Thin film assemblies of branched polyethylenimine (PEI) and poly(acrylic acid) (PAA), deposited using the layer-bylayer technique, were studied in an effort to produce all-polymer thin films with low oxygen permeability. Altering the pH of PEI and PAA results in large thickness variations (from 90 nm to 4.74 μm for 30-bilayer films). Cross-linking these films with glutaraldehyde (GA) create an inhibition of polymer interdiffusion, causing exponential film growth to be reset. AFM images show the surface morphology of PEI/PAA assemblies can be controlled by the pH and the final polymer layer deposited, with surface roughness ranging from 2.1 to 49.1 nm. Most pH combinations failed to produce a film with high oxygen barrier, but 8 bilayers of PEI at pH 10 and PAA at pH 4 produce a 305 nm thick film with an oxygen transmission rate below 0.005 cm 3 /(m 2 day). This unique thin film barrier (P O 2 < 3.2 Â 10 -21 cm 3 (STP) cm/(cm 2 s Pa)) is a promising alternative to current polymeric membranes, flexible electronics, and food packaging materials.

Thin films of polyethylenimine (PEI) and poly(acrylic acid) (PAA), deposited using layer-by-layer assembly, were studied to understand the influence of various crosslinking methods on their oxygen and water vapor barrier. Glutaraldehyde (GA), 1-[3-(dimethylamino)propyl]-3-ethylcarbodiimide methiodide (EDC) and thermal crosslinking were evaluated with respect to film thickness and barrier properties. The thickness of an 8 bilayer PEI/PAA assembly decreased from 640 to 551 nm after crosslinking with a 0.1 M aqueous GA solution and obtained an oxygen permeability one order of magnitude better than an SiO x thin film (,5.9 6 10 221 cm 3 cm cm 22 s 21 Pa 21 ) This same crosslinking treatment reduced the oxygen transmission rate (OTR), measured at 23 uC and 100% RH, from 0.61 to 0.09 (cm 3 m 22 day 21 ). Increasing the number of bilayers and heating time with thermal crosslinking also reduces the water vapor transmission rate (WVTR). These nanocoatings are a promising alternative to currently used barrier layers for flexible electronics and food packaging.

Polyurethane foam found in household furnishings and bedding creates a severe fire hazard, resulting in loss of life and property each year. In an effort to reduce the flammability of polyurethane foam, a polyelectrolyte multilayer (PEM) coating, comprised of polyethylenimine and polyacrylic acid-stabilized aluminum hydroxide (ATH), was deposited onto foam using layer-by-layer (LbL) assembly. PEM coatings with and without incorporation of ATH were deposited and compared to assess the effectiveness of ATH on flame suppression. All recipes resulted in conformal coatings, maintaining the open cellular structure of the foam. Only three bilayers of PEI/ PAA-ATH retained the shape of foam after exposure to a butane torch flame for 10 s. With six bilayers, the flame was extinguished, which prevented flashover. Cone calorimetry revealed that this 6 BL coated foam exhibited a 64 % reduction in peak heat release rate and a 44 % reduction in maximum average rate of heat emission. This work demonstrates an extraordinarily effective flame-retardant nanocoating that uses environmentally benign chemistry and relatively few deposition steps, prepared using LbL assembly.

Multilayer films deposited from water using layer-by-layer (LbL) assembly have shown extraordinarily low oxygen permeability and are of high interest due to their robustness, tailorability, and ease of fabrication. [5][6][7][8][9] Despite all the advantages associated with LbL assembly, the large number of processing steps remains a considerable challenge for commercial use. [10,11] In order to apply films composed of oppositely charged polyelectrolytes to substrates using just one or two deposition steps, solutions containing polyelectrolyte complexes (PECs) can be employed. [12][13][14][15] PECs are formed by the entropy-driven association of oppositely charged polyelectrolytes in water and can exist as stable colloids, flocculants, or metastable coacervates. [16][17][18][19][20] Governed by conditions such as pH and ionic strength, PEC coacervation is marked by a liquid-liquid phase separation, where a polymer rich coacervate phase is in equilibrium with a polymer poor solution phase. PEC coacervates are composed of weakly bound polyelectrolytes, and have viscous liquid-like behavior that can be exploited to quickly apply them as thin films. [18,[21][22][23][24] In the present study, polyelectrolyte coacervates were applied to substrates using Meyer rod coating (a common type of blade coating), [25] in an effort to quickly fabricate thin oxygen barrier films in a single step. The Meyer rod is drawn across a substrate, doctoring off coating fluid, Multilayer coatings consisting of oppositely charged polyelectrolytes have proven to be extraordinarily effective oxygen barriers but require many processing steps to fabricate. In an effort to prepare high oxygen barrier thin films more quickly, a polyelectrolyte complex coacervate composed of polyethylenimine and polyacrylic acid is prepared. The coacervate fluid is applied as a thin film using a rod coating process. With humidity and thermal posttreatment, a 2 µm thin film reduces the oxygen transmission rate of 0.127 mm poly(ethylene terephthalate) by two orders of magnitude, rivalling conventional oxygen barrier technologies. These films are fabricated in ambient conditions using low-cost, water-based solutions, providing a tremendous opportunity for single-step deposition of polymeric high barrier thin films.

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