Weak Polyelectrolyte Research Articles

Conformation and Ionization Behavior of Charge-Regulating Polyelectrolyte Brushes in a Poor Solvent.

Understanding the structural response of weak polyelectrolyte brushes upon external stimuli is crucial for their applications ranging from modifying surface properties to the development of smart and intelligent materials. In this work, coarse-grained molecular dynamics simulations were carried out to investigate the conformation and ionization behavior of charge-regulating polyelectrolyte brushes under poor solvent conditions, using an implicit solvent model. The results show that, while the thickness of a sparse polyelectrolyte brush shows a similar behavior to that of a single chain, namely, a monotonic change as a function of solvent quality (modeled by an effective segment-segment attraction strength parameter) and solution pH, a dense polyelectrolyte brush exhibits more complex behavior. An unexpected reexpansion is observed when the effective segment-segment attraction strength is further increased, especially in the case of a high pH. In the latter case, strong attraction in polymer segments promotes the formation of large, interchain, cylindrical aggregates, leading to an increase in brush thickness.

Transfer Matrix Model of pH Effects in Polymeric Complex Coacervation.

Oppositely charged polyelectrolytes can undergo an associative phase separation, in a process known as polymeric complex coacervation. This phenomenon is driven by the electrostatic attraction between polyanion and polycation species, leading to the formation of a polymer-dense coacervate phase and a coexisting polymer-dilute supernatant phase. There has been significant recent interest in the physical origin and features of coacervation; yet notably, experiments often use weak polyelectrolytes the charge state of which depends on solution pH, while theoretical or computational efforts typically assume strong polyelectrolytes that remain fully charged. There have been only a few efforts to address this limitation, and thus there has been little exploration of how pH can affect complex coacervation. In this paper, we modify a transfer matrix theory of coacervation to account for acid-base equilibria, taking advantage of its ability to directly account for some local ion correlations that will affect monomer charging. We show that coacervation can stabilize the charged state of a weak polyelectrolyte via the proximity of oppositely charged monomers, and can lead to asymmetric phase diagrams where the positively and negatively charged polyelectrolytes exhibit different behaviors near the pKa of either chain. Specifically, there is a partitioning of one of the salt species to a coacervate to maintain electroneutrality when one of the polyelectrolytes is only partially charged. This results in the depletion of the same salt species in the supernatant, and overall can suppress phase separation. We also demonstrate that, when one of the species is only partially charged, mixtures that are off-stoichiometric in volume fraction but stoichiometric in charge exhibit the greatest propensity to form coacervate phases.

Reconfiguring nanostructures in magnetic fluids using pH and magnetic stimulus for tuning optical properties

In this paper, we demonstrate tuning the optical properties of magnetic fluids through pH and magnetic field control. The role of reconfigurable nanostructures in the temporal and switching behavior of transmitted light is also studied. An oil-in-water magnetic nanoemulsion with an average diameter of 200 nm, containing Fe3O4 superparamagnetic nanoparticles of 10 nm average diameter, is used in this study. To make the emulsion pH reconfigurable, the droplets are covered with a weak polyelectrolyte, poly (acrylic acid). The swelling of the PAA molecules, due to ionization of the carboxylic acid groups, resulted in an increase in adlayer thickness at increase in pH. The change in transmitted intensity is found to scale linearly with the applied magnetic field strength, and at higher fields, the formed structures took more time to reconfigure themselves after the removal of magnetic field. At a pH 5, the increase in transmitted light intensity is seen even at a very low magnetic field strength due to the lower interdroplet spacing. On the contrary, at pH 9, the interdroplet spacing between the emulsion drops were large, due to the extension of PAA molecules, which led to a weaker magnetic interaction between the droplets and formation of shorter chains, where a lower transmission is seen due to multiple scattering. A straight-line pattern was observed in the transmitted intensity, due to the scattering of light from linear chains that are aligned perpendicular to the light propagation direction. Bridging of polymer covered surfaces was seen at pH 3, due to hydrogen bonding between PAA molecules. Our results show that tuning optical properties of magnetic emulsion (switching between transparent and opaque states) is possible through pH control. Our findings open up new opportunities to develop optical devices and accessing of diverse structures by tuning pH and magnetic field.

