Water sorption and diffusion in poly-(3-hydroxybutyrate) films

For improved understanding of the transport of low-molecular substances in moderately hydrophobic polymers, the temperature dependence of water vapor sorption and diffusion in poly-(3-hydroxybutyrate) (PHB) was studied for th e first time. Equilibrium sorption and diffusion kinetics were determined b y a quartz McBain's vacuum microbalance technique in the temperature range 303-333 K. The water molecule interaction with the polymer matrix was analy zed for wet PHB films by the Fourier transform infrared spectroscopy techni que. Water sorption isoterms are interpreted as a superposition of free wat er sorption estimated by the Flory-Huggins equation and water immobilized o n carbonyl groups of PHB. The immobilization effect was described by a Lang muir-type equation. The dependence of diffusivity on water concentration wa s described in the framework of Fujita's immobilization model in which the growing function Dw versus Cw characterized the filling degree of carbonyl groups as sites of immobilization in the polymer. Enthalpy of free water so rption (12 kJ/mol) and water immobilization (42 kJ/mol) as well as the acti vation energy of water diffusion coefficients (71 kJ/mol) in noncrystalline areas of PHB were determined.