Abstract
This thesis work was supported by the French ANR LOOP4PACK project, which aims to develop a process for the valorization of agro-industrial residues into biosourced and biodegradable microbial bio-polyesters: P(3HB-co-3HV)s belonging to the polyhydroxyalkanoates (PHAs) family. With a low content of 3HV units in the copolymer, the resulting material is brittle and rigid. Increasing the 3HV content can be a key factor to improve the material ductility although it is not systematic. Therefore, this thesis aims to explore the possible tools identified to increase the ductility of P(3HB-co-3HV)-based materials. This work focuses on two scientific questions: (i) to what extent can the specific carbon feeding rate (propionic acid) control the 3HV content in the copolymer? (ii) to what extent do the extraction and melt processing of copolymers with defined proportions of 3HV influence the mechanical properties of the produced films? Additionally, this work also explored the feasibility of on-line monitoring of PHAs biosynthesis by flow cytometry techniques with respect to the operating conditions of the biosynthesis implemented.Under phosphorus deficiency conditions, the maximum propionic acid uptake capacities in Cupriavidus necator were determined at 0.24 Cmol/Cmol/h (qPAcrit). Below this treshold, the limiting carbon flux leads to the regular incorporation of the 3HV monomer at a content of 23 ± 4 mol%. At a limiting flux but close to qPAcrit, biosynthesis performances are maximized in terms of conversion yields and productivity. Beyond this treshold (qPAcrit) leading to carbon overflow, a content of at least 44 mol% of 3HV is incorporated into the copolymer but with a reduction in terms of conversion yields and productivity. On the other hand, the feeding strategy has no influence on the molar mass of the random P(3HB-co-3HV)s copolymers (about 1000 g/mole) and on their distribution, which remains homogeneous (polydispersity index, about 1.5).Thanks to this work, the link between the presence of residual propionic acid in the broth and the enrichment of 3HV units in the copolymer was demonstrated. Furthermore, the link between this 3HV enrichment and the improvement of the ductility of P(3HB-co-3HV)-based materials was confirmed whatever the polymer recovery method used (high pressure homogenisation (HHP) or chloroform extraction). Based on a differential scanning calorimetry (DSC) in-depth study, the optimal processing temperature could be determined to modulate the P(3HB-co-3HV) crystallization behavior and thus the mechanical properties of the material. As a promising result, an elongation at break of 62 % was obtained for a self-supported and non-formulated film made from a chloroform-extracted P(3HB-co-3HV) containing 28 mol% of 3HV. Finally, the monomeric composition cannot be considered as the only tool to improve the final mechanical properties of PHAs. The whole process from bioproduction to melt processing must therefore be taken into account and evaluated to better consider potential applications of these copolymers.