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PubMed2004

Creating effective refractive index gradients within polyelectrolyte multilayer films: molecularly assembled rugate filters.

We demonstrate the use of aqueous-based layer-by-layer (LbL) processing and in situ nanoreactor chemistry for creating polyelectrolyte multilayer (PEM) nanocomposites in which the concentration of a nanoparticle species is modulated throughout the depth of the film in a complex way. Using the in situ growth of silver nanoparticles to selectively increase the refractive index of precisely defined regions of the PEM film, we have produced a digitized rugate filter, a special type of dielectric mirror possessing many thin layers that approximate a continuous, periodic refractive index profile. The design of the digital rugate from an initial smooth index profile was aided by a computer, and construction of the device was accomplished entirely by automated means. Poly(allylamine hydrochloride) (PAH) and poly(acrylic acid) (PAA) bilayer regions within the film supported subsequent nanoreactor chemistry and created the high refractive index regions of the film, while PAH and poly(styrenesulfonate) (PSS) bilayer regions remained unmodified and served as the low index regions. Spectroscopic results obtained from the assembled device confirm the presence of a reflection band that grows in amplitude with increasing silver incorporation, reaching a peak reflectance of 75% after five silver loading and exchange cycles. The rugate structure suppresses reflection sidebands, features common to Bragg reflectors and simpler dielectric mirror designs. Cross-sectional TEM reveals nanoscale control of the size and location of silver-containing domains within the PEM film. The ability of this method to control the placement of nanoparticles within PEMs enables the inexpensive, easy production of conformable optical coatings with tunable properties. In addition, these nanocomposite films may have applications in a number of other fields where precise control of effective gradients in physical properties throughout thin films has been shown to affect the overall performance in a desirable way.

Journal Article↗
PubMed2005

Interactions of hydrophobically modified polyelectrolytes with nonionic surfactants.

Interactions of surfactants with hydrophobically modified polyelectrolytes in aqueous solutions are important in several applications such as detergency, cosmetics, food, and paints. Complexes formed in these systems raise some fundamental questions about the polymer-surfactant interactions that control their behavior. In this work, the interactions of a nonionic surfactant, penta-ethyleneglycol mono n-dodecyl ether (C(12)EO(5)), with a hydrophobically modified anionic polymer, poly(maleic acid/octyl vinyl ether) (PMAOVE), in aqueous solutions were studied using surface tension, viscosity, electron paramagnetic resonance (EPR) spectroscopy, light scattering, and fluorescence spectroscopic techniques. When the nonionic surfactant C(12)EO(5) was added to aqueous solutions of the anionic polymer PMAOVE, it was incorporated into the hydrophobic nanodomains of PMAOVE far below the the critical micelle concentration (cmc) of the surfactant. Two inflection points were observed corresponding to the critical complexation concentration (formation of mixed micelles composed of C(12)EO(5) and the octyl chains of PMAOVE) and the saturation concentration (saturation of the polymer with C(12)EO(5) molecules). Above the saturation concentration, the coexistence of pure C(12)EO(5) micelles and mixed micelles of PMAOVE and C(12)EO(5) was observed. Such a coexistence of complexes has major implications in their performance in colloidal processes.

Journal Article↗
PubMed2005

Formation of intra- and interparticle polyelectrolyte complexes between cationic nanogel and strong polyanion.

