Studies On Conformational Behavior And Complex Flow Of Polyelectrolytes

Polyelectrolytes are an important branch of soft matter. In the present work, structures and complex flows of several polyelectrolyte systems (including planar and spherical polyelectrolyte brushes, bottle-brush polyelectrolytes, single nucleosomes and chromatin fibre) are studied based on molecular simulation methods, as well as modulation of neutral polymer brushes on electroosmotic flows and their structural characteristics in shear flows. The polyelectrolyte brushes exist in nature extensively, and it becomes possible to synthesize polyelectrolyte brushes due to advances of synthetic chemistry. They can be applied in colloid stability, lubrication, smart surfaces, biological sensors and valves. Therefore, the investigations of these systems have important scientific interest and practical value. In the control of electroosmotic flows, neutral polymer and polyelectrolyte brushes have wide application prospects. Additionally, deep investigations into non-equilibrium dynamics of nucleosomes are of great significance to understanding of the nature of life activity. In this work, physical mechanisms of these systems in different aspects were analyzed extensively, and some new phenomena were found. Compared to experiments and theories, computer simulations can provide a direct visualization on the physical process, and further help us understanding the essence of some complex phenomena. The main contents are listed as follows:(1) We used molecular dynamics simulations to study the effects of counterion valence on static and dynamic behavior of polyelectrolyte brushes. When two opposing polyelectrolyte brushes do not contact each other, increasing the counterion valence leads to a collapsed conformation of the brushes. Under normal compression, the osmotic pressure is examined in detail by decomposing it into various virial terms. It has been found that at small wall separations the increase of the osmotic pressure can be ascribed to the increase of the short-range virial term. At large wall separations, the negative osmotic pressure is observed in trivalent systems. It is noteworthy that though the magnitudes of the normal and shear stress components for the trivalent case are significantly less than those for the monovalent case, the friction coefficient is larger in trivalent systems.(2) The conformational transition of a single bottle-brush polyelectrolyte with charged and neutral side chains is studied. The structure of the bottle-brush polyelectrolyte and the counterion condensation are found to depend greatly on the Bjerrum length. The neutral side chains in a poor solvent can condense into clusters with variable size. This transition is quite different from that in the good solvent case under which there are not observable clusters and only a globular structure is formed at large Bjerrum length. For bottle-brush polyelectrolyte brushes, the effect of the grafting density and the Bjerrum length on the conformational behavior of the brush is examined in detail. At weak electrostatic interaction, the main chains of bottle-brush polyelectrolytes adopt a strongly stretched conformation, and the monomer density profiles of side chains exhibit a clear oscillatory behavior. Compared to the bottle-brush polyelectrolyte brushes, the counterion condensation effect is stronger for the LPE brushes, regardless of whether the electrostatic interaction is weak or strong and of whether the grafting density is low or high.(3) Molecular dynamics simulations are applied to investigate the control of electroosmotic flow by polymer brushes. The effects of grafting density, electric field strength and solvent quality on electroosmotic flow velocity, counterion distribution and conformational characteristics of grafted chains have been studied. The simulation results indicate that in the range of grafting densities investigated, the change of electroosmotic flow velocity is demonstrated to be related to counterion distribution, polymer coverage and interactions between monomers and solvent particles. It is found that counterions tend to move towards the interface between polymer layer and solvent as the grafting density increases. Especially in the poor solvent case, most of counterions gather near the interface at high grafting densities. Additionally, dissipative particle dynamics simulations are used to study electroosmotic flow in a nanopore under strong electrostatic screening. Our results indicate that the screening effect becomes stronger when the polymer layer is in a good solvent. The maximum flow velocity occurs near the wall other than the channel center.(4) The interaction of polyelectrolyte brushes with oppositely charged surfactants has been studied. For planar and spherical polyelectrolyte brushes, we examined the effects of grafting density, surfactant concentration and length on density profiles, adsorption characteristics, end-monomer distributions and structures of surfactant/brush complexes. For bottle-brush polyelectrolytes, the morphology of the complex as functions of backbone stiffness and surfactant concentration was mainly investigated. Our results reveal that the formation of the complex is driven by electrostatic binding interactions of polyelectrolytes with surfactants and hydrophobic interactions of surfactants. It is worth emphasizing that the cooperative binding effect plays a critical role in the formation of the complex. Under different parameter conditions, it was found that polyelectrolyte brushes with surfactants self-assemble into many novel complex structures.(5) Particle-continuum coupling simulations are applied to investigate the interplay between the shear flow of solvent and the polymer conformation. Molecular dynamics and NS equations are used to describe the fluid motion in the particle region and the continuous one, respectively. In the present work, two kinds of shear flows including Couette and oscillatory shear flows are considered. It is found that the particle velocity and the continuous velocity in the coupling region join smoothly. The comparison between the coupling simulation and full molecular simulation indicates the coupling simulation not only can save computational time, but also ensure the validity of simulation results.(6) Monte Carlo (MC) simulations are applied to investigate 30nm chromatin fibre restricted in a nanofluidic channel with square cross-section. The effects of the channel width and Enax on the configuration of the chromatin fibre are investigated in detail. Our results show that the channel width has a considerable influence on conformational characteristics of the chromatin fibre. It is found that at large Emax, the chromatin fibre undergoes a conformational transition from stretched state to coiled state with increasing the channel width. The wrapping and unwrapping of the complex of double-stranded DNA and an oppositely charged sphere (or the coarse-grained model of the octameric histone) are investigated using the coarse-grained model method. It is observed that when increasing the number of charged particles, the wrapping degree of DNA increases and it undergoes a transition from ordering fold to disordering fold. Additionally, the influence of external stretching on the DNA conformation is addressed. Three regions can be found in the extension-force curve. In the intermediate force region, increasing external force has weak effect on the extension of DNA; however, when external force exceeds a certain critical value, internal DNA will unwrap from the sphere.