Water sorption behaviour of two series of PHA/montmorillonite films and determination of the mean water cluster size

Biodegradable polyester-based films constituted of poly(hydroxyalkanoates) (PHA) were successfully extruded with various Cloisite 30B contents. The morphology was highly dependent on the matrix, poly(3-hydroxybutyrate-co-3-hydroxyvalerate) and poly(3-hydroxybutyrate-co-4-hydroxybutyrate), the polymer crystalline phase fraction, the matrix/nanoclay interfacial regions as well as the nanoclay content. Water vapour resistance was investigated through sorption kinetics, isotherms, modelling aspect, and diffusivity. A typical sigmoid-shaped isotherm was obtained in every case. It emerges that the nanoclay highly contributed to the increase of water solubility of matrices. The dependence of polymer crystallinity on the affinity of the nanocomposite films for water was highlighted. Thermodynamic and kinetic contributions of the sorption process were also correlated with the film morphology. According to the matrix used, water diffusivity in films was differently impacted by the sorbed water amount. The access of sorbed water molecules within films was examined through a mathematical modelling approach and the deduced mean cluster size of water vs. its activity was corroborated by sorption kinetics.