Transport water and molecular mobility in novel barrier membranes with different morphology features

Bacterial poly(3-hydroxybutyrate) (PHB) — a new biopolymer and its derivatives are being extensively applied in medicine, biotechnology and membrane technologies. The relationship between structure–transport parameters and molecular mobility of PHB was investigated by quartz McBain's microbalance, X-ray diffraction and ESR method. Two types of the PHB membranes with different structural organisation—isotropic and textured—were studied. The specific features of the sorption and diffusion characteristics of PHB are related to the structural organisation. The polymer matrixes are characterised by significantly microstructural heterogeneity as it was shown by the spin probe (TEMPO/TEMPOL) measurements. A non-crystalline phase of PHB includes two kinds of microfields with different molecular mobility. Moisture is found to change both the velocity of molecular motion and the mole content of such microfields. The diffusion behaviour of the membranes has been explained in accordance with dynamic information based on the ESR method.

Fig.4. ESR spectra for spin probe TEMPOL in the textured (a) and isotropic (b) samples of PHB at 293 K. 1, wet samples; 2, dry samples. The diffusion transferring mechanism of low molecular substances, particularly, water molecules, in a polymer matrix depends on intensity of interaction between water molecules with macromolecules and structural features of the polymer matrix. The influence of the last factor on the sorption/diffusion properties dominated in polymers is characterised by a high degree of crystallinity, and one of them is PHB. As it has been shown in the present work, the PHB membranes with different sorption/diffusion behaviour may be obtained by forming a definite structural organisation at the membranes preparation stage. In that case sorption features of the membranes have been regulated by PHB crystallites packaging and their orientation between themselves in the matrix. The orientation and packaging density of the crystallites in the membranes strongly affected non-crystalline area accessibility of polar sorption sites for water molecules. Rates of diffusion transport of water molecules in the polymer matrix depend on an intensity of molecular mobility in the PHB noncrystalline areas. Our dynamic research has shown that there are microfields with different intensity of segmental motion in the PHB noncrystalline areas. Water transport occurs on the non-crystalline microfields. In the presence of water in the PHB membranes, both molar ratio of such microfields and intensity of molecular motion therein are changed. In our opinion, such change is a cause of complicated behaviour of the diffusivity as the water concentration function (see Fig. 2) in the membranes. The results of this study give the basis for challenging the understanding of the structure– transport relationships and molecular mobility in non-crystalline regions of PHB with water and organic solvent permeability through moderately hydrophilic barrier membranes.