Behavior of Cationic Surfactants in Poly(styrene sulfonate) Brushes

Tuning the Light Response of Strong Polyelectrolyte Brushes with Counterions

In this work, a negatively charged poly(3-sulfopropyl methacrylate potassium) (PSPMA) brush has been employed as a model system to demonstrate the tuning of the light response of strong polyelectrolyte brushes (SPBs) with counterions. The substitution of K⁺ counterions by azobenzene-containing counterions (Azo-N⁺) renders the PSPMA brush light-responsive in aqueous solutions. Nevertheless, the strength of the light response of the PSPMA brush is weak due to the inefficient disassembly of the micelle-like aggregates in the brush upon irradiation with ultraviolet light. Counterion mixtures of Azo-N⁺ and K⁺ are employed to realize a strong light response of the PSPMA brush by incorporating a reasonable amount of Azo-N⁺ counterions into the brush. The strength of the light response of the PSPMA brush can be tuned by the mole ratio of Azo-N⁺ to K⁺. Furthermore, properties including the hydration and conformation of the PSPMA brush can be reversibly switched via alternating ultraviolet and visible light irradiation. This work opens up the opportunities available for the use of counterions to tune the light response of SPBs.

Interaction of synthetic and lignin-based sulfonated polymers with hydrophilic, hydrophobic, and charged self-assembled monolayers

There is a need to understand the role of polymer structure on its interaction with surfaces to produce effective functional surfaces. In this work, we produced two anionic polymers of lignin-3-sulfopropyl methacrylate (L-S) and poly(vinyl alcohol-co-vinyl acetate)-3-sulfopropyl methacrylate (PVA-S) with similar charge densities and molecular weights. On the gold-coated surface, we deposited self-assembled monolayers (SAM) bearing different terminal moieties namely, hydroxyl, carboxyl, methyl, and amine groups of alkanethiols. This study highlighted the difference between the interaction of L-S and PVA-S and functionalized self-assembled surfaces. The information was generated using advanced tools, such as an X-ray photoelectron spectroscopy (XPS), and a quartz crystal microbalance with dissipation (QCM-D), which facilitated the correlation development between polymer properties and deposition performance on the functionalized surfaces. The higher deposition of PVA-S than L-S onto OH and COOH surfaces was observed due to its greater hydrogen bonding development and higher solubility. The solubility and structure of PVA-S were also beneficial for its higher adsorption than L-S onto CH₃ and NH₂ surfaces. However, the variation in pH, temperature, and salt significantly affected the adsorption of the macromolecules.

The interaction mechanism of synthetic and lignin based sulfonated materials with well-designed functional surfaces was investigated systematically.

Mass production of polymer nano-wires filled with metal nano-particles

Despite the ongoing progress in nanotechnology and its applications, the development of strategies for connecting nano-scale systems to micro- or macroscale elements is hampered by the lack of structural components that have both, nano- and macroscale dimensions. The production of nano-scale wires with macroscale length is one of the most interesting challenges here. There are a lot of strategies to fabricate long nanoscopic stripes made of metals, polymers or ceramics but none is suitable for mass production of ordered and dense arrangements of wires at large numbers. In this paper, we report on a technique for producing arrays of ordered, flexible and free-standing polymer nano-wires filled with different types of nano-particles. The process utilizes the strong response of photosensitive polymer brushes to irradiation with UV-interference patterns, resulting in a substantial mass redistribution of the polymer material along with local rupturing of polymer chains. The chains can wind up in wires of nano-scale thickness and a length of up to several centimeters. When dispersing nano-particles within the film, the final arrangement is similar to a core-shell geometry with mainly nano-particles found in the core region and the polymer forming a dielectric jacket.

Ionic Surfactant Binding to pH-Responsive Polyelectrolyte Brush-Grafted Nanoparticles in Suspension and on Charged Surfaces

The interactions between silica nanoparticles grafted with a brush of cationic poly(2-(dimethylamino) ethyl methacrylate) (SiO2-g-PDMAEMA) and anionic surfactant sodium dodecyl sulfate (SDS) is investigated by dynamic light scattering, electrophoretic mobility, quartz crystal microbalance with dissipation, ellipsometry, and atomic force microscopy. SiO2-g-PDMAEMA exhibits pH-dependent charge and size properties which enable the SDS binding to be probed over a range of electrostatic conditions and brush conformations. SDS monomers bind irreversibly to SiO2-g-PDMAEMA at low surfactant concentrations (∼10(-4) M) while exhibiting a pH-dependent threshold above which cooperative, partially reversible SDS binding occurs. At pH 5, SDS binding induces collapse of the highly charged and swollen brush as observed in the bulk by DLS and on surfaces by QCM-D. Similar experiments at pH 9 suggest that SDS binds to the periphery of the weakly charged and deswollen brush and produces SiO2-g-PDMAEMA/SDS complexes with a net negative charge. SiO2-g-PDMAEMA brush collapse and charge neutralization is further confirmed by colloidal probe AFM measurements, where reduced electrosteric repulsions and bridging adhesion are attributed to effects of the bound SDS. Additionally, sequential adsorption schemes with SDS and SiO2-g-PDMAEMA are used to enhance deposition relative to SiO2-g-PDMAEMA direct adsorption on silica. This work shows that the polyelectrolyte brush configuration responds in a more dramatic fashion to SDS than to pH-induced changes in ionization, and this can be exploited to manipulate the structure of adsorbed layers and the corresponding forces of compression and friction between opposing surfaces.

