Preparation of elastic poly (R-3-hydroxybutyrate-co-R-3-hydroxyhexanoate) fiber with high strength and regulation of its mechanical properties

In this study, first, the preparation process of poly(R-3-hydroxybutyrate- co-R-3-hydroxyhexanoate) (PHBHHx) (with 12 mol% HHx content) original fiber was optimized by lowering down the extrusion temperature to 120°C, which could greatly accelerate the crystallization rate and shorten the subsequent processing time. Then, PHBHHx fibers with excellent mechanical properties were successfully obtained following an "isothermal crystallization-drawing- annealing with fixed ends" processing technique. The average tensile strength could reach up to 262 MPa, with an elongation at break of still larger than 120%. Two-dimensional X-ray diffraction measurements and Raman spectroscopy showed that the polymer chains in the high-strength PHBHHx fibers were highly oriented along the drawing direction both in crystalline and amorphous regions, and no β crystal modification was observed in the highly oriented PHBHHx fibers with or without annealing procedure, which was quite different from poly (R-3-hydroxybutyrate) (PHB) fiber and poly(R-3-hydroxybutyrate-co-R-3- hydroxyvalerate) (PHBV) fiber. Dynamic mechanical analysis (DMA) results demonstrated that annealing procedure was beneficial for improving the storage module (C) of fibers. For example, after annealing at 70°C for 3 h, the G' could be doubled at 30°C and increased by five folds at 40°C. Further study revealed that the mechanical properties of fibers could be easily regulated by changing the annealing conditions, which would guide the further study and lead to wide potential applications of PHBHHx materials.