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     

Responsive Polyelectrolyte Multilayers Assembled at High Ionic Strength with an Unusual Collapse at Low Ionic Strength

Responsive polyelectrolyte multilayers (PEMs) of poly(diallyl dimethyl ammonium chloride) (PDADMAC) and poly(styrene sodium sulfonate) (PSS) with thicknesses between 350 and 400 nm for 11 deposited polyelectrolyte layers were fabricated assembling the polyelectrolytes at 3 M NaCl. When the 3 M NaCl bulk solution is replaced by water, the PEMs release water, approximately a 46% of the total mass, and experience a thickness reduction of more than 200 nm. Changes in thickness and water content are fully reversible. The film recovers its original thickness and water content when it is exposed again to a 3 M NaCl solution. A responsive polymer film is achieved with the capability of swelling at high ionic strength and collapsing in water with variations in thickness of hundred of nanometers.     

Influence of the nature of the polycation on the adsorption kinetics and on exchange processes in polyelectrolyte multilayer films

The deposition of polyelectrolyte multilayer films (PEMs) appears more and more as a versatile tool to functionalize a broad range of materials with coatings having controlled thicknesses and properties. To increase the control over the properties of such coatings, a good knowledge of their deposition mechanism is required. Since Cohen Stuart et al. (Langmuir 18 (2002) 5607-5612) showed that the adsorption of one polyelectrolyte could induce desorption of polyelectrolyte complexes instead of regular deposition, more and more findings highlight peculiarities in the deposition of such films. Herein we demonstrate that the association of sodium polyphosphate (PSP) as the polyanion and either poly(-L-lysine hydrobromide) (PLL) or poly(allylamine chloride) (PAH) as the polycations may lead to non-monotonous film deposition as a function of time. Complementary, films containing PSP and PLL can be obtained from a (PLL-HA)(n) template films after the exchange of HA (hyaluronic acid) from the sacrificial template by PSP from the solution. This exchange is accompanied by pronounced film erosion. However, when starting from a (PAH-HA)(n) template, the film erosion and exchange due to the contact with PSP is by far less pronounced, nevertheless the film morphology changes. These findings show that the nature of the polycation used to deposit the PEM film may have a profound influence of the film's response to a competing polyanion.     

Assembly of multilayer films from polyelectrolytes containing weak and strong acid moieties

Multilayer films were assembled from a copolymer containing both weakly and strongly charged pendant groups, poly(4-styrenesulfonic acid-co-maleic acid) (PSSMA), deposited in alternation with poly(allylamine hydrochloride) (PAH). The strongly charged groups (styrene sulfonate, SS) are expected to form electrostatic linkages (to enhance film stability), while the weakly charged groups (maleic acid, MA) can alter multilayer film properties because they are responsive to external pH changes. In this study, we varied several assembly conditions such as pH, SS/MA ratio in PSSMA, and the ionic strength of the polyelectrolyte solutions. The multilayer films were also treated by immersion into pH 2 and 11 solutions after assembly. Quartz crystal microgravimetry and UV-visible spectrophotometry showed that the thickness of PSSMA/PAH multilayers decreases with increasing assembly pH regardless of whether salt was present in the polyelectrolyte solutions. When no salt was added, the multilayers are thinner, smoother, and grow less regularly. Atomic force microscopy images indicate that the presence of salt in polyelectrolyte solutions results in rougher surface morphologies, and this effect is especially significant in multilayers assembled at pH 2 and pH 11. When both polyelectrolytes are adsorbed at conditions where they are highly charged, salt was necessary to promote regular multilayer growth. Fourier transform infrared spectroscopy studies show that the carboxylic acids in the multilayers are essentially ionized when assembled from different pHs in 0.5 M sodium chloride solutions, whereas some carboxylic acids remain protonated in the multilayers assembled from solutions with no added salt. This resulted in different pH stability regimes when the multilayers were exposed to different pH solutions, post assembly.     

