Characterization of MMC-produced PHAs with tailored chemical composition

In recent years, the problems associated with the solid waste management and the dependence on petroleum-based plastics have created great interest, mainly focused on the development of bio-derived and biodegradable polymers. One of the most promising group of biopolymers that can be used as a fossil plastic substitute is the polyhydroxyalkanoates family (PHAs). PHAs are polyesters that can be naturally accumulated as intracellular granules by many species of bacteria in nature [1]. The stored copolymer is a fully biodegradable thermoplastic material with mechanical properties comparable to some conventional fossil fuel derived plastics. Poly(3-hydroxybutyrate) [P(3HB)] and its copolymers, mainly with 3-hydroxyvalerate comonomeric repeating unit [P(3HB-co-3HV)], are among the most investigated biopolymers of this class. The large-scale diffusion of these materials is hampered by problems related mainly to their production costs and poor processability. In fact, the production takes place in pure microbial culture, which means the adoption of sterile conditions and selected feedstock, thus entailing high maintenance costs. In addition, P(3HB) and P(3HB-co-3HV) copolymers with low unit content of 3HV are highly crystalline, fragile, and characterized by high sensitivity to thermal degradation because of their high melting temperatures close to that of degradation. These features make them difficult to process with the common industrial processes. On the other hand, the use of mixed microbial culture (MMC) and renewable feedstock allows to reduce the costs of the production. Moreover, it has been recently found that it is possible to obtain P(3HB-co-3HV) copolymers in a wide range of different monomeric unit content ratio by varying not only the carbon substrate composition but also its concentration and the feeding schedule [2]. This study aims at investigating the correlation between feeding strategy and the final properties of MMC-produced P(3HB-co-3HV). The copolymers characterized in this work was produced in a laboratory scale sequencing batch reactor (SBR) under controlled feeding conditions, tuned to obtain a 3HV content spanning from 20 to 60 mol%. This composition range is very interesting because in between is located pseudo-eutectic point (about 40 mol % HV), at which the materials show low melting temperature and crystallinity as well as brittleness. The variation of composition (in terms of 3HV content) and molecular weight was evaluated as a function of the applied organic load rate (OLR) and SBR cycle length. A strong correlation between process parameters and copolymer properties was observed. As an example, by decreasing OLR and cycle length, the 3HV content and the molecular weight of copolymer increase. Thermal, morphological, and mechanical properties were studied by thermogravimetric analysis (TGA), differential scanning calorimetry (DSC), polarized optical microscopy (POM) and stress-strain tests. All the PHAs show a broad and complex low temperature melting process which suggests that the samples are a blend of homo- or copolymer with different structure and composition. The mechanical properties were strictly related to the polymer composition and for intermediate 3HV contents (about 40 mol %) a reduction of brittleness was recorded. The results obtained from this work highlight the possibility to control the operating conditions to obtain MMC-produced PHAs with customized chemical composition and consequent improvement of thermal as well as mechanical properties and, therefore, the processability. Acknowledgements The financial support of USABLE Packaging, Bio-Based Industries Joint Undertaking under the Horizon 2020 (Grant Agreement No. 836884) 1. [1] B. Laycock, P. Halley et al, Progress in Polymer Science, 2014, 39, 397-442. 2. [2] A. Ferre-Guell and J. Winterburn, Biomacromolecules, 2018, 19, 996-1005.