Blowing tough polylactide film enabled by the in situ construction of covalent adaptive networks with epoxidized soybean oil as dynamic crosslinks

Abstract

Because of the rather low melt strength, poor processability, and brittleness, polylactide (PLA) is yet to fulfill its promise as an advanced biobased and biodegradable film to replace fossil-based polymer films. In this work, through the grafting of maleic anhydride (MA) on PLA chains and then based on epoxy-anhydride chemistry and Zn²⁺-catalyzed transesterification exchange, PLA covalent adaptive networks (CANs) with biobased epoxidized soybean oil (ESO) as dynamic crosslinks are in situ constructed in a twin-screw extruder, which can be scaled up for industrial production. The ionic dipole interaction between Zn²⁺ and MA weakens the complexation of Zn²⁺ with the ester groups on PLA chains, maintaining the thermal stability of PLA CANs. PLA CANs show strain-hardening behavior under extensional flow, indicating enhanced melt strength. The combination of accelerated exchange of dynamic covalent bonds and decoupled ionic dipole interaction by elevated temperature and mechanical activation prompts the macroscopic flow, providing rapid reprocessibility for PLA CANs. Thus, PLA CANs can be stably processed by extrusion film blowing, and a high blow-up ratio of 3.71 is reached. Interestingly, the flexible crosslinked network that is highly elastically stretched during film blowing allows the film to exhibit unexpectedly enhanced ductility in both take-up direction (MD) and transverse direction (TD), and the film shows elongation at break of 132.1% in MD and 111.0% in TD at an ESO content of 6.42 wt%. Furthermore, the PLA CAN film presents lower environmental impacts than petroleum-based plastic films in terms of low CO₂ emissions, low fossil resource scarcity, and potential to biodegrade. Such a strategy combining enhanced melt strength and toughness enabled by PLA CANs with ESO as dynamic crosslinks will open up a new avenue toward advanced PLA films in response to severe plastic film pollution.