Highly Tunable Nanostructures in a Doubly pH‐Responsive Pentablock Terpolymer in Solution and in Thin Films

AbstractMultiblock copolymers with charged blocks are complex systems that show great potential for enhancing the structural control of block copolymers. A pentablock terpolymer PMMA‐b‐PDMAEMA‐b‐P2VP‐b‐PDMAEMA‐b‐PMMA is investigated. It contains two types of midblocks, which are weak cationic polyelectrolytes, namely poly(2‐(dimethylamino)ethyl methacrylate) (PDMAEMA) and poly(2‐vinylpyridine) (P2VP). Furthermore, these are end‐capped with short hydrophobic poly(methyl methacrylate) (PMMA) blocks in dilute aqueous solution and thin films. The self‐assembly behavior depends on the degrees of ionization α of the P2VP and PDMAEMA blocks, which are altered in a wide range by varying the pH value. High degrees of ionization of both blocks prevent structure formation, whereas microphase‐separated nanostructures form for a partially charged and uncharged state. While in solutions, the nanostructure formation is governed by the dependence of the P2VP block solubility of the and the flexibility of the PDMAEMA blocks on α, in thin films, the dependence of the segregation strength on α is key. Furthermore, the solution state plays a crucial role in the film formation during spin‐coating. Overall, both the mixing behavior of the 3 types of blocks and the block sequence, governing the bridging behavior, result in strong variations of the nanostructures and their repeat distances.

Switchable Electron Transfer in Single Thermoresponsive Core-Shell Plasmonic Hybrid Nanoelectrodes

Active control of electrochemical processes at the nanoscale is promising for applications in switchable electronics. It has been demonstrated that stimuli-responsive polymers can serve as logic gates to switch between “on” and “off” states in response to environmental changes at both the macro- and nanoscale. As the majority of these stimuli-responsive polymers are weak polyelectrolytes, metal ions or conductive monomers are often incorporated into the stimuli-responsive matrix for electrochemical applications. However, the incorporation of conductive components may reduce the gating efficacy of these stimuli-responsive materials. In this work, we investigated the efficacy of switchable electron transfer in nonconductive stimuli-responsive core-shell hybrid nanoelectrodes consisting of thermoresponsive poly(N-isopropylacrylamide) grafted from plasmonic gold nanoparticles, termed pNIPAM@AuNPs. Under applied electrochemical potentials, electron transfer to the plasmonic core can be monitored optically through changes in the linewidth and peak position of the longitudinal surface plasmon resonance. Using single-particle dark-field spectroelectrochemistry, we observed that expanded pNIPAM@AuNPs exhibited hindered electron transfer, but, upon polymer collapse, electron transfer substantially increased. We also determined that, although heterogeneous, the extent of electron transfer in pNIPAM@AuNPs was reversibly switched through temperature cycling. Furthermore, we established that the gating efficiency of the polymer is dependent on the core morphology as a result of the size and shape dependent strength of the plasmonic electric field. These results provide insight into reversibly gated electron transfer in stimuli-responsive hybrid nanoelectrodes and suggest that a conductive polymer matrix is not necessarily required to facilitate electron transfer.

Towards enhanced oil recovery: Effects of ionic valency and pH on the adsorption of hydrolyzed polyacrylamide at model surfaces using QCM-D

Adding polymers to the injection water in water-flooding oil recovery increases the viscosity and thereby enhances recovery factor. However, the beneficial effects are often counteracted by adsorption of polymer to the ambient mineral surface. In here, we used Quartz Crystal Microbalance with Dissipation (QCM-D) measurements to study the adsorption of the frequently used hydrolyzed polyacrylamide (HPAM), a weak polyelectrolyte, to silica and alumina model surfaces representing sandstone reservoirs. At pH 6 and 8 and concentrations from 1 mM to 100 mM of NaCl and CaCl2, we find that negatively charged HPAM molecules generally adsorb more strongly to positively charged alumina surfaces. For silica surfaces, the presence of Ca2+ ions, which form strong complexes with the carboxyl groups on the polymer, as evidenced by titration measurements, strongly enhances HPAM adsorption. This effect is attributed to the formation of Ca2+-mediated ion bridges with the substrate. Kinetic measurements suggest a two-step adsorption process consisting of a primary polymer adsorption onto the surface followed – in most cases – by a slower secondary process, which involves rearrangements of previously adsorbed polymer molecules, including polymer-on-polymer adsorption.

