A comprehensive platform investigating thermal, mechanical and rheological properties for biocomposite based on poly (3-hydroxybutyrate-co-3-hydroxyvalerate) reinforced with biomass by-products

Interest in developing polyhydroxyalkanoates (PHAs) -based biocomposites with biomass fillers (biofillers), derived from local organic by-products, to mitigate global plastic pollution has increased significantly in the last decades. These biocomposites not only offer a more cost-effective solution but also contribute to reducing the carbon footprint, enhancing product circularity, and accelerating biodegradation kinetics. However, optimising their mechanical properties and processability to effectively replace conventional plastics remains a challenge. This study holistically investigates key factors influencing the thermal, mechanical, and rheological properties of poly(3-hydroxybutyrate- co -3-hydroxyvalerate) (PHBV) -based biocomposites. The range of biofillers used in this work, including wood sawdust, macadamia / walnut nutshell flour, agave piña fibres, sorghum stem/leaf fibres, wheat bran flakes and treated wood flour, differ significantly in origin, composition and shape. Except for the wood, the addition of biofillers resulted in a decrease in T m by 1 to 3 °C, T c by 1 to 6 °C, and T g by 0.5 to 13.5 °C. The greatest reduction in T g was observed with the Walnut biofiller. A significant gap of 60 % in tensile stress and modulus was noted between the best and worst performing biofillers, highlighting the importance of carefully selecting the biomass for biocomposite manufacturing. Consistent correlations were found between mechanical properties and the cellulose content, crystallinity, and the hydrophilic surface of the biofillers. The findings suggest that superior PHBV-based biocomposite performance can be attributed to the presence of hydroxyl groups from carbohydrates on the biofillers' surface and a robust internal cellulose structure. Additionally, the shape of the biofillers, particularly the aspect ratio, shows a stronger correlation with rheological behaviour at low angular frequency than the interactions between biofillers and PHBV. At high angular frequencies, the addition of biofillers leads to a reduction in PHBV viscosity by 40 % to 80 %, depending on biofiller used. To support the development of a comprehensive platform for PHAs-based biocomposites, this study compares and validates various characterisation methods ( 13 C ssNMR, TGA, carbohydrate and lignin analysis) that more effectively assess the composition of reinforcing biofillers. • Strong correlation on biofillers composition assessed by Chemical Method, TGA, ssNMR. • 60 % difference in mechanical properties depending on biofillers loaded in PHA. • Key factor for mechanical performance of composite: biofiller hydrophilicity, %CI. • Key factor for rheology properties of composite: biofiller aspect ratio, compositions.