Polyelectrolyte brushes

We survey recent progress made in the field of polyelectrolyte brushes. These systems consist of long polyelectrolyte chains that are grafted densely to planar or curved surfaces. The main feature of all polyelectrolyte brushes is the strong confinement of the counterions within the brush layer. The high osmotic pressure which is thus built up explains the unusual features of these systems. Here we focus on the most recent experimental developments which are rationalized on the basis of existing theoretical predictions and opens new challenging problems. In particular, we shall discuss briefly the experimental systems used for comparing theory and experiment lately. Moreover, we review various aspects related to the experimental analysis of polyelectrolyte brushes. As a final point, we survey trends in recent applications which demonstrate that polyelectrolyte brushes have an excellent prospect for future nanotechnology.     

Polyelectrolyte entry and transport through an asymmetric alpha-hemolysin channel

We study the entry and transport of a polyelectrolyte, dextran sulfate (DS), through an asymmetric alpha-hemolysin protein channel inserted into a planar lipid bilayer. We compare the dynamics of the DS chains as they enter the channel at the opposite stem or vestibule sides. Experiments are performed at the single-molecule level by using an electrical method. The frequency of current blockades varies exponentially as a function of applied voltage. This frequency is smaller for the stem entrance than for the vestibule one, due to a smaller coupling with the electric field and a larger activation energy for entry. The value of the activation energy is quantitatively interpreted as an entropic effect of chain confinement. The translocation time decreases when the applied voltage increases and displays an exponential variation which is independent of the stem or vestibule sides.     

High elongation of polyelectrolyte chains in the osmotic limit of spherical polyelectrolyte brushes: a study by cryogenic transmission electron microscopy

We investigate the conformation of long polyelectrolyte chains attached to colloidal latex particles by cryogenic transmission electron microscopy (cryo-TEM). The dense grafting of the polyelectrolyte chains ("polyelectrolyte brush") leads to a confinement of the counterions and a concomitantly high osmotic pressure within the polyelectrolyte layer attached to the core particles. Cryo-TEM has provided first model-independent direct proof for the strong stretching of the polyelectrolyte chains by direct visualization. If salt is added, cryo-TEM clearly shows how chains collapse because of the strong screening of the electrostatic interaction. Moreover, the analysis of interacting particles by cryo-TEM shows that the polyelectrolyte chains retract at close contact. Hence, we demonstrate how cryo-TEM can be used to analyze directly the spatial structure of polyelectrolyte brushes in situ.     

Coil-stretch Transition of DNA Molecules in Slit-like Confinement

We experimentally investigate the influence of slit-like confinement on the coil-stretch transition of single DNA molecules in a homogeneous planar elongational electric field. We observe a more gradual coil-stretch transition characterized by two distinct critical strain rates for DNA in confinement, different from the unconfined case where a single critical strain rate exists. We postulate that the change in the coil-stretch transition is due to a modified spring law in confinement. We develop a dumbbell model to extract an effective spring law by following the relaxation of an initially stretched DNA. We then use this spring law and kinetic theory modeling to predict the extension and fluctuations of DNA in planar elongational fields. The model predicts that a two-stage coil-stretch transition emerges in confinement, in accord with experimental observations.     

Brownian dynamics simulations of polyelectrolyte adsorption in shear flow with hydrodynamic interaction

