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Using Langevin dynamics simulation, we study the surface switching properties of mixed polyelectrolyte brushes made of 4-arm stars and linear chains. The length and average charge fraction of the linear chains, Bjerrum length (a characteristic length to quantify the strength of the electrostatic interaction), the solvent quality as well as external electric fields were used as controlling parameters to induce sharp transitions of molecular conformations, leading to brush surfaces dominated by end monomers of linear chains or stars. Phase diagrams in terms of the length and charge fraction of linear chains demarcating different regimes of the composition of brush surfaces were constructed under different external electric fields. Besides the different regimes in the phase diagram of mixed brushes under a stretching electric field or in the absence of electric fields, a new regime, which is featured by the linear chains with very low charge fraction sandwiched between the collapsed and un-collapsed stars, emerges in the phase diagram under a collapsing electric field. The stratification within the brush layer of the mixed brushes under external electric fields was also investigated. [ABSTRACT FROM AUTHOR]

The adsorption of flexible polyelectrolytes (PEs) with the annealed charge distribution onto an oppositely charged sphere immersed in a PE solution is studied numerically with the continuum self-consistent field theory. The numerical study reveals interesting scaling relationships between the boundary layer thickness and the surface charge density of the sphere as well as the degree of ionization of the monomers of the PE chains in the bulk solution. The dependences of the degree of charge compensation of the total amount of charges on adsorbed PE chains over the surface charges upon various system parameters are investigated. In particular, the effect of the radius of the charged sphere on the degree of charge compensation is carefully examined. The numerical study indicates that the curvature effect is closely related to the surface electric potential of the charged sphere. Moreover, in this study of the adsorption of PEs with the annealed charge distribution, a comparison with the corresponding case for PEs with the smeared charge distribution in terms of the boundary layer scaling law and the degree of charge compensation is also presented. [ABSTRACT FROM AUTHOR]

The continuum self-consistent field (SCF) theory is applied to the study of the adsorption of flexible polyelectrolytes (PEs) onto the surfaces of two parallel and infinitely long charged columns, taking into account the short-range monomer-surface non-Coulombic interaction. Due to the complex interplay between the electrostatic and surface interactions, very interesting PE adsorption behaviors in terms of the degree of charge compensation and the bridging chain conformation are found from the numerical solution of the SCF equations. The screening-enhanced salt effect and the permanent adsorption of PEs, irrespectively of the salt concentration, emerge in the presence of the monomer-surface non-electrostatic interaction. The numerical results reveal that, for relatively weak monomer-surface interactions, the degree of charge compensation decreases with increasing monomer-surface interaction. Numerical result shows that, for the strength of monomer-surface interaction above the desorption-adsorption critical value and in a salt-free solution, the total amount of the adsorbed PE chains is linearly proportional to the surface charge density in the high PE charge fraction regime. [ABSTRACT FROM AUTHOR]

We study the dynamics and equilibrium profile shapes of contact lines for wetting in the case of a spatially inhomogeneous solid wall with stripe defects. Using a phase-field model with conserved dynamics, we first numerically determine the contact line behavior in the case of a stripe defect of varying widths. For narrow defects, we find that the maximum distortion of the contact line and the healing length is related to the defect width, while for wide defects, it saturates to constant values. This behavior is in quantitative agreement with the experimental data. In addition, we examine the shape of the contact line between two stripe defects as a function of their separation. Using the phase-field model, we also analytically estimate the contact line configuration and find good qualitative agreement with the numerical results. [ABSTRACT FROM AUTHOR]

The phase-separation dynamics of a ternary mixture (A, B and C) coupled with a reversible chemical reaction between the two constituents A and B is presented. It is demonstrated that the free-energy functional form of time-dependent-Ginzburg-Landau equation describing the phase-separation dynamics of the ternary mixture coupled with a reversible chemical reaction is similar to that of the mixture composed of a block copolymer and a homopolymer. Our simulation study reveals that for the case of equal forward and backward reaction rates, the lamellar thickness scales with the reaction rate constant as a single power law λ[sub L]∼Γ[sup -0.22], regardless of high or low reaction rate regimes. This study sheds insight to the unique features of the involvement of chemical reaction in the phase separation of the ternary mixture. If chemical reaction and phase separation take place simultaneously, the different pattern evolutions at high and low reaction rate constants are originated from the balance between the domain coarsening due to phase separation and the breakup of the continuous phase due to the chemical conversion. The different pattern evolution at high and low reaction rate constants when chemical reaction lags behind phase separation can be interpreted in terms of the discrepancy between the domain sizes at the time step immediately before the turning on of the chemical reaction and the inherent lamellar thickness. It is also pointed out that the crossover of the ternary mixture from one phase region to another, due to the concentration change between A and B, might generate interesting steady-state domain patterns. © 2002 American Institute of Physics. [ABSTRACT FROM AUTHOR]