Fibronectin adsorption onto polyelectrolyte multilayer films

The Layer-by-layer deposition of positively and negatively charged macromolecular species is an ideal method for constructing thin films incorporating biological molecules. We investigate the adsorption of fibronectin onto polyelectrolyte multilayer (PEM) films using optical waveguide lightmode spectroscopy (OWLS) and atomic force microscopy (AFM). PEM films are formed by adsorption onto Si(Ti)O2 from alternately introduced flowing solutions of anionic poly(sodium 4-styrenesulfonate) (PSS) and cationic poly(allylamine hydrochloride) (PAH). Using OWLS, we find the initial rate and overall extent offibronectin adsorption to be greatest on PEM films terminated with a PAH layer. The polarizability density of the adsorbed protein layer, as measured by its refractive index, is virtually identical on both PAH- and PSS-terminated films; the higher adsorbed density on the PAH-terminated film is due to an adsorbed layer of roughly twice the thickness. The binding of monoclonal antibodies specific to the protein's cell binding site is considerably enhanced to fibronectin adsorbed to the PSS layer, indicating a more accessible adsorbed layer. With increased salt concentration, we find thicker PEM films but considerably thinner adsorbed fibronectin layers, owing to increased electrostatic screening. Using AFM, we find adsorbed fibronectin layers to contain clusters; these are more numerous and symmetric on the PSS-terminated film. By considering the electrostatic binding of a segmental model fibronectin molecule, we propose a picture of fibronectin adsorbed primarily in an end-on-oriented monolayer on a PAH-terminated film and as clusters plus side-on-oriented isolated molecules onto a PSS-terminated film.     

Polymer-metal complexes in polyelectrolyte multilayer films as catalysts for oxidation of toluene

We report on the binding of metal ions (Me(2+); Co(2+) and Cu(2+)) with weak polyelectrolyte multilayers (PEMs), as well as on catalytic activity of PEM-Me(2+) films for oxidation of toluene. Using several types of PEM films constructed using branched polyethyleneimine (BPEI) or quaterinized poly-4-vinylpyridines (QPVPs) as polycations and poly(acrylic acid) (PAA) or poly(styrene sulfonate) (PSS) as polyanions, we found that binding of Co(2+) and Cu(2+) ions with a PEM matrix can occur both through coordination to polycationic amino groups and/or ionic binding to polyacid groups. The amount of metal ions loaded within the film increased linearly with film thickness and was strongly dependent on polyelectrolyte type, film assembly pH, and fraction of permanent charge in polymer chains. Among various PEM-Me(2+) systems, BPEI/PAA-Co(2+) films assembled at pH 8.5 show the best catalytic performance, probably because of the preservation of high mobility of Co(2+) ions coordinated to amino groups of BPEI in these films. With BPEI/PAA-Co(2+) films, we demonstrated that films were highly permeable to reagents and reaction products within hundreds of nanometers of the film bulk; i.e., film catalytic activity increased linearly with layer number up to 30 bilayers and slowed for thicker films.     

Dynamics of ion exchange between self-assembled redox polyelectrolyte multilayer modified electrode and liquid electrolyte

A probe beam deflection (PBD) study of ion exchange between an electroactive polymer poly(allylamine)-bipyridyl-pyridine osmium complex film and liquid electrolyte is reported. The PBD measurements were made simultaneously to chronoamperometric oxidation-reduction cycles, to be able to detect kinetic effects in the ion exchange. Layer-by-layer (LbL) self-assembled redox polyelectrolyte films with osmium bipyridyl complex covalently attached to poly(allylamine) (PAH-Os) and poly(styrene sulfonate) (PSS) have been built by alternate electrostatic adsorption from soluble polyelectrolytes. The ionic exchange during initial conditioning of the film ("break-in") undergoing oxidation-reduction cycles and recovery after equilibration in the reduced state have shown an exchange of anions and cations with time lag between them. The effect of the nature of cation on the ionic exchange has been investigated with dilute HCl, LiCl, NaCl, and CsCl electrolytes. The ratio of anion to cation exchanged at the film-electrolyte interface has a strong dependence on the nature of charge in the topmost layer, that is, when negatively charged PSS is the capping layer, a larger proportion of cation exchange is observed. This demonstrates that the electrical potential distribution at the redox polyelectrolyte multilayer (PEM)/electrolyte interface determines the ionic flux in response to charge injection in the film.     

