Balancing Polyelectrolyte Diffusion and Clay Deposition for High Gas Barrier

The use of clay nanoplatelets in layer-by-layer assembled thin films (50–200 nm thick) has been shown to reduce the oxygen transmission rates of polymer thick film substrates (>12 µm) by many orders of magnitude. In an effort to reduce the thickness and number of deposited layers required to achieve high gas barrier, the pH of the montmorillonite (MMT) clay aqueous suspension was reduced. Growth of a repeating sequence of polyethylenimine (PEI)/poly(acrylic acid) (PAA)/PEI/MMT was reduced when the MMT pH was lowered from 10 to 6. This reduced pH causes the preceding PEI layer to be more charged, which causes more clay to be deposited per layer. Just three PEI/PAA/PEI/MMT quad layers (44 nm thick) exhibit an undetectable oxygen transmission rate (<0·005 cm³/(m² d atm)) on a 179 µm thick poly(ethylene terephthalate) (PET) substrate. As the clay deposition pH is altered, there is a compromise between high polymer diffusion (at pH 10) and high clay deposition that increases with decreasing pH. Clay at pH 6 provides the best compromise of dense packing and adequate spacing between layers. The three-quad layer film on PET exhibits an oxygen permeability orders of magnitude below silicon oxide (SiO_x) and metal thin films (<5 × 10⁻²² cm³ cm/(cm² s Pa)).