Nicole Zacharia

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.

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.

Ethylene-co-methacrylic acid (EMAA) ionomers are incorporated into polyelectrolyte complexes and thin films fabricated with the layer-by-layer technique using mixed solvent systems of THF and water. EMAA ionomers have been reported to have self healing properties. The thin films were optically clear and can be made as a coating or freestanding. Their composition was determined with elemental analysis. DSC showed these polymer blend materials to have suppressed polyethylene crystallinity compared to bulk EMAA and an increased amount of energy required to create the order-to-disorder transition of disrupting the associations between the ionic groups of the ionomer.

The mechanism of the transition from a continuous morphology to a porous morphology within polyelectrolyte multilayers (PEMs) of linear poly(ethylene imine) (LPEI) and poly(acrylic acid) (PAA) and poly(allylamine hydrochloride) (PAH) and PAA assembled by the layer-by-layer (LbL) technique is examined. These morphological changes were created by both acidic and basic postassembly treatments. Basic postassembly treatment is shown to create different types of porosity than acidic postassembly treatment. The morphological variation from the introduction of porosity to the collapse of these porous structures and the dissolution of films under postassembly treatments was observed by AFM, optical microscopy, quartz crystal microbalance (QCM), and SEM. These morphological transitions which are a result of structural rearrangement of weak polyelectrolytes due to pH changes are closely related to the neutralization of the polycations and the ionization of polyanions. Results obtained from FTIR spectroscopy and QCM confirm that polyelectrolytes are being selectively or partially released from the polyelectrolyte multilayers thin films (PEMs) in response to the pH treatment as a function of exposure time. In conclusion, here new information is presented about the structural reorganization found in a number of weak polyelectrolyte systems. This information will be useful in designing functional materials based on polyelectrolytes.

Layer-by-layer assembly of films containing metal ions was investigated. A complex between various metal ions and branched polyethyleneimine is formed in solution and then assembled into multilayer films with poly(acrylic acid). The metal ligand complex formation results in brightly colored materials that deposit as thick layers. Cu2+-containing films were chosen as a model for studying the disassembly of these films in response to various stimuli, including pH, salt, and surfactants. The range of pH instability corresponds to the pH range over which pores are formed in the film. We demonstrate controllable disassembly of these materials, which could be used for antifungal or antibacterial applications.

The authors have recently introduced the use of hydrogel stamp materials to pattern polyelectrolyte multilayer (PEM) films. It has been demonstrated that using a stamp equilibrated in either low or high pH can cause local swelling in these films, leading to patterns. It has also been shown that stamps soaked in high ionic strength salt solutions are able to locally etch PEM films. This hydrogel stamping technique gives both lateral control of surface properties and depth control over the film's properties. This technique is a promising way to pattern chemical reactions within PEM, phase transformation, and physical properties such as film thickness, Young's modulus, and swelling. By using hydrogels for the stamp material, stamping becomes a process of continuously delivering aqueous reagent of interest to a film, instead of merely a single layer of material, as is the case when using hydrophobic stamp materials such as PDMS. While chemical modification of only the surface may be desirable in some cases, the hydrogel stamping technique is more versatile. By creating local variations in swelling, we are able to pattern mechanical stiffness, and in turn cell adhesion. We demonstrate the creation of gradients in mechanical stiffness which we are able to use to direct cell growth and adhesion on these films.

We investigated the creation of porous morphologies from polyelectrolyte multilayers (PEMs) consisting of linear poly(ethylenimine) and poly(acrylic acid), and poly (allylamine hydrochloride) and poly (acrylic acid) as a function of pH and immersion time under post-base assembly treatment. The porous transition is linked to the neutralization of the polycations electrolytes as well as ionization of PAA by the exposing LbL films to high pH. This causes PEMs to undergo spinodal decomposition, creating pores and an increase in film thickness. By using reactive wet stamping technique, we were able to locally cause porosity changes under high pH conditions in the LbL films. Further investigation of the mechanical properties of patterned LbL films was done by performing nano-indentation analysis. The results showed clear difference of physical properties such as hardness and modulus between stamped and unstamped regions based on porous transition.

The recently developed practice of spraying polyelectrolyte solutions onto a substrate in order to construct thin films via the layer-by-layer technique has been further investigated and extended. Here we describe a fully automated system capable of depositing thin polymer films from atomized mists of solutions containing species of complementary functionality. Film growth is shown to be similar to that in conventional "dipped" LbL assembly, whereas the reported technology allows us to realize 25-fold decreases in process times. Furthermore, complete automation removes human interaction and the possibility of operator-induced nonuniformities. We extend the versatility of the spray LbL technology by depositing both weak and strong polyelectrolyte films, hydrogen-bonded films, and dendritic compounds and nanoparticles, broadening its range of future applications. Finally, the technology is used to uniformly coat an otherwise hydrophobic substrate from aqueous solutions. ESEM images indicate that the atomization process produces a conformal coating of individual nanofibers within the substrate, dramatically changing the hydrophilicity of the macroscopic surface. Such an automated system is easily converted to an array of nozzle banks and could find application in the rapid, uniform coating of large areas of textile materials.

Omniphobic and slippery coatings from lubricant-infused, textured surfaces have recently been shown to have superior properties including low contact angle hysteresis and low sliding angles. Here, we present an omniphobic slippery surface prepared by infusing a fluorinated lubricant into a porous polyelectrolyte multilayer. These surfaces repel water and decane with sliding angles as low as 3°. One advantage of polyelectrolyte multilayers is the ease with which they can coat nonplanar surfaces, demonstrated here.

