Lignin-derivable block copolymer micelle for effectively reinforcing and toughening polylactic acid

The development of high-performance biodegradable polylactic acid (PLA) materials integrating high strength, malleability and toughness is desired but an ongoing challenge. In this work, a novel full-biobased block copolymer was designed and synthesized by grafting L (+)-lactide (L-LA) and ε-caprolactone (ε-CL) onto lignin via ring-opening polymerization. The obtained lignin-PLA-PCL block copolymer was composed of rigid lignin and poly (LA-CL) rubber segment, could self-assemble into uniform nano-micelles with average diameters of 80-100 nm regulated by simply altering copolymer content. The incorporation of lignin-PLA-PCL copolymers into PLA matrix induced the formation of many cavities, promoted free volume between PLA matrix and copolymer to accelerate chain mobility, achieving excellent ductility and stretchability with maximum stretching deformation of 64.8 %. The resultant PLA composites with the copolymer content as low as 5 wt% displayed simultaneously improved strength (41.84 MPa) and toughness (8.1 MJ/m³), 6.7 % and 1520 % increment than those of neat PLA, respectively. The reinforcing and toughening mechanisms were explored and verified that the combination of cavity growth and fibrillation, followed by extensive shear yielding of matrix, causing substantial plastic deformation. This study extended the design strategy and the foundation for simultaneous reinforcing and toughening PLA plastics using lignin-derived rubbery micelles.