Preparation of uniformly sized interpenetrating polymer network polyelectrolyte hydrogel droplets from a solid-state liquid crystal shell

Uniformly sized interpenetrating polymer network (IPN) polyelectrolyte hydrogel droplets were produced with solid-state liquid crystal (LCsolid) shells, which had been produced by a microfluidic method. The weak polyelectrolyte hydrogel and the temperature-responsive poly(N-isopropylacrylamide) (PNIPAM) networks were intertwined in the LCsolid shell to produce an IPN structure; the anionic poly(acrylic acid) (PAA) and cationic poly(N-[3-(dimethylamino)propyl] methacrylamide) (PDMAPMA) were utilized for the weak polyelectrolyte hydrogel networks. The anionic PNIPAM/PAA, cationic PNIPAM/PDMAPMA, and zwitterionic PAA/PDMAPMA IPN droplets were successfully produced by the LCsolid shell templates. The PNIPAM/PAA IPN droplets were applied to a Ca2+ sensor. Further, the anionic PNIPAM/PAA and cationic PNIPAM/PDMAPMA IPN droplets could adsorb the cationic methylene blue and anionic Acid Red 37 dyes, respectively, while the zwitterionic PAA/PDMAPMA IPN droplets could adsorb both dyes. This method of producing uniformly sized IPN hydrogel droplets from LCsolid shell templates is simple and does not require any sophisticated machines.

Phase transition in hydrophobic weak polyelectrolyte gel utilized for water desalination

By this article, we continue studying weak polyelectrolyte hydrogels and their application as a desalination agent. We modeled the desalination process as a four-step reversible thermodynamic cycle transferring ions from a low salinity solution to a high salinity one. The cycle implies reversibility at any stage, therefore the method can achieve the maximum thermodynamic efficiency, comparable to the reverse osmosis. As a driving force for ions movement, we use the fact that compression of the gel leads to a decrease in the gel ionization degree, and therefore to a release of ions entrapped in the gel. We considered the gel composed of ionogenic units modified by hydrophobic pendants and showed that this modification significantly increases the number of transferred ions. This increase is caused by a first-order phase transition originated from an interplay between repulsive electrostatic and attractive steric interactions. The transition happens during hydrogel compression. At a certain pressure, the gel collapses abruptly, changing its volume almost to that of a dry state, and releases almost all ions collected inside. Employing the phase transition allows us to model the desalination cycle, which transfers a much larger number of ions and works at rather low pressures <10bar.

PH/Thermo-Responsive Grafted Alginate-Based SiO2 Hybrid Nanocarrier/Hydrogel Drug Delivery Systems.

We report the preparation of mesoporous silica nanoparticles covered by layer by layer (LbL) oppositely charged weak polyelectrolytes, comprising poly(allylamine hydrochloride) (PAH) and a sodium alginate, highly grafted by N-isopropylacrylamide/N-tert-butylacrylamide random copolymers, NaALG-g-P(NIPAM90-co-NtBAM10) (NaALG-g). Thanks to the pH dependence of the degree of ionization of the polyelectrolytes and the LCST-type thermosensitivity of the grafting chains of the NaALG-g, the as-prepared hybrid nanoparticles (hNP) exhibit pH/thermo-responsive drug delivery capabilities. The release kinetics of rhodamine B (RB, model drug) can be controlled by the number of PAH/NaALG-g bilayers and more importantly by the environmental conditions, namely, pH and temperature. As observed, the increase of pH and/or temperature accelerates the RB release under sink conditions. The same NaALG-g was used as gelator to fabricate a hNP@NaALG-g hydrogel composite. This formulation forms a viscous solution at room temperature, and it is transformed to a self-assembling hydrogel (sol-gel transition) upon heating at physiological temperature provided that its Tgel was regulated at 30.7 °C, by the NtBAM hydrophobic monomer incorporation in the side chains. It exhibits excellent injectability thanks to its combined thermo- and shear-responsiveness. The hNP@NaALG-g hydrogel composite, encapsulating hNP covered with one bilayer, exhibited pH-responsive sustainable drug delivery. The presented highly tunable drug delivery system (DDS) (hNP and/or composite hydrogel) might be useful for biomedical potential applications.

