Electrolyte-induced collapse of a polyelectrolyte brush

We have investigated the electrolyte-induced collapse of a polyelectrolyte brush covalently attached to a planar solid surface. Positively charged poly-4-vinyl [N-methyl-pyridinium] (MePVP) brushes were prepared in situ at the surface by free radical chain polymerization using a surface-immobilized initiator monolayer ("grafting from" technique) and 4-vinylpyridine as the monomer, followed by a polymer-analogous quaternization reaction. The height of the brushes was measured as a function of the external salt concentration via multiple-angle null ellipsometry. As predicted by mean-field theory, the height of the MePVP brushes remains unaffected by the addition of low amounts of external salt. At higher salt concentrations the brush height decreases. The extent to which the brush shrinks strongly depends on the nature of the salt present in the environment. MePVP brushes collapse to almost the dry layer thickness upon the addition of potassium iodide to a contacting aqueous medium. In contrast, the collapse of MePVP brushes having bromide or chloride counterions is much less pronounced. These brushes remain in a highly swollen state even after large amounts of salt have been added to the solution.     

Bridging contributions to polyelectrolyte brush collapse in multivalent salt solutions

Polyelectrolyte brushes are essential in many aspects of surface functionality, particularly for colloidal stabilization and lubrication in biological and materials science applications. It has been shown experimentally that the brushes undergo an abrupt shrinkage in the presence of multivalent counter-ions. This transition is studied here using a phenomenological mean-field approach with a model that specifically includes bridging of the polyelectrolyte chains by the multiple charges on the multivalent counter-ions. Using an energy balance represented by the sum of electrostatic, polymeric and entropic mean-field terms, additional parameterized phenomenological terms are introduced for counter-ion condensation and for the attractive interaction between adjacent polyelectrolyte chains to account for the bridging effect. The free energy is minimized with respect to the counter-ion populations and the brush height. In agreement with experimental observations, increasing the concentration of multivalent ions leads to a sharp collapse of the polyelectrolyte brush height. © 2015 Wiley Periodicals, Inc. J. Polym. Sci., Part A: Polym. Chem. 2016, 54, 284–291     

Nanostructure and transition of a strong polyelectrolyte brush at the air/water interface

The strong polyelectrolyte layer in the monolayer of ionic amphiphilic diblock copolymers at the air/water interface consists of carpet and brush layers when the brush density is satisfactorily high like that of the weak acid polymer. Also, a drastic structural change was induced by the addition of salt to the brush layer. In this study, the critical brush density for the transition between "carpet-only" and "carpet + brush" structures for the strongly ionic amphiphilic diblock copolymer, poly(hydrogenated isoprepene)-b-poly(styrene sulfonic acid) sodium salt, monolayer was measured by an in situ X-ray reflectivity technique. The critical brush density was found to be about 0.12 nm-2, which is lower than that observed for a weak acid polymer and, unlike the weak acid polymer, is independent of the hydrophilic chain length. This difference might be attributed to the strong ionic nature of the brush chain. In addition, the reversibility of the transition was confirmed. The effect of salt addition to the nanostructure of the carpet layer was examined in detail. No structural change was found, indicating that most of the ionic groups in the carpet layer do not show an ionic nature because of counterion condensation.     

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.     

Directed motion of a polyelectrolyte micelle along tethered chains of oppositely charged polyelectrolyte brush

The complexation of different single polyelectrolyte (PE) micelles formed by linear diblock copolymers with oppositely charged brushes with varying grafting densities and charge content was studied by means of molecular dynamics simulations using the primitive model. We found that all micelles perform a directed motion along the vertical z axis on the grafted surface where they trapped while on the other axes the motion is restricted in a circle in which the diameter decreases with the increase of the hydrophilic length of the linear diblock copolymer. The motion of micelles is characterized as super diffusion inside brushes with low densities and low charge content. At high grafting densities and charge content the diffusion becomes Fickian or slightly subdiffusive. The number of the absorbed brush chains on the micelle corona increases almost monotonically with the increase of brush grafting density or with the decrease of charge content. The distance from the surface in which the micelle is trapped can be controlled by the charge density along the grafted PE chain. © 2019 Wiley Periodicals, Inc. J. Polym. Sci., Part B: Polym. Phys. 2019 , 57, 621–631     

