Process Improvements for Gas Barrier Thin Films Deposited Via Layer-By-Layer Assembly



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Thin layers of aluminum have provided good oxygen barrier for food packaging for many years, but aluminum coatings can easily crack, are completely opaque, and are not environmentally friendly. One gas barrier solution for food, to flexible electronics, and pressurized bladders is to create polymer nanocomposite thin-films using layer-by-layer (LbL) assembly. These non-metal, water-based thin films contain a tortuous path through which a gas molecule must navigate. The work in this dissertation focuses on improving the process of creating these thin films to optimize their performance and achieve lower transmission rates with fewer layers.

Excellent gas barrier was achieved in a layer-by-layer thin film with fewer layers by optimizing deposition time of cationic polyethylenimine (PEI) and anionic poly(acrylic acid) [PAA]. Substantial deposition occurs with short deposition times for the first four PEI/PAA bilayers, while thicker deposition occurs with longer deposition times beyond 4 bilayers. Eight bilayers (650 nm) were required to achieve an undetectable oxygen transmission rate (<0.005 cm^3/(m^2?day)) using 1 min deposition steps, but this barrier was obtained with only 6 BL (552 nm) using 1s deposition of the first four bilayers, reducing total deposition time by 73%.

Polymer?clay bilayer films show good oxygen barrier properties due to a nanobrick wall structure consisting of clay nanoplatelets within polymeric mortar. Super oxygen barrier trilayer thin films have been deposited using two successive anionic layers of montmorillonite (MMT) clay and polymer (PAA) following every cationic polymer (PEI) layer during layer-by-layer assembly. It is shown here that adding an anionic polymer layer reduces free volume of the film by filling in gaps of the similarly charged clay layer, which increases the barrier performance by at least one order of magnitude.

Barrier improvement can also be achieved by reducing the pH of the clay suspension in the PEI/MMT system. The charge of the deposited PEI layer increases in the clay suspension environment as the pH decreases, attracting more clay. This enables a 5? improvement in the gas barrier for a 10 PEI/MMT bilayer thin film (85 nm) made with pH 4 MMT, relative to the same film made with pH 10 MMT (57 nm).