Zuzana Limpouchová

Molecular dynamics simulations of a polyelectrolyte star in poor solvent

We used Molecular Dynamics (MD) simulations to investigate the conformations of a star polyelectrolyte in poor solvent. We observed several distinct morphologies: at low number of arms, pearl-necklace structures are formed on individual arms; at higher number of arms inter-arm bundling is the dominant structural motif.

Polyelectrolyte shells of copolymer micelles in aqueous solutions: A Monte Carlo study

Multimolecular micelles, formed by polystyrene-block-poly(methacrylic acid) in water, are studied by lattice Monte Carlo method. Electrostatic interactions are calculated in the mean-field approximation by solving the Poisson-Boltzmann equation. The model is parametrized according to available experimental data. The dependence of micellar size on pH and ionic strength is calculated and compared with experimental data. A special attention is devoted to the behavior in solutions with a low ionic strength.

Monte Carlo simulation of polymer brushes in narrow pores

We present the model of a cylindrical pore with chains tethered by one end to its inner surface. The conformational and “quasistatic” (under low and medium shear rates) hydrodynamic properties of the system are studied. Since it was shown by other researchers that the density profile of the polymer layer is only slightly affected by flow in the region of low and medium shear rates, we do not take flow into account when calculating chain conformations. The conformational properties of the concave layer are very similar to those of the flat one. They are characterized by the layer thickness h. The hydrodynamic properties are characterized by the hydrodynamic thickness Lh. We show that Lh depends strongly on the chain surface density σ under conditions of the “mushroom” regime in contrast to h. Finally, we predict that the net permeability of the cylinder could decrease with the increasing temperature. It enables us, principally, to control the pore permeability with temperature.

A Monte Carlo study of shells of hydrophobically modified amphiphilic copolymer micelles in polar solvents

Multimolecular micelles in polar solvents formed by polystyrene-block-poly(methacrylic acid) (PS-PMA), hydrophobically modified by a naphthalene tag between blocks and an anthracene tag at the end of PMA block, are studied by a lattice Monte Carlo method. The model is parametrized according to available experimental data and several structural characteristics of the PMA shell together with the fluorometric decay functions are calculated and compared with results obtained for system without anthracene tags. The hydrophobic tags at the ends of shell-forming blocks try to avoid the polar medium and bury in the shell forcing the chains to loop back. The resulting distribution of traps is bimodal with a smaller peak in the vicinity of the core and a larger one farther from the core than the maximum on the distribution of segments.

Solubilization and release of hydrophobic compounds from block copolymer micelles. II. Release of pyrene from polyelectrolyte micelles under equilibrium conditions

The kinetics of the release of pyrene from poly(tert-butyl acrylate)-block-poly(2-vinylpyridine) polyelectrolyte micelles into aqueous media was studied by two independent experimental fluorometric techniques. Mathematical treatment of the time-dependent fluorescence intensities based on a model of diffusion from a sphere allowed the evaluation of the diffusion coefficient of pyrene in the dense poly(tert-butyl acrylate) micellar core. Assumptions necessary for mathematical treatment of experimental curves were tested and their validity was proved. Fluorometric measurements suggest that a considerable fraction of pyrene is solubilized also in poly(2-vinylpyridine) micellar shells.

Self-assembly and co-assembly of block polyelectrolytes in aqueous solutions. Dissipative particle dynamics with explicit electrostatics

ABSTRACT This topical review outlines the principles of dissipative particle dynamics (DPD) and discusses its use for studying electrically charged systems – particularly its application for investigation of the self-assembly of polyelectrolytes in aqueous solutions. Special emphasis is placed on DPD with incorporation of explicit electrostatic forces (DPD-E). At present, this empowered method is being used by only a few research groups and most studies of polyelectrolyte self-assembly are based on the ‘implicit solvent ionic strength’ approach which completely ignores electrostatics. The inclusion of electrostatics in the DPD machinery not only complicates the calculations and considerably slows down the simulation run, but it also generates some problems of primary importance that have to be solved prior to employing DPD-E to study practically important systems. In the introductory parts, we describe the principles of DPD-E, analyse all the problematic issues and show how they can be resolved or overcome. The later parts demonstrate the successful application of DPD-E. We discuss papers that study the self-assembling behaviour of two different practically important systems and show that they not only closely reproduce all the decisive features of the behaviour, but also reveal new details that are difficult to access for experimentalists. The topical review shows that the tedious calculations are worthwhile: (1) DPD-E simulations are concerned with the true principles of the behaviour of polyelectrolyte systems and therefore provide reliable data and (2) the practically important advantage of computer simulations, i.e. their predictive power (at the level of the employed coarse-graining), which is a questionable aspect in simulations that use physically impoverished models, is not endangered in the case of DPD-E.

