Poly(β‐hydroxyalkanoates): Biorefinery polymers in search of applications

Abstract

Abstract 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 13 C 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.