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

Abstract

Ultra-high-molecular-weightpoly[(R)-3-hydroxybutyrate-co-3.2 mol%-(R)-3-hydroxyhexanoate] (UHMW-PHBH),withaweight-average molecular weight (M_w) 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 M_w above 62 × 10⁴. The crystallization half time (t_1/2), 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.