The adsorption of single polyelectrolyte molecules in shear flow is studied using Brownian dynamics simulations with hydrodynamic interaction (HI). Simulations are performed with bead-rod and bead-spring chains, and electrostatic interactions are incorporated through a screened Coulombic potential with excluded volume accounted for by the repulsive part of a Lennard-Jones potential. A correction to the Rotne-Prager-Yamakawa tensor is derived that accounts for the presence of a planar wall. The simulations show that migration away from an uncharged wall, which is due to bead-wall HI, is enhanced by increases in the strength of flow and intrachain electrostatic repulsion, consistent with kinetic theory predictions. When the wall and polyelectrolyte are oppositely charged, chain behavior depends on the strength of electrostatic screening. For strong screening, chains get depleted from a region close to the wall and the thickness of this depletion layer scales as N(1/3)Wi(2/3) at high Wi, where N is the chain length and Wi is the Weissenberg number. At intermediate screening, bead-wall electrostatic attraction competes with bead-wall HI, and it is found that there is a critical Weissenberg number for desorption which scales as N(-1/2)kappa(-3)(l(B)|sigmaq|)(3/2), where kappa is the inverse screening length, l(B) is the Bjerrum length, sigma is the surface charge density, and q is the bead charge. When the screening is weak, adsorbed chains are observed to align in the vorticity direction at low shear rates due to the effects of repulsive intramolecular interactions. At higher shear rates, the chains align in the flow direction. The simulation method and results of this work are expected to be useful for a number of applications in biophysics and materials science in which polyelectrolyte adsorption plays a key role.     

Ordered polyelectrolyte multilayers. Rules governing layering in organic binary multilayers

We study the growth and internal structure of polyelectrolyte multilayers obtained by combining three polyanions with nine polycations of the ionene family, of systematically varied chemical architecture. We find that, contrary to a generally held belief, ordered organic multilayers are by no way exceptional, provided one of the polyelectrolytes bears groups which induce structure in water, such as long hydrophobic segments or mesogenic groups. However, this condition is not sufficient, as order will or will not emerge in the multilayer depending on the specific pairing of the polyelectrolytes. The results support the notion that layering in the multilayer results from some degree of prestructuring of a water-swollen layer adsorbed during each step of deposition. These findings pave the way to new possible uses of polyelectrolyte multilayers, for example, for applications requiring preferential alignment or strong confinement of specific functional groups.     

Confined polyelectrolytes: The complexity of a simple system

The interaction between polyelectrolytes and counterions in confined situations and the mutual relationship between chain conformation and ion condensation is an important issue in several areas. In the biological field, it assumes particular relevance in the understanding of the packaging of nucleic acids, which is crucial in the design of gene delivery systems. In this work, a simple coarse‐grained model is used to assess the cooperativity between conformational change and ion condensation in spherically confined backbones, with capsides permeable to the counterions. It is seen that the variation on the degree of condensation depends on counterion valence. For monovalent counterions, the degree of condensation passes through a minimum before increasing as the confining space diminishes. In contrast, for trivalent ions, the overall tendency is to decrease the degree of condensation as the confinement space also decreases. Most of the particles reside close to the spherical wall, even for systems in which the density is higher closer to the cavity center. This effect is more pronounced, when monovalent counterions are present. Additionally, there are clear variations in the charge along the concentric layers that cannot be totally ascribed to polyelectrolyte behavior, as shown by decoupling the chain into monomers. If both chain and counterions are confined, the formation of a counterion rich region immediately before the wall is observed. Spool and doughnut‐like structures are formed for stiff chains, within a nontrivial evolution with increasing confinement. © 2015 Wiley Periodicals, Inc.     

Structure of flexible and semiflexible polyelectrolyte chains in confined spaces of slit micro/nanochannels

Understanding the behavior of a polyelectrolyte in confined spaces has direct relevance in design and manipulation of microfluidic devices, as well as transport in living organisms. In this paper, a coarse-grained model of anionic semiflexible polyelectrolyte is applied, and its structure and dynamics are fully examined with Brownian dynamics (BD) simulations both in bulk solution and under confinement between two negatively charged parallel plates. The modeling is based on the nonlinear bead-spring discretization of a continuous chain with additional long-range electrostatic, Lennard-Jones, and hydrodynamic interactions between pairs of beads. The authors also consider the steric and electrostatic interactions between the bead and the confining wall. Relevant model parameters are determined from experimental rheology data on the anionic polysaccharide xanthan reported previously. For comparison, both flexible and semiflexible models are developed accompanying zero and finite intrinsic persistence lengths, respectively. The conformational changes of the polyelectrolyte chain induced by confinements and their dependence on the screening effect of the electrolyte solution are faithfully characterized with BD simulations. Depending on the intrinsic rigidity and the medium ionic strength, the polyelectrolyte can be classified as flexible, semiflexible, or rigid. Confined flexible and semiflexible chains exhibit a nonmonotonic variation in size, as measured by the radius of gyration and end-to-end distance, with changing slit width. For the semiflexible chain, this is coupled to the variations in long-range bond vector correlation. The rigid chain, realized at low ionic strength, does not have minima in size but exhibits a sigmoidal transition. The size of confined semiflexible and rigid polyelectrolytes can be well described by the wormlike chain model once the electrostatic effects are taken into account by the persistence length measured at long length scale.     