Normal and shear forces between a polyelectrolyte brush and a solid surface

The diblock copolymer poly(methyl methacrylate)‐b‐poly(sodium sulfonated glycidyl methacrylate) (PMMA‐b‐PSGMA) was end‐attached by its hydrophobic block (PMMA) onto mica hydrophobized by a stearic trimethylammonium iodide (STAI) layer, to form a polyelectrolyte brush immersed in water. With a surface force balance (SFB), we extended earlier measurements between two such brush layers for the case of normal and shear forces at different shear rates, surface separation, and compressions between one mica surface coated with STAI or a STAI‐diblock layer against a bare mica surface. After coating one of the surfaces with STAI, a long range attraction that results in a jump into an adhesive flat contact between the hydrophobic and hydrophilic surfaces was observed. A very different behavior was seen after forming the polyelectrolyte brush on the STAI‐coated surface. The long range attraction was replaced by repulsion, accompanied by very low friction during shear (ca. three orders of magnitude lower than with adsorbed polyelectrolytes). On further compression, a weak attraction to the adhesive contact was observed. From the final surface–surface contact separation, we deduce that most of the polyelectrolyte diblock brush layer was squeezed out from the gap, leaving the STAI layer and a small amount of the polymer attached to the surface. Stick‐sliding behavior was seen while applying shear, suggesting a dissipation mechanism caused by the trapped polyelectrolyte. © 2004 Wiley Periodicals, Inc. J Polym Sci Part B: Polym Phys 43: 193–204, 2005     

Ionic effects in collapse of polyelectrolyte brushes

We investigated the effect of counterion valence on the structure and swelling behavior of polyelectrolyte brushes using a nonlocal density functional theory that accounts for the excluded-volume effects of all ionic species and intrachain and electrostatic correlations. It was shown that charge correlation in the presence of multivalent counterions results in collapse of a polyelectrolyte brush at an intermediate polyion grafting density. At high grafting density, the brush reswells in a way similar to that in a monovalent ionic solution. In the presence of multivalent counterions, the nonmonotonic swelling of a polyelectrolyte brush in response to the increase of the grafting density can be attributed to a competition of the counterion-mediated electrostatic attraction between polyions with the excluded-volume effect of all ionic species. While a polyelectrolyte brush exhibits an "osmotic brush" regime at low salt concentration and a "salted brush" regime at high salt concentration regardless of the counterion valence, we found a smoother transition as the valence of the counterions increases. As observed in recent experiments, a quasi-power-law dependence of the brush thickness on the concentration ratio can be identified when the monovalent counterions are replaced with trivalent counterions at a fixed ionic strength.     

Thin adsorbed films of a strong cationic polyelectrolyte on silica substrates

The adsorption of poly(diallyldimethyl ammonium chloride) (DADMAC) on planar silica substrates was examined as a function of ionic strength and pH. The study was carried out with reflectometry in an impinging-jet cell and complemented by atomic force microscopy (AFM) and ellipsometry investigations. The adsorption process is initially transport limited, whereby the adsorption rate increases somewhat with increasing ionic strength. This effect is caused by a simultaneous decrease of the hydrodynamic radius of the polymer. After a transient period, the adsorption process saturates and leads to an adsorption plateau. The plateau value increases strongly with increasing ionic strength. This increase can be explained by progressive screening of the electrostatic repulsion between the adsorbing polyelectrolyte chains, as can be rationalized by a random sequential adsorption (RSA) model. The adsorbed amount further increases with increasing pH, and this effect is probably caused by the corresponding increase of the surface charge of the silica substrate.     