Interactions of poly(2-methacryloyloxyethyl phosphorylcholine) with various salts studied by size exclusion chromatography

The objective of this research is to develop a relationship between salt type and concentration to poly(2-methacyloyloxyethyl phosphorylcholine) (PMPC) zwitterionic polymer solution behaviors. In particular, polyelectrolyte hydrodynamic volumes were analyzed through size exclusion chromatography in relation to the addition of various salts at various concentrations. The salt properties examined were salt concentration, ionic strength, solution pH, cation type/size, anion type/size, valency, and configuration. It was found that the effect of ion properties is related to mechanisms associated with the geometry of the polyelectrolyte. The negative charge group of the polyelectrolyte situated closer to the backbone (inside) is less important to the change in hydrodynamic volume than the positive charge group situated at the end of the side chain (outside). The extensive amount of data generated in this study provides a strong background for possible accurate formulation of a theory based on the salt effect on PMPC polyelectrolyte solution behavior.     

Relationship between the growth regime of polyelectrolyte multilayers and the polyanion/polycation complexation enthalpy

The alternate deposition of polyanions and polycations leads to the formation of films called polyelectrolyte multilayer films (PEMs). Two types of growth processes are reported in the literature, leading to films that grow either linearly or exponentially with the number of deposition steps. In this article we try to establish a correlation between the nature of the growth process and the heat of complexation between the polyanions and the polycations constituting the PEM film. Isothermal titration microcalorimetry experiments performed on several polyanion/polycation systems seem to indicate that an endothermic complexation process is characteristic of an exponential film growth, whereas a strongly exothermic process corresponds to a linear growth regime. Finally, weakly exothermic processes seem to be associated with weakly exponentially growing films. These results thus show that exponentially growing processes are mainly driven by entropy. This explains why the exponential growth processes are more sensitive to temperature than the linear growing processes. This temperature sensitivity is shown on the poly-L-glutamic acid/poly(allylamine) system which grows either linearly or exponentially depending on the ionic strength of the polyelectrolyte solutions.     

Functionalization of organic membranes by polyelectrolyte multilayer assemblies: application to the removal of copper ions from aqueous solutions

The functionalization of an organic polyethersulfone membrane (PES) was performed by alternating deposition of poly(allylamine hydrochloride) (PAH) and poly(sodium 4-styrene sulfonate) (PSS), leading to the formation of a polyelectrolyte multilayer film (PEM). The resulting assembly was characterized by tangential streaming potential measurements to determine the charge of the modified membranes as a function of the polyelectrolyte solution concentration and as a function of the immersion time of the membrane in the polyelectrolyte solutions. Then, the modified membranes were used to perform the ultrafiltration of aqueous solutions containing copper(II) ions. Different operating conditions were tested including: polyelectrolyte concentration, polyelectrolyte nature, thickness of the PEM film or pH of the Cu(2+) solutions. These filtration experiments demonstrated that it was possible to obtain a satisfactory retention of the copper ions (88%), thus proving that this type of assembly can be useful for the removal of copper ions from contaminated aqueous solutions.     