Superhydrophilic polyelectrolyte brush layers with imparted anti-icing properties: effect of counter ions

This work demonstrates the feasibility of superhydrophilic polyelectrolyte brush coatings for anti-icing applications. Five different types of ionic and nonionic polymer brush coatings of 25-100 nm thickness were formed on glass substrates using silane chemistry for surface premodification followed by polymerization via the SI-ATRP route. The cationic [2-(methacryloyloxy)ethyl]trimethylammonium chloride] and the anionic [poly(3-sulfopropyl methacrylate), poly(sodium methacrylate)] polyelectrolyte brushes were further exchanged with H+, Li+, Na+, K+, Ag+, Ca2+, La3+, C16N+, F-, Cl-, BF4-, SO4(2-), and C12SO3- ions. By consecutive measurements of the strength of ice adhesion toward ion-incorporated polymer brushes on glass it was found that Li+ ions reduce ice adhesion by 40% at -18 °C and 70% at -10 °C. Ag+ ions reduce ice adhesion by 80% at -10 °C relative to unmodified glass. In general, superhydrophilic polyelectrolyte brushes exhibit better anti-icing property at -10 °C compared to partially hydrophobic brushes such as poly(methyl methacrylate) and surfactant exchanged polyelectrolyte brushes. The data are interpreted using the concept of a quasi liquid layer (QLL) that is enhanced in the presence of highly hydrated ions at the interface. It is suggested that the ability of ions to coordinate water is directly related to the efficiency of a given anti-icing coating based on the polyelectrolyte brush concept.

Nondestructive light-initiated tuning of layer-by-layer microcapsule permeability

A nondestructive way to achieve remote, reversible, light-controlled tunable permeability of ultrathin shell microcapsules is demonstrated in this study. Microcapsules based on poly{[2-(methacryloyloxy)ethyl] trimethylammonium iodide} (PMETAI) star polyelectrolyte and poly(sodium 4-styrenesulfonate) (PSS) were prepared by a layer-by-layer (LbL) technique. We demonstrated stable microcapsules with controlled permeability with the arm number of a star polymer having significant effect on the assembly structure: the PMETAI star with 18 arms shows a more uniform and compact assembly structure. We observed that in contrast to regular microcapsules from linear polymers, the permeability of the star polymer microcapsules could be dramatically altered by photoinduced transformation of the trivalent hexacyanocobaltate ions into a mixture of mono- and divalent ions by using UV irradiation. The reversible contraction of PMETAI star polyelectrolyte arms and the compaction of star polyelectrolytes in the presence of multivalent counterions are considered to cause the dramatic photoinduced changes in microcapsule properties observed here. Remarkably, unlike the current mostly destructive approaches, the light-induced changes in microcapsule permeability are completely reversible and can be used for light-mediated loading/unloading control of microcapsules.

Formation of hybrid wormlike micelles upon mixing cetyl trimethylammonium bromide with poly(methyl methacrylate-co-sodium styrene sulfonate) copolymers in aqueous solution

The association of cetyltrimethylammonium bromide, CTAB, with a series of P(MMAx-co-SSNa) random copolymers of sodium styrene sulfonate (SSNa) with methyl methacrylate (MMA) was explored in aqueous solution as a function of the MMA molar content, x, of the copolymers. The polyelectrolyte/surfactant complexation in aqueous solution was verified through pyrene fluorescence probing. In addition, turbidimetry studies in dilute or more concentrated aqueous solutions elucidated the phase separation behavior of the P(MMAx-co-SSNa)/CTAB systems as a function of the copolymer composition x and the surfactant to polyelectrolyte mixing charge ratio. It is found that practically phase separation is completely suppressed within the studied mixing range when the MMA content of the copolymers is ∼30-40 mol%. For lower MMA contents the polyelectrolyte/surfactant complex separates out from water, while for higher x values the solubility limits of the copolymers in water are attained. For the intermediate MMA contents, viscoelastic systems are obtained in more concentrated polymer/surfactant solutions provided that the polyelectrolyte is fully complexed with the cationic surfactant ((1)H NMR results). Moreover, the (1)H NMR studies indicate that hybrid P(MMAx-co-SSNa)/CTAB wormlike micelles are formed in water under these conditions. Finally, it is shown that addition of salt prevents syneresis problems and facilitates the rheological investigation.

