Tuning Thermal and Mechanical Properties of Poly(hydroxybutarate) Copolymers through Aminolysis with Functionalized Polyurethanes

Growing demand for safe, sustainable, and high-performance biomaterials─alongside the global shift away from petrochemical plastics─has accelerated interest in biodegradable polymers derived from renewable sources. Poly(hydroxyalkanoates) (PHAs), biosynthesized by bacteria, have emerged as promising sustainable materials for biomedical and high-value applications. Among them, poly(3-hydroxybutyrate- co -4-hydroxybutyrate) P(3HB- co -4HB) is an attractive candidate for biomedical and advanced manufacturing applications. However, its inherent brittleness and high melting temperature restrict its processing flexibility and broader commercial adoption. This study introduces a chemical modification strategy to improve the thermal and processing properties of P(3HB- co -4HB). The approach involves aminolysis-driven grafting of amine-functionalized polyurethane (PU-PEI) onto the polymer backbone, enabling controlled disruption of crystallinity and tunable changes in molecular architecture. Successful modification was confirmed using FTIR, 1 H NMR, and XPS spectroscopy. Size exclusion chromatography indicated a reduction in molecular weight, attributed to ester bond cleavage during aminolysis. Thermal and structural characterization using DSC, TGA, and XRD revealed significant changes in polymer behavior. Incorporation of only 6.9% (w/w) PU-PEI reduced the melting temperature by 12 °C compared to the unmodified polymer, enhancing thermal processability. Rheological and mechanical testing suggested partial cross-linking induced by aminolysis, resulting in increased tensile strength and Young’s modulus. In vitro enzymatic degradation studies demonstrated that low levels of modification preserved biodegradability, whereas higher modification degrees reduced degradation rates, likely due to excessive cross-linking. Importantly, the modified polymers exhibited substantially improved thermal processing performance, enabling successful melt-extrusion into uniform fine fibers─an outcome unattainable with the pristine material. Overall, this work presents a scalable chemical modification strategy that overcomes key processing limitations of PHA-based biopolymers, advancing their potential for next-generation biodegradable materials in biomedical and sustainable high-value applications.