Highly Moisture Resistant Super Gas Barrier Polyelectrolyte Thin Films

Flexible, thin coatings with high barrier to oxygen are widely used for improving the lifespan and shelf life of flexible organic electronics and perishable food products, respectively. While metalized plastic and metal oxide thin films exhibit excellent barrier, they also have unique drawbacks. The lack of transparency and recyclability of metalized plastic, and the rigidity and poor adhesion of metal oxide films (e.g., SiOx and AlxOy), severely limit their benefit, especially in food packaging. The utilization of polyelectrolytes to develop high gas barriers with exceptional transparency, adhesion, and flexibility has long been realized. These barriers can be deposited utilizing layer-by-layer (LbL) assembly, providing nanoscale control over film structure and a high degree of tunability in barrier performance. Another approach utilizes a one-pot polyelectrolyte complex deposited via rod or dip coating in a single step. This film is then buffer cured, which generates ionic crosslinks between the polyelectrolytes and therefore minimizes oxygen diffusion through the film. While polyelectrolyte-based barriers typically lose their efficacy in high humidity, a LbL generated barrier, which is only 132 nm thick, provides a 99x reduction of the oxygen transmission rate of PET at 0% and 90% relative humidity (RH). This barrier retains nearly 81% of its performance at high humidity, a feat that is typically only possible with chemical crosslinking. Another impressive barrier was prepared via a simple dip-coating process of polyethyleneimine (PEI) and poly(acrylic acid) (PAA), which was then cured with a citric acid buffer solution. Through simple ionic crosslinking, undetectable oxygen transmission rate (OTR < 0.005 cc/m²*day), at both 0% and 90% RH, can be achieved with a PEI:PAA molar ratio of 1:1. The strong complexation from ionic crosslinking creates an unusually dense thin film that is promising for various packaging applications (food, electronics, etc.). These thin film exhibit some of the best-ever polymer-based oxygen barriers at high humidity.