Dissipative Particle Dynamics Study On Structure And Dynamics Of Confined Complex Fluids

Study of complex fluid is an interdisciplinary field, which includes physics, chemistry, biology and material science. Chemical reactions and spatial dimensions can restrict the phase behavior and dynamics of polymers. Confined polymer complex fluids can control the phase structure and the chain structure. Polymer chain translocation is also in confined enviroment. Structure and dynamics of complex fluids play an important role in the preparation and processing of polymer materials, in which exists a competition between reversible reaction and phase separation. Accompanied with the reversible reaction, phase separation will strongly affect the mechanical properties of polymer materials. Therefore, it is necessary to study the coupling between the two dynamic processes. Moreover, association reaction is a basic reaction in material science. Some new product morphology can be obtained through the association reaction in polymer solution. These complex structures will change the mechanical properties, thermal stability and solvent resistance of the polymer materials. Therefore, the study of the association reaction is able to predict the research and development of new materials. In addition, translocation of a polymer chain through a nanopore in polymer solution is a complex dynamical process which is related to numerous biological relevant mechanisms. To its biological application, the translocation dynamics become a challenging topic in polymer physics.Computer simulations can finely deliver the details of the physical processes. It helps us to understand and explore the laws in these natural phenomena clearly. In our study, DPD method and the coarse-grained MD method are used to study the binary mixture phase separation coupled with a simple reversible reaction, the process of association reaction in telechelic polymer solution and the passage for a polymer chain through a nanopore in the absence of any external driving force. The main results are as follows:(1) The influence of reversible reaction on the phase separation of binary immiscible mixtures has been investigated by DPD method with Lowe-Andersen temperature controlling technique in two dimensions. Apparent suppressing effects due to reversible reactions on the phase separation are well illustrated in our simulations. The influences of quench depth, reaction rates and viscosity on the phase separation of the binary fluids have also been considered in this reversible reaction system. The domain growth scaling exponent is related to both the reaction rate and the quench depth. Viscosity of the component is also an important factor which is considered. For low viscosities, there is a 0.4 scaling in the cases of low reaction rates and a 0.25 scaling in the cases of high reaction rates. In the cases of high viscosities, there is a 0.25 scaling in the cases of low reaction rates and a 0.5 scaling in the cases of high reaction rates. Therefore the domain sizes with high viscosities and low reaction rates are very similar to those with low viscosities and high reaction rates.(2) The process of association reaction in telechelic polymer solution is investigated by DPD simulation method. According to the characteristics of telechelic polymer, an association reaction model has been constructed and the reaction environment and the reaction conditions are also been provided in detail. There is an important parameter Ps to control the association reaction. We study the effect of some master factors such as reaction degree, length of telechelic polymer and the concentration of telechelic polymer on the product morphology. The primary factors in association reaction include the concentration of polymer, Ps which controls the microstructure of system and the length of telechelic polymer which control the connectivity of product. In addition, we also study the energy of association and the number of characteristic structures in steady state. Besides, we find there is a linear relationship between the number of characteristic structures and the concentration of polymer.(3) The polymer translocation through a nanopore in the absence of an external driving force is investigated using both WCA potential and DPD potential in two-dimensional simulations. A polymer chain is placed symmetrically in the middle of a pore initially. In DPD model, density fluctuation nearby the wall is eliminated, and bond crossing is reasonably avoided by using an analytic geometry method. There are two statistical techniques used to present the results. One is the mean translocation time tE, and the other is the most probable escape time tM. By comparing the results calculated by tM and tE, we find the adoption of the technique does not change the scaling behavior of the chain Accurate estimates for the scaling exponents of the mean escape time tE as a function of N are obtained. The effect due to hydrodynamic interactions is the major factor in determining the scaling exponents with increasing pore sizes. In WCA model, for small pore sizes, the scaling close to 5/2 implies the condition without HI effect, and for larger pore sizes the scaling close to 9/4 indicates the condition with HI effect. However, in DPD model the scaling is close to 2 for small pore size, and the scaling is close to 7/4 for larger pore size.