Barrier properties of polyamide 12/montmorillonite nanocomposites: Effect of clay structure and mixing conditions

Hybrid films, composed of a PA12 polyamide matrix filled with 5 wt% of clays were melt processed under different mixing conditions to classically obtain montmorillonite-filled microcomposites and organo-modified montmorillonite-filled nanocomposites. The change in mixing shear rates allowed to notably modulating the fragmentation level for montmorillonite and the exfoliation level for modified montmorillonite. Barrier properties to nitrogen and water were altered with a reduction in PA12 chain mobility by confinement effect and geometrical constraints. Tortuosity effects induced by fillers were also highlighted. The water permeability and diffusivity were significantly decreased with exfoliated clays, while an increase was measured with an increased water solubility in presence of clay aggregates, both counterbalanced tortuosity effects. This trend was amplified when using low shear rates. According to filler used and mixing conditions, water sorption measurement has successfully demonstrated an increase in water solubility, due to filler affinity, which was surprisingly inconsistent with the change of water permeability. The occurrence of water plasticization phenomenon was also pointed out through the non-constant diffusivity, reflecting an increased free volume with water activity. Again, a decrease in water diffusivity was unexpectedly obtained, explained by tortuosity effects and ascribed to local water retention at the vicinity of clays, which creates water aggregates in films. These findings were also confirmed by a mathematical modeling approach of sorption process, discussing about the sorbed water access.