Polyelectrolyte/Nanosilicate Thin-Film Assemblies: Influence of pH on Growth, Mechanical Behavior, and Flammability Yu-Chin Li, Jessica Schulz, and Jaime C. Grunlan* Department of Mechanical Engineering, Texas A&M University, College Station, Texas 77843 ABSTRACT Thin composite films of branched polyethylenimine (BPEI) and Laponite clay platelets were prepared using layer-by- layer assembly. The film thickness was tailored by altering the pH of the aqueous mixtures used to deposit these films, resulting in growth that ranged from 0.5 to 5 nm/bilayer (BL). High-pH BPEI and low-pH clay produced the thickest films. The microstructure of tilted Laponite clay platelet stacks is observed with transmission electron microscopy when using unadjusted BPEI (pH 10.3) and pH 6 Laponite. This recipe resulted in a film with 83 wt % clay and a hardness of 0.5 GPa. In all films, the clay platelets are uniformly deposited and look analogous to a cobblestone path in atomic force microscopy surface images. Several 40-BL films, with thicknesses of 100 nm or more, exhibit reduced moduli ranging from 7 to 10 GPa and hardness of around 0.5 GPa, suggesting that these transparent films could be useful as hard coatings for plastic films. These thin coatings were also deposited onto cotton fabric. Each individual cotton fiber was uniformly coated, and the fabric has significantly more char left after burning than the uncoated fabric. Thermogravimetric analysis results reveal that fabric coated with 10 BLs of BPEI/Laponite produces up to 6 wt % char at 500 °C, which is almost 1 order of magnitude greater than that of untreated fabric. This initial study demonstrates that polymer/clay assemblies could improve the thermal stability of cotton and may be useful for fire safety applications. KEYWORDS: layer-by-layer assembly • clay • TEM • nanocomposites • flame suppression INTRODUCTION L ayer-by-layer (LbL) assembly has become a popular method to fabricate multifunctional films that are typically less than 1 μm thick (1-3). These thin films are assembled by alternating the deposition of positively and negatively charged layers on a substrate. Deposition often involves the soaking of a charged (or at least polar) substrate in aqueous mixtures of charged polymers and/or particles, alternating between cationic and anionic species. The pri- mary means of multilayer buildup is electrostatic attractions, but a variety of other interactions have been successfully exploited. For example, assemblies have been built through donor/acceptor interactions (4-6), hydrogen bonding (7, 8), and covalent bonds (9, 10). In the case of electrostatic growth, each pair of positively and negatively charged layers is referred to as a bilayer (BL). Typical BL thicknesses range from 1 to 100 nm, depending on factors that include chemistry (11), molecular weight (12), temperature (13, 14), counterions (15), ionic strength (16), and pH (14, 17). A variety of LbL-assembled functional thin films are currently being evaluated for properties that include antimicrobial (18, 19), antireflection (20), electrochromic (21-23), sensing (24-26), oxygen barrier (27), and biomedical applications (28). In many cases, solid nanoparticles, such as clay (27-31), are one type of the charged species imparting a desired property. Smectite clays have been widely studied in LbL thin films, especially montmorillonite (MMT) (27, 32, 33) and Laponite (34-40). These platelet structures are a class of phyllosili- cates, which exhibit swelling and exfoliation in water (29). Each individual clay platelet (elementary sheet) is composed of one central layer of Al 3+ and Mg 2+ octahedra, sandwiched between two layers of Si 4+ tetrahedra. The thickness of these clay units is approximately 1 nm. They typically organize face-to-face into aggregates, but in aqueous dispersion, these faces become negatively charged as water intercalates and exfoliates the clay layers. At the edges, the clay elementary sheets contain many oxygen atoms and hydroxyl groups that can accept or release protons, depending on the pH of the suspension (29). Laponite is a synthetic clay with uniform disk-shaped particles that are approximately 25 nm in diameter (41, 42). Composite films of poly(diallyldimethyl- ammonium chloride) (PDDA) and Laponite sorb water rap- idly and reversibly, which led to their evaluation as humidity sensors (34). Additionally, modeling has indicated that PDDA/Laponite film formation results in significantly higher surface coverage than that of natural clay films (35). Film preparations with various Laponite contents (15-60 wt %) and poly(ethylene oxide) (PEO) have resulted in high degrees of orientation in both the polymer and clay platelets (37). Bulky, hydrophobic, and amphiphilic polyelectrolytes can be incorporated while still keeping the clay platelets aligned parallel to the substrate (38). Laponite, PEO, and linear polyethylenimine have also been used to form trilayers, whose anisotropic structure results in significant ion trans- port (40). * Corresponding author. Tel: +1 979 845 3027. Fax: +1 979 862 3989. E-mail: jgrunlan@tamu.edu. Received for review July 21, 2009 and accepted September 10, 2009 DOI: 10.1021/am900484q © 2009 American Chemical Society ARTICLE 2338 VOL. 1 • NO. 10 • 2338–2347 • 2009 www.acsami.org Published on Web 09/25/2009