Thermomechanical characterization of biosynthesized P(3HB-co-4HB) copolymers

Polyhydroxyalkanoates (PHAs) are biodegradable and biocompatible polyesters produced through biosynthesis, potentially from a renewable feedstock. To date, more than 150 different hydroxyalkanoic acids have been described in literature. The thermomechanical properties of PHAs can be regulated by varying the comonomer composition, which allows to tune the properties to application-specific needs. One of the most promising PHAs is poly(3-hydroxybutyrate-co-4-hydroxybutyrate) (P(3HB-co-4HB)), which is a an ideal candidate for the
development of high value-added applications in the biomedical industry. However, fundamental knowledge on the structure-property relations of P(3HB-co-4HB) is still lacking. A systematic study on this promising material,
although complicated due to the lack of commercial sample availability, is thus needed. Therefore, shake-flask cultivation was carried out for the biosynthesis of P(3HB-co-4HB) by Cupriavidus sp. USMAA2-4. Copolymer samples with varying 4HB molar fractions were obtained by using combinations of various carbon sources. Thermomechanical analysis of these samples was performed and the influence of comonomer composition was studied. Moreover, since slow crystallization is known to be an issue for the industrial melt processing of PHA
materials, the kinetics of P(3HB-co-4HB) crystallization were studied. Experimental results were modelled by an adapted Haudin-Chenot numerical model, which was found to accurately describe the crystallization kinetics of
P(3HB-co-4HB) with varying composition. In addition, the effect of processing temperature and time on the subsequent crystallization rate of the copolymer was studied. Characterization of the crystallization as function of composition, time and temperature is crucial to acquire fundamental understanding of the relation between structure, processing and properties.