Polyelectrolyte complex formation of a strong polyanion, potassium poly(vinyl alcohol) sulfate (KPVS), with positively charged nanogels was studied at 25 degrees C in aqueous solutions with different KCl concentrations (C(s)) as a function of the polyion-nanogel mixing ratio based on moles of anions versus cations. Used as the gel sample was a polyampholytic nanogel consisting of lightly cross-linked terpolymer chains of N-isopropylacrylamide, acrylic acid, and 1-vinylimidazole; thus, the complexation was performed at pH 3 at which the imidazole groups are fully protonated to generate positive charges. Turbidimetric titration was employed to vary the mixing ratio. Also employed for studies of the resulting complexes at different stages of the titration were dynamic light scattering (DLS) and static light scattering (SLS) techniques. It was found from the titration as well as DLS and SLS that there is a critical mixing ratio (cmr) at which both the size and molar mass of the complexed gel particles abruptly increase. The value of the cmr at C(s) = 0 or 0.01 M (mol/L) was observed at approximately 1:1 mixing ratio of anions versus cations but at lower mixing ratios than the 1:1 ratio under conditions of C(s) = 0.05 and 0.1 M. At the mixing ratios less than the cmr, the molar mass of the complex agrees with that of one gel particle with the calculated amount of the bound KPVS ions, indicating the formation of an "intraparticle" KPVS-nanogel complex, by the aggregation of which an "interparticle" complex is formed at the cmr. During the process of the intraparticle complex formation, both the hydrodynamic radius by DLS and the radius gyration by SLS decreased with increasing mixing ratio, demonstrating the gel collapse due to the complexation. At C(s) = 0 or 0.01 M and under conditions where the amount of KPVS bindings was less than half of the nanogel cations, however, the decrease of the hydrodynamic radius was very small, while the radius gyration fell monotonically. These results were discussed in connection with a collapse of dangling chains attached to the nanogel surface by the binding of KPVS.

Journal Article↗
PubMed2005

Salt softening of polyelectrolyte multilayer capsules.

The changes in the morphology and the mechanical properties of hollow polyelectrolyte multilayer capsules made from poly(styrenesulfonate)/poly(allylamine hydrochloride) in response to added salt were investigated. We found that capsules shrink in response to salt exposure. The effect depends strongly on the nature of the salt added and follows trends of the Hoffmeister series, with weakly hydrated cations inducing the strongest shrinking. For NaCl, we have investigated additional effects on capsule mechanical properties that are occurring above a 3 M salt concentration and we found that the morphological changes are accompanied by a pronounced softening of the capsule wall material, which we can quantify by analyzing the force response of capsules in the prebuckling regime. This shows that salts can act as plasticizers in the multilayers and induce annealing effects.

Journal Article↗
PubMed2005

Polyelectrolyte-induced peeling of charged multilamellar vesicles.

We study mixtures of charged surfactants, which alone in solution form uni- and multilamellar vesicles, and oppositely charged polyelectrolytes (PEs). The phase behavior is investigated at fixed surfactant concentration as a function of the PE-to-surfactant charge ratio, x. We find that, for x > 0, aggregates form. Light microscopy and X-ray scattering experiments show that the isoelectric point plays a crucial role, since the morphology and the microscopic structure of the aggregates are different before (x < or = 1) and after the isoelectric point (x > 1). To better understand the dynamics for the formation of PE/surfactant complexes, we perform light microscopy experiments where we follow in real time the effect of a PE solution on one multilamellar vesicle (MLV). We find that the PE induces a peeling of the bilayers of the MLV one by one. The peeling is accompanied by strong shape fluctuations of the MLV and leads ultimately to a pile of small aggregates. This novel phenomenon is analyzed in detail and discussed in terms of PE-induced tension and pore formation and growth in a surfactant bilayer.

Journal Article↗
PubMed2005

Effect of the mobility of charged units on the microphase separation in amphiphilic polyelectrolyte hydrogels.

The effect of the migration of charged units on the structure of hydrophobically modified polyelectrolyte gels swollen by D(2)O was studied by small-angle neutron scattering on an example of gels of terpolymers of acrylic acid, n-dodecylacrylate, and 2-acrylamido-2-methyl-1-propanesulfonic acid (quenched charged units) and gels of copolymers of partially neutralized acrylic acid and n-dodecylacrylate (annealed charged units). The content of charged units did not exceed 20 mol %, so that the electrostatic repulsion was too weak to disrupt the hydrophobic domains formed by self-assembled n-dodecyl chains, which was evidenced by NMR data. It was shown that upon increasing the charge content both types of gels undergo microphase separation with the formation of hydrophobic clusters consisting of several densely packed hydrophobic domains and hydrophilic regions swollen by water, where most of the charged repeat units and counterions are located. The dimensions of the nanostructure of the gels with quenched and annealed charged groups were compared. It was shown that the size of clusters in the gels with annealed charged units is much bigger than that in the gels with the same fraction of quenched charged units. This effect was attributed to a much weaker electrostatic repulsion in the corona of the hydrophobic clusters in the gels with annealed charged groups, because the charged units repelling each other are able to move farther apart.

