Search

We demonstrate that a highly charged polyelectrolyte confined in a spherical cavity undergoes reversible transformations between amorphous conformations and a four-fold symmetry morphology as a function of dielectric mismatch between the media inside and outside the cavity. Surface polarization due to dielectric mismatch exhibits an extra "confinement" effect, which is most pronounced within a certain range of the cavity radius and the electrostatic strength between the monomers and counterions and multivalent counterions. For cavities with a charged surface, surface polarization leads to an increased amount of counterions adsorbed in the outer side, further compressing the confined polyelectrolyte into a four-fold symmetry morphology. The equilibrium conformation of the chain is dependent upon several key factors including the relative permittivities of the media inside and outside the cavity, multivalent counterion concentration, cavity radius relative to the chain length, and interface charge density. Our findings offer insights into the effects of dielectric mismatch in packaging and delivery of polyelectrolytes across media with different relative permittivities. Moreover, the reversible transformation of the polyelectrolyte conformations in response to environmental permittivity allows for potential applications in biosensing and medical monitoring.

The transport of polyelectrolytes confined by oppositely charged surfaces and driven by a constant electric field is of interest in studies of DNA separation according to size. Using molecular dynamics simulations that include the surface polarization effect, we find that the mobilities of the polyelectrolytes and their counterions change non-monotonically with the confinement surface charge density. For an optimum value of the confinement charge density, efficient separation of polyelectrolytes can be achieved over a wide range of polyelectrolyte charge due to the differential friction imparted by oppositely charged confinement on the polyelectrolyte chains. Furthermore, by altering the placement of the charged confinement counterions, enhanced polyelectrolyte separation can be achieved by utilizing the surface polarization effect due to dielectric mismatch between the media inside and outside the confinement. [ABSTRACT FROM AUTHOR]

Understanding nanoscale interactions at the interface between two media with different dielectric constants is crucial for controlling many environmental and biological processes, and for improving the efficiency of energy storage devices. In this contributed paper, we show that polarization effects due to such dielectric mismatch remarkably influence the double-layer structure of a polyelectrolyte solution confined between two charged surfaces. Surprisingly, the electrostatic potential across the adsorbed polyelectrolyte double layer at the confining surface is found to decrease with increasing surface charge density, indicative of a negative differential capacitance. Furthermore, in the presence of polarization effects, the electrostatic energy stored in the double-layer structure is enhanced with an increase in the charge amplification, which is the absorption of ions on a like-charged surface. We also find that all of the important double-layer properties, such as charge amplification, energy storage, and differential capacitance, strongly depend on the polyelectrolyte backbone flexibility and the solvent quality. These interesting behaviors are attributed to the interplay between the conformational entropy of the confined polyelectrolytes, the Coulombic interaction between the charged species, and the repulsion from the surfaces with lower dielectric constant. [ABSTRACT FROM AUTHOR]

The swelling of polyelectrolyte (PE) gels by 1:1 and 2:1 salts is studied via osmotic ensemble Monte Carlo simulations at constant osmotic pressure and electrolyte chemical potential of a reservoir phase in equilibrium with a model PE network. Large molecular weight gels exhibit a remarkable swelling response to small changes in 2:1 salt concentration. Gel collapse is accompanied by the formation of a previously unknown heterogeneous nanostructure, predicted by theory and observed in simulations, consisting of regions dense in monomers coexisting with regions rich in mono- and divalent ions. [ABSTRACT FROM AUTHOR]

Surfaces formed by charged polymeric species are highly abundant in both synthetic and biological systems, for which maintaining an optimum contact distance and a pressure balance is paramount. Here we investigate interactions between surfaces of two same-charged and highly swollen polyelectrolyte gels, using extensive molecular dynamic simulations and minimal analytical methods. The external-pressure responses of the gels and the polymer-free ionic solvent layer separating two surfaces are considered. Simulations confirmed that the surfaces are held apart by osmotic pressure resulting from excess charges diffusing out of the network. At a threshold pressure, the counterion-induced osmotic pressure in the gel interiors becomes nearly equal to that in solvent gap, and the gels begin to deform. At the threshold pressure, the distance between surfaces is equivalent to the electrostatic screening length imposed by excess charges. Both the solvent layer and pressure dependence are well described by an analytical model based on the Poisson-Boltzmann solution for low and moderate electrostatic strengths. Scaling descriptions for the threshold pressure and critical distance compare well with the simulations. The threshold pressure decreases with increasing polymerization degree of network chains, similar to sparsely grafted polyelectrolyte brushes. As the electrostatic coupling strength is increased, the deviations from the analytical model also increase due to condensed counterions. Our results are of great importance for systems where charged gels or gel-like structures interact in various solvents, including systems encapsulated by gels and microgels in confinement. [ABSTRACT FROM AUTHOR]

