Fiber Structure Development of PHBH through Stress-Induced Crystallization in High-Speed Melt Spinning Process

High-speed melt spinning of poly(3-hydroxybutyrate-co-3-hydroxyhexanoate) (PHBH) with the 3- hydroxyhexanoate composition of 5.4 mol% was carried out. Melt processing of PHBH is known to be difficult because of the combination of its low glass transition temperature being lower than the ambient temperature and low crystallization rate. With the aim of overcoming such difficulty, the high-speed melt spinning process was applied to utilize the effect of tensile stress on the acceleration of crystallization rate. Melt spinning experiment in this study revealed that the crystallization of PHBH proceeded in the spin-line at high take-up velocities even when the extrusion temperature was higher than the Tm of pure PHB where the nucleus of PHB crystals are reported to disappear. WAXD analysis of as-spun fibers showed that the crystalline orientation of α-form crystals increased with an increase in the take-up velocity. A small amount of β-form crystals started to appear at high take-up velocities. This was another evidence for the occurrence of crystallization under high tensile stress. Results of WAXD and birefringence measurements also suggested that the fiber structure development was promoted if the extrusion condition of higher throughput rate was adopted. This was due to the suppressed thermal decomposition originated from the shorter residence time of polymer in the extrusion system. Mechanical properties of as-spun fibers increased with an increase in the take-up velocity. The highest tensile strength and tensile modulus of 156 MPa and 2.43 GPa were obtained under the conditions of take-up velocity 6 km/min, extrusion temperature 180 ̊C and total through-put rate for four filaments 10 g/min.