Journal Article↗
PubMed2004

Coulombic interactions on the deposition and rotational mobility distributions of dyes in polyelectrolyte multilayer thin films.

We employed negatively charged fluorescein (FL), positively charged rhodamine 6G (R6G), and neutral Nile Red (NR) as molecular probes to investigate the influence of Coulombic interaction on their deposition into and rotational mobility inside polyelectrolyte multilayer (PEM) films. The entrapment efficiency of the dyes reveals that while Coulombic repulsion has little effect on dye deposition, Coulombic attraction can dramatically enhance the loading efficiency of dyes into a PEM film. By monitoring the emission polarization of single dye molecules in polyethylenimine (PEI) films, the percentages of mobile R6G, NR, and FL were determined to be 87 +/- 4%, 76 +/- 5%, and 68 +/- 3%, respectively. These mobility distributions suggest that cationic R6G enjoys the highest degree of rotational freedom, whereas anionic FL shows the least mobility because of Coulombic attraction toward cationic PEI. Regardless of charges, this high percentage of mobile molecules is in stark contrast to the 5-40% probe mobility reported from spun-cast polymer films, indicating that our PEI films contain more free volume and display richer polymer dynamics. These observations demonstrate the potential of using isolated fluorescent probes to interrogate the internal structure of a PEM film at a microscopic level.

Coloring Agents↗
PubMed2005

"Like-charge attraction" between anionic polyelectrolytes: molecular dynamics simulations.

"Like-charge attraction" is a phenomenon found in many biological systems containing DNA or proteins, as well as in polyelectrolyte systems of industrial importance. "Like-charge attraction" between polyanions is observed in the presence of mobile multivalent cations. At a certain limiting concentration of cations, the negatively charged macroions cease to repel each other and even an attractive force between the anions is found. With classical molecular dynamics simulations it is possible to elucidate the processes that govern the attractive behavior with atomistic resolution. As an industrially relevant example we study the interaction of negatively charged carboxylate groups of sodium polyacrylate molecules with divalent cationic Ca2+ counterions. Here we show that Ca2+ ions initially associate with single chains of polyacrylates and strongly influence sodium ion distribution; shielded polyanions approach each other and eventually "stick" together (precipitate), contrary to the assumption that precipitation is initially induced by intermolecular Ca2+ bridging.

Journal Article↗
PubMed2005

Macromolecule encapsulation in diazoresin-based hollow polyelectrolyte microcapsules.

A stable enzyme encapsulation technique based on the conversion of weak interactions between diazo resin/poly(styrene sulfonate) to covalent bonds was explored. Photosensitive diazoresin-based polyelectrolyte microcapsules were prepared via layer-by-layer electrostatic self-assembly of poly(styrene sulfonate) and diazoresin on MnCO(3) templates. UV-vis and zeta-potential measurements confirmed the alternate deposition of {PSS/DAR} multilayers on the micrometer-sized dissolvable templates. The DAR-based microcapsules were demonstrated to be permeable to enzymes prior to UV irradiation, while the permeability of the multilayer wall was changed substantially after photo-cross-linking. Encapsulated molecules were stably entrapped after UV irradiation, as shown by confocal microscopy and atomic force microscopy images. Activity assays revealed that encapsulated glucose oxidase possessed 52.8% of the catalytic activity exhibited by the same amount of free enzyme, proving the preservation of native conformation and accessibility of substrate. This encapsulation technique is promising for many biomedical and biotechnological applications, particularly enzyme biosensors, which require stable immobilization of functional components while allowing sufficient transport rates for substrate molecules.

Azo Compounds↗
PubMed2004

Novel solid-state polymer electrolyte consisting of a porous layer-by-layer polyelectrolyte thin film and oligoethylene glycol.