Polyelectrolytegels are known to exhibit strong response to changes in salt concentration,an effect attributed to osmotic effects. Traditional theories areinsufficient to describe all the properties observed in experiments;namely, reentrant swelling effects at high salt concentrations andcomplete polyelectrolyte collapse at high salt valencies. We incorporateliquid-state integral equation theories to account for the many-bodycorrelations between finite-sized ions in a strongly charged polyelectrolytegel. This represents a theory for such systems that goes well beyondwell-known Debye–Huckel (DH) and Poisson–Boltzmann (PB)theories. We demonstrate that charge correlations give rise to enhancedgel deswelling, a first-order collapse to a highly correlated stateat large valencies, and counterion size-dependent reentrant swelling.Such ion-specific effects provide routes for practical control ofstimuli-responsive polymeric materials based on polyelectrolyte gels,and deeper understanding of biological systems. [ABSTRACT FROM AUTHOR]

We demonstrate periodic nanostructures emerged in homogeneouslycross-linked thin gels made of strongly charged polymers. Free-energyfunctional of polyelectrolyte gels with density and charge inhomogeneitiesis developed and numerical energy minimization is performed over candidatenanostructures. The two-dimensional calculations indicate the possibilityof realizing lamellar, square, solvent-rich hexagonal, and polymer-richhexagonal phases, by a fine interplay of polymer and solvent characteristics.Nonlinearities in network elasticity and electrostatic energy arefound to be the deciding factor in the thermodynamic selection ofnanostructures. In particular, the nonlinear network elasticity suppressesthe stability of solvent-rich hexagonal nanostructures as solventquality increases, which results in exchange of stabilities betweennanostructures and leads to stable nanostructures of square symmetryin two-dimensional gels. The phase transition between nanostructuresby changes in environmental stimuli suggests a possible means to controlbulk properties of polyelectrolyte gels. [ABSTRACT FROM AUTHOR]

Transport of ionic species in heterogeneous polymeric media is highly dependent on the charge distributions and interactions between mobile and immobile groups. Here we perform coarse-grained molecular dynamics simulations to study the ion dynamics in swollen polyelectrolyte gels under external electric fields. A nonlinear response of the ionic conductivity to an applied electric field, for field strengths that are comparable to the ionic coupling strength, is observed. This behavior correlates to a broadening of the ionic distribution around the polymer backbone under an increasing electric field. Also, we find that the weak-field ionic mobility in gels increases with density, which is opposite to the behavior of simple electrolytes. This relates to the mean coupling between charges that decreases in gels, but increases in simple electrolytes, with increasing density. These results provide more insights into the electric response of polyelectrolyte gels to support the development of applications that combine electric and mechanical properties of polyelectrolyte gels for energy storage, sensing, selective transport, and signal transfer. [ABSTRACT FROM AUTHOR]

Advanced materials for energy storage and process technology are globally in high demand. Promising components for such materials are polymer blends and copolymers, because of their ability to self-assemble into nanostructured materials that combine desirable material properties, such as high mechanical stability with ion-selective conductivity. It remains a challenge to predict the nanoscale structure in charged polymers, because of intricate ionic correlations that can influence the structure at many length scales. Here we present a free energy analysis of charged polymer blends with electrolytes of the primitive model, by combining a Flory-Huggins-type theory with liquid-state methods, relying on the Ornstein-Zernike equation. We find that different mechanisms, driven by entropy and electrostatics, can stimulate or repress phase separation in blends. These mechanisms can enable the existence of a triple point, where three phases can coexist in an effective two-component blend. We analyze the influence of chain length and electrostatic coupling strength, set by the ion type and dielectric properties of the polymer, on the coexistence lines and nanoscale structure within the blend. Our predictions may add resolution to the perspective on multiphase coexistence in blends of charged polymers, and the formation of nanoscale structure in ionic gels and ionomers such as Nafion. [ABSTRACT FROM AUTHOR]

Abstract: Polyelectrolyte gels are intelligent materials that undergo large reversible volume changes for a range of environmental stimuli. Although the strength of electrostatic interactions have a strong influence on the gel response, these interactions are not properly accounted in the classical mean-field theories that assume a homogeneous charge-neutral gel. Using Poisson–Boltzmann theory and theoretically informed coarse-grained simulations, we emphasize the importance of charge inhomogeneities and the associated Coulomb interactions in determining the response of gels. Our analysis reveals that nanometer-sized gels, collapsed gels, and gels in media with low salinity or high dielectric constant, have large regions of excess charge. We also observe that the addition of salt can induce collapse in swollen gels by compensating the polymer charge. [Copyright &y& Elsevier]