Alexey A. Polotsky

Sharp and fast: sensors and switches based on polymer brushes with adsorption-active minority chains.

We propose a design for polymer-based sensors and switches with sharp switching transition and fast response time. The switching mechanism involves a radical change in the conformations of adsorption-active minority chains in a brush. Such transitions can be induced by a temperature change of only about ten degrees, and the characteristic time of the conformational change is less than a second. We present an analytical theory for these switches and support it by self-consistent field calculations and Brownian dynamics simulations.

Stimuli-Responsive Brushes with Active Minority Components: Monte Carlo Study and Analytical Theory

Using a combination of analytical theory, Monte Carlo simulations, and three-dimensional self-consistent field calculations, we study the equilibrium properties and the switching behavior of adsorption-active polymer chains included in a homopolymer brush. The switching transition is driven by a conformational change of a small fraction of minority chains, which are attracted by the substrate. Depending on the strength of the attractive interaction, the minority chains assume one of two states: an exposed state characterized by a stem-crown-like conformation and an adsorbed state characterized by a flat two-dimensional structure. Comparing the Monte Carlo simulations, which use an Edwards-type Hamiltonian with density-dependent interactions, with the predictions from self-consistent-field theory based on the same Hamiltonian, we find that thermal density fluctuations affect the system in two different ways. First, they renormalize the excluded volume interaction parameter vbare inside the brush. The prope...

Polymer adsorption onto random planar surfaces: interplay of polymer and surface correlations.

We study the adsorption of homogeneous or heterogeneous polymers onto heterogeneous planar surfaces with exponentially decaying site-site correlations, using a variational reference system approach. As a main result, we derive simple equations for the adsorption-desorption transition line. We show that it is preferable to have a small amount of strongly adsorbing sites or monomers rather than a greater amount of weakly adsorbing ones. The results are discussed with respect to their implications for the physics of molecular recognition.

Collapse of a weak polyelectrolyte star in a poor solvent

We present a theory of intramolecular collapse transition in a weak (pH sensitive) polyelectrolyte (PE) star induced by a decrease in the solvent strength and/or variation in the ionic strength in the solution. Our system mimics conformational coil-to-globule transitions in individual star-shaped thermo- and pH-sensitive (e.g. poly(dimethylaminoethyl methacrylate)) macromolecules in dilute aqueous solutions. Systematic comparison with the behaviour of non-ionic and strong (quenched) polyelectrolyte stars in poor solvents enables us to unravel specific features of the collapse transition in weak polyelectrolyte stars. We demonstrate that, depending on temperature and the ionic strength of the solution, a vast diversity of different scenarios for the salt- or temperature-induced collapse transitions may take place. Both at high or low ionic strength a collapse transition induced by a decrease in the solvent strength occurs continuously resembling the collapse of a neutral polymer star. On the contrary, at intermediate salt concentrations the collapse of a weak polyelectrolyte star may feature a first order phase transition, which involves the co-existence of a collapsed, weakly ionized state with a swollen, strongly ionized state. At a fixed temperature the collapse transition can be triggered by an increase in salt concentration. In the latter case the transition may occur via a sequence of two co-existence regimes separated by continuous though non-monotonous variation in the star dimensions as a function of salt concentration.

Mechanical unfolding of a homopolymer globule studied by self-consistent field modeling

We present results of numerical self-consistent field (SCF) calculations for the equilibrium mechanical unfolding of a globule formed by a single flexible polymer chain collapsed in a poor solvent. In accordance with earlier scaling theory and stochastic dynamics simulations findings we have identified three regimes of extensional deformation: (i) a linear response regime characterized by a weakly elongated (ellipsoidal) shape of the globule at small deformations, (ii) a tadpole structure with a globular “head” coexisting with a stretched “tail” at intermediate ranges of deformations, and (iii) an uniformly stretched chain at strong extensions. The conformational transition from the tadpole to the stretched chain is accompanied by an abrupt unfolding of the depleted globular head and a corresponding jump-wise drop in the intrachain tension. The unfolding-refolding cycle demonstrates a hysteresis loop in the vicinity of the transition point. These three regimes of deformation, as well as the first-order li...

A Quantitative Theory of Mechanical Unfolding of a Homopolymer Globule

We propose the quantitative mean-field theory of mechanical unfolding of a globule formed by long flexible homopolymer chain collapsed in poor solvent and subjected to extensional deformation. We demonstrate that depending on the degree of polymerization and solvent quality (quantified by the Flory−Huggins χ parameter) the mechanical unfolding of the collapsed chain either may occur continuously (by passing a sequence of uniformly elongated configurations) or involves intramolecular microphase coexistence of a collapsed and a stretched segment followed by an abrupt unraveling transition. The force−extension curves are obtained and quantitatively compared to our recent results of numerical self-consistent-field (SCF) simulations. The phase diagrams for extended homopolymer chains in poor solvent comprising one- and two-phase regions are calculated for different chain length or/and solvent quality.

Influence of sequence correlations on the adsorption of random heteropolymers onto homogeneous planar surfaces

Using a reference system approach, we develop an analytical theory for the adsorption of random heteropolymers with exponentially decaying and/or oscillating sequence correlations on planar homogeneous surfaces. We obtain a simple equation for the adsorption--desorption transition line. This result as well as the validity of the reference system approach is tested by a comparison with numerical lattice calculations.

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.

Theory of Mechanical Unfolding of Homopolymer Globule: All-or-None Transition in Force-Clamp Mode vs Phase Coexistence in Position-Clamp Mode

Equilibrium mechanical unfolding of a globule formed by long flexible homopolymer chain collapsed in a poor solvent and subjected to an extensional force f (force-clamp mode) or extensional deformation D (position-clamp mode) is studied theoretically. Our analysis, like all previous analysis of this problem, shows that the globule behaves essentially differently in two modes of extension. In the force-clamp mode, mechanical unfolding of the globule with increasing applied force occurs without intramolecular microphase segregation, and at certain threshold value of the pulling force the globule unfolds as a whole (“all-or-none” transition). The value of the threshold force and the corresponding jump in the distance between the chain ends increase with a deterioration of the solvent quality and/or with an increase in the degree of polymerization. In the position-clamp mode, the globule unfolding occurs via intramolecular microphase coexistence of globular and extended microphases followed by an abrupt unrav...

Mixed supercrystalline structures in mixtures of ABC-triblock and AB(BC)-diblock copolymers, 1. Lamellar structures in bicomponent mixtures

The theory of lamellar superstructures in binary mixtures of AB diblock and linear ABC triblock copolymers under the condition of strong segregation between chemically different blocks is developed. This system is considered using Alexander-de Gennes (box-model) and self-consistent field (SCF) models. The formation of a mixed lamellar superstructure comprising both mixture components is proved. It is shown that a mixed lamella may be the only type of lamellae in the mixture, or it may coexist with pure diblock lamellae, depending on the mixture composition, local characteristics of blocks (thickness and Kuhn segment length), and surface tension coefficients at the A/B and B/C interfaces. Preliminary experimental results provide support of these theoretical estimations. The formation of mixed lamellae in a mixture of linear ABC and branched (AB) 2 C block copolymers is also considered.