A novel solid-state polymer electrolyte was constructed using layer-by-layer (LbL) polyelectrolyte assembly of linear poly(ethylenimine) (LPEI) and poly(acrylic acid) (PAA), combined with a plasticization step using oligoethylene glycol dicarboxylic acid (OEGDA). This composite film exhibits a relatively high ionic conductivity of 9.5 x 10(-5) S/cm at 25 degrees C and 22% relative humidity. Detailed characterization of the composite was undertaken using grazing-angle Fourier transform infrared (GA-FTIR), atomic force microscopy (AFM), scanning electron microscopy (SEM), differential scanning calorimetry (DSC), and impedance spectroscopy. After immersing the LPEI/PAA films into OEGDA aqueous solutions, the films exhibited a swelling behavior and increased surface roughness indicative of porosity induced by reorganization of ionic interactions between LPEI and PAA in acidic solution. This internal porous structure allows inclusion of OEGDA within the multilayer and increased ionic conductivity under ambient conditions due to the combined effects of plasticization of the LbL matrix by atmospheric water as well as the added mobility of ions in molten OEGDA within the composite.

Journal Article↗
PubMed2004

Critical aggregation concentration in mixed solutions of anionic polyelectrolytes and cationic surfactants.

We have examined the polymer/surfactant interaction in mixed aqueous solutions of cationic surfactants and anionic polyelectrolytes combining various techniques: tensiometry, potentiometry with surfactant-selective electrodes, and viscosimetry. We have investigated the role of varying polymer charge density, polymer concentration, surfactant chain length, polymer backbone rigidity, and molecular weight on the critical aggregation concentration (Cac) of mixed polymer/surfactant systems. The Cac of these systems, estimated from tensiometry and potentiometry, is found to be in close agreement. Different Cac variations with polymer charge density and surfactant chain length were observed with polymers having persistence lengths either smaller or larger than surfactant micelle size, which might reflect a different type of molecular organization in the polymer/surfactant complexes. The surfactant concentration at which the viscosity starts to decrease sharply is different from the Cac and probably reflects the polymer chain shrinkage due to surfactant binding.

Journal Article↗
PubMed2004

Modification of a supported lipid bilayer by polyelectrolyte adsorption.

Addition of a weak polyelectrolyte, poly(methacrylic acid) (PMA), to a supported phospholipid bilayer made from 1,2-dimyristoyl-sn-glycero-3-phosphocholine (DMPC) depresses the melting temperature and alters the morphology of the bilayer in the gel phase. Ellipsometry measurements show that PMA adsorption lowers the phase transition temperature by 2.4 degrees C. Atomic force microscopy (AFM) showed no visible contrast in the fluid phase (above the melting temperature) but a rich morphology in the gel phase. In the gel phase, adsorption leads to formation of significantly less mobile phospholipid islands and other defects. One consequence of this lower mobility is a decrease in the implied cooperativity number of the phase transition, N, when polymer is added. Additionally, AFM images of the gel-phase bilayer show a highly defected structure that anneals significantly more slowly than in the absence of adsorbed polymer. Tentatively, we suggest that PMA preferentially decorates island and defect edges of the DMPC bilayer.

Adsorption↗
PubMed2004

Large apparent interfacial slippage at polyelectrolyte-perfluorocarbon interfaces on a quartz crystal resonator.

The apparent negative areal mass densities obtained for a polyelectrolyte multilayer on a quartz crystal resonator in contact with four different perfluorocarbon liquids are explained by the interfacial slippage between the multilayer and the liquids. It is shown that the zone of interfacial slipping can be conveniently treated as a separate layer with distinct physical parameters. Three models of slippage were taken into a closer study. In the first model, the so-called de Gennes model, a very thin gas-filled cavity is formed between the moving phase and the stationary phase. The second model is based on the slipping layer consisting of water. In the third model, the so-called "true slipping" model, it is assumed that the particle velocity has a discontinuity at the interface. In each case, the slipping admittances and slippage lengths as well as the corrected areal mass densities were calculated from the experimental data. Although no unambiguous experimental evidence was found to favor strongly any of these three models, the slightly smaller variation in the slipping admittance and the areal mass density seems to give more credibility to the de Gennes model.

Journal Article↗
PubMed2004

Layer by layer self-assembled polyelectrolyte multilayers with embedded phospholipid vesicles.

We describe a method to embed phospholipid vesicles into polyelectrolyte multilayers built up by the alternate deposition of polyanions and polycations. Before deposition, the vesicles are rigidified by polycation adsorption onto their surface avoiding their fusion once deposited on the multilayer surface. The vesicles adsorb to form a compact and "hard" monolayer as imaged by atomic force microscopy. The thickness of the adsorbed vesicle layer, of the order of 250 nm, is very close to the diameter of the vesicles in solution. This work should open the route to the buildup of multilayer films containing phospholipid vesicles that could act as "reservoirs" for drugs or enzymatic nanoreactors.

