Layer-by-layer (LbL) assembly is a promising technique for depositing multi-functional thin films from dilute aqueous solutions. These films have found use as environmentally-benign flame retardants, replacing halogenated flame retardants, and as flexible gas barrier thin films, replacing vacuum-deposited inorganic oxide thin films. Unfortunately, LbL assembly has drawbacks that have not been adequately addressed, such as stiffening of coated substrates and the high number of deposition steps required. Thin films of chitosan and poly(sodium phosphate) were deposited on cotton fabric via LbL assembly to reduce flammability. The fabric was rinsed in an ultrasonication bath between deposition steps to improve the softness (i.e., hand) of the coated fabric. Ultrasonication is believed to remove weakly adhered polyelectrolyte and eliminate bridging of individual fibers, preventing the fabric from becoming stiff while improving the flame retardant behavior. Incorporating amine salts into the cationic polyelectrolyte and its associated rinse enables LbL clay-containing films to achieve large thickness (>1 um) with relatively few deposition steps. This technique is potentially universal, exhibiting thick growth with multiple types of nanoclay, including montmorillonite and vermiculite, a variety of amine salts (e.g., hexylamine and tris), and a host of cationic polyelectrolytes, such as poly(allylamine) and chitosan. The characteristic ordered structure found in LbL-assembled films is maintained despite the increased thickness. These films display extraordinary gas barrier and flame resistance with fewer than 8 deposition cycles.
