Sodium dodecyl sulfate modulates the structure and rheological properties of Pluronic F108-poly(acrylic acid) coacervates).

Micelles formed within coacervate phases can impart functional properties, but it is unclear if this micellization provides mechanical reinforcement of the coacervate whereby the micelles act as high functionality crosslinkers. Here, we examine how sodium dodecyl sulfate (SDS) influences the structure and properties of Pluronic F108-polyacrylic acid (PAA) coacervates as SDS is known to decrease the aggregation number of Pluronic micelles. Increasing the SDS concentration leads to larger water content in the coacervate and an increase in the relative concentration of PAA to the other solids. Rheological characterization with small angle oscillatory shear (SAOS) demonstrates that these coacervates are viscoelastic liquids with the moduli decreasing with the addition of the SDS. The loss factor (tan δ) initially increases linearly with the addition of SDS, but a step function increase in the loss factor occurs near the reported CMC of SDS. However, this change in rheological properties does not appear to be correlated with any large scale structural differences in the coacervate as determined by small angle X-ray scattering (SAXS) with no signature of Pluronic micelles in the coacervate when SDS concentration is >4 mM during formation of the coacervate, which is less than that observed (6 mM SDS) in initial Pluronic F108 solution despite the higher polymer concentration in the coacervate. These results suggest that the mechanical properties of polyelectrolyte-non-ionic surfactant coacervates are driven by the efficicacy of binding between the complexing species driving the coacervate, which can be disrupted by competitive binding of the SDS to the Pluronic.

Non-destructive determination of functionalized polyelectrolyte placement in layer-by-layer films by IR ellipsometry.

Layer-by-layer (LbL) assembly facilitates controlled coatings on a variety of surfaces with the ability to manipulate the composition through the thickness by selection of the complementary pairs. However, the characterization of these composition profiles tends to be destructive and requires significant compositional differences that can limit their utility. Here, we demonstrate the ability to non-destructively quantify the depth dependence of the allyl content associated with the selective incorporation of poly(sodium acrylate-co-allylacrylamide) (84 : 16 mol : mol) (allyl-PAA) in LbL films based on the assembly of poly(diallyldimethylammonium chloride) (PDAC)/poly(acrylic acid) (PAA) and PDAC/allyl-PAA. Although the atomic composition of the film is not dramatically influenced by the change between PAA and allyl-PAA, the absorption in the IR near 1645 cm-1 by the allyl group provides sufficient optical contrast to distinguish the LbL components with spectroscopic ellipsometry. The use of IR spectroscopic ellipsometry can determine the thickness of layers that contain allyl-PAA and also gradients that develop due to re-arrangements during the LbL process. With multiple films fabricated simultaneously, the location of the gradient between the 1st and 2nd series of multilayers (e.g., first PDAC/PAA bilayers and then PDAC/allyl-PAA bilayers) can be readily assessed. The results from a variety of different multilayer architectures indicate that the gradient is located within the thickness expected for the 1st deposited bilayer stack (PDAC/PAA or PDAC/allyl-PAA). These results are indicative of a dynamic dissolution-deposition process (in- and out- diffusion) during the fabrication of these LbL films. These results provide additional evidence into the mechanisms for exponential growth in LbL assemblies. The ability to quantify a gradient with the low contrast system examined indicates that spectroscopic IR ellipsometry should be able to non-destructively determine compositional gradients for most polymer films where such gradients exist.

Polyelectrolyte-micelle coacervates: intrapolymer-dominant vs. interpolymer-dominant association, solute uptake and rheological properties.

