COMPLEX COMPOSITION DISTRIBUTION OF POLY(3-HYDROXYBUTYRATE-CO-3-HYDROXYVALERATE)

Four commercially available samples of bacterial poly(3-hydroxybutyrat e-co-3-hydroxyvalerate) [P(3HB-co-3HV)] containing from 6.5 to 21.8 mo l % 3HV have been fractionated by chloroform/n-heptane mixed solvent. They have been separated into several fractions with wide composition ranges. These results show that the composition distribution of as rec eived bacterial P(3HB-co-3HV)s is extremely broad and/or has many peak s over a wide composition range. The effects of the complex compositio n distribution on physical properties have been analyzed through the c omparison of melting and crystallization behavior between samples befo re and after fractionation. Three as received P(3HB-co-3HV)s show the behavior corresponding to the average composition in spite of their co mplex composition distribution. The melting temperature and spherulite growth rate correspond to the values expected from extrapolation of t he data from the fractionated samples. In these copolyesters, cocrysta llization of the chains within the wide composition range occurs. Howe ver, one as received P(3HB-co-3HV) has a higher melting temperature an d faster growth rate than might be expected. The apparent crystallizat ion behavior corresponds to that of P(3HB-co-3HV) with lower 3HV conte nt. These data suggest that only the component chains of relatively lo w 3HV content are crystallized in this as received P(3HB-co-3HV). The crystallization of components of high 3HV content is significantly res tricted. The extremely broad composition distribution of this copolyes ter affects the crystallization (melting) behavior.