Self-assembled nanostructured biohybrid coatings by an integrated ‘sol–gel/intercalation’ approach
The combination of sol–gel technology and intercalation was investigated in this study as a strategy to develop bionanocomposite hybrid materials in the form of coatings with specifically intended oxygen barrier properties. To this goal, the exopolysaccharide pullulan was used as the organic phase, whereas tetraethyl orthosilicate (TEOS) and Na+–montmorillonite (MMT) were used as the metal alkoxide precursor and the nanobuilding blocks (NBB) for the sol–gel technology and the intercalation process, respectively. Complementary information from XRD and TEM analyses disclosed a new supramolecular organization arising from the self-assembly of NBB and pullulan, with the latter apparently intercalated between the clay platelets. Although affected by a rise in haze, the hybrid coatings exhibited outstanding oxygen barrier properties, with permeability coefficient values (P′O2) ranging from 0.89 mL μm m−2 (24 h)−1 atm−1 for a filler volume fraction (ϕ) = 0.017 to an impressive 0.15 mL μm m−2 (24 h)−1 atm−1 (ϕ = 0.095) under dry conditions. Modeling of P′O2 suggested a very tight structure under dry conditions, which yielded an apparent clays aspect ratio (α) ∼50, whereas in the hydrated state a more realistic α ∼ 100 was restored. This finding was further supported by SEM analysis, which also highlighted partial embrittlement of the final hybrid coatings.