Brushes of Dendritically Branched Polyelectrolytes

Polyelectrolyte Brushes with Protein-Like Nanocolloids

Electrostatic interaction of ampholytic nanocolloidal particles (NPs), which mimic globular proteins, with polyelectrolyte brushes is analyzed within mean-field Poisson-Boltzmann approximation. In accordance with experimental findings, the theory predicts that an electrostatic driving force for the particle uptake by the brush may emerge when the net charge of the particle in the buffer and the charge of the brush are of the same sign. The origin of this driving force is change in the ionization state of weak cationic and anionic groups on the NP surface provoked by interaction with the brush. In experimental systems, the ionic interactions are complemented by excluded-volume, hydrophobic, and other types of interactions that all together control NP uptake by or expulsion from the brush. Here, we focus on the NP-brush ionic interactions. It is demonstrated that deviation between the buffer pH and the NP isoelectric point, considered usually as the key control parameter, does not uniquely determine the insertion free energy patterns. The latter depends also on the proportion of cationic and anionic groups in the NPs and their specific ionization constants as well as on salt concentration in the buffer. The analysis of the free energy landscape proves that a local minimum in the free energy inside the brush appears, provided the NP charge reversal occurs upon insertion into the brush. This minimum corresponds either to a thermodynamically stable or to a metastable state, depending on the pH offset from the IEP and salt concentration, and is separated from the bulk of the solution by a free energy barrier. The latter, being fairly independent of salt concentration in height, may strongly impede the NP absorption kinetically even when it is thermodynamically favorable. Hence, change reversal is a necessary but insufficient condition for the uptake of the NPs by similarly charged polyelectrolyte brushes.

Non-linear elasticity effects and stratification in brushes of branched polyelectrolytes

Brushes formed by arm-tethered starlike polyelectrolytes may exhibit internal segregation into weakly and strongly extended populations (stratified two-layer structure) when strong ionic intermolecular repulsions induce stretching of the tethers up to the limit of their extensibility. We propose an approximate Poisson-Boltzmann theory for analysis of the structure of the stratified brush and compare it with results of numerical self-consistent field modeling. Both analytical and numerical models point to the formation of a narrow cloud of counterions (internal double electrical layer) localized inside a stratified brush at the boundary between the layers.

Simulations of 3-arm polyelectrolyte star brushes under external electric fields

Langevin dynamics (LD) simulations have been performed to study the conformations and stratification of grafted three-arm polyelectrolyte (PE) stars in response to external electric fields. The grafted chains with neutral stems and fully charged branches were immersed in a salt-free solution sandwiched between the grafting electrode and a second oppositely charged electrode. The branching points of neutral-stem PE brushes at low grafting densities exhibit a bimodal distribution normal to the grafting electrode. With increasing grafting density, the molecular conformations in the brush layer become more complex with the emergence of multi-mode distributions of the branching point monomers. Under strong electric fields, the fraction of grafted chains with either nearly completely stretched stems or collapsed branches onto the grafting electrode gradually decreases with increasing grafting density due to the stronger electrostatic screening from counterions and monomer charges at higher grafting densities. Simulation results revealed that a collapsing electric field promotes the stratification within the brush layer, leading to high degrees of charge overcompensation from charged monomers collapsed onto the oppositely charged grafting electrode. An approximate analytical self-consistent field model was developed to examine the stratification within the brush layer. Regarding the fraction of grafted chains with the free branches in the upper layer, the prediction of the analytical model qualitatively agrees with the simulation results.

Interaction Forces between Pegylated Star-Shaped Polymers at Mica Surfaces

We present a study focused on characterizing the interaction forces between mica surfaces across solutions containing star-shaped polymers with cationic ends. Using the surface forces apparatus, we show that the interaction forces in pure water between surfaces covered with the polymers can be adequately described by the dendronized brush model. In that framework, our experimental data suggest that the number of branches adsorbed at the surface decreases as the concentration of polymer in the adsorbing solution increases. The onset of interaction was also shown to increase with the concentration of polymer in solution up to distances much larger than the contour length of the polymer, suggesting that the nanostructure of the polymer film is significantly different from that of a monolayer. High compression of the polymer film adsorbed at low polymer concentration revealed the appearance of a highly structured hydration layer underneath the polymer layer. These results support that charged polymer chains do not necessarily come into close contact with the surface even if strong electrostatic interaction is present. Altogether, our results provide a comprehensive understanding of the interfacial behavior of star-shaped polymers and reveal the unexpected role of hydration water in the control of the polymer conformation.

Polymer-brush lubrication: a review of recent theoretical advances

This review compiles recent theoretical advances to describe compressive and shear forces of polymer-brush bilayers, which consist of two opposing brushes in contact. Such model systems for polymer-brush lubrication are frequently used as a benchmark to gain insight into biological problems, e.g., synovial joint lubrication. Based on scaling theory, I derive conformational and collective properties of polymer-brush bilayers in equilibrium and out-of-equilibrium situations, such as shear forces in the linear and nonlinear response regimes of stationary shear and under non-stationary shear. Furthermore, I discuss the influence of macromolecular inclusions and electrostatic interactions on polymer-brush lubrication. Comparisons to alternative analytical approaches, experiments and numerical results are performed. Special emphasis is given to methods for simulating polymer-brush bilayers using molecular dynamics simulations.