Anna A. Mercurieva

Conformations of polymer and polyelectrolyte stars

The structure of polymer and polyelectrolyte stars in solution was studied by means of joint analysis of the results of analytical consideration, allowing for nonlocal effects, and numerical simulation based on the Scheutjens-Fleer self-consistent field approach. A limitation of the theoretical treatment is the assumption that all ends of polymer chains are fixed onto the external surface and its benefit is the possibility of obtaining compact and interpretable results. The Scheutjens-Fleer approach makes it possible to study conformations without introduction of additional limitations. The combination of analytical methods and direct numerical calculation turns out to be especially informative.

Liquid-crystalline polymer brushes: deformation and microphase segregation☆

Abstract A statistical theory of the structure and thermodynamics of a planar brush (‘accordion’) formed by bridged polymer chains containing mesogenic segments and immersed in a solvent is developed. It is shown that deformation of an accordion can lead to the formation of a two-phase structure with coexisting liquid-crystalline (LC) and swollen microphases. Phase diagrams for accordions with different grafting densities are obtained. The influence of anisotropic interaction between mesogenic segments on the structure of phase diagrams is investigated.

Microphase segregation in bridging polymeric brushes : Regular and singular phase diagrams

A mean-field theory of deformation-induced microphase segregation in bridging polymeric brushes anchored to two parallel surfaces is presented. Models with isotropic and orientation-dependent liquid-crystalline interactions between segments are considered. For the first model, the problem is similar to that of classical liquid-vapor phase separation, and the phase diagram in the P-T plane has a line of first-order transitions terminating at the critical point. We show that the critical pressure is negative implying that a free brush tethered only to one surface always exists at supercritical conditions and hence cannot undergo the collapse phase transition. In the second model, the free energy density depends on two coupled order parameters, one related to segment density and the other to the orientational order, which strongly modifies the phase behavior. Depending on the grafting density the system is described by a phase diagram of a regular or a singular type. In the regular phase diagram the first-order transition line terminates at the critical point. In a singular diagram, the first-order transition line extends to infinity; the critical point corresponds to infinite pressure so that the system undergoes the phase transition at arbitrary external pressures. Regular phase diagrams correspond to dense grafting, and singular ones to sparse grafting. The change from a regular phase behavior to another occurs at a certain marginal value of the grafting density. On approaching this value the critical point on the regular diagram moves to infinity, logarithmically with the deviation from the critical grafting density. We relate the analytical properties of the free energy density as a function of the segment concentration to the type of the phase diagram and the shape of the coexistence curve in the temperature-concentration plane.

Amphiphilic polymer brush in a mixture of incompatible liquids

An analogue of the Alexander-DeGennes box model is used for theoretical investigation of polymer brushes in a mixture of two solvents. The basic solvent A and the admixture B are assumed to be highly incompatible (Flory-Huggins parameter X AB = 3.5). Thermodynamics of a polymer in the solvents A and B are described by parameters X B < X A ≤ 1/2. The equilibrium behavior of a brush is investigated in dependence on solvent composition, grafting density, polymer-solvents and solvent-solvent interactions. The possibility of a phase transition related with a strong preferential solvation of a brush by a minor solvent component with higher affinity to polymer is shown and examined. Microphase segregation inside a brush is also demonstrated despite overestimating of the brush homogeneity given by the box model. A further simplification of the model permits to obtain scaling formulas and to investigate main regularities in the brush behavior. This offers a clear physical picture of the phase segregation inside a brush in correlation with the phase state of a bulk solvent.