A variety of functional thin films can be produced using the layer-by-layer (LbL) assembly technique. Thin films, typically < 1µm thick, are created by alternately exposing a substrate to positively- and negatively-charged molecules or particles.
The movie below illustrates the layer-by-layer assembly process using a polycation (positively-charged polymer) and anionic clay platelets, which is the basis for our gas barrier and anti-flammable technologies. Each individual layer may be 1 – 100+ nm thick depending on: chemistry, molecular weight, charge density, temperature, deposition time, counter ion, and pH of the species being deposited.
The ability to control coating thickness down to the nm-level, easily insert variable thin layers without altering the process, economically use raw materials (due to thin nature), self-heal, and process under ambient conditions are some of the key advantages this deposition technique has.
The PNC Lab is currently studying LbL-based thin films as a super gas barrier (i.e. foil replacement material), an anti-flammable coating for foam and fabric, antimicrobial surfaces, electrochromic thin films and electrically conductive layers that can be patterned using traditional photolithography.
Super Gas Barrier
The processing of clay-based composites is a challenge, and it is nearly impossible to incorporate more than a few weight percent (without causing significant aggregation) in bulk composites. Depositing clay in a layer-by-layer fashion creates thin films that are fully dense, completely transparent, and contain as much as 80 wt. % of completely exfoliated clay (ACS Applied Materials & Interfaces 2010).
These “nano brick wall” films (see figure below) act as impressive gas barriers, with many film structures yielding an oxygen transmission rate (OTR) below 0.005 cm3/m2 day (0%RH, 23oC). Even with short (5 second) dip times, films created with 30-bilayers of polyethylenimine (PEI) and natural montmorillonite (MMT) clay have been shown to maintain these super barrier properties (Industrial & Engineering Chemistry Research 2010).
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