Simulation Study Of Polymer Diffusion On Rough Surfaces

The diffusion behavior of polymers on solid surfaces is directly related to many applications,ranging from traditional surface coatings and lubricants,lithography and polymer composites filled with particles,to packaging of viruses and DNA segregation of bacteria in biological systems.A thorough understanding of the dynamics of polymer on solid surfaces is not only conducive to the clarification of the molecular mechanism of confined polymer property deviating from that in the bulk,but also to the selection of the processing and application conditions of polymer materials.Hence,it has been the frontier and focus of polymer in recent years.Although the diffusion behavior of polymers on solid surfaces has been extensively explored via theoretical,experimental and computer simulation methods,there still exist some crucial problems that need to be solved:a)During the process of theoretically studying the dynamics of polymers on surfaces,the surfaces are usually assumed as absolutely smooth.However,surfaces in practical applications and in real systems are not completely smooth.And the surface roughness is difficult to be systematically adjusted with current experimental technology.Additionally,previous computer simulations typically consider only two extremes(i.e.,absolutely smooth and completely rough).An evaluation mechanism of roughness of solid surface is urgently needed;b)Owing to the difficulty of precision synthesis of polymer with other architectures,most of previous experimental researches on polymer diffusion behavior on solid surface focus on linear chains.However,the scarce experimental results about ring polymers conflict with simulation results.The clear demonstrations from microscopic physical images are absent;c)The orientation of polymer chains in confined melts can induce the disentanglement,which further accelerates their dynamics.While,obstacles involved by rough surfaces aggravate the entanglement and confinement.Hence,it is necessary to systemically assess the effects of the competition between disentanglement induced by confinement and extra entanglements introduced by obstacles on the diffusion dynamics of polymer chains.In this dissertation,we systemically explore the effects of surface roughness on the diffusion and relaxation dynamics of polymer chains using molecular dynamics simulation method with the combination of coarse grained-polymer chains and rough surface model with adjustable roughness,as well as layer resolved entanglement and dynamics analyses for confined polymer melt,where the roughness is characterized as the height of obstacles and the distance between obstacles.The simulation results obtain the physical images of polymer diffusing on surfaces within different systems,establishing an evaluation mechanism of roughness with the reference of polymer chain size,clarifying the relativity of roughness to polymer with different architecture,demonstrating the desynchrony of diffusion and relaxation of ring polymers on rough surfaces,discovering the intrinsic connection between surface roughness and influenced extent and scope of entanglement and dynamics of polymer chains in confined melt.It is hoped that the results can provide theoretical guidance for the potential applications.The productive results are as follows:1 Effects of surface roughness on the diffusion dynamics of polymer chains:In this section,we employ molecular dynamics simulations to explore the diffusion behavior of a single polymer adsorbed on surfaces with different roughness,where the roughness is characterized as the height of obstacles and the separation distance between obstacles.According to the changes of diffusion dynamic with the increase of obstacle height,we divide surfaces into three types for strongly adsorbed polymer in dilute solution:smooth surfaces(Rouse dynamics),rough surfaces(reptationlike dynamics)and transitional surfaces.These results provide a semiquantitative criterion for the definition of smooth and rough surfaces with respect to strongly adsorbed dilute polymer solution,and provide helpful theoretical guidance for polymer separation by using diffusion on surfaces.2 Unusual diffusion dynamics of polymer chains on transitional surfaces:In this section,we employ molecular dynamics simulations to thoroughly explore the diffusion behavior of polymer chains with an extensive length scale on rough surfaces and transitional surfaces.The simulation results demonstrate that on transitional surfaces,the dynamics of polymer chains separately satisfies the Rouse dynamics(lateral size less than a half of obstacle distance)and the reptationlike dynamics(lateral size beyond a half of obstacle distance).With chain length beyond a certain critical length,polymer chains have sufficient free energy to form loop conformation that could stride obstacles in vertical direction.The dynamics further degrade to a quasi-Rouse dynamics.These results illustrate the complete dynamics for the diffusion of polymer chains on different surfaces,which is of great important to the establishment of the theoretical system of polymer diffusion dynamics on surfaces.3 Diffusion dynamics of ring polymers on rough surfaces:In this section,weemploy molecular dynamics simulations and coarse-grained chain model of ring polymer to investigate the effects of the unique closed architecture of polymer on the diffusion dynamics on surfaces.The simulation results demonstrate that with the increase of chain size,ring polymers are gradually anchored on obstacles.The closed architecture introduces additional confinement from rough surfaces,which further induces the uncorrelation of diffusion and relaxation of ring polymers:the diffusion dynamics can be divided into three regimes;the relaxation dynamics only exhibits two regimes.These results clarify the relativity of surface roughness on polymers with different topological structure,and reveal the unique dynamics induced by the closed architecture of ring polymers on rough surface,which is helpful to deepen the understanding of the nature of ring chain and rough surface.4 Dynamics of confined polymer melts on rough surfaces:In this section,we employ molecular dynamics simulations,ZI-code method and layer resolved mean squared displacement of a central monomer and the center-of-mass to explore the entanglement and dynamics of chains in rough surface confined polymer melt.The simulation results reveal that the disentanglement of polymer chains near the smooth surface is caused by the formation of train conformation induced by surface.When the surfaces involve obstacles,these train conformations can entangle with obstacles to restrain(involving sparse obstacles)or reverse(involving dense obstacles)the disentanglement tendency.However,the short-lived entanglements between the polymer chains and short obstacles exert only a slight influence on the relaxation and diffusion because the chains can stride these obstacles easily during diffusion.With the increase of obstacle height,the effects on confined melt are further extended.The chains that are totally confined in the array of the obstacles severely entangle with these obstacles.And the distance between obstacles directly controls the relaxation and diffusion of the confined polymer chains.As the polymer chains move away from surfaces,the influenced degree of entanglement and dynamics decreases rapidly.When the polymer chains have little contact with the surfaces or the obstacles,the entanglement state and dynamics of polymer chains restore that in the bulk.These results clarify the effects of the surface roughness on the influence scope and degree of the static and dynamic properties of polymer chains in confined melt,which is helpful to further understand the phenomenon that the polymer film properties in the actual system do not conform to the theoretical prediction and simulation results.The simulation results in this dissertation provided clear physical images for polymer conformation and dynamics on rough surfaces with different roughness,which is helpful to qualitatively or semi-quantitatively analyze the roughness of realistic surfaces.Additionally,the results also provided beneficial theoretical guidance to explore the potential application.