Dissipative Particle Dynamics Simulation Study On The Structure And Dynamics Of Polymer In Confinements

For the structure and the dynamics of polymers in the confined condition, many problems have not been solved. We use dissipative particle dynamics (DPD) simulations to capture the essential physical and chemical properties of the system with a particular emphasis on the role played by the confinement that is fundamental to the living systems and important to novel technology developments. For the polymer translocation through protein channels and nuclear pores, we have built up a model based on DPD to study the possible influences of such as external field and the polymer-surface interaction on the translocation. We study the structure and dynamics of polymer in nanoscale slit pores, especially in the uniform shear flow and the pressure driven flow. The results may enhance our understanding of single chain dynamics, the ability to control the migration of polymer in confined dilute solution and the design of various applications of polymeric and colloidal systems in microfluidic and nanofluidic devices. We try to investigate the influence of the patterned confinement on the microphase separation of diblock copolymer which is helpful for the formation of novel materialsComputer simulations can visualize these physical processes directly. In this dissertation, we carry out DPD simulations to study the physical and chemical preperties of polymer in confinements. 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 steps can be very large. It's also due to the soft repulsions, we can Coarse-grain some molecules or polymer segments into one DPD bead, thus the DPD model can be used to study the systems at mesoscopic length scale. Hydrodynamic interactions (HI) is very important for the structure and dynamics of polymer in dilute solution and it is intrinsically embedded in the DPD model because of the pari-wise interactions which result the momentum of the system being conserved. DPD method has been applied on the study of polymer blends, microphase separation of the block copolymers, complex fluid, self-organizing of amphiphilic molecules into memberane, and the budding and fission of bionic micelles.In our study, DPD method is used to study how the polymer translocates the pore under flow, the structure and dynamics of polymer in confined dilute solution under flow, and the microphase separation of diblock copolymer in different confinement conditions. The main contents are as follows:(1) DPD simulations are carried out to study the translocation of a single polymer chain through a pore under fluid field. The influences of the field strength E, the chain length N, the solvent qualityαsp, and the pore size h on the translocation time are evaluated. The translocation timeτ, which is defined as the time that the chain moves through the pore completely in the direction of the driving force, scales with the field strength E asτ~ E?0.48±0.01. We find that the translocation time is proportional to the chain length, which is in agreement with the experimental results and theoretical predictions. Tracing the variation of the radius of gyration, Rg, and the polymer configuration during translocation, we observe that the chain is elongated when it is passing through the pore, which manifests that the chain is not in equilibrium during the translocation process. We also find that the worse the solvent quality is, the less time it will take to translocate, no matter what the size of the pore is. This is because the free energy is decreased through the pore in the poor solvent, and the chain is easier to translocate. If the size of the pore is enlarged, the translocation time will be shorter. The information we gain from this study may benefit to the DNA sequencing.(2) The structure and dynamics of confined single polymer chain in dilute solution, either in equilibrium or subjected to different flow fields, are investigated by means of dissipative particle dynamics simulations. The no-slip boundary condition without density fluctuation near the wall is taken into account to mimic the environment of a confined plate. The dependence of the radius of gyration and the diffusion of the chain on the strength of the confinement and the solvent quality is studied. In non-equilibrium systems, both the simple uniform shear flow and the pressure driven flow acting on a dilute polymer solution are investigated. The effect of the interaction between polymer and solvent in these two flow conditions are found to be the same: the polymer migrates to the center of the channel when the interaction is reduced. With increase of the flow strength, there are two peaks with a dip in the center of the polymer density profile in the pressure driven flow and only one peak in the center in the uniform shear flow, which are in agreement with previous investigations.In three dimentional system, we study the structure and dynamics of confined single polymer chain in dilute solution, either in equilibrium or subjected to different shear rates in the uniform shear flow field. The dependence of the radius of gyration, especially in three different directions, and the density profile of the chain mass center on the strength of the confinement and the Weissenberg number (Wn) is studied. The effect of the interaction between polymer and solvent on the density profile is also investigated in the cases of moderate and strong Wn. In the high shear flow, the polymer migrates to the center of the channel with increasing Wn. There is only one density profile peak in the channel center in the uniform shear flow, which is in agreement with the results of experiments and theory.(3) The effect of the solid surface on the diblock copolymer microphase separation is investigated by means of dissipative particle dynamics simulations. If the surface is smooth and possesses the same interaction with the two components of the diblock, we can obtain lamellar phases which are perpendicular to the solid surface and parallel to each other. When the surface has different interactions with the two diblock components, the lamellar phase is parallel to the surface. The surface with different patterns is designed by changing the selectivity of the patch of the surface for the diblock copolymer, and the microphase structure under patterned surface confinement is more complex. These results can provide references for forming long range nanometer structure, and provide direction for designing and making the block copolymer membrane.