A neutron reflectivity study of surfactant self-assembly in weak polyelectrolyte brushes at the sapphire-water interface

Poly(2-(dimethylamino)ethyl methacrylate) (PDMAEMA) brushes grown by surface-initiated polymerization from a polyanionic macroinitiator adsorbed at the sapphire-water interface have been used as a substrate to study the interaction between the weak polyelectrolyte PDMAEMA and the oppositely charged surfactant sodium dodecyl sulfate (SDS) with neutron reflectivity. At pH 3, multilayered structures are formed in which the interlayer separation (∼40 Å) is comparable to the dimensions of a SDS bilayer or micelle. The number of repeating layers that form depends on brush thickness, ranging from three layers in a relatively thin brush (5 nm dry thickness) to 15 layers in a relatively thick brush (17 nm dry thickness). In the 5 nm brush, addition of 0.01 mM SDS leads to brush deswelling, and the distinct layered structure only forms when the SDS concentration reaches 1 mM, with the brush reswelling slightly at 5 mM SDS. In the thicker (11 and 17 nm) brushes, distinct layered structures form at 0.1 mM SDS, in which the molar SDS/DMAEMA ratio is greater than unity. Exposing the 17 nm brush/SDS complex to 1 M NaNO(3) results in the complete removal of the surfactant and recovery of the bare brush structure. At pH 9, there is significant surfactant uptake by the brush, but no multilayer structures are formed. The brush presents a high concentration of DMAEMA segments that are localized to within 500-1000 Å of the sapphire interface. At pH 9 the high local concentration of hydrocarbon segments in the brush screens the hydrophobic tails of the surfactants from the unfavorable interaction with water, leading to significant surfactant uptake by the brush. At pH 3 the high local concentration of charges inside the brush additionally screens the repulsive interactions between the surfactant headgroups, making surfactant uptake even more favorable, leading to the formation of multilayered surfactant aggregates confined within the brush.

Quantitative in situ attenuated total internal reflection Fourier transform infrared study of the isotherms of poly(sodium 4-styrene sulfonate) adsorption to a TiO2 surface over a range of cetylpyridinium bromide monohydrate concentration

Quantitative in situ attenuated total internal reflection Fourier transform infrared (ATR FTIR) spectroscopy has been used to study the isotherm of poly(sodium 4-styrene sulfonate), PSS, adsorption to a TiO(2) surface in aqueous solution at a pH of 3.5. The effect of adding surfactant cetylpyridinium bromide monohydrate (CPBM) on the adsorption isotherm of PSS was investigated at CPBM concentrations of 3.60 x 10(-7), 1.02 x 10(-5), and 1.04 x 10(-4) M. The use of in situ ATR FTIR allowed for the calculation of the concentration of both PSS and CPBM at the TiO(2)/water interface over the entire course of all experiments. It was found that the addition of a small amount of CPBM, 3.60 x 10(-7) M, to PSS solutions resulted in 23 +/- 3% less PSS accumulating at the TiO(2)/water interface compared to isotherm studies with no CPBM present. The mole ratio of CPBM to PSS varies from 4 +/- 1 to 1 to 20 +/- 4 to 1 in a stepwise manner as the solution concentration of PSS is increased for solutions with a CPBM concentration of 3.60 x 10(-7). The addition of CPBM at concentrations of 1.02 x 10(-5) and 1.04 x 10(-4) M showed distinct differences in the behavior of the PSS isotherm, but at the highest solution PSS concentrations, the amount of PSS at the TiO(2)/water interface compared to that of PSS solutions with no CPBM added is indistinguishable within the experimental uncertainties. For these higher concentrations of CPBM, both PSS and CPBM appear to come to the TiO(2) surface as aggregates and the mole ratio of CPBM to PSS at the TiO(2)/water interface decreases as the concentration of PSS is increased. For a CPBM concentration of 1.02 x 10(-5) M, the mole ratio of CPBM to PSS changes from 139 +/- 29 to 1 to 33 +/- 7 to 1 as the solution PSS concentration is increased. For a CPBM concentration of 1.04 x 10(-4) M, the mole ratio of CPBM to PSS changes from 630 +/- 130 to 1 to 110 +/- 21 to 1 as the solution PSS concentration is increased. Despite the large differences in the CPBM to PSS mole ratios, the amount of PSS that adsorbs to the surface is statistically indistinguishable for CPBM concentrations of 0, 1.02 x 10(-5), and 1.04 x 10(-4) M, indicating that the structure of the PSS molecules in each of the systems does not significantly change in the presence of CPBM.