# Barriers and Chemistry in a Bottle: Mechanisms in Today's Oxygen Barriers for Tomorrow's Materials
## 1 Introduction  
In recent years, the world of packaging has been shifting continuously towards the application of more plastic-based materials as replacements for, e.g., aluminum and glass. These traditional materials have a high production cost and high carbon footprint compared to polymeric containers [1; 2; 3]. Additionally, plastics are less fragile and extremely lightweight. However, while glass and aluminum are highly impermeable to gases, small molecules may still be able to move in between polymer chains. Therefore, the plastics industry is forced to develop new technologies to meet the packaging requirements of certain products, such as juices, beers and wines. As such, specific barrier solutions are employed in order for these products to retain an appropriate lifetime. Depending on the contents of the packaging, demands can be imposed for retaining light, water, CO\({}_{2}\) and/or O\({}_{2}\), etc. These demands are quantified in a single parameter, namely the shelf life. The shelf life is defined as the period of time ranging from the packaging of a product to the moment wherein the product is no longer of acceptable quality to be sold. A wide range of solutions has been brought onto the market in order to provide the product's specific shelf life demands. These can be subcategorized into active and passive barriers, depending on the type of molecular interactions (i.e., chemical reactions, adsorption, desorption, etc.) that occur between the barrier compound and the targeted molecule [4; 5]. For many applications, the exclusion of oxygen from the package is critical. The oxidative nature of this molecule can quickly deteriorate the quality of a packaged product [6; 7]. Depending onthis product, the requirements for the oxygen barrier to achieve a sufficient shelf life may vary. Fruit preservation thrives best under oxygen levels of 2-5%, imposing the need for very slow scavengers and the creation of a steady state between permeation and oxygen removal [8,9]. In metals and in many beverages (ex. juice, beer, wine, etc.) any oxygen could be harmful. Packagers should therefore aim for the exclusion of as much oxygen as possible until other contaminations will limit the shelf life. For example, in solid products such as meats, microbial threats limit the shelf life to 8-9 weeks anyway, so oxygen barriers that provide protection for at least this long are sufficient [10]. Aseptic filling of liquid products provides a much better protection from microbial contaminants, so a longer potential shelf life can be achieved in comparison to the modified atmospheric packaging (MAP) of solid foods. Therefore, the most potent oxygen barriers in the state-of-the-art are preferred for beverages to achieve maximum durability [11]. Aside from foodstuffs, other technical applications for polymeric oxygen barriers can comprise the protection of organic electronics (e.g., organic light-emitting diodes (OLEDs) and solar cells) [12, 13, 14]. OLED-structures comprise molecules with high conjugation that may excite and subsequently luminesce when an electric current is applied. At these excited energy levels, it may readily interact with water, but also oxygen, as will be further explained below. The substrate on which the electrodes and the organic semi-conductor phase of OLEDs are grown should therefore provide long-lasting protection from oxygen and water. The encapsulations in the traditional set-up are glass plates and metal lids, which achieve excellent performances. Now the industry is focusing on producing its next generation, flexible OLEDs. The targeted flexibility can only be achieved with the exclusion of the rigid glass plates. Transparent polymers with high barrier properties would offer an appropriate alternative, but the current solutions are still expensive and insufficient for long lifetimes. Applications encompass, amongst others, the incorporation of flexible computer screens in clothing.