In this work, the morphological transitions in weak polyelectrolyte (PE) multilayers (PEMs) assembled from linear poly(ethylene imine) (LPEI) and poly(acrylic acid) (PAA) upon application of an electric field were studied. Exposure to an electric field results in the creation of a porous structure, which can be ascribed to local changes in pH from the hydrolysis of water and subsequent structural rearrangements of the weak PE constituents. Depending on the duration of application of the field, the porous transition gradually develops into a range of structures and pore sizes. It was discovered that the morphological transition of the LbL films starts at the multilayer-electrode interface and propagates through, the film. First an asymmetrical structure forms, consisting of microscaled pores near the electrode and nanoscaled pores near the surface in contact with the electrolyte solution. At longer application of the field the porous structures become microscaled throughout. The results revealed in this study not only demonstrate experimental feasibility for controlling variation in pore size and porosity of multilayer films but also deepens the understanding of the mechanism of the porous transition. In addition, electrical potential is used to release small molecules from the PEMs.

We present the fabrication of nanoscale electroactive thin films that can be engineered to undergo remotely controlled dissolution in the presence of a small applied voltage (+1.25 V) to release precise quantities of chemical agents. These films, which are assembled by using a nontoxic, FDA-approved, electroactive material known as Prussian Blue, are stable enough to release a fraction of their contents after the application of a voltage and then to restabilize upon its removal. As a result, it is possible to externally trigger agent release, exert control over the relative quantity of agents released from a film, and release multiple doses from one or more films in a single solution. These electroactive systems may be rapidly and conformally coated onto a wide range of substrates without regard to size, shape, or chemical composition, and as such they may find use in a host of new applications in drug delivery as well as the related fields of tissue engineering, medical diagnostics, and chemical detection.

Here we present a layer-by-layer (LbL) assembled device architecture that serves as a model heterostructure to study the atypical assembly that can result from the bottom-up combination of multiple multilayers of different compositions. Heterostructure assembly is disrupted by diffusion of linear poly(ethylene imine) (LPEI) within an LPEI/poly(acrylic acid) (PAA) polyelectrolyte multilayer and the exchange of LPEI with an aromatic polycation (poly(hexyl viologen) or PXV) that was assembled with PAA in an underlying multilayer. We illustrate this diffusion/exchange mechanism by showing that the assembly of an LPEI/PAA multilayer on to a PXV/PAA multilayer causes the heterostructure film to roughen and become opaque, indicating significant morphological changes. FTIR analysis confirms that LPEI diffuses into the underlying polyelectrolyte multilayer and displaces PXV. Exchange experiments of the constructed PXV/PAA multilayers in the presence of LPEI solutions were completed. Molecular weight dependence on the rate of exchange is shown through the use of high molecular weight LPEI, which is shown to undergo displacement at rates much slower than the time frame of the typical layer adsorption cycle. Finally, we prevent LPEI diffusion by incorporating a thin blocking layer of cross-linked LbL film, resulting in a discrete, compartmentalized multilayer structure. We explain these phenomena by the strength of acid-base interactions between LPEI and PAA, the ability of hydrophilic LPEI molecules to move
through a multilayer, and the tendency of weak polyelectrolytes to redistribute their ionization in response to changes in immediate environment. The success of assembling a heterostructured LbL film can be predicted by whether or not the individual component layers grow superlinearly in isolation. Our system provides a model example of LbL assemblies in which interdiffusion destabilizes film growth, and by understanding the mechanism of this destabilization, we are able to control it.

One promising aspect of the electrostatic multilayer assembly techniques is the ability to consistently and predictably create controlled heterostructures that may be of interest for active devices, designed biomaterials, membranes, or other composite thin film structures. ...

ABSTRACT Highly reactive layer-by-layer (LbL) films have been developed as protective coatings intended for application on fibers worn by military personnel. In this work, the anionic species are titanium dioxide nanoparticles ranging from 5 to 10 nm in size, which are prepared in a stable colloidal solution specifically designed for this application, while the cationic species can be one of several traditional synthetic polycations, including weak and strong polyelectrolytes. The resulting coatings are mechanically stable and offer selective protection when the wearer is exposed to UV radiation (e.g.,sunlight); whereas the inherent water transmissive nature of the multilayers allows for much greater water vapor transport rates as compared to an inert rubber barrier material. Permeation tests of coated materials were conducted in a specially engineered cell by exposing the materials to a CWA simulant. In the extreme case, when a coated material is subjected to a saturated vapor of the CWA simulant, UV exposure resulted in a 95% decrease in toxic agent permeation. Furthermore, the coating can be deposited via a spray-LbL technique developed specifically for rapid, uniform deposition over large areas of textile materials at ambient temperatures and moderate pressures.

The fabrication of multifunctional nanomaterials and their subsequent use for novel applications in various branches of nanotechnology has been under intense scrutiny. Particularly in the area of nanomechanics, the design of multicomponent nanostructures with an integrated multifunctionality would enable the construction of building blocks for nanoscale analogues of macroscopic objects. Here, we introduce a new class of flexible nanostructures: metallic nanorods with polyelectrolyte hinges, synthesized using layer-by-layer electrostatic self-assembly of oppositely charged polyelectrolytes on barcode metal nanorods followed by segment-selective chemical etching. Nanorods with hinges that consist of one polyelectrolyte bilayer display considerable flexibility, but with a greater number of bilayers the flexibility of the hinge is significantly reduced. Magnetically induced bending about the polymer hinge is illustrated through the incorporation of nickel segments into the barcodes and the application of an external fluctuating magnetic field.

... Author: Zacharia, Nicole Suzan. Other Contributors: Massachusetts Institute of Technology. Dept. ... PAH, the most basic of the polycations, forms films that are the thinnest and the most ionized while PAMAM films have the most free free acid groups and form the thickest films. ...