Microbeam X-ray Diffraction and Enzymatic Degradation of Poly[(R)-3-hydroxybutyrate] Fibers with Two Kinds of Molecular Conformations

Abstract

A new core−sheath structure constructed by two kinds of molecular conformations2/1 helix (α-form) and planar zigzag (β-form) conformationsin biodegradable poly[(R)-3-hydroxybutyrate] (P(3HB)) fiber was revealed by 0.5 μm microbeam X-ray diffraction in synchrotron radiation. P(3HB) fiber processed by a method combining cold-drawing and two-step-drawing has both α-form and β-form molecular chains in the core region, while the sheath region consists of only α-form chains. The crystallinity and orientation of α-form crystals in the core region were higher than those in the sheath region. Microbeam X-ray diffraction during two-step-drawing of cold-drawn fiber at room temperature revealed the generation mechanism of this interesting core−sheath structure at the highly ordered structural level. The β-form in core region was generated from amorphous chains between α-form lamellar crystals by the strong stretching of two-step-drawing. However, in the sheath region, since the α-form lamellar crystals rotated along the two-step-drawing direction, molecular chains between lamellar crystals could not elongate sufficiently. The enzymatic degradation of P(3HB) fibers was performed with an extracellular poly(hydroxybutyrate) depolymerase purified from Ralstonia pickettii T1. The intensity of a reflection derived from the β-form decreased faster than that of the α-form in X-ray fiber diagram, despite the β-form existing in the core region. The enzymatic degradation progressed from (1) the amorphous chains between α-form lamellar crystals in the sheath region, (2) the β-form molecular chains in the core region, and (3) the α-form lamellar crystals in the whole fiber. The result that the rate of enzymatic erosion of the β-form was faster than that of the α-form indicates that the rate of enzymatic degradation can be controlled by the molecular conformation, despite the same chemical structure.