Fluorescence Spectroscopy as a Tool for Investigating the Self-Organized Polyelectrolyte Systems

In this article, we outline the principles and application of several time-resolved fluorescence techniques for studying the behavior of stimuli-responsive self-assembled polymer systems. We demonstrate the high research potential of fluorescence using results of several published studies performed by the research team at the Charles University in Prague in the framework of the Marie Curie Research Training Network “Self-Organized Nanostructures of Amphiphilic Copolymers” (MRTN-CT-2003-505027). We have chosen several interesting examples of complex self-assembling systems, the behavior of which could not have been understood without the help of targeted fluorescence studies. We have chosen four different techniques, two of them relatively popular (fluorescence anisotropy and nonradiative excitation energy transfer) and two only little used in polymer science (the solvent relaxation method and fluorescence correlation spectroscopy). The last part of the article is devoted to computer simulations (Monte Carlo and molecular dynamics) aimed at the interpretation of fluorescence data.

Monte Carlo study of chain conformations in the swollen middle layer of onion-skin polymeric micelles

Conformations of chains in swollen middle layers of onion-skin micelles were studied by Monte Carlo simulations on a tetrahedral lattice under conditions that mimie real systems of swolten onion-skin micelles. Polymer blocks are modeled as tethered self-avoiding chains, enclosed in a narrow spherical layer. Average density of segments, ca. 0.6, corresponds to swollen micellar systems. Only the excluded volume effect was taken into account since it plays the most important role in dense polymer systems. Individual chains are described by equivalent ellipsoids of gyration. Distributions of the ellipsoid half-axes were calculated during simulations. Results based on a large series of simulations indicate that the middle layer forming blocks may be described as prolonged ellipsoids oriented preferentially perpendicular to the radial direction. Analysis of the data concerning the orientations of end-to-end vectors and distributions of segments within one chain indicates that individual chains are strongly interpenetrated and the multi-chain system is fairly disordered.

Chromatographic study of the conformational behavior of graft copolymers with a broad distribution of grafting densities in dilute solutions in selective solvents for grafts

ABSTRACT A graft copolymer of poly(4-methylstyrene-graft-2-vinylpyridine) was prepared by the living “grafting onto” method. Its molecular weight and composition was analyzed by size-exclusion chromatography, liquid chromatography under limiting conditions of desorption, 1H NMR, and light scattering. The results indicated a non-negligibly broad distribution of grafting density. Its conformational behavior was studied by reversed phase liquid chromatography with gradient elution. Targeted studies provided two discrete base-line separated fractions. Their compositions were estimated by pyrolysis gas chromatography. The results suggest that distinct chain conformations (differing in grafting density and interacting differently with stationary phase) exist in studied solutions and can be separated by well-tuned chromatographic techniques. Experimental data were analyzed and interpreted on the basis of theoretical and computer studies of the conformational behavior of graft copolymers in selective solvents. GRAPHICAL ABSTRACT

Double-exponential decay of orientational correlations in semiflexible polyelectrolytes

In this paper we revisited the problem of persistence length of polyelectrolytes. We performed a series of Molecular Dynamics simulations using the Debye-Hückel approximation for electrostatics to test several equations which go beyond the classical description of Odijk, Skolnick and Fixman (OSF). The data confirm earlier observations that in the limit of large contour separations the decay of orientational correlations can be described by a single-exponential function and the decay length can be described by the OSF relation. However, at short countour separations the behaviour is more complex. Recent equations which introduce more complicated expressions and an additional length scale could describe the results very well on both the short and the long length scale. The equation of Manghi and Netz when used without adjustable parameters could capture the qualitative trend but deviated in a quantitative comparison. Better quantitative agreement within the estimated error could be obtained using three equations with one adjustable parameter: 1) the equation of Manghi and Netz; 2) the equation proposed by us in this paper; 3) the equation proposed by Cannavacciuolo and Pedersen. Two characteristic length scales can be identified in the data: the intrinsic or bare persistence length and the electrostatic persistence length. All three equations use a single parameter to describe a smooth crossover from the short-range behaviour dominated by the intrinsic stiffness of the chain to the long-range OSF-like behaviour.