ОСОБЕННОСТИ ДИСТАНЦИОННОГО ВЗАИМОДЕЙСТВИЯ ГИДРОГЕЛЕЙ ПОЛИАКРИЛОВОЙ КИСЛОТЫ И ПОЛИЭТИЛЕНИМИНА

The remote interaction between a weak polyacrylic acid polyelectrolyte (gpac) and a weak polyethyleneimine polybase (gpei) is studied as a function of time at their different molar ratios and states in an aqueous medium. To predict the possibility of activation of the studied hydrogels, electrochemical properties were studied by conductometry and pH metry. During pH measurement, it was found that 24 hours of pH have the lowest values, indicating a high content of H+ ions in the aqueous medium. The dependence of the specific electrical conductivity, the maximum value at the ratio of 3:3 (PAC:PEI) and it coincides with the result of pH from the ratio of components. The obtained results indicate that significant changes in the electrochemical and conformational values of the initial macromolecules in the interpolymer system occur in this interpolymer system. Thus, studies have shown on the presence of a remote interaction between hydrogels and their mutual activation. With an increase in the content of one of the hydrogels in the solution, a significant increase in the swelling of hydrogels is observed in proportion to the concentration of the second component, which indicates their mutual activation.

Modeling of weak polyelectrolyte hydrogels under compression – Implications for water desalination

Hydrogels immersed in salty solutions induce partitioning of small ions that can be exploited for water desalination. This partitioning also affects the H+ ions, leading to a decrease of the pH in the gel as compared to supernatant solution. When using weak polyelectrolyte gels, this lower pH may decrease their ionization degree, and hence diminish their desalination capacity. Using the recently developed grand-reaction ensemble, we performed particle-based computer simulations of a gel in contact with a reservoir of salt solution at pH ≃ 7. We showed that the effect of ion partitioning on the ionization degree is negligible, as long as the salinity of the reservoir is high. However, when the reservoir salinity approaches that of fresh water, the ionization degree of weak polyelectrolyte gel is significantly decreased, thus making it a less efficient desalination agent. Furthermore, we demonstrated that the ionization degree of the gel is decreased upon compression. However, up to pressures of approximately 30 bar this does not significantly affect the ion partitioning. Our result predict that weak polyelectrolyte hydrogels, such as the often employed poly(acrylic acid), can be efficiently used to lower the salinity of sea- or brackish water, while they will be inefficient in desalinating water of low salinity.

Single-molecule studies reveal the distinction of strong and weak polyelectrolytes in aqueous solutions.

Polyelectrolytes are an important class of functional polymers that have the advantages of both polymers and electrolytes due to the presence of charges, and have prospective applications in many fields. The charge of the backbone is an important factor affecting the properties of polyelectrolytes. Therefore, the complex interactions caused by the charges in polyelectrolyte solutions pose a challenge to the study of polyelectrolyte systems, and there is no consensus on the distinction between the behavior of strong and weak polyelectrolytes in solution. Based on single-molecule force spectroscopy (SMFS), the distinction of strong and weak polyelectrolytes is clarified for the first time at the single molecular level by comparing the single-chain elasticity in different environments. It is expected that the single-molecule study will provide the theoretical and experimental basis for the further application of polyelectrolytes.

PH-Dependent complexation and polyelectrolyte chain conformation of polyzwitterion-polycation coacervates in salted water.

The phase behavior and chain conformational structure of biphasic polyzwitterion-polyelectrolyte coacervates in salted aqueous solution are investigated with a model weak cationic polyelectrolyte, poly(2-vinylpyridine) (P2VP), whose charge fraction can be effectively tuned by pH. It is observed that increasing the pH leads to the increase of the yielding volume fraction and the water content of dense coacervates formed between net neutral polybetaine and cationic P2VP in contrast to the decrease of critical salt concentration for the onset of coacervation, where the P2VP charge fraction is reduced correspondingly. Surprisingly, a single-molecule fluorescence spectroscopic study suggests that P2VP chains upon coacervation seem to adopt a swollen or an even more expanded conformational structure at higher pH. As the hydrophobicity of P2VP chains is accompanied by a reduced charge fraction by increasing the pH, a strong pH-dependent phase and conformational behaviors suggest the shift of entropic and enthalpic contribution to the underlying thermodynamic energy landscape and chain structural dynamics of polyelectrolyte coacervation involving weak polyelectrolytes in aqueous solution.