Conformational properties and interaction of polyelectrolyte pH-sensitive stars

Changes in the conformations of two polyelectrolyte stars with amphiphilic segments during their convergence are investigated by the Scheutjens-Fleer method (a numerical method of solving self-consistent field equations). The stars are placed in an aqueous-salt solvent at a pH close to the pK of the segments of polyelectrolyte arms. Individual stars have a two-phase quasi-micellar conformation at a sufficiently strong hydrophobicity of segments in the uncharged state and at a sufficiently low salt concentration. This conformation is formed via separation of star arms into two groups: One consists of uncharged arms and forms a dense core in the star center, while the other consists of arms coming through and forming a corona with charged arms. When stars in this conformation converge, the transition of arms from the corona on the side of the oncoming star to the core occurs inside both stars. When stars touch each other with their cores, the stars merge into a united quasi-micelle. The free energy of interaction of stars is a nonmonotonic function of distance D between their centers. This value grows with a decrease in D until the cores come into contact. Upon the contact and merging of the cores, the free-energy values are characterized by a local minimum corresponding to an ellipsoid-shaped united quasi-micelle. With an increase in the concentration of salt in the solution, individual stars adopt extended conformations. In this case, the interaction of two stars is repulsive at all D values. However, if the salt concentration very slightly exceeds the threshold value of formation of a core in an isolated star, then the initially extended stars during their approach toward each other adopt (at first, each of them separately) a quasi-micellar conformation, while their cores grow as they converge and merge upon contact.     

Shear of a planar polyelectrolyte brush

The behavior of a planar polyelectrolyte brush subjected to normal and tangential external forces is considered. A new “polyelectrolyte effect” is predicted: shear of a free polyelectrolyte brush leads to a decrease in brush thickness contrary to the case of a free neutral brush. Such behavior is equivalent to that of a neutral brush subjected to an external normal stretching force. In the case of a polyelectrolyte brush this force is created by the osmotic pressure of mobile counterions neutralizing grafted chain charges. Addition of salt diminishes the polyelectrolyte effect and changes the sign of correlation between brush thickness and tangential deformation.     

Counterion localization in solutions of starlike polyelectrolytes and colloidal polyelectrolyte brushes: a self-consistent field theory

A quantitative analysis of the distribution of counterions in salt-free solutions of colloidal polyelectrolyte brushes and starlike polyelectrolytes is performed on the level of the Poisson-Boltzmann approximation. Exact numerical solutions are obtained for starlike polyelectrolyte molecules composed of f = 20, . . ., 50 arms with a fixed fractional charge alpha per segment by applying the self-consistent field method of Scheutjens and Fleer (SF-SCF). The Wigner-Seitz cell dimension defines the concentration of polyelectrolyte stars in the system. The numerical results are compared to predictions of an analytical mean field theory and related to experimental observations on the osmotic pressure in solutions of starlike polyelectrolytes and colloidal polyelectrolyte brushes.     

