Samples of bacterial poly(d-hydroxybutyrate-co-fl-hydroxyvalerate) (P(HB-co-HV)) were analyzed by 250-MHz 1H nuclear magnetic resonance (NMR) and Fourier transform infrared (FTIR) spectroscopy. The compositions of 40 copolyester samples, as determined by NMR, ranged from 0 to 47 mol % ß-hydroxyvalerate (HV). The shapes and intensities of numerous IR bands, particularly those at 1279,1228, and 1185 cm"1, were found to be sensitive to the degree of crystallinity. FTIR bands sensitive to composition include the C-H bands around 2900 cm'1 and the C-C band at 977 cm"1. By use of the 2900-cm"1 bands, methods were developed for composition analysis of as-received samples of equal degree of crystallinity in the solid state and in solution. Relative areas, in which the C-H area was normalized to the compositioninsensitive C=0 area, were calibrated to the copolymer compositions determined by 250-MHz 1H NMR. The experimental uncertainties of composition analysis of P(HB-co-HV) by NMR and FTIR were estimated to be ±1 and ±2-3 mol % HV, respectively. FTIR and wide-angle X-ray diffraction (WAXS) studies on solution-cast and melt-quenched samples showed a dramatic trend in that the rate of crystallization of these copolyesters decreases with increasing HV content. A two-stage crystallization process was identified for P(HB-co-HV) cast from solution and was attributed to changes in both nucleation rate and rate of crystal growth, whereas crystallization from the melt is largely nucleation rate limited. The crystallinity index (Cl) determined by FTIR for as-received samples of P(HB-co-HV) was nearly independent of HV content, indicating that the copolyesters are as highly crystalline as PHB homopolymer. The degree of crystallinity (Xc) derived from X-ray diffraction was used to follow crystallization from the melt. The final value of Xc ranged from 62 to 69% for equilibrated copolyesters quenched from the melt. Crystallization of these copolyesters from the melt is faster than for solution-cast samples. However, both processes are slow when compared to the rate of crystallization of solution-precipitated samples.

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Poly(β‐hydroxyalkanoates) are nature's high molecular weight thermoplastic polyesters. They occur as storage granules in a variety of bacteria. From a plastics, film, and fibres perspective, the statistically random copolyesters based on β‐hydroxybutyrate (HB) and β‐hydroxyvalerate (HV) repeating units have high potential for commercial exploitation because of their biodegradability. They provide a range of melting points from 180°C down to 80°C and all compositions exhibit high crystallinity due to isodimorphism. The latter is due to the similar crystalline conformations of poly(β‐hydroxybutyrate) (PHB) and poly(β‐hydroxyvalerate) (PHV). The nascent granules of a 21 mole % HV copolyester sample in freeze‐dried bacterial cells were examined by 13C solid‐state NMR. A disorder in the HV ethyl side group was noted but backbone carbons for HB and HV units showed evidence of crystalline order which was confirmed by x‐ray diffraction. In keeping with the isodimorphous properties of this system, electron diffraction of copolyester single crystals for compositions up to 21 mole % HV confirmed a lattice expansion previously observed for bulk crystallized P(HB‐co‐HV). Solution or melt crystallized films showed decreasing rates of crystallization with increasing HV content. Homogeneous blends of PHB with P(HB‐co‐HV) could be formed which showed a single melting peak by differential scanning calorimetry. The potential of P(HB‐co‐HV) as a source of value‐added small molecules is discussed. Depending on the method of degradation (i.e., chemical or pyrolytic) chiral synthons or vinylic small molecules are obtainable in nearly quantitative yields. Because their physical properties resemble those of polyolefins this family of chiral thermoplastics will probably find wide use in biomedical applications where compatibility and absorbability are essential features.

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