The effects of polyelectrolyte charge density, polyelectrolyte-to-surfactant ratio, and micelle species on coacervation were studied by turbidity, dynamic light scattering, and zeta potential measurements to examine the coacervation of the weak polyelectrolyte branched polyethylenimine (BPEI) and oppositely charged sodium dodecyl sulfate (SDS) micelles as well as BPEI and mixed micelles composed of SDS and poly(ethylene glycol) 4-nonylphenyl 3-sulfopropyl ether potassium salt (PENS). The results of dynamic light scattering and zeta potential measurements are discussed in terms of pH and BPEI-to-surfactant ratio. An intrapolymer-dominant to interpolymer-dominant association model for the BPEI-micelle coacervates was proposed based on the variation of size and zeta potential of coacervate particles by their BPEI-to-surfactant ratio. The partition coefficient of solutes into BPEI-micelle coacervates was determined using UV-vis measurements as a function of pH, BPEI-to-surfactant ratio, and mixed micelle composition. Both the hydrophobicity of solutes and micelles, as well as the association mode of coacervates, impact the solute uptake efficiency. Dynamic rheological measurements on the coacervates suggest that the rheological properties of the complex coacervates are impacted by the association mode of the coacervates as well as the charge density on BPEI chains during coacervation.

Protein encapsulation via polyelectrolyte complex coacervation: Protection against protein denaturation.

Complex coacervation can be used as a route to compartmentalize a variety of solutes such as organic small molecules, inorganic nanoparticles, and proteins within microscale coacervate droplets. To obtain insight into the accumulation of proteins within complex coacervate phases, the encapsulation of Bovine Serum Albumin (BSA) within complex coacervates containing cationic polyelectrolyte poly(allylamine hydrochloride) (PAH) and anionic polyelectrolyte poly(acrylic aid) (PAA) was investigated as a function of mixing sequence, total polyelectrolyte concentration, BSA overall concentration, and the mixing molar ratio of PAA/PAH. Mixing BSA having a negative net charge with the polycation PAH before coacervation, increasing the total polyelectrolyte concentration and PAA/PAH molar ratio, or decreasing the BSA overall concentration led to more efficient protein encapsulation. Preservation of the secondary structure of BSA during the complex coacervation process was confirmed using circular dichroism spectroscopy. Our study shows that PAA-PAH coacervates can serve as a protective system against the denaturation of BSA when exposed to extremes of pH, high temperatures, as well as in solution of urea. Additionally, it was found that by encapsulation of proteins within coacervates via complex coacervation, the complexation between proteins and heavy metal can be efficiently inhibited. Protection of BSA against severe environmental conditions via encapsulation within polyelectrolyte coacervates provides new insights and methods to issues of maintaining stability and function of proteins.

Mechanical properties of bulk graphene oxide/poly(acrylic acid)/poly(ethylenimine) ternary polyelectrolyte complex.

Ternary complexes formed in a single pot process through the mixing of cationic (branched polyethylenimine, BPEI) and anionic (graphene oxide, GO, and poly(acrylic acid), PAA) aqueous solutions exhibit superior mechanical performance in comparison to their binary analogs. The composition of the ternary complex can be simply tuned through the composition of the anionic solution, which influences the water content and mechanical properties of the complex. Increasing the PAA content in the complex decreases the overall water content due to improved charge compensation with the BPEI, but this change also significantly improves the toughness of the complex. Ternary complexes containing ≤32 wt% PAA were too brittle to generate samples for tensile measurements, while extension in excess of 250% could be reached with 57 wt% PAA. From this work, the influence of GO and PAA on the mechanical properties of GO/PAA/BPEI complexes were elucidated with GO sheets acting to restrain the viscous flow and improve the mechanical strength at low loading (<12.6 wt%) and PAA more efficiently complexes with BPEI than GO to generate a less swollen and stronger network. This combination overcomes the brittle nature of GO-BPEI complexes and viscous creep of PAA-BPEI complexes. Ternary nanocomposite complexes appear to provide an effective route to toughen and strengthen bulk polyelectrolyte complexes.

Slippery Liquid-Infused Porous Surfaces (SLIPS) Using Layer-by-Layer Polyelectrolyte Assembly in Organic Solvent.

Slippery liquid-infused porous surfaces (SLIPS) have potential impact on a wide range of industries, including healthcare, food packaging, and automobile. A tremendouseffort has been focused on developing novel fabrication methods for making SLIPS. However, current fabrication methods usually involve harsh conditions and complicated postfabrication modifications or are limited to specific substrates. Presented here is a novel method for the fast and facile fabrication of SLIPS. Layer-by-layer (LBL) assembly of branched polyethylenimine and Nafion, a perfluorinated polyelectrolyte, is performed with methanol as the solvent. Hierarchically rough and superhydrophobic surface is obtained directly without further modification on various substrates. The surface properties are shown to highly depend on the LBL assembly parameters, including deposition cycles, dipping time, rinsing time, and drying time between baths. The polyelectrolyte multilayers obtained with this method are infused with Krytox 100 to form SLIPS surfaces, which show excellent omniphobic, antifouling, self-cleaning, flexible, and optical properties. The result of this study not only simplifies the fabrication of SLIPS surfaces, but also provides great insight for making LBL films with specific morphologies.

