Effect of Molecular Weight on Physical and Crystallization Properties of UHMW-poly [(R)-3-hydroxybutyrate-co-(R)-3-hydroxyhexanoate]

Abstract

Ultra-high-molecular-weight poly[(R)-3-hydroxybutyrate-co-3.2 mol%-(R)-3-hydroxyhexanoate] (UHMW-PHBH), with a weight-average molecular weight (Mw) of approximately 400 104, is extracted from genetically engineered microbial cells via an optimized solvent extraction method. PHBH polymers with various molecular weights (various MW-PHBHs) are prepared from the UHMW-PHBH using H2SO4 treatment. The thermal properties and spherulite growth morphology of the UHMW-PHBH and various MW-PHBHs are analyzed in order to investigate the properties of UHMW-PHBH and the effect of the molecular weight on the above properties. The value of Tm and Tg were same regardless of Mw above 62 104. The crystallization half time (t1/2), as an index of overall crystallization speed, of UHMW-PHBH did not change, compared with that of normal molecular weight PHBH. The observation of crystallization morphology revealed that molecular chains with high-molecular-weight lead to decrease the growth rate of spherulite and increase the crystal nuclei forming frequency. Drawn films of UHMW-PHBH are prepared by one-step or two-step cold-draw methods, and their mechanical properties and highly ordered structure are analyzed by tensile tests and X-ray measurements, respectively. One-step cold-drawn UHMW-PHBH films exhibit enhanced properties relative to those of normal-molecular-weight PHBH. Wide-angle X-ray diffraction and small-angle X-ray scattering reveal that the one-step cold-drawn UHMW-PHBH possesses the rare β-form crystal structure. Additionally, the β-form crystal are increased drastically after two-step drawing process with increasing tensile stress. The crystal structure of those films were also investigated by synchrotron X-ray. (Received 31 July, 2017; Accepted 14 November, 2017).