Molecular dynamics simulations of polyelectrolyte brushes: from single chains to bundles of chains

Using molecular dynamics simulations in combination with scaling analysis, we have studied the effects of the solvent quality and the strength of the electrostatic interactions on the conformations of spherical polyelectrolyte brushes in salt-free solutions. The spherical polyelectrolyte brush could be in one of four conformations: (1) a star-like conformation, (2) a "star of bundles" conformation in which the polyelectrolyte chains self-assemble into pinned cylindrical micelles, (3) a micelle-like conformation with a dense core and charged corona, or (4) a conformation in which there is a thin polymeric layer uniformly covering the particle surface. These different brush conformations appear as a result of the fine interplay between electrostatic and monomer-monomer interactions. The brush thickness depends nonmonotonically on the value of the Bjerrum length. This dependence of the brush thickness is due to counterion condensation inside the brush volume. We have also established that bundle formation in poor solvent conditions for the polymer backbone can also occur in a planar polyelectrolyte brush. In this case, the grafted polyelectrolyte chains form hemispherical aggregates at low polymer grafting densities, cylindrical aggregates at an intermediate range of the grafting densities, and vertically oriented ribbon-like aggregates at high grafting densities.     

Assembly and mechanical properties of phosphorus dendrimer/polyelectrolyte multilayer microcapsules

We report the preparation, characterization, and mechanical properties of polyelectrolyte/phosphorus dendrimer multilayer microcapsules. The shells of these microcapsules are composed either by alternating poly(styrenesulfonate) (PSS) and positively charged dendrimer G4(NH+Et2Cl-)96 or by alternating poly(allylamine hydrochloride) (PAH) and negatively charged dendrimer G4(CH-COO-Na+)96. The same multilayers were constructed on planar support to examine their layer-by-layer growth and to measure the multilayer thickness. Surface plasmon resonance spectroscopy (SPR) showed regular linear growth of the assembly upon each bilayer deposited. We probe the mechanical properties of these polyelectrolyte/dendrimer microcapsules by measuring force-deformation curves with the atomic force microscope (AFM). The experiment suggests that they are much softer than PSS/PAH microcapsules studied before. This softening is attributed to an enhanced permeability of the polyelectrolyte/dendrimer multilayer shells as compared with multilayers formed by linear polyelectrolytes. In contrast, Young's modulus of both dendrimer-based multilayers was found to be on the same order as that of PSS/PAH multilayers.     

Diffusion of cationic polyelectrolytes into cellulosic fibers

The penetration of cationic polyelectrolytes into anionic cellulosic fibers was evaluated with fluorescent imaging techniques in order to clarify the mechanism and time scales for the diffusion process. The bulk charge of the cellulosic fibers indirectly creates a driving force for diffusion into the porous fiber wall, which is entropic in nature due to a release of counterions as the polyelectrolyte adsorbs. The individual bulk charges in the fiber cell wall also interact with the diffusing polyelectrolyte, such that the polyelectrolyte diffuses to the first available charge and consequently adsorbs and remains fixed. Thus, subsequent polyelectrolyte chains must first diffuse through the adsorbed polyelectrolyte layer before adsorbing to the next available bulk charges. This behavior differs from earlier suggested diffusion mechanisms, by which polyelectrolytes were assumed to first adsorb to the outermost surface and then reptate into the pore structure. The time scales for polyelectrolyte diffusion were highly dependent on the flexibility of the chain, which was estimated from calculations of the persistence length. The persistence length ultimately depended on the charge density and electrolyte concentration. The charge density of the polyelectrolyte had a greater influence on the time scales for diffusion. High charge density polyelectrolytes were observed to diffuse on a time scale of months, whereas the diffusion of low charge density polyelectrolytes was measured on the order of hours. An influence of the chain length, that is, steric interactions due the persistence length of the polyelectrolyte and to the tortuosity of the porous structure of the fiber wall, could only be noted for low charge density polyelectrolytes. Increasing the electrolyte concentration increased the chain flexibility by screening the electrostatic contribution to the persistence length, in turn inducing a faster diffusion process. However, a significant change in the diffusion behavior was observed at high electrolyte concentrations, at which the interaction between the polyelectrolyte charges and the fiber charges was almost completely screened.     