Effect of small molecules on the phase behavior and coacervation of aqueous solutions of poly(diallyldimethylammonium chloride) and poly(sodium 4-styrene sulfonate).

HYPOTHESIS: Complex coacervates are capable of easily partitioning solutes within them based on relative affinities of solute-water and solute-polyelectrolyte pairs, as the coacervate phase has low surface tension with water, facilitating the transport of small molecules into the coacervate phase. The uptake of small molecules is expected to influence the physicochemical properties of the complex coacervate, including the hydrophobicity within coacervate droplets, phase boundaries of coacervation and precipitation, solute uptake capacity, as well as the coacervate rheological properties. EXPERIMENTS: Phase behavior of aqueous solutions of poly(diallyldimethylammonium chloride) (PDAC) and poly(sodium 4-styrene sulfonate) (SPS) was investigated in the presence of various concentrations of two different dyes, positively charged methylene blue (MB) or non-charged bromothymol blue (BtB), using turbidity measurements. These materials were characterized with UV-vis spectroscopy, zeta potential measurements, isothermal titration calorimetry (ITC), fluorescence spectroscopy, and dynamic rheological measurements. FINDINGS: The presence of MB or BtB accelerates the coacervation process due to the increased hydrophobicity within coacervates by the addition of MB or BtB. The encapsulated MB or BtB tends to reduce the ionic crosslink density in the PDAC-SPS coacervates, resulting in a much weaker interconnecting network of the PDAC-SPS coacervates.

Tuning Wet Adhesion of Weak Polyelectrolyte Multilayers.

Weak polyelectrolyte multilayers (PEMs) assembled by the layer-by-layer method are known to become tacky upon contact with water and behave as a viscoelastic fluid, but this wet adhesive property and how it can be modified by external stimuli has not yet been fully explored. We present here a study on the wet adhesive performance of PEMs consisting of branched poly(ethylene imine) and poly(acrylic acid) under controlled conditions (e.g., pH, type of salt, and ionic strength) using a 90° peel test. The multilayers demonstrate stick-slip behavior and fail cohesively in nearly all cases. The peel force is the highest at neutral pH, and it decreases in both acidic/basic environments because of inhibited polyelectrolyte mobility. The addition of salts with various metal ions generally reduces the peel force, and this effect tracks with the ionic strength. When transition metal ions are used, their ability to form coordination bonds increases the peel force, with two exceptions (Cu2+ and Zn2+). With a transition metal ion such as Fe3+, the peel force first increases as a function of the concentration and then eventually decreases. The peel force increases proportionally to the peel rate. The films are also characterized via zeta potential (when assembled onto colloidal particles) and shear rheometry. This work provides insight into both the wet adhesive properties of PEMs and the interactions between PEMs and metal ions.

Surface Functionalization for a Nontextured Liquid-Infused Surface with Enhanced Lifetime.

Liquid-infused surfaces (LISs) are a new class of self-cleaning surfaces having superior properties compared to other self-cleaning surfaces. One challenge regarding these is the eventual washing away or drainage of the lubricant, limiting their longevity. Presented here is a surface functionalization strategy to compatibilize the lubricant and surface, enhancing the ability of the lubricant to remain on the surface even during washing. The strategy used here is the grafting of a layer of polydimethylsiloxane (PDMS) to the surface, which stabilizes a layer of silicone oil. The effectiveness of this layer is studied as a function of PDMS molecular weight. The stable liquid layer can exist even in the absence of texture on the surface that is generally used to "lock" the lubricant in place. This strategy is shown to be effective on both flat and textured surfaces. One advantage of a flat surface is that the composite liquid/solid surface can be studied using optical techniques such as ellipsometry, which are difficult to employ in the presence of a rough solid surface. This method of surface compatibilization shows an enhanced lifetime when used on textured surfaces as well. This is a promising strategy for the enhanced longevity of LISs required for real-world applications.