Thermodynamic non-ideality in charge regulation of weak polyelectrolytes

Polymer ionization differs from that for their monomeric counterparts due to intramolecular correlations. Such effects are conventionally described in terms of the site-binding model that accounts for short-range interactions between neighboring sites. With an apparent equilibrium constant for each ionizable group and the nearest-neighbor energy as adjustable parameters, the site-binding method is useful to correlate experimental titration curves when the site-site interactions are insignificant at long ranges. This work aims to describe the electrostatic behavior of weak polyelectrolytes in aqueous solutions on the basis of the intrinsic equilibrium constants of the individual ionizable groups and solution conditions underlying the thermodynamic non-ideality. A molecular thermodynamic model is proposed for the protonation of weak polyelectrolytes by incorporating classical density functional theory into the site-binding model to account for the effects of the local ionic environment on both inter-chain and intra-chain correlations. By an extensive comparison of theoretical predictions with experimental titration curves, we demonstrate that the thermodynamic model is able to quantify the ionization behavior of weak polyelectrolytes over a broad range of molecular architectures and solution conditions.

Molecular weight and dispersity affect chain conformation and pH-response in weak polyelectrolyte brushes

The pH-dependence of the conformation of annealed polyelectrolyte brushes can be tuned by varying the molecular weight distribution, as characterized via weight-average molecular weight and dispersity.

Structure and properties of polyelectrolyte complexes of various type (chitosan chloride – polyacrilic acid) and triple polyelectrolyte-metalic complexe with cations Cu2+

Using FT-IR-spectroscopy, X-ray diffraction and thermomechanical analysis structure and thermomechanical properties of two nonstoichiometry and one stoichiometry polyelectrolyte complexes (PEC) based on opposite charged polyelectrolytes – strong cationic polyelectrolyte (chitosan chloride) and weak anionic polyelectrolyte (polyacrylic aсid) and triple polyelectrolyte-metal complexes (TPMC) based on stoichometry polyelectrolyte complexes and cations Cu(II) were investigated. It was shown, that chitosan chloride has amorphous-cristallinity structure, which is significantly different from the structure of neat chitosan, and polyacrylic acid posses amorphous structure. Meantime, all PEC samples have amorphous structure, differing from the structure of weak anionic polyelectrolyte, at the same time amorphous structure of nonstoichiometric PECs insignificantly different from that of stoichiometric polyelectrolyte complexes. Additionally, amorphous structure of TPMC has another structure, compared to all PEC. According to thermomechanical analysis, all PECs have one temperature transition from glassy to highly elastic state (from 77 to 84 °C). The deformation value of the samples of nonstoichiometric PEC is similar and somewhat less than the deformation of the stoichiometric PEC. The TPMC sample has two glass transitions (81 and 226 °C), and his high-temperature transition characterizes the segmental mobility of fragments of macromolecules of one stoichiometric PEC, the polar groups of which form chelate circles with Cu (II) cations. Deformation parameter of the TPMC is higher in comparison with the stoichiometric polyelectrolyte complexes. Keywords: structure, properties, deformation, ionic force, cationic Cu(II), polyelectrolyte, polyelectrolyte-metal complexes, chitosan chloride, polyacrylic acid, X-ray diffraction, thermomechanical analysis.

Ultrathin clay-containing layer-by-layer separator coating enhances performance of lithium-sulfur batteries

