Polyelectrolyte/nanosilicate thin-film assemblies: influence of pH on growth, mechanical behavior, and flammability

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 degrees 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.