Assembly of large area crack free clay porous films.

Porous materials with well-defined porosity have advantages in a wide range of applications, including filtration media, catalysis, and electrodes. The bottom-up fabrication of inverse opals have promised to provide those nanostructures, but fabrication of these materials is often plagued with large numbers of defects and macro-scale cracks. Here, we present a method for making nanostructured porous clay films with well defined pore size that are crack free over a large area (multiple cm2).

A family of mechanically adaptive supramolecular graphene oxide/poly(ethylenimine) hydrogels from aqueous assembly.

Composite hydrogels containing graphene oxide (GO) offer advantageous mechanical properties, but tuning these properties generally requires the synthesis of new hydrogels or if the hydrogel is thermally responsive, utilization of a chemistry determined temperature window. Here, we demonstrate a simple route to generate a family of GO-based hydrogels from aqueous solution based assembly of GO with polycationic poly(ethylenimine), PEI, without any secondary chemical crosslinking. Tuning the ratio of GO : PEI during the assembly produces a family of hydrogels that responds to mechanical compression by irreversibly altering their equilibrium water content and mechanical properties in a controllable manner. Despite the lack of chemical crosslinks, the hydrogels are stable when stored in an excess of water or NaCl solutions (up to 1 M) and exhibit a tunable swelling ratio (mass hydrogel : mass solid) between 44 and 162 based on both composition and compression history. Consequently, the storage modulus from shear rheology can be increased by more than 3 orders of magnitude from this irreversible mechanical compression of the hydrogel. This stiffening of the hydrogels in response to mechanical stimuli enables the prior compression loading of the hydrogel to be determined. We demonstrate that this strategy is generalizable to other anionic 2D materials such as clay (cloisite). This family of mechanically adaptive hydrogels enables facile fabrication and tuning of physical properties that could be advantageous for sensing, energy dissipation, and other applications.

Self-Healing of Bulk Polyelectrolyte Complex Material as a Function of pH and Salt.

Self-healing materials are an emerging class of modern materials gaining importance due to environmental and energy concerns. Materials based on the complexation of oppositely charged polyelectrolytes, usually in the form of coatings and films, have been shown to have water activated self-healing properties. In this work, the self-healing of bulk branched poly(ethylene imine) and poly(acrylic acid) (BPEI/PAA) complex is studied as a function of the aqueous solutions used to activate the self-healing. Specifically, exposure to different salt solutions and solutions of different pH was examined including sodium and copper ion containing solutions as well as acidic and basic solutions. By applying NaCl treatment, especially followed by exposure to DI water, the self-healing ability of the BPEI/PAA complex was enhanced. In contrast, after treated by CuCl2, the BPEI/PAA complex lost its self-healing ability, showing an ability to modulate the ability to self-heal as a function of external stimulus. In addition to improving the ability to self-heal using salt as compared to using DI water alone, acidic and basic solutions can also improve the ability to self-heal. The self-healing is caused by chain mobility at the cut interface of the polyelectrolyte complex material which is controlled by charge density along the polyelectrolyte backbone as well as ionic cross-link density, and correlation between this mobility to rheological behavior is made. Tensile testing and determination of fracture toughness were used to characterize self-healing.

Accelerated Amidization of Branched Poly(ethylenimine)/Poly(acrylic acid) Multilayer Films by Microwave Heating.

