Molecular Dynamics Insights into Gas Transport in Multilayer Polymer Films

Abstract

Coextruded multilayer polymer films represent promising materials for packaging applications because of their controlled architecture and continuous processing. In this work, molecular dynamics (MD) simulations were performed to obtain a better understanding of the factors that can potentially influence the transport and distribution of gas molecules in multilayer polymer films. Three types of multilayer systems were considered: polylactide/poly(butylene succinate) (PLA/PBS), polylactide/poly-3-hydroxybutyrate (PLA/PHB), and polylactide/polyethylene (PLA/PE). A comprehensive detailed analysis of O2, CO2, and H2O transport in the multilayer systems with varied composition and compatibility of the polymers in the layers, different layer thicknesses, and crystallinity was carried out. In the case of PLA/PE films, significant anisotropy of gas diffusion for O2 and CO2 molecules was observed: the diffusion coefficient in the normal direction to the plane of the layers was much lower than that in the in-plane direction. This effect suggests that, in some cases, transport of gas molecules through a multilayer film perpendicular to the plane of the layers can be limited. It was found that the observed anisotropy of gas diffusion is caused by a combined effect of both the difference in size of free volume elements and the mobility of polymers in neighboring layers. Particularly, the passage of gas molecules from the PE layer to the PLA layer was hindered due to the difference in the size and speed of rearrangement of free volume elements for these polymers. This effect is purely interfacial and connected to the amorphous dynamics at the interfaces in the multilayer films.