The effects of the nature and strength of the wall-polymer interaction, chain length and film thickness on the entanglement and dynamics of confined chains are explored. Simulation results demonstrate that under critical adsorption condition the size and the mobility of chains are maximized due to minimized confinement effects. In non-entangled polymer films, interfacial interaction affects the dynamics of short chains through changing the confinement imposed by the neighboring particles. For entangled polymer films, chains become disentangled under strong confinement, irrespective of the particular wall-polymer interaction. In thinner films, the disentanglement effect dominates and largely accelerates the dynamics of long chains under weak attractive or repulsive wall-polymer interactions. However, when the wall-polymer interaction becomes strong, the disentanglement effect is screened by enhanced confinement of neighboring particles. Therefore, chain dynamics rapidly slows down in thinner films. As the film thickness increases, the degree of chain entanglement hardly changes, and the confinement of neighboring particles gradually weakens, which leads to a slow acceleration of chain movement.3. Chain entanglement and dynamics in bidisperse polymer films: In this section, the entanglement and dynamics of the short-chain and long-chain components in bidisperse polymer melts confined by neutral walls are investigated using Monte Carlo simulation combined with the primitive path analysis method. The effects of film composition and film thickness on the entanglement and dynamics of both chain components are explored. Lengthbased migrations of chains are observed in bidisperse films. That is, longer chains reside away from the walls, and the shorter chains are found nearer the walls. Irrespective of the film composition, the confinement of neighboring particles dominates the dynamics of chains that are shorter than the bulk entanglement length and makes it decrease with the film thickness. The dynamics of chains that are much longer than the bulk entanglement length is closely related to the length and the weight fraction of the short-chain component, which is discussed in terms of reptation and constraint release dynamics combined with the tube dilation model. For bidisperse film compositions with higher weight fractions of chains that are shorter than the bulk entanglement length, the fast relaxation of the short chains results in the long chains reptating inside a dilated tube, and the confinement of neighboring particles dominates the long-chain dynamics that decreases monotonically with the decrease of film thickness. In contrast, the dynamics of the long chains with higher weight fractions is not affected by constraint release and varies nonmonotonically with the film thickness due to the competition between the disentanglement effect and the confinement of neighboring particles. This is in agreement with the situation when the short-chain component is much longer than the bulk entanglement length. However, for compositions where constraint release contributes significantly to the relaxation mechanism, the critical weight fraction of the longchain component at which its dynamics changes is higher than that predicted by the tube dilation model, which may be attributed to the confinement effects caused by the walls.These simulation results can provide a better understanding for the microscopic chain behaviors in complex polymer films and improve the development of related theoretical models, which would confer insight into the technological applications of polymer thin films capped between two solid surfaces such as lubricants.