A model for water vapor permeability reduction in poly(lactic acid) and nanoclay nanocomposites

Poly(lactic acid) (PLA) is used in packaging applications, but its moisture barrier properties are inferior to poly(ethylene terephthalate) and polystyrene. One objective of the study was to improve these by dispersing nanoclay in PLA. It was found that Cloisite 30B nanoclay showed the best dispersion based on both permeability and transmission electron microscopy results. Compression molded nanocomposite films were amorphous, and moisture permeability measurements revealed that, at the highest loading level of 5.3 vol % organoclay, permeability was reduced by 69% compared to neat PLA. Additionally, independent experiments demonstrated that moisture solubility in the polymer remains unchanged even as solubility in the nanocomposite increases with increasing clay content. A second objective was to explain the measured permeability reduction. A new model is proposed where both the mass flux and area for mass transfer are reduced due to a tortuous path around the impermeable barriers. It is shown that the permeability decreases by a factor of (1+phi)2 where h/t is the aspect ratio of the nanoplatelets, and phi is their volume fraction. Model predictions agree quantitatively with the measured permeability values when data are obtained as a function of filler volume fraction, temperature of measurement, and the concentration driving force. (c) 2018 Wiley Periodicals, Inc. J. Appl. Polym. Sci. 2018, 135, 46506.