Synergistic effects of PHAs and biomass fillers on the thermal, rheological, and mechanical properties of biocomposites

This study examines polyhydroxyalkanoate (PHA)-based biocomposites incorporating locally sourced, biomass-derived fillers as a sustainable alternative to conventional plastics. While environmentally advantageous, optimisation of their mechanical properties and processability remains challenging due to the complex interplay of factors influencing biocomposite behaviour. Various commercial PHA polymers with different types of comonomer and content, were compounded with Australian lignocellulosic biofillers, wood, macadamia, and walnut shells, characterised by distinct cellulose and lignin contents. Gel permeation chromatography assessed PHA molecular weight changes post-processing, indicating thermo-mechanical degradation as a function of biofillers. Differential scanning calorimetry revealed correlations between biofiller cellulose content and crystallisation behaviour, glass transition temperature, and crystal perfection, which aligned with tensile strength and elastic modulus trends. The degree of crystallinity in neat PHAs emerged as a key determinant of mechanical performance in biocomposites. Oscillatory rheometry revealed that neat poly(3-hydroxybutyrate-co-4-hydroxybutyrate) exhibited higher viscosity, while poly(3-hydroxybutyrate-co-hexanoate) showed lower viscosity, independent of comonomer content and molecular weight. Rheological analysis also showed viscosity correlated with biofiller cellulose content at low shear rates and with PHA molecular weight at higher shear rates. Overall, this work provides a comprehensive platform and guidance for the development of PHA-based biocomposites.