Titre de la dissertation: « Design of microbial polyhydroxyalkanoates with advanced properties using the platform of functionalized polysaccharide nanocrystals »

Résumé de la dissertation: Due to their renewability and elevated biodegradability in any natural compartments (soil, water, etc.), polyhydroxyalkanoates (PHAs) are more and more considered as a promising alternative to replace non-biodegradable fuel-derived plastics from a circular economy viewpoint. Interestingly, these polyesters can be obtained through a microbial fermentation process using cheap substrates. In function of the substrates used, they can exhibit a large range of thermomechanical properties upon the comonomer composition achieved along the backbone of these microbial polyesters. However, the most commercially available PHAs, i.e., polyhydroxybutyrate-co-hydroxyvalerate (PHBV), present a too narrow processability window and elevated brittleness related to the formation of large and ill-defined spherulites, hampering the vast implementation of these microbial polyesters at a large scale. To span the range of thermomechanical properties of these PHAs-based materials and in the frame of the ARC project PHASYN, this thesis aims to design polyhydroxyalkanoates (PHAs) with advanced thermomechanical properties using polysaccharide nanocrystals as nucleating and reinforcing agents. In the first step, cellulose nanocrystals (CNCs) were chosen as a nanofiller due to their renewability and biodegradability, as well as outstanding mechanical properties achieved at low loading content. To improve the compatibility of hydrophilic cellulose against hydrophobic PHBV and therefore their fine dispersion, esterified CNCs were extracted through the one-pot Fischer esterification method using a catalytic amount of hydrochloric acid and an excess of organic acid as esterification agent (lactic acid, etc.). This method allowed us the extraction of nanocrystals from any cellulose source and their direct surface-functionalization in a single step. Subsequently, these CNCs were incorporated in PHBV through solvent casting, and we proved that modified CNCs, particularly butyrate-based CNCs, improved the PHBV mechanical properties due to a good dispersion and strong interaction at the interface. In a second step, we are encouraged to improve the melt-processability of PHBV using chitin nanocrystals (ChNCs) as another green (nano)filler for their high thermal stability and more hydrophobic surface compared to CNCs. To achieve a fine dispersion, modified ChNCs were again produced using the same method applied for CNCs and could be readily incorporated in the PHBV matrix using melt-processing techniques. Interestingly, the resulting PHBV nanocomposites presented both enhanced thermal stability and thermomechanical properties.To explain the improvement of the thermomechanical properties of these PHBV nanocomposites, we demonstrated the nucleating role of our modified nanocrystals via melt-crystallization kinetics using isothermal DSC analyses and polarized light microscopy as main tools. Based on the Avrami model, we have shown that these nucleating agents for PHBV were able to reduce the size of spherulites and the crystallization time, impacting the thermomechanical properties of PHBV positively. Interestingly, we demonstrated the nucleating effect of these modified CNCs and ChNCs in homemade PHAs of variable (co)monomer composition in collaboration with the ProteoMic laboratory. These PHAs were obtained using Rhodospirillum Rubrum which enables to produce PHAs under photosynthetic redox stress using cheap volatile fatty acids as a carbon source. An extraction protocol valid for all the obtained PHAs was designed as well.Finally, and to close the loop given our circular approach, some attempts of chemical recycling were successfully performed on these bionanocomposites obtained from commercial PHBV.

Promoteur: Monsieur Jean-Marie Raquez et Madame Giada Lo Re (Chalmers University of Technology, Sweden)