Ionizable polyelectrolyte brushes: brush height and electrosteric interaction

Semi-analytical scaling theory is used to describe quenched and annealed (weakly charged, ionizable, charge-regulating) polyelectrolyte brushes in electrolyte solutions of arbitrary salt concentration. An Alexander-De Gennes box model with homogeneous distribution of polymer segments and the free ends located at the edge of the brush is assumed, as is local electroneutrality in the brush. For annealed polyelectrolyte and in the low-salt regime, the theory predicts that for sufficiently dense brushes, the salt concentration has a small influence on brush height, while the brush expands with increasing grafting density, in agreement with experiment. Expressions are presented for the interaction free energy of compressed ionizable and quenched polyelectrolyte brushes (proportional to the force between particles or curved surfaces). In all cases, the required prefactors are explicitly stated. The theory is compared directly with published experiments on the influence of salt concentration, pH, and grafting density on the thickness and interaction force of polystyrene sulfonate (quenched) and poly(meth)acrylic acid (annealed) brushes. In general, trends are well reproduced but significant deviations remain.     

Poisson-Boltzmann theory of pH-sensitive (annealing) polyelectrolyte brush

We present a self-consistent field analytical theory of a polymer brush formed by weakly charged pH-sensitive (annealing) polyelectrolytes tethered to a solid-liquid interface and immersed in buffer solution of low molecular weight salt. We use the Poisson-Boltzmann framework, applied by us previously to polyelectrolyte (PE) brushes with quenched charge (Zhulina, E. B.; Borisov, O. V. J. Chem. Phys. 1997, 107, 5952). This approach allows for detailed analysis of the internal structure of annealing PE brush in terms of polymer density distribution, profiles of electrostatic potential and of local degree of chain ionization as a function of buffer ionic strength and pH without any assumptions on mobile ion distribution imposed in earlier scaling-type models. The presented analytical theory recovers all major asymptotic dependences for average brush properties predicted earlier. In particular, a nonmonotonic dependence of brush thickness on ionic strength and grafting density is confirmed and specified with accuracy of numerical coefficients including crossover regions. Moreover, the theory predicts qualitatively new effects, such as, e.g., disproportionation of tethered polyions into weakly charged concentrated proximal and strongly charged sparse distal brush domains at low salt and moderate grating densities. The presented results allow us to quantify responsive features of annealing PE brushes whose large-scale and local conformational properties can be manipulated by external stimuli.     

Potentiometric titration of polyelectrolyte gels equilibrated with a solution of a strong polyelectrolyte: Influence of gel swelling

A weak acid gel (carboxylmethyldextran) was titrated with sodium hydroxide while equilibrated with a solution sodium poly(vinyl sulfate), a salt of a strong acid polyelectrolyte. It was found that the quantity in which V/V₀ is the volume of gel relative to its volume in acid form in pure water, is independent of the concentration C_p of the polyelectrolyte solution and of the relative volume of gel. The relation where α is the degree of ionization, pK′ is the apparent pK of the gel, γ is the activity coefficient of the counterion in the gel phase, and v is the number of ionizable groups, is found to be constant for all values of C_p and for 0.1 < α < 0.9. These results can be extended to highly crosslinked ionexchange resins.     

Influence of the counterion size on swelling and collapse of polyelectrolyte gel

A theory that predicts the effect of the counterion size on the swelling and collapse of a weakly charged polyelectrolyte gel was developed. In addition to excluded-volume interactions between monomer units of the gel, the theory involves the counterion-monomer unit and counterion-counterion interactions in terms of the virial approximation. The character of interactions between different units in the system varies from repulsion to attraction depending on the type of solvent, counterion, and dielectric permittivity of the solvent. For solvents with a low permittivity, the effect of condensation of counterions resulting in the formation of ion pairs is taken into account.     

Periodic swelling and collapse of polyelectrolyte brushes driven by chemical oscillation

Poly(acrylic acid) (PAA) chains exhibit conformational change in response to pH, whereas bromate-sulfite-ferrocyanide (BSF) solution shows pH oscillation between 3.2 and 6.6. By use of quartz crystal microbalance with dissipation (QCM-D), we have investigated the pH induced conformational change of PAA brushes under a continuous flow of BSF solution in real time. The changes in frequency and dissipation clearly indicate the periodic swelling and collapse of brushes with pH oscillation. The changes in thickness, viscosity, and elastic modulus further indicate the oscillation of the interface.     

