Formation of Highly Ordered Structure in Poly[(R)-3-hydroxybutyrate-co-(R)-3-hydroxyvalerate] High-Strength Fibers

Biodegradable fibers of poly[(R)-3-hydroxybutyrate-co-8%-(R)-3-hydroxyvalerate] (P(3HB-co-8%-3HV)) produced from wild-type bacteria were prepared by melt-spinning, followed in an amorphous state by quenching near the glass transition temperature (Tg). High tensile strength fibers of over 1 GPa were prepared by one-step drawing with small crystal nuclei grown by isothermal crystallization near the Tg. This new drawing technique is a very attractive method for obtaining strong fibers from low-molecular-weight polyesters produced by wild-type bacteria. The highly ordered structure of strong P(3HB-co-8%-3HV) fibers was investigated by tensile measurement, scanning electron microscopy, and wide- and small-angle X-ray scatterings (WAXD and SAXS) with synchrotron radiation (SPring-8, Japan). The WAXD patterns of strong fibers showed sharp reflections corresponding to highly oriented α-form (21 helix conformation) and β-form (planar zigzag conformation). The weak reflections along the meridian in the SAXS patterns of strong fibers indicate that the density between α- and β-form crystals is nearly identical and that the fibers have highly oriented structure with various thicknesses of α-form lamellar crystals. Furthermore, the microbeam wide-angle X-ray diffraction with Fresnel zone plate optics in synchrotron radiation was performed to investigate the inner structure of the monofilament. It was revealed that strong P(3HB-co-8%-3HV) fibers have a uniform structure consistent with two types of molecular conformations: the α-form and β-form crystals. From the results of WAXD and SAXS patterns of one-step-drawn fibers after isothermal crystallization, a new mechanism for the development of β-form crystals is proposed as follows: Many small crystal nuclei in the amorphous region grow slowly during isothermal crystallization near the Tg. The β-form is produced during drawing by the stretching of molecular chains in the constrained amorphous region between small crystal nuclei which act as cross-linking points, and then the α-form lamellar crystals are generated from small crystal nuclei by the crystallization during annealing.