On the heterogeneous composition of bacterial polyhydroxyalkanoate terpolymers

•Waste glycerol was up-graded to P(3HB-4HB-3HV) terpolymers.•Terpolymers of similar monomeric composition have different physical behavior.•P(3HB-4HB-3HV) has lower elasticity than P(3HB-4HB) with the same 4HB molar ratio.•Bioproduced P(3HB-4HB-3HV) is a complex mixture of polymeric chains.•Fractionation and blend adjustment are essential to reach reproducible properties. Poly(3-hydroxybutyrate-4-hydroxybutyrate-3-hydroxyvalerate) (P(3HB-4HB-3HV)) terpolymers of low 3-hydroxyvalerate (3HV) content (1.7–6.4%) with 4-hydroxybutyrate (4HB) molar fractions from 1.8% to 35.6% were produced by fed-batch cultivation of Cupriavidus necator DSM545. Waste glycerol, γ-butyrolactone and propionic acid were used as main carbon source, 4HB and 3HV precursors, respectively. Uniaxial tensile tests were performed on the corresponding biopolymers. The Young’s modulus and tensile strength of P(3HB-4HB-3HV) decreased, whereas the elongation at break increased with the 4HB molar%, following the general trend described for poly(3-hydroxybutyrate-4-hydroxybutyrate) (P(3HB-4HB)) but with pronounced lower elasticity. Differential scanning calorimetry results indicate that the temperature of crystallization and enthalpy of melting decreased as the 4HB% increased. No crystallization was observed in terpolymers containing more than 30% of heteromonomers (4HB and 3HV) even though multiple melting events were detected. Terpolymer fractions of different composition were obtained by solvent-fractionation of the original bacterial terpolymers.