Microstructural reorganisation and mechanical property enhancement in PHAs and their wood biocomposites through post-fabrication heat treatment

Abstract Polyhydroxyalkanoates (PHAs) are biodegradable bioplastics with strong environmental benefits, yet their inherent brittleness and high production cost limit their broader adoption. Blending PHAs with lignocellulosic biofillers offers a circular and cost-effective pathway but often compromises mechanical performance. This study investigates post-fabrication heat treatment, e.g. annealing (< 150 °C) and partial-melting (> 150 °C) conditions, as a scalable strategy to tailor the properties of PHAs such as poly(3-hydroxybutyrate-co-3-hydroxyvalerate) (PHBV), their blends with poly(3-hydroxybutyrate-co-4-hydroxybutyrate) (P34HB), and their wood fibre biocomposites. Annealing improved ductility by enhancing the mobile amorphous fraction (MAF) and reducing the rigid amorphous fraction (RAF), while partial melting promoted crystal perfection but also induced thermal degradation, increasing RAF and material stiffness. Optimal mechanical performance was achieved after 30 min at 150 °C, with tensile strain at break increasing by ~ 650% for neat PHAs and ~ 200% for wood/PHAs biocomposite variants. This was accompanied by a 20–30% reduction in modulus and ≤ 16% drop in tensile stress for both materials. Notably, shrinkage above 175 °C was significant in neat and blended PHAs but was strongly mitigated by wood biofillers. The results highlight post-fabrication heat treatment as a simple, effective method to enhance the mechanical behaviour and dimensional stability of PHAs-based materials for rigid packaging and other demanding applications.