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 fractionate d by fractional precipitation methods using a solution of acetone-wate r. Both samples were mixtures of random copolymers having a wide varie ty of 3HV composition. The melting points of the fractionated samples showed pseudoeutectic changes with minimum breaking point at ca. 40 mo l% 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 copolyme r was of either P(3HB) or P(3HV) type depending on whether the 3HV com position was lower or higher than this range, respectively. The crysta l phase transition of isodimorphism for P(3HB-co-3HV), therefore, occu rred in a broad composition range from 36 to 56 mol% 3HV, which corres ponded to the pseudoeutectic composition range. The amorphous density estimated by density measurement decreased linearly as the 3HV composi tion increased, showing a breaking point at ca. 30 mol% 3HV. The cryst alline density estimated by the X-ray method was obtained over the who le range of 3HV composition. Crystallinity estimated by the density me thod was almost parallel to that evaluated by the X-ray method.