Ultrasound-Assisted Pullulan/Montmorillonite
Bionanocomposite
Coating with High Oxygen Barrier Properties
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Abstract
In this paper, the preparation and characterization of
oxygen barrier
pullulan sodium montmorillonite (Na<sup>+</sup>-MMT) nanocomposite
coatings are presented for the first time. Full exfoliation of platelets
during preparation of the coating water dispersions was mediated by
ultrasonic treatment, which turned out to be a pivotal factor in the
oxygen barrier performance of the final material even at high relative
humidity (RH) conditions [oxygen permeability coefficients ∼1.43
± 0.39 and 258.05 ± 13.78 mL·μm·m<sup>–2</sup>·(24 h)<sup>−1</sup>·atm<sup>–1</sup> at
23 °C and 0% RH and 70% RH, respectively]. At the micro- and
nanoscale, the reasons are discussed. The final morphology of the
coatings revealed that clay lamellae were stacked on top of one another,
probably due to the forced confinement of the platelets within the
coating thickness after solvent evaporation. This was also confirmed
by modeling the experimental oxygen permeability data with the well-known
Nielsen and Cussler permeation theoretical models, which suggested
a reasonable aspect ratio (α) of ∼100. Electron microscopic
analyses also disclosed a peculiar cell-like arrangement of the platelets.
The stacking of the clay lamellae and the cell-like arrangement create
the excellent oxygen barrier properties. Finally, we demonstrated
that the slight haze increase in the bionanocomposite coating materials
arising from the addition of the clays depends on the clay concentration
but not so much on the sonication time, due to the balance of opposite
effects after sonication (an increase in the number of scattering
centers but a reduction in their size)