Reversible electrochemical switching of polyelectrolyte brush surface energy using electroactive counterions

Polyelectrolyte brushes with electroactive counterions provide an effective platform for surfaces with electrochemically switchable wetting properties. Polycationic poly(2-(methacryloyloxy)-ethyl-trimethyl-ammonium chloride) (PMETAC) brushes with ferricyanide ions ([Fe(CN)6] 3-) were used as the electrochemically addressable surface. After a negative potential of -0.5 V was applied to the [Fe(CN)6](3-)-coordinated PMETAC brushes, the [Fe(CN)6](3-) species were reduced to [Fe(CN)6](4-), and the surface became more hydrophilic. By application of alternating negative and positive potentials, PMETAC brushes were switched reversibly between the reduced state ([Fe(CN)6]4-) and oxidized state ([Fe(CN)6]3-), resulting in reversible changes in water contact angles. The time required for a complete contact angle change can be tuned from 1 to 20 s, by changing the brush thickness and the concentration of supporting electrolyte. We present an electrochemical brush transport model that includes the electrochemical reaction at the charged electrode and describes ion transport through the brush phase covering the electrode. The model quantitatively describes the response of the contact angle (hydrophilicity) to the applied voltage as a function of background ionic strength and brush thickness, supporting the proposed mechanism of ion transport through the brush and electrochemical reaction at the electrode. A typical diffusion constant for ferricyanide in a PMETAC brush of any thickness in 5 mM KCl supporting electrolyte was found to be 2 x 10(-15) m2 s(-1), 5 to 6 orders of magnitude smaller than its bulk solution value.     

Synthesis and nanostructure of strong polyelectrolyte brushes in amphiphilic diblock copolymer monolayers on a water surface

We synthesized an ionic amphiphilic diblock copolymer, poly(hydrogenated isoprene)-b-poly(styrenesulfonic acid) (PIp-h2-b-PSS), by living anionic polymerization, and the nanostructure of its monolayer spread on a water surface was directly investigated by the in situ X-ray reflectivity technique. The monolayer of the diblock copolymer on a water surface had a smooth hydrophobic PIp-h2 layer on water and a "carpet"/polymer brush double layer in a hydrophilic sodium polystyrene sulfonate (PSSNa) layer under the water. The surface pressure dependence and PSSNa chain length dependence of the PIp-h2 layer thickness and the brush nanostructure were quantitatively studied. The effect of salt concentration in the subphase was also investigated in aqueous solutions containing 0-2 M NaCl. The salt effect on monolayer structure occurred at around 0.2 M. The thickness of the PSS brush layer decreased at salt concentrations above 0.2 M, while no structural change was observed below 0.2 M. This critical salt concentration is thought to be related to the balance of ionic concentrations inside the brush and in bulk solution.     

Influence of a strong polyelectrolyte block on the formation and properties of polymer micelles with a mixed corona

Joint micellization of two amphiphilic diblock copolymers is studied by velocity sedimentation, transmission electron microscopy, electrophoretic mobility measurements, and static light scattering. One of the diblock copolymers is a strong polyelectrolyte (polystyrene-block-poly(N-ethyl-4-vinylpyridinium bromide)), while the second one is a weakly charged or uncharged copolymer (polystyrene-block-poly(acrylic acid) or polystyrene-block-poly(4-vinylpyridine)). It is shown that the mixing of the diblock copolymers in a selective aqueous-organic solvent (DMF-methanol-water) leads to the formation of joint (hybrid) micelles and that the composition of these micelles is close to the composition of the polymer mixture. Micelles consist of an insoluble polystyrene core and a mixed corona composed of blocks of a strong polyelectrolyte and a weakly charged or uncharged copolymer. Aqueous dispersions of mixed micelles are obtained with the use of the dialysis technique, the spherical morphology of the micelles is ascertained, and their three-layered structure is proposed. The nonlinear dependence of the molecular mass of micelles on their composition is found. The decisive effect of electrostatic repulsion between strong polyelectrolyte units on the thermodynamics of micellization and the dispersion stability and molecular-mass characteristics of the mixed micelles is demonstrated.     