Adsorption↗
PubMed2004

Interfacial microgels formed by oppositely charged polyelectrolytes and surfactants. Part 2. Influence of surfactant chain length and surfactant/polymer ratio.

The adsorption and complexation of polystyrene sulfonate (a highly charged anionic polyelectrolyte) and a series of cationic surfactants, alkyltrimethylammonium bromide, CnTAB, n = 8-16, at the air-water interface has been studied by combining surface tension and ellipsometry measurements. We find that increasing the chain length of the surfactant from 8 to 10 carbons leads to a sharp increase in adsorption of PSS/CnTAB complexes. When the surfactant tail length is further increased to 12 and 14 carbons, surface adsorption becomes less favored than macroscopic phase separation, resulting in a partial surface depletion. Furthermore, we find that when surface tensions are plotted against surfactant/monomer molar concentration ratio, all data collapse to a single curve. This result shows that the surfactant-polymer molar ratio, s/p, is a key parameter for tuning the surface activity of the complexes formed.

Journal Article↗
PubMed2005

Solubilization of phenols in anionic polyelectrolyte gels with adsorbed cationic surfactant.

Solubilization isotherms for various phenols in cetylpyridinium chloride (CPC)-polyelectrolyte gel aggregates have been determined in order to compare solubilization within these aggregates with that in free micelles and to examine the effects of gel chemistry and structure on solubilization. The isotherms describing solubilization are quite similar to those found for free surfactant in solution. Solutes that are more hydrophobic give rise to larger solubilization constants with trends similar to what is seen for hydrophobic effects in adsorption from aqueous solutions onto hydrophobic solids. The solubilization constants decrease as the fraction of solute in the aggregates increases, indicating that the solutes partition into the palisade region of the aggregates. Solubilization is found to be quite insensitive to changes in gel structure (cross-linker varying from 1% to 3%) and chemistry (poly(acrylic acid) versus poly(methacrylic acid) and neutralization from 50% to 100%). However, the switch from poly(acrylic acid) to poly(methacrylic acid) did give rise to a slight decrease in magnitude of the slope of the isotherm. The most significant factors appear to be the initial concentration of surfactant in solution and the ratio of surfactant solution to gel amount. A decrease in surfactant concentration (especially combined with an increase in solution volume) gives rise to a decrease in solubilization constants.

Journal Article↗
PubMed2005

Self-assembly of a metallosupramolecular coordination polyelectrolyte in the pores of SBA-15 and MCM-41 silica.

It is shown that intrinsically stiff chain aggregates of a metallosupramolecular coordination polyelectrolyte (MEPE) can form in the cylindrical nanopores of MCM-41 and SBA-15 silica by self-assembly of its constituents (metal ions and organic ligand). The UV/vis spectra of the resulting MEPE-silica composites exhibit the characteristic metal-to-ligand charge transfer band of the MEPE complex in solution. For the MEPE-silica composite in SBA-15 an iron content of 1.2 wt % was found, corresponding to ca. 10 MEPE chains disposed side by side in the 8 nm wide pores of the SBA-15 matrix. In the case of MCM-41 (pore width < 3 nm), where only one MEPE chain per pore can be accommodated, an iron content of 0.3 wt % was obtained, corresponding to half-filling of the pores. It was also found that MEPE chains spontaneously enter the pores of SBA-15, when a solution of MEPE is exposed to the silica matrix.

Journal Article↗
PubMed2005

Evidence of surface charge at the air/water interface from thin-film studies on polyelectrolyte-coated substrates.

The stability of thin water films on silicon substrates coated with cationic and anionic polyelectrolytes was investigated by the thin film pressure balance technique. Depending on the surface charge of the substrate, the water films are either stable (on negatively charged wafers) or rupture rapidly (on positively charged wafers). It is supposed that this behavior is due to a negative surface charge of the free water surface. The underlying assumption that the films' stability is due to electrostatic interactions is supported by measurements of the disjoining pressure on silicon wafers with a native oxide layer, which indicates a decrease of the film thickness, and thus decreasing repulsive interaction between the two film interfaces, with increasing ionic strength.

Journal Article↗