Solid-State Structures and Enzymatic Degradabilities for Melt-Crystallized Films of Copolymers of (R)-3-Hydroxybutyric Acid with Different Hydroxyalkanoic Acids

Abstract

The films of random copolymers of (R)-3-hydroxybutyric acid with different hydroxyalkanoic acids such as (R)-3-hydroxypentanoic acid, (R)-3-hydroxyhexanoic acid, 4-hydroxybutyric acid, 6-hydroxyhexanoic acid, and (S,S)-lactide were prepared by the melt-crystallized method at various crystallization temperatures. The solid-state structures and thermal properties of melt-crystallized films were characterized by means of wide-angle X-ray diffraction, small-angle X-ray scattering, differential scanning calorimetry, optical microscopy, and scanning electron microscopy. Both the long period distance and the lamellar thickness of melt-crystallized polyester films were increased with an increase in the crystallization temperature. The melting temperature of polyester films also increased with an increase in the crystallization temperature. From the relationship between lamellar thickness and melting temperature, it has been concluded that randomly distributed second monomer units except for the (R)-3-hydroxypentanoic acid unit in copolyesters act as defects of poly[(R)-3-hydroxybutyric acid] crystal and are excluded from the crystalline lamellae. The enzymatic degradation of melt-crystallized films was carried out at 37 °C in an aqueous solution (pH 7.4) of PHB depolymerase from Alcaligenes faecalis. After enzymatic degradation, the smooth and rough planes were observed mutually along the radial direction of spherulites on the surface of melt-crystallized films, suggesting that PHB depolymerase predominantly hydrolyzes polymer chains on the edges of crystalline lamellar stacks. The enzymatic erosion rate of crystalline region in polyester films decreased with an increase in the lamellar thickness.