Sorption and Transport of Water Vapor In Poly(3-hydroxybutyrate-co-3-hydroxyvalerate) Membranes Via Various Isothermal Crystallization Conditions

The water transport properties of poly(3-hydroxybutyrate-co-3-hydroxyvalerate) (PHBV) membranes prepared via melt- and cold-crystallization procedures were investigated. The thermal properties and crystalline structure were determined by DSC, WAXD and SAXS, the free volume properties were obtained by positron annihilation lifetime spectroscopy (PALS) and the sorption isotherm and diffusivity of water in the membranes were measured by the gravimetrical method with a recording microbalance. The results showed that the crystallinity of melt-crystallized PHBV membranes was higher than that of the cold-crystallized ones with the same crystallization temperature. The sorption data evidenced that water molecules could form clusters in the polymer matrix.  The water clustering phenomenon would decrease the diffusivity of water in melt- and cold-crystallized PHBV membranes at higher activities. The water diffusivity was successfully correlated with the relative fractional free volume (FFV) and the crystallinity using the modified free-volume theory.