Impact of Ultraconfinement on Composite Barriers

Flexible optoelectronic packaging is required to provide an ultrahigh barrier to oxygen under ambient conditions, meaning at a relative humidity above 50%. Many polymeric packaging materials, however, adsorb water vapor and the consequential softening is detrimental for the barrier properties. Despite its importance, systematic studies on the impact of the relative humidity (RH) on the oxygen permeability (OP) of clay nanocomposite barriers and convincing evidence for a potential hydrophobization due to compounding with nanosheets are scarce. Especially at filler contents greater than 30 vol %, as required for ultrahigh barriers, a severe confinement is imposed on interlayered polymer and thus its permeability properties are expected to be significantly modified as compared to the bulk. A systematic study of the relation between permeability and RH requires nanocomposite films that differ in filler content but at the same time are comparable with respect to aspect ratio, filler type, quality of texture, and one-dimensional crystallinity. By applying water-soluble polyvinylpyrrolidone (PVP) and ultrahigh-aspect-ratio synthetic clay (sodium fluorohectorite), we were able to prepare hybrid samples that meet these requirements for the first time. By spray coating, the components self-assemble into hybrid films of one-dimensional crystalline Bragg stacks. Two such hybrid films with filler contents of 31 and 40 vol % were fabricated. Indeed, the filler content was found to greatly affect the dependence of the oxygen permeability on RH. Comparing the performance of these two films, the OP in the 40 vol % sample was four times lower than would be expected because of the increase in filler content. To the best of our knowledge, this is the first convincing evidence for a pronounced confinement effect on the permeability.