Miscibility and enzymatic degradability of poly(3-hydroxybutyrate-co-3-hydroxyhexanoate)-based polyester blends by PHB depolymerase and lipase

Poly(3-hydroxybutyrate-co-3-hydroxyhexanoate) (PHBH) containing 5 and 17 mol% 3-hydroxyhexanoate has been blended with atactic poly(3-hydroxybutyrate) (PHB), polycaprolactone (PCL), and polybutylene succinate (PBS) at different ratios by the solvent-casting method. PHBH-5%/atactic PHB and PHBH-17%/atactic PHB showed a single glass-transition temperature (Tg), indicating miscibility in the amorphous state. Immiscibility was observed in the PHBH-5%/PCL and PHBH-17%/PCL blends based on the non-transitioning Tg of the PHBH component, presence of two distinct Tg values for some of the blend films, and individual crystallization manner. The thermal properties and spherulitic morphologies of the PHBH-5%/PBS blend films suggested partial miscibility. The enzymatic degradability of the three types of blends by Ralstonia pickettii T1 PHB depolymerase and Burkholderia cepacia lipase was investigated. Depending on the enzyme–substrate specificity, miscibility, and phase structure of the blend, the variation of the enzymatic degradation rate with the blend ratio was different among the three types of blends. The weight loss rate of PHBH-5%/atactic PHB was higher than that of PHBH-17%/atactic PHB when degraded by PHB depolymerase. The PCL component accelerated the weight loss rate of PHBH-17%/PCL but decreased that of PHBH-5%/PCL. Lipase barely eroded PHBH-5%/PBS, but the weight loss rate increased as the PCL content increased. From quantitative analysis of the water-soluble products liberated during degradation, the degradation manners of the blends by PHB depolymerase and lipase are discussed. © 2021 Elsevier Ltd