Using Langevin dynamics simulations, the response of neutral polymer brushes with charged terminal monomers to external electric fields has been investigated. The external electric field is equivalent to the field generated by the opposite surface charges on two parallel electrodes. The effects of charge valence of terminal monomers on the structure of double layers and overall charge balance near the two electrodes were examined. Using the charge density distributions obtained from simulations, the total electric field normal to the electrodes was calculated by numerically solving the Poisson equation. Under external electric fields, the total electric field across the two electrodes is highly non-uniform and in certain regions within the brush, the total electric field nearly vanishes. The probability distribution of electric force acting on one charged terminal monomer was obtained from simulations and how it affects the probability density distribution of terminal monomers was analyzed. The response of polymer brushes with charged terminal monomers to a strongly stretching external electric field was compared with that of uniformly charged polymer brushes.

The response of strong polyelectrolyte (PE) brushes grafted on an electrode to electric fields generated by opposite surface charges on the PE-grafted electrode and a second parallel electrode has been numerically investigated by self-consistent field theory. The influences of grafting density, average charge fraction, salt concentration, and mobile ion size on the variation of the brush height against an applied voltage bias were investigated. In agreement with molecular dynamics simulation results, a higher grafting density requires a larger magnitude of voltage bias to achieve the same amount of relative change in the brush height. In the experimentally relevant parameter regime of the applied voltage, the brush height becomes insensitive to the voltage bias when the grafting density is high. Including the contribution of surface charges on the grafting electrode, overall charge neutrality inside the PE brushes is generally maintained, especially for PE brushes with high grafting density and high average charge fraction. Our numerical study further reveals that the electric field across the two electrodes is highly non-uniform because of the complex interplay between the surface charges on the electrodes, the charges on the grafted PE chains, and counterions.

The complex microstructures of drug particle/ABA star triblock copolymer in dilute solutions have been investigated by a theoretical approach which combines the self-consistent field theory and the hybrid particle-field theory. Simulation results reveal that, when the volume fraction of drug particles is smaller than the saturation concentration, the drug particle encapsulation efficiency is 100%, and micelle loading capacity increases with increasing particle volume fraction. When the volume fraction of drug particles is equal to the saturation concentration, the micelles attain the biggest size, and micelle loading capacity reaches a maximum value which is independent of the copolymer volume fraction. When the volume fraction of drug particles is more than the saturation concentration, drug particle encapsulation efficiency decreases with increasing volume fraction of drug particles. Furthermore, it is found that the saturation concentration scales linearly with the copolymer volume fraction. The above simulation results are in good agreement with experimental results.

The effective interactions between two test counterions and two test solvent dipoles in a semidilute/concentrated weakly charged polyelectrolyte solution are studied using the field-theoretical approach on the mean-field level. From the effective Hamiltonians in terms of the two test counterions and the two test solvent dipoles, respectively, analytical expressions for the effective interactions in the real space are derived. It is unambiguously demonstrated that, at a Theta solvent condition, both the effective interactions between two counterions and two parallel-oriented solvent dipoles consist of an attractive part at intermediate distances of separation. As the electrostatic screening effect from counterions and salt ions quantified by the Debye-Hückel screening parameter becomes stronger, the magnitude of the attraction decreases and the minimum of the attractive profile shift to a shorter distance of separation. On the other hand, when the excluded volume effect is dominant, the effective interactions are purely repulsive. This nontrivial and seemingly counterintuitive result originates from the finite size effect of the monomer units of the polymer chains on the electrostatics of the polyelectrolyte solution. As the size of the monomer units goes to zero, at the Theta solvent condition, the effective interactions between two counterions and two parallel-oriented solvent dipoles are purely repulsive.

We employ self-consistent-field and density-functional theories to simulate the phase behaviors of diblock copolymer-nanoparticle mixtures confined in a two-dimensional circular pore. By varying the block ratio, the size of the pore, and the particle concentration, rich phase structures are discovered. It is shown that the structural frustration, the loss of conformational entropy of the polymer chains under confinement, the curvature of the pore, and the steric packing effect of the particles play important roles in determining the morphologies of the nanocomposites under circular confinement. It is found that the increase in the particle concentration can promote the transformation of concentric lamellas to the cylindrical domains. Our results suggest effective ways to stabilize the phase orderings of diblock copolymer-nanoparticle mixtures under two-dimensional circular confinement.