Light-scattering study of polyelectrolyte complexes: Heparin with partially aminoacetalized poly(vinyl alcohol) in aqueous solution

The structure of soluble polyelectrolyte complexes composed of heparin (Hep) and partially aminoacetalized poly(vinyl alcohol) (PVAA) in aqueous solution was investigated by light scattering. The pH was fixed at 3.2 while the ionic strength and mixing ratio were varied. At high ionic strength (0.5), polyelectrolyte complexes were not formed owing to the screening effect of simple salts on polyion charges. At low ionic strength (0.005), polyelectrolyte complexes formed were stable and dispersed when either the polycation or the polyanion was in great excess, whereas the complexes became unstable and coagulated when the concentrations of polycation and polyanion approached each other. At intermediate ionic strength (0.1), when PVAA was in excess, complex formation was similar to that at low ionic strength (0.005); but with an excess of Hep, polyelectrolyte complexes with similar structure (i.e., roughly spherical with average diameters about 2,700 Å) were formed over a wide range of mixing ratio. This observation is of interest in connection with the physiological activity of Hep in vivo.     

Adhesion of polyelectrolyte multilayers: sealing and transfer of microchamber arrays

Polyelectrolyte multilayer (PEM) films with array of responsive microchambers are promising candidates for site-specific release of chemicals in small and precisely defined quantities on demand. It requires effective sealing of the microchambers toward a support to prevent leakage of a cargo. In this paper, we study the pressure-induced adhesion of poly(allylammonium)-poly(4-styrenesulfonate) (PAH-PSS) multilayers assembled on different templates toward the poly(4-styrenesulfonate)-poly(diallyldimethylammonium) multilayer. The tensile bond strength increases from 0.4 to 3.5 MPa upon the increase of PAH-PSS bilayers from 10 to 40, if assembled on a silicon template. Weaker tensile bond strength of 0.35 MPa between the PAH-PSS multilayer and a poly(methylmethacrylate) (PMMA) template results in adhesive break at this interface and allows mechanical removal of the template. The successful PEM transfer is demonstrated for templates of various geometrical patterns, while the tensile break of a multilayer film happens for the others.     

Recent advances in the use of the quartz crystal microbalance with dissipation for the study of the collapse of polyelectrolyte brushes

The combination of Quartz Crystal Microbalance and ellipsometry to quantify polyelectrolyte brush hydration and how this impacts on the study of brush collapse is discussed here. Recent articles study collapse after quantifying hydration of poly (2-methacryloyloxy ethyl trimethyl ammonium chloride) (PMETAC) and poly(potassium sulfo propyl methacrylate). Water lost increases with ionic strength but only a 50% of their water content is lost at 1 M NaCl. For PMETAC brushes the exchange of Cl⁻ by ClO₄⁻ is more effective, causing 50% lost at 0.05 M. The hydration and water lost for PMETAC brushes synthesized from thiol monolayers with variable density of initiators from 1 to 100% show that the freely entrapped water of the brushes is around a 10%. Irrespectively of initiator percentage the brush does not lose at 1 M NaCl more than 50% of the original water content. © 2013 Wiley Periodicals, Inc. J. Polym. Sci., Part B: Polym. Phys. 2013, 51, 1068–1072     

Binding of oppositely charged surfactants to spherical polyelectrolyte brushes: a study by cryogenic transmission electron microscopy

The formation of a complex between an anionic spherical polyelectrolyte brush (SPB) and the cationic surfactant cetyltrimethylammonium bromide (CTAB) is investigated. The SPB consists of long chains of the strong polyelectrolyte poly(styrene sulfonate) (PSS), which are bound chemically to a solid poly(styrene) core of 56 nm in radius. The SPB are dispersed in water, and the ionic strength is adjusted by addition of NaBr. The resulting complexes are investigated in dilute solution by dynamic light scattering, by electrophoretic light scattering, and by cryogenic transmission electron microscopy (cryo-TEM). The formation of the complex between the SPB and the surfactant can be monitored by a strong shrinking of the surface layer when adding CTAB to dilute suspensions (0.01 wt %) and by a decrease of the effective charge of the complexes. Complex formation starts at CTAB concentrations lower than the critical micelle concentration of this surfactant. If the ratio r of the charges on the SPB to the charge of the added surfactant is exceeding unity, the particles start to flocculate. Cryo-TEM images of the complexes at r = 0.6 measured in salt-free solution show that the surface layer composed of the PSS chains and the adsorbed CTAB molecules is partially collapsed: A part of the chains form a dense surface layer while another part of the chains or aggregates thereof are still sticking out. This can be deduced from the cryo-TEM micrographs as well as from the hydrodynamic radius, which is still of appreciable magnitude. The 1:1 complex (r = 1.0) exhibits a fully collapsed layer formed by the PSS chains and CTAB. If the complex is formed in the presence of 0.05 M NaBr, r = 0.6 leads to globular structures directly attached to the surface of the core particles. All structures seen in the cryo-TEM images can be explained by a collapse transition of the surface layer brought about by the hydrophobic attraction between the polyelectrolyte chains that became partially hydrophobic through adsorption of CTAB.     

