STRUCTURAL-CHANGES OF POLY(3-HYDROXYBUTYRATE-CO-3-HYDROXYVALERATE) FRACTIONATED WITH ACETONE-WATER SOLUTION

Biosynthesized poly(3-hydroxybutyrate-co-3-hydroxyvalerate) (P(3HB-co-3HV)) copolymers with 3HV mole fraction of 36 and 61% were fractionated by fractional precipitation methods using a solution of acetone-water. Both samples were mixtures of random copolymers having a wide variety of 3HV composition. The melting points of the fractionated samples showed pseudoeutectic changes with minimum breaking point at ca. 40 mol% 3HV. The fractionated samples whose 3HV composition range was from 36 to 56 mol% showed a single melting peak and the coexistence of both P(3HB) and P(3HV) crystal phases. The crystal lattice of the copolymer was of either P(3HB) or P(3HV) type depending on whether the 3HV composition was lower or higher than this range, respectively. The crystal phase transition of isodimorphism for P(3HB-co-3HV), therefore, occurred in a broad composition range from 36 to 56 mol% 3HV, which corresponded to the pseudoeutectic composition range. The amorphous density estimated by density measurement decreased linearly as the 3HV composition increased, showing a breaking point at ca. 30 mol% 3HV. The crystalline density estimated by the X-ray method was obtained over the whole range of 3HV composition. Crystallinity estimated by the density method was almost parallel to that evaluated by the X-ray method.