Crystallisation and fractionation of selected polyhydroxyalkanoates produced from mixed cultures

• PHA biopolymers from mixed microbial cultures contain a broad mix of copolymers.
• Differential scanning calorimetry studies demonstrated the presence of blends.
• Solvent fractionation studies confirmed this by isolating subfractions.
• The crystallisation rate of higher melting fractions is faster than for lower melting.
• Mechanical properties will be controlled by crystallisation kinetics and morphology.

Poly[R-3-hydroxybutyrate-co-(R-3-hydroxyvalerate)] (PHBV) copolymers were produced from mixed cultures of biomass (activated sludge) fed with acetic acid (HAc) and propionic acid (HPr). Feeding was performed in such a way as to produce materials with a wide range of monomer compositions and microstructures. Solvent-cast thin films of these materials have recently been shown to exhibit a narrow range of mechanical properties similar to those of the homopolymer poly(R-3-hydroxybutyrate) (PHB) [1]. In this work, more detailed analyses of the thermal and crystallisation properties of these mixed-culture polyesters have revealed that they like comprise complex blends with broad compositional distribution of random and/or blocky copolymers of very different 3-hydroxyvalerate (3HV) contents and melting temperatures and thus have very different respective crystallisation kinetics. This blend complexity was confirmed by solvent fractionation of selected samples. The findings support the hypothesis that overall mechanical properties of these complex copolymer blend materials will be strongly influenced by the more rapidly crystallising components that form the matrix within which the slower crystallising components exist as microdomains. New opportunities in the material development of PHAs are likely to be found in establishing and exploiting such structure–function relationships.

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