Supramolecular structures and conformational transitions in polyelectrolyte gels

Three actual problems connected with the collapse transition of polyelectrolyte gels in poor solvent are reviewed. (i) The first topic is the appearance of microstructures for polyelectrolyte gels in poor enough solvents. Three reasons for microsegregation are discussed: effect of polyelectrolyte – hydrophobic competition, effect of the formation of multiplets from ionic pairs in the ionomer regime and the effect of partial formation of glassy kinetically frozen polymer rich regions. (ii) Second we analyze the interaction of the gels with polymers which are capable to form complexes with the gel chains. Main experimental results are presented for poly(methacrylic acid) gels interacting with poly(ethylene oxide), (iii) Finally, we consider the problem of interaction of polyelectrolyte gels with the sufactants. The micelle formation of surfactants inside the gel and its consequences for the conformational properties of the gel are discussed in detail.     

Electrical monitoring of polyelectrolyte multilayer formation by means of capacitive field-effect devices

The semiconductor field-effect platform represents a powerful tool for detecting the adsorption and binding of charged macromolecules with direct electrical readout. In this work, a capacitive electrolyte–insulator–semiconductor (EIS) field-effect sensor consisting of an Al-p-Si-SiO₂ structure has been applied for real-time in situ electrical monitoring of the layer-by-layer formation of polyelectrolyte (PE) multilayers (PEM). The PEMs were deposited directly onto the SiO₂ surface without any precursor layer or drying procedures. Anionic poly(sodium 4-styrene sulfonate) and cationic weak polyelectrolyte poly(allylamine hydrochloride) have been chosen as a model system. The effect of the ionic strength of the solution, polyelectrolyte concentration, number and polarity of the PE layers on the characteristics of the PEM-modified EIS sensors have been studied by means of capacitance–voltage and constant-capacitance methods. In addition, the thickness, surface morphology, roughness and wettabilityof the PE mono- and multilayers have been characterised by ellipsometry, atomic force microscopy and water contact-angle methods, respectively. To explain potential oscillations on the gate surface and signal behaviour of the capacitive field-effect EIS sensor modified with a PEM, a simplified electrostatic model that takes into account the reduced electrostatic screening of PE charges by mobile ions within the PEM has been proposed and discussed.

Effect of the Ionic Strength on Collapse Transition in Star-like Polyelectrolytes

We present an analytical theory describing intramolecular conformational transition in a star-shaped polyelectrolyte (PE) molecule induced by a decrease in the solvent strength or/and increase in the ionic strength in the solution. We apply a mean-field Flory-type analysis in a box-like approximation, that is we assume fairly uniform distribution of densities of charged monomer units and all mobile ions (co- and counterions) in the intra-molecular volume of the star. We demonstrate that depending on the number of branches, fraction of charged monomers in the star and the ionic strength of the solution different scenario of the collapse transition may take place. At high ionic strength or low degree of ionization collapse transition occurs continuously resembling the collapse of a neutral polymer star. 1 On the contrary, at low ionic strength and high degree of ionization collapse transition features first order phase transition, that is, occurs discontinuously and involves coexistence of collapsed and stretched states.     

