Enhanced compatibility and toughness of poly(3‐hydroxybutyrate‐co‐3‐hydroxyvalerate)/polycaprolactone blends by ring‐opening polymerization and hydrogen bonding of epoxy‐terminated hyperbranched polyester

Melt extrusion process was followed in order to improve the high crystallinity and poor toughness of poly(3‐hydroxybutyrate‐co‐3‐hydroxyvalerate) (PHBV) blend materials. It was achieved by the incorporation of epoxy‐terminated hyperbranched polyester (EHBP) elastomer into PHBV and polycaprolactone (PCL). EHBP cross‐links PHBV and PCL through ring‐opening polymerization of epoxy‐terminated and carboxyl groups. The mass percentages of EHBP in the PHBV/PCL blends are 1, 2, 3, and 4mass%, which are expressed as 1, 2, 3, and 4 phr in the following text. Therefore, when the EHBP content was 3 phr, the Young's modulus and tensile strength of the blends are increased to 750 and 15 MPa, respectively, which was comparable to the biodegradable polymers used for packaging. Simultaneously, compatibility between PHBV and PCL has been improved and the particle size reduction of blends can be obviously observed in scanning electron microscope (SEM) images. Dynamic mechanical analysis (DMA) analysis revealed that PHBV and PCL showed improved compatibility with each other by the addition of EHBP. Differential scanning calorimetry (DSC) revealed that the decrease of crystallinity of the blend was consistent with the increase in mechanical properties. Additionally, all the bio‐blends show good thermal stability. Food overall migration studies showed that the amount of migration of composite materials in contact with food was also far lower than the national standard value. Therefore, PHBV/PCL/EHBP blends are expected to be used in the field of food packaging. In this study, epoxy‐terminated hyperbranched polyester (EHBP) was synthesized by epoxy‐terminated modification of hyperbranched polyester, which cross‐links poly(3‐hydroxybutyrate‐co‐3‐hydroxyvalerate) (PHBV), and polycaprolactone (PCL) via epoxy‐terminated ring‐opening, while hydroxyl‐terminated groups form hydrogen bonding effect with PHBV and PCL. Together, they improved the toughness and compatibility of PHBV and PCL.