Conformational characteristics of poly(3-hydroxyvalerate) (P3HV) and structure-property relationships of P3HV and poly(3-hydroxybutyrate)

Conformational analysis of biosynthetic and biodegradable poly(3-hydroxyvalerate) (P3HV), an analog of poly(3-hydroxybutyrate) (P3HB), was performed. The only structural difference between the two polymers consists in the side chain, which is either methyl (P3HB) or ethyl (P3HV). Molecular orbital calculations and NMR experiments were conducted on a monomeric model compound to determine the bond conformations of the main and side chains of P3HV. The refined rotational isomeric state (RIS) scheme was applied to derive configurational properties of P3HV. The characteristic ratio of P3HV (2.1-3.0) is smaller than that of P3HB (5.4-5.6), indicating greater conformational flexibility of the P3HV chain. The increased flexibility due to the internal rotation of the ethyl side group of P3HV results in a lower equilibrium melting point (130 °C) compared with P3HB (203 °C). The RIS calculations on P3HV also suggested its potential for rubber-like properties. Periodic density functional theory calculations were used to optimize the crystal structures of P3HB and P3HV and obtain their elastic moduli. Three-dimensional Young's modulus distributions of both crystals are closer to isotropic than anisotropic. In conclusion, the material design and usage of poly(3-hydroxybutyrate- co -3-hydroxyvalerate) are discussed. The small structural difference between these two biodegradable polymers, whether the side group is methyl or ethyl, leads to marked divergences in their physical properties, despite their similar crystal structures.