Structural change of poly(3-hydroxybutyrate-co-3-hydroxyvalerate) fibers during heating process

Poly (3-hydroxybutyrate-co-3-hydroxyvalerate) fibers were prepared under different drawing and annealing conditions. Three types of crystal structure were observed in each fiber. Those are orthorhombic crystals with two different orientation modes and a new crystal form represented by one strong reflection on the equator in the wide-angle X-ray diffraction (WAXD) pattern. Changes in the WAXD pattern, birefringence and dynamic mechanical properties of these fibers with increasing temperature were investigated. The structural change of the fibers began to occur when the temperature exceeded the annealing temperature of respective starting fibers. Results on the changes in the amount of three crystal structures agreed with the change in the birefringence if intrinsic birefringence of each crystal structure was considered. Comparing the structural changes of different fibers during heating, it was speculated that all the three crystal structures in the starting fibers were formed in the drawing process and their thermal stability depends on drawing temperature.