Chemical cross-linking of layer-by-layer assembled films promotes mechanical stability and robustness in a wide variety of environments, which can be a challenge for polyelectrolyte multilayers in saline environments or for multilayers made from weak polyelectrolytes in environments with extreme pHs. Heating branched poly(ethylenimine)/poly(acrylic acid) (BPEI/PAA) multilayers at sufficiently high temperatures drives amidization and dehydration to covalently cross-link the film, but this reaction is rather slow, typically requiring heating for hours for appreciable cross-linking to occur. Here, a more than one order of magnitude increase in the amidization kinetics is realized through microwave heating of BPEI/PAA multilayers on indium tin oxide (ITO)/glass substrates. The cross-linking reaction is tracked using infrared spectroscopic ellipsometry to monitor the development of the cross-linking products. For thick films (∼1500 nm), gradients in cross-link density can be readily identified by infrared ellipsometry. Such gradients in cross-link density are driven by the temperature gradient developed by the localized heating of ITO by microwaves. This significant acceleration of reactions using microwaves to generate a well-defined cross-link network as well as being a simple method for developing graded materials should open new applications for these polymer films and coatings.

Sequestration of Methylene Blue into Polyelectrolyte Complex Coacervates.

Polyelectrolyte complex coacervation is a process that has been proposed as a model for protocell formation due to its ability to compartmentalize chemicals in solution without a membrane. During the liquid-liquid phase separation that results in water rich and polyelectrolyte rich phases, small molecules present in solution selectively partition to one phase over the other. This sequestration is based on relative affinities. Here, a study of the sequestration of methylene blue (MB) into the complex coacervate phase of three pairs of synthetic polyelectrolytes is presented; branched polyethylene imine with polyacrylic acid, polyvinyl sulfonate, or poly(4-styrenesulfonic acid). These materials are characterized with UV-vis, zeta potential measurements, and dynamic light scattering. The branched polyethylene imine/poly(4-styrenesulfonic acid) system is shown to have a significantly higher sequestration capacity for the MB as compared to either of the other two systems, based on π-π interactions which are not possible in the other systems.

Response of Swelling Behavior of Weak Branched Poly(ethylene imine)/Poly(acrylic acid) Polyelectrolyte Multilayers to Thermal Treatment.

Weak polyelectrolyte multilayers (PEMs) prepared by the layer-by-layer technique have attracted a great deal of attention as smart responsive materials for biological and other applications in aqueous medium, but their dynamic behavior as a function of exposure to a wide temperature range is still not well understood. In this work, the thermally dependent swelling behavior of PEMs consisting of branched poly(ethylenimine) and poly(acrylic acid) is studied by temperature controlled in situ spectroscopic ellipsometry. Because of diffusion and interpenetration of polyelectrolytes during film deposition, the PEMs densify with increasing bilayer number, which further affects their water uptake behavior. Upon heating to temperatures below 60 °C, the worsened solvent quality of the PEM in water causes deswelling of the PEMs. However, once heated above this critical temperature, the hydrogen bonds within the PEMs are weakened, which allows for chain rearrangement within the film upon cooling, resulting in enhanced water uptake and increased film thickness. The current work provides fundamental insight into the unique dynamic behavior of weak polyelectrolyte multilayers in water at elevated temperatures.

Contraction of weak polyelectrolyte multilayers in response to organic solvents.

Weak polyelectrolyte multilayers (PEMs) prepared by the layer-by-layer assembly technique have recently been found to demonstrate a unique contraction upon exposure to organic solvents. This response is dependent upon which organic solvent is employed, and fundamental questions have not been clarified regarding the correlation of the magnitude of the film contraction with solvent type. In this work, we used solubility parameters to analyze the response of branched poly(ethylene imine)/poly(acrylic acid) (BPEI/PAA) multilayers when exposed to a variety of solvents. BPEI/PAA multilayers were immersed in a series of 16 different organic solvents and solvent mixtures. Immersion in organic solvent caused film dehydration and therefore contraction and also induced changes in the mechanical properties of PEMs. The film thickness was the best predictor of how a film swelled in water or contracted in organic solvent when using different batches of commercially available polyelectrolytes, rather than polyelectrolyte assembly pH conditions. The degree of film contraction was correlated with Hansen and Kamlet-Taft solubility parameters as well as solvent dielectric constant. In most cases, the hydrogen bonding ability of solvents is the primary factor to determine the magnitude of film contraction. For these solvents, increasing the temperature which corresponds to decreasing the strength of hydrogen bonding, also decreases the ability to dehydrate the films. For solvents that do not follow these trends with the strength of hydrogen bonding, a stronger correlation was found between contraction and dielectric constant, indicating that both traditional solvent quality arguments and electrostatics are important to understanding the contraction of PEMs in organic solvents.

