Effect of Radical Initiators on the Rheological Properties and Toughness of Poly(Lactic Acid)/Poly(3‐Hydroxybutyrate‐Co‐4‐Hydroxybutyrate) Blends

In this study, poly(lactic acid) (PLA) and poly(3‐hydroxybutyrate‐co‐4‐hydroxybutyrate) [P(3HB‐4HB), a type of polyhydroxyalkanoate (PHA)], were melt‐blended at an 80/20 (wt/wt) ratio, and radical initiators were employed as reactive compatibilizers to enhance their mechanical performance. Azobisisobutyronitrile (AIBN), benzoyl peroxide (BPO), and dicumyl peroxide (DCP) were introduced within the range of 0.1–0.5 wt%. Depending on the initiator type and concentration, chain scission and grafting or crosslinking reactions occurred, governing both viscosity evolution and interfacial adhesion between PLA and PHA. Rheological and morphological analyses, along with Izod impact resistance measurements, revealed that 0.1 wt% DCP generated optimal compatibilization, yielding reduced PHA domain size and markedly enhanced toughness. Furthermore, by implementing a two‐step mixing protocol, a broadened PHA domain size distribution with a characteristic tail toward larger domain sizes was achieved, where the large cavitation‐prone domains cooperatively interacted with smaller stabilizing domains. This synergistic deformation mechanism enabled a substantial increase in impact strength relative to the conventional single‐step process, demonstrating that domain size tuning via controlled mixing provides an effective pathway for high‐performance, fully bio‐based PLA/PHA blends.