STUDIES OF COCRYSTALLIZATION OF POLY(3-HYDROXYBUTYRATE-CO-3-HYDROXYVALERATE) BY SOLID-STATE HIGH-RESOLUTION C-13 NMR-SPECTROSCOPY AND DIFFERENTIAL SCANNING CALORIMETRY

The solid-state structures of bacterially synthesized copolymers of 3-hydroxybutyrate (HB) and 3-hydroxyvalerate (HV) (P(3HB-3HV)) have been studied in detail by means of solid-state high-resolution C-13 NMR spectroscopy and differential scanning calorimetry. The observed C-13 chemical shifts of P(3HB-3HV) containing more than 40 mol % HV are different from those of P(3HB-3HV) containing less than 40 mol % HV; this indicates that the structural transition in the P(3HV-3HV) crystals occurs at the comonomer composition of ca. 40 mol % HV. It is shown that the higher (< 40%) and lower (< 40%) HV-containing copolymers crystallize in the crystalline lattices similar to those of P(3HB), respectively. From the partitioning of comonomer units between the crystalline and noncrystalline regions, it is concluded that the two kinds of comonomer units cocrystallize in the P(3HB-3HV) crystals. The heat of fusion of the P(3HB-3HV) crystals depends on comonomer composition, while their crystallinity is almost unchanged. These results also support the occurrence of cocrystallization in the P(3HB-3HV) crystals. In the P(3HB) lattice, the HB units are easier to crystallize than the HV units and vice versa in the P(3HV) lattice. The crystallizing ability of the HV units in the P(3HB) crystalline lattice was determined to be about twice that of the HB units in the P(3HV) lattice.