Layer-by-layer rose petal mimic surface with oleophilicity and underwater oleophobicity.

Surfaces designed with specific wetting properties are still a key challenge in materials science. We present here a facile preparation of a surface assembled by the layer-by-layer technique, using a colloidal dispersion of ionomer particles and linear polyethylene imine. The colloidal ethylene-co-methacrylic acid (EMAA) particles are on the order of half a micron in size with surface features from 40 to 100 nm in width. The resultant surface has roughness on two length scales, one on the micron scale due to the packing of particles and one on the nanoscale due to these surface features on the EMAA particles. This hierarchical structure results in a hydrophobic surface with good water pinning properties (∼550 µN). We show that there is a balance between maximizing contact angle and water pinning force. Furthermore, this surface is oleophilic with regard to many organic solvents, also demonstrating underwater oleophobicity, and given the difference in wetting between aqueous and organic phases, this material may be a candidate material for oil/water separations.

Large-scale solvent driven actuation of polyelectrolyte multilayers based on modulation of dynamic secondary interactions.

Polyelectrolyte multilayers (PEMs), assembled from weak polyelectrolytes, have often been proposed for use as smart or responsive materials. However, such response to chemical stimuli has been limited to aqueous environments with variations in ionic strength or pH. In this work, a large in magnitude and reversible transition in both the swelling/shrinking and the viscoelastic behavior of branched polyethylenimine/poly(acrylic acid) multilayers was realized in response to exposure with various polar organic solvents (e.g., ethanol, dimethyl sulfoxide, and tetrahydrofuran). The swelling of the PEM decreases with an addition of organic content in the organic solvent/water mixture, and the film contracts without dissolution in pure organic solvent. This large response is due to both the change in dielectric constant of the medium surrounding the film as well as an increase in hydrophobic interactions within the film. The deswelling and shrinking behavior in organic solvent significantly enhances its elasticity, resulting in a stepwise transition from an initially liquid-like film swollen in pure water to a rigid solid in pure organic solvents. This unique and recoverable transition in the swelling/shrinking behaviors and the rheological performances of weak polyelectrolyte multilayer film in organic solvents is much larger than changes due to ionic strength or pH, and it enables large scale actuation of a freestanding PEM. The current study opens a critical pathway toward the development of smart artificial materials.

Silver nanoparticle aided self-healing of polyelectrolyte multilayers.

Self-healing is the ability of a material to repair mechanical damage. The lifetime of a coating or film might be lengthened with this capacity. Water enabled self-healing of polyelectrolyte multilayers has been reported, using systems that grow via the interdiffusion of polyelectrolyte chains. Due to high mobility of the polyelectrolyte chains within the assembly, it is possible for lateral diffusion to heal over scratches. The influence of metal ions and nanoparticles on this property has, however, not been previously studied. Here we demonstrate that the incorporation of silver nanoparticles reduced in situ within the branched poly(ethyleneimine)-poly(acrylic acid) polyelectrolyte multilayer structure speeds the ability of the multilayer assembly to self-heal. This enhancement of property seems to not be due to changes in mechanical properties but rather in enhanced affinity to water and plasticization that enables the film to better swell.

Facile Assembly Enhanced Spontaneous Fluorescent Response of Ag+ Ion Containing Polyelectrolyte Multilayer Films.

Fluorescent organic-inorganic composite materials exhibiting "turn-on" response are often based on conjugated small molecules. Conjugated polymers, however, often exhibit a "turn-off" response in combination with metal ions. Here we present fluorescent turn-on behavior of a branched poly(ethylene imine)-poly(acrylic acid)-Ag+ ion complex in a thin film. The material is characterized by UV-vis, spectrofluorometry, XPS, and ICP-MS. The turn-on response is exhibited only with all three components present, implying that the optically active metal coordination complex contains amine and carboxylic acid groups. This behavior is observed in the solid state, meaning this material could be easily integrated into devices. We demonstrate sensing of formaldehyde vapor as well as halide containing solutions based on fluorescence quenching. This fluorescent material is simply made using the layer-by-layer technique and commercially available polymers.