Surface interactions during polyelectrolyte multilayer build-up. 2. The effect of ionic strength on the structure of preformed multilayers

Interactions between surfaces bearing multilayer films of poly(allylamine hydrochloride) (PAH) and poly(styrenesulfonate sodium salt) (PSS) were investigated across a range of aqueous KBr solutions. Three layer films (PAH/PSS/PAH) were preassembled on mica surfaces, and the resulting interactions were measured with the interferometric surface force apparatus (SFA). Increasing the ionic strength of the medium resulted in a progressive swelling of the multilayer films. Interactions in solutions containing more than 10(-3) M KBr were dominated by a long-ranged steric repulsion originating from compression of polyelectrolyte segments extending into solution. In 10(-1) M KBr, repeated measurements at the same contact position showed a considerable reduction of the range and the strength of the steric force, indicating a flattening of the film during initial approach. Furthermore, this flattening was irreversible on the time scale of the experiments, and measurements performed up to 72 h after the initial compression showed no signs of relaxation. These studies aid in understanding the dominant interactions between polyelectrolyte multilayers, including polyelectrolyte films deposited on colloidal particles, which is important for the preparation of colloidally stable nanoengineered particles.     

Unusual salt stability in highly charged diblock co-polypeptide hydrogels

The stability and properties of dilute solution hydrogels, synthesized by transition metal mediated polymerization of amino acid N-carboxyanhydrides (NCAs), have been studied in deionized (DI) water as well as various ionic media. These hydrogels are diblock amphiphilic copolymers of hydrophilic, charged segments of poly(l-lysine HBr) or poly(l-glutamic acid sodium salt), and helical, hydrophobic segments of poly(l-leucine). While many of these samples are able to form strong gels in deionized water at polymer concentrations as low as 0.25 wt %, stability in salt or buffer solutions was found to be only achieved at moderately higher polymer concentrations ( approximately 3.0 wt %). We have adjusted relative copolymer compositions and molecular weights to optimize hydrogel strength and polymer solubility in salt concentrations up to 0.5 M NaCl, as well as in cell growth media and aqueous buffers of varying pH. These materials are unique since they do not collapse in high ionic strength media, even though gel formation is contingent upon the presence of highly charged polyelectrolyte segments. The remarkable properties of these hydrogels make them excellent candidates for use as scaffolds in biomedical applications, such as tissue regeneration.     

Redox and acid-base coupling in ultrathin polyelectrolyte films

A single layer of poly(allylamine) with a covalently attached osmium pyridine-bipyridine complex adsorbed onto a Au surface modified by mercaptopropanesulfonate has been studied theoretically with a molecular approach and experimentally by cyclic voltammetry. These investigations have been carried out at different pHs and ionic strengths of the electrolyte solution in contact with the redox polyelectrolyte modified electrode. The theory predicts strong coupling between the acid-base and redox equilibria, particularly for low ionic strength, pH close to the pKa, and high concentration of redox sites. The coupling leads to a decrease in the peak potential at pH values above the apparent pKa of the weak polyelectrolyte, in good agreement with the experimental pH dependence at 4 mM NaNO3. Theoretical calculations suggest that the inflection point in the peak position versus pH curves can be used to estimate the apparent pKa of the amino groups in the polymer. Comparison of the apparent pKa for PAH-Os in the film with that of poly(allylamine) reported in the literature shows that the underlying charged thiol strongly influences charge regulation in the film. A systematic study of the film thickness and the degree of protonation in sulfonate and amino groups for solutions of different pH and ionic strength shows the coupling between the different interactions. It is found that the variation of the film properties has a non-monotonic dependence on bulk pH and salt concentration. For example, the film thickness shows a maximum with electrolyte ionic strength, whose origin is attributed to the balance between electrostatic amino-amino repulsions and amino-sulfonate attractions.