Structure and phase behavior of polyelectrolyte star solutions

Using the recently developed effective interaction potentials between polyelectrolyte stars, we examine the structure and phase behavior of solutions of the same. The effective interaction is ultrasoft and density dependent, owing to the integration of the counterionic degrees of freedom. The latter contribute extensive volume terms that must be taken into account in drawing the phase diagram of the system. The structural behavior of the uniform fluid is characterized by anomalous structure factors, akin to those found previously for solutions of uncharged star polymers. The phase diagram of the system is very rich, featuring a fluid phase at low arm numbers of the stars, two reentrant melting regions, as well as a variety of crystal structures with unusual symmetry. The physical origin of these features can be traced back to the ultrasoft nature of the effective interaction potential.     

Controlled unidirectional energy transfer in luminescent self-assembled conjugated polymer superlattices

We report the synthesis and characterization of multi-layered organic superlattices made by polyelectrolyte self-assembly. Self-assembled films were formed from a water-soluble form of poly(phenylene vinylene) with high-photoluminescence quantum efficiency (QE). We observed a self-quenching of the luminescence with increasing film thickness. This quenching can be reversed by inserting spacer layers between each active conjugated layer. A red shift of the luminescence was also observed as additional poly(phenylene vinylene) layers were added. We attribute the red shift and increasing QE to changing polymer conformation, together with efficient unidirectional energy transfer. We rule out quantum confinement as the origin of the red shift.     

Brownian dynamics simulations of polyelectrolyte adsorption onto topographically patterned surfaces

The effect of patterned surface topography on the adsorption of single polyelectrolyte molecules is explored using Brownian dynamics simulations. The polyelectrolyte is modeled as a free-draining, freely jointed bead-rod chain, and electrostatic interactions are incorporated using a screened Coulombic potential with excluded volume interactions accounted for by the repulsive part of a Lennard-Jones potential. Topography consisting of periodically spaced valleys of square cross section separated by flat hills is considered. Chain conformations are characterized for a wide range of valley widths, depths, and spacings as well as for several different types of surface charge distributions. Depending on the parameter values describing the topography, the chains are found to adopt conformations ranging from flat and extended to those associated with bridge-, brush-, or semi-bridge-like structures. The formation of these structures is rationalized on the basis of a free-energy model that takes into account the increase in free energy due to entropic confinement, excluded volume interactions, and chain stretching as well as the decrease in free energy due to bead-surface electrostatic attraction. The results of this work are expected to be useful in designing patterned surface topography to control the conformations of adsorbed polyelectrolyte molecules.     

Photochemically and thermally tunable planar defects in colloidal photonic crystals

We report a bottom-up synthesis of a photochemically and thermally active azobenzene-based polyelectrolyte multilayer (PEM) planar defect embedded in a colloidal photonic crystal (CPC). Both photoisomerization and thermal cycling lead to a precise tuning of an intragap transmitting state induced by the PEM structural defect.     

Monomer volume fraction profiles in pH responsive planar polyelectrolyte brushes

Spatial dependencies of monomer volume fraction profiles of pH responsive polyelectrolyte brushes were investigated using field theories and neutron reflectivity experiments. In particular, planar polyelectrolyte brushes in good solvent were studied and direct comparisons between predictions of the theories and experimental measurements are presented. The comparisons between the theories and the experimental data reveal that solvent entropy and ion‐pairs resulting from adsorption of counterions from the added salt play key roles in affecting the monomer distribution and must be taken into account in modeling polyelectrolyte brushes. Furthermore, the utility of this physics‐based approach based on these theories for the prediction and interpretation of neutron reflectivity profiles in the context of pH responsive planar polyelectrolyte brushes such as polybasic poly(2‐(dimethylamino)ethyl methacrylate) (PDMAEMA) and polyacidic poly(methacrylic acid) (PMAA) brushes is demonstrated. The approach provides a quantitative way of estimating molecular weights of the polymers polymerized using surface‐initiated atom transfer radical polymerization. © 2016 Wiley Periodicals, Inc. J. Polym. Sci., Part B: Polym. Phys. 2016 , 54, 956–964