Structure and protein binding capacity of a planar PAA brush

We performed neutron reflectometry (NR) and total internal reflection fluorescence (TIRF) spectroscopy to characterize the structure and the protein binding capacity of a planar poly(acrylic acid) (PAA) brush at different temperatures. A PAA brush was prepared by spin-coating planar quartz or silicon wafers with a thin film of poly(styrene). Then, the diblock copolymer poly(styrene)-poly(acrylic acid) was deposited on these modified wafers using the Langmuir-Sch?fer or Langmuir-Blodgett technique. PAA grafting densities of about 0.1 chains per nm2 were obtained. The NR experiments indicate a remarkable swelling of the PAA brush in contact with a buffer solution, when it is heated to 40 degrees C for several hours. The swollen brush structure remains upon cooling back to 20 degrees C suggesting a disentanglement of the initially formed PAA brush by the temporary heating. At pD = 6.7, the protein bovine serum albumin (BSA) with a negative net charge is strongly adsorbed to the swollen PAA brush. From the scattering length density profiles obtained from the NR curves, an almost homogeneous filling of the whole PAA brush space with BSA molecules can be deduced corresponding to an average BSA volume fraction of about 7-10% and an adsorbed protein mass of about 1.4 mg m-2. By analyzing the TIRF experiments, it is found that BSA adsorption is enhanced when increasing the temperature which represents an evidence for an entropic driving force for protein adsorption. However, the mechanism of BSA adsorption at a PAA brush appears to be different at 20 and 40 degrees C.     

A modified box model including charge regulation for protein adsorption in a spherical polyelectrolyte brush

Recent experiments showed significant adsorption of bovine serum albumin (BSA) in spherical polyelectrolyte brushes (SPB) consisting of polyacrylic acid, even for pH values above the isoelectric point of the protein, when both protein and polyion are negatively charged. To describe these experimental findings theoretically, we have constructed a spherical box model for an annealed brush consisting of a weak polyelectrolyte that includes the adsorption of BSA. At equilibrium the chemical potential of BSA in solution equals that at each location in the brush, while the net force on the polyions (including osmotic, stretching, and excluded volume terms) is zero at each location. Protein adsorption is predicted above the isoelectric point and--in agreement with experimental data--is a strong function of ionic strength and pH. Adsorption of protein in the brush is possible because the pH in the brush is below the isoelectric point and protein reverses its charge from negative to positive when it adsorbs.     

Unusual collapse of highly hydrated polyelectrolyte multilayers with the ionic strength

Highly hydrated polyelectrolyte multilayers (PEMs) were fabricated by “layer by layer” (LBL) assembly of poly (diallyl dimethyl ammonium chloride) (PDADMAC) and poly (sodium 4‐styrene sulfonate) (PSS) in 0.5 M NaCl. Both thickness and hydration of the film were determined in situ as the multilayer was assembled by means of the quartz crystal microbalance with dissipation (QCM‐D) and the Spectroscopic Ellipsometry techniques combined in a single device. For PEMs of 17 total layers in water, a final thickness of up to 300 nm and a hydration of 69% were measured. The response towards the ionic strength was then studied by means of QCM‐D. PEMs of 17 layers, with PDADMAC as last layer, shrunk dramatically and lost water when exposed to aqueous NaCl solutions of increasing concentration. Indeed, a thickness variation up to 100 nm and reduction in the 50% of the water content were observed when the PEM was exposed to 1 M NaCl. On the contrary, PEMs where PSS appears on top showed no measurable change upon the variation in the ionic strength. This brings the possibility to control the responsive character of the PEMs simply by selecting the last polyelectrolyte layer (PDADMAC or PSS) deposited. © 2011 Wiley Periodicals, Inc. J Polym Sci Part A: Polym Chem, 2011