Research paperCrystallisation and fractionation of selected polyhydroxyalkanoates produced from mixed cultures
Graphical abstract
Introduction
Poly(R-3-hydroxybutyrate) (PHB) and poly[R-3-hydroxybutyrate-co-(R-3-hydroxyvalerate)] (PHBV) copolyesters are thermoplastic biopolyesters that are promising biobased materials that can be extruded, moulded and spun on conventional processing equipment and that are suitable for many applications, particularly those where biodegradability and biocompatibility are required [2], [3]. The use of mixed microbial cultures from activated sludge for the production of these polyhydroxyalkanoate (PHA) copolymers is a developing area, emerging from the traditional waste treatment technologies as a method to produce value added polymers from services of residuals management. Mixed cultures have several advantages over pure cultures, such as the robust integration of the biomass production into complex waste treatment processes [4], adaptation to changes in substrate/operating conditions owing to microbial diversity, and nonsterile operation [5], [6].
The physical properties of pure random PHA copolyesters are known to vary depending on the chemical structure/monomer unit distribution as well as the composition of the comonomers. With increasing proportions of 3-hydroxyvalerate (3HV) units in PHBV copolymers, the flexibility and toughness are reported to be greatly improved [5], [6]. The melting temperature (Tm) and the glass transition temperature (Tg) also decrease [3], [7]. It has been widely reported [3], [8] that the monomer composition of the PHA produced using mixed cultures is directly proportional to the composition of the feedstock, and models have been developed to simulate the evolution of PHA composition in mixed cultures based on feed [9]. It has generally been assumed that the material properties of these copolymers can then be inferred from the resulting composition and thermal properties. However, we have found that, unexpectedly, the mechanical properties of thin films of PHBV copolymers produced using enriched mixed cultures fed with acetic acid (HAc) and propionic acid (HPr), with 3HV unit contents ranging from 12 mol% to 72 mol%, were in general similar to those of the homopolymer PHB [1]. This surprising outcome was in spite of intentionally attempting to generate a wide range of microstructural variation through varying feeding strategies. Only one material, produced using an alternating feeding strategy, exhibited material properties that corresponded to those reported for higher 3HV content materials. There were some indications that, for all materials produced, the PHA extracted from the biomass may have been heterogeneous blends of random and/or blocky copolymers of broad compositional distribution.
It is well known that material/mechanical properties of polymers are strongly influenced by their crystallisation properties such as nucleation density, optimum crystallisation temperature and rate, and final crystalline morphology, which ultimately result from changes in molecular conformation and intermolecular packing. PHAs are semicrystalline polymers containing a high proportion of chains in crystalline structures. The large spherulite size, high crystallinity and slow secondary crystallisation of PHB is associated with its brittleness and rigidity [2], [3], and PHBV copolymers are known to retain a high crystallinity (>50%) throughout the full range of compositions from 0 to 95 mol% 3HV [3], [10], [11]. However, it is also known that the presence of blends strongly affects the crystallisation properties of materials, depending on the relative degree of compatibility of the blend components in the melt. In addition, block copolymers can have unique crystalline structures due to the constraint introduced with microphase separation, that is in turn controlled by the block lengths of the different chains within the copolymer [12]. Therefore, this paper reports on a fundamental investigation of the thermal and crystallisation properties of these mixed culture PHA materials, in order to better understand the governing factors in structure–function relationships of PHBV copolymers recovered from activated sludge where the recovered material is a complex copolymer blend.
Section snippets
Materials
HAc and HPr were of 98% purity. Chloroform was of HPLC grade (99.9% purity) and was obtained from Sigma–Aldrich. All other chemicals were of at least 98% purity and obtained from Sigma–Aldrich.
Samples
The full details of polymer production and characterisation are given in Arcos-Hernandez et al. [1], although a number of additional reference samples are included in this analysis. These have been prepared in the same fashion as previously described, and the production methods are summarised in Table 1.
Thermal properties of as-produced copolyesters
The thermal properties of the as-produced materials are summarised in Table 2 and Figure 1, Figure 2. In this study, the thermal histories arising from the extraction/production procedure were removed before analysis by pre-melting and storage of the samples for 2 weeks to allow for full crystallisation of slower crystallising components. While some of this data has been previously introduced [1], a more detailed analysis is provided here. It should be noted that the first heating scan was used
Conclusions
This study sought to determine the underlying cause for unexpected mechanical properties of as-produced bacterial copolyesters of PHBV, produced using mixed microbial cultures and having widely differing 3HV contents and in theory differing comonomer unit distributions. DSC studies pointed towards the presence of blends, and solvent fractionation confirmed that, for at least two of these samples, blends of different PHBVs of very broad compositional distribution were present. Isothermal
Acknowledgements
The authors would like to thank the Australian Research Council for funding through grant ARC LP0990917. The ARC had no role in the study design, collection, analysis or interpretation of the data. The authors also thank AnoxKaldnes Sweden for funding through grant ARC LP0990917, and acknowledge gratefully the support provided by Dr Alan Werker (a coauthor on this study) in the interpretation of the data. In addition, we thank Lamija Karabegovich, Per Magnusson and Peter Johansson for their
Glossary
- γ
- ratio of final to initial lamellar thickness
- ρ
- density of polymer
- ρa
- density of polymer in the amorphous state
- ρc
- density of polymer in the crystalline state
- ΔHc
- enthalpy of crystallisation
- ΔHcc
- enthalpy of cold crystallisation
- ΔHm
- enthalpy of fusion
- D
- = FBBFVV/FBVFVB
- ∑G
- sum of molar-attraction constants for all the atoms and groupings in the repeating unit
- FBB
- relative fraction of the BB diad sequence
- FBV
- relative fraction of the BV diad sequence
- FVV
- relative fraction of the VV diad sequence
- FVB
- relative fraction of
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2023, Chemical Engineering JournalCitation Excerpt :Therefore, it can be highlighted that RC did not substantially affect the polymer composition and the high HV content obtained in this work (more than 20 %, wt/wt) was certainly due to the substrate composition (65 % acetic acid and 35 % propionic acid) since the propionic acid was the only precursor of the HV monomer available in the feeding solution and it was incremented (from 15 % to 35 %) in this study compared to other studies performed with the same acids mixture [44]. Other researchers have also highlighted the importance of the HV fraction in the PHBV copolymer in view of obtaining PHA for several applications, including food packaging [45–47]. In this context, Melendez-Rodriguez and colleagues [47] demonstrated that a copolymer with an HV content of 40 % (molar fraction), similar to the compositions obtained in this study, showed better characteristics for the processing of a biopaper in food packaging.