Dissolution of polysulfides upon cycling of the sulfur cathode and their diffusion through the separator membrane ‒ the shuttle effect ‒ is a major factor which deteriorates performance and safety of lithium-sulfur (Li-S) batteries. Here, ultrathin, light-weight polyelectrolyte-clay layer-by-layer assemblies have been developed as versatile nanocoatings on the polypropylene (PP) separator to suppress the shuttle effect. The use of weak polyelectrolytes (polyethyleneimine, PEI, and polyacrylic acid, PAA) and montmorillonite (MMT) or halloysite (Hal) clay nanoparticles enabled full control over mass and charge balance of polyelectrolyte/clay film components, which was critical for achieving ion selectivity and reduction of the polysulfide diffusion. The differences in the geometry and structure of MMT and Hal (nanoplatelets vs. nanotubes), as well as in the pH-controlled charge density of PEI and PAA greatly affected the thickness, morphology, and the ultimate performance of the nanocoatings. In particular, deposition of 450 nm MMT-based coatings at pH 3 and 6 lead to an increase in electrolyte uptake, delayed the lithium dendrite growth and enhanced the discharge capacity of the cell from 300 mAh g−1 for bare to 690 mAh g−1 for coated PP separator over 200 charge-discharge cycles 0.5 C. At the same time, 250 nm-thick Hal-based coatings deposited at pH 3 resulted in an increase in the Coulombic efficiency from 50% for the bare PP separator to ~99% for the coated one, showing an outstanding performance. Additionally, the polyelectrolyte-clay nanocoatings significantly improve thermal and dimensional stability of the PP separator. The results demonstrate that ultrathin LbL clay-containing coatings on the PP separator membrane can drastically improve the cyclability of the Li-S cells.

A comparison between the structures of reconstituted salivary pellicles and oral mucin (MUC5B) films

HypothesisSalivary pellicles i.e., thin films formed upon selective adsorption of saliva, protect oral surfaces against chemical and mechanical insults. Pellicles are also excellent aqueous lubricants. It is generally accepted that reconstituted pellicles have a two-layer structure, where the outer layer is mainly composed of MUC5B mucins. We hypothesized that by comparing the effect of ionic strength on reconstituted pellicles and MUC5B films we could gain further insight into the pellicle structure. ExperimentsSalivary pellicles and MUC5B films reconstituted on solid surfaces were investigated at different ionic strengths by Force Spectroscopy, Quartz Crystal Microbalance with Dissipation, Null Ellipsometry and Neutron Reflectometry. FindingsOur results support the two-layer structure for reconstituted salivary pellicles. The outer layer swelled when ionic strength decreased, indicating a weak polyelectrolyte behavior. While initially the MUC5B films exhibited a similar tendency, this was followed by a drastic collapse indicating an interaction between exposed hydrophobic domains. This suggests that mucins in the pellicle outer layer form complexes with other salivary components that prevent this interaction. Lowering ionic strength below physiological values also led to a partial removal of the pellicle inner layer. Overall, our results highlight the importance that the interactions of mucins with other pellicle components play on their structure.

Non-monotonic behavior of weak-polyelectrolytes adsorption on a cationic surface: A Monte Carlo simulation study

In this work, the weak polyelectrolyte (PE) adsorption on a strong cationic surface is studied with constant pH Monte Carlo simulations using a coarse-grained model. When a large number of PE chains is added to the system, the PE adsorbed amount vs pH curve exhibits a non-monotonic behavior, with the appearance of a maximum close to the intrinsic pKa0-value of the PE titratable groups. The apparent pKa-value of the PE chains shows a non-trivial tendency depending on the pH-value and the surface coverage degree. In increasing the pH-value, the small anions that accompany the cationic surface are replaced by PE chains and small cations. For pH>pKa0+1, an evident charge reversion of surface is observed. These results are explained analyzing the interplay between the attractive and repulsion electrostatic interactions between the different components of the system (inter- and intra-charged monomers of PE chains, the strong cationic surface and small ions) and their effects on the PE chain ionization.

Influence of Charge Regulation and Charge Heterogeneity on Complexation between Weak Polyelectrolytes and Weak Proteins Near Isoelectric Point

AbstractA combination of constant pH method and single chain in mean field simulations is employed to probe the influence of charge regulation and charge heterogeneities on the bridging characteristics of dissociable polyelectrolytes on globular proteins. By adopting a coarse‐grained representation of proteins as spherical particles, the influence of charge patches on the polyelectrolyte bridging probabilities near protein isoelectric points and in regimes in which the net charge of the protein is the same sign as that of the polyelectrolytes is probed. The results demonstrate that in the presence of dissociable polyelectrolytes, the probability of bridging of proteins capable of charge regulation is higher relative to proteins which are completely dissociated. For homogeneously charged proteins and/or proteins with weak charge heterogeneities, partially dissociated polyelectrolytes are seen to exhibit enhanced bridging characteristics compared to completely dissociated polyelectrolytes. In contrast, for proteins exhibiting strong charge heterogeneities, dissociable polyelectrolytes are seen to exhibit weaker bridging compared to completely dissociated polyelectrolytes.