Immobilization influence on the water sorption and diffusion in poly(3-hydroxybutyrate)

The temperature dependency of water vapor sorption and diffusion in poly(3- hydroxybutyrate) (PHB) was studied for the first time. Equilibrium sorption and diffusion kinetics were determined by a quartz McBain's vacuum microba lance technique in the temperature range of 303-333 K. A probability of wat er molecule interaction with the polymer matrix was analyzed for wet PHB fi lms by FTIR spectroscopy technique. Sorption isotherms are interpreted as t he solution of free water molecules estimated by the Flory-Huggins equation and the sorption of water molecules immobilized on the carbonyl groups of PHB. The immobilization effect was described by a Langmuir-type equation. T he dependency of diffusivity on water concentration was described in the fr ames of Fujita's immobilization model in which the growing function D-w ver sus C-w characterized the filling degree of carbonyl groups as sites of imm obilization in the polymer. Enthalpy of free water sorption (12 kJ/mol) and water immobilization (42 kJ/mol), as well as the activation energy of wate r diffusion coefficients (71 kJ/mol), in noncrystalline areas of PHB were d etermined. (C) 1999 John Wiley & Sons, Inc.