Diffusion and sorption of water in moderately hydrophilic polymers: From segmented polyetherurethanes to poly-3-hydroxybutyrate

Abstract

In matrices of moderately hydrophilic polymers (MHP) the balance of dispersive and polar interactions determines the structure-morphological peculiarities and the mechanism of diffusion and sorption. Segmented polyetherurethanes (SPEU) were prepared from polytetramethylene oxide or copolymers of ethylene and propylene oxides with 4,4′-diphenylmethane diisocyanate with tetramethylendiol as chain extender. High-molecular weight poly-3-hydroxybutyrate (PHB) was produced by two-stage biotechnological synthesis. Isotopic accessibility and sorption-diffusion experiments were carried out using a suitable designed permeability cell for H-D exchange and vacuum balance techniques, respectively. FTIR/ATR spectroscopy and mechanical thermal analysis were used for MHP structure investigations. The main goal of the paper is to investigate the interrelation between chemical and supermolecular structure and transport processes in SPEU and PHB membranes. SPEU is characterized by a decrease of water diffusion coefficients, D w, with increasing relative humidity and in a rise of accessibility of urethane groups in accordance with partial immobilization model of water diffusion. In the presence of water (1–2 wt.%) α-relaxation maximum is not shifted (plastification is not observed) but water molecules are immobilized in an intermediate layer between soft region and hard domain in SPEU. The latter are not penetrated by water. For PHB free and tightly immobilized forms of water were discovered by FTIR and sorption-diffusion methods. The results of this study give the basis for revealing structure-transport relationship in novel perspective polymers with a moderate hydrophilicity.