Integration Design of Toughening Agent for Poly(lactic acid) with Compatibility, Transparency, and Environmental Friendliness

The toughening modification of polylactic acid (PLA), one of the most promising sustainable plastics, usually comes at the expense of other characteristic properties, such as mechanical strength, transparency, and environmental friendliness. In this work, a polyurethane copolymer elastomer (PCLA-PLA-U) containing three different blocks, i.e., PLA, copolymer synthesized from random copolymerization of ε-caprolactone and lactide (PCLA), and polyurethane (PU), with different functions has been designed and synthesized. The elastomer's compatibility with the matrix is enhanced by the addition of PLA blocks, therefore effectively regulating the size and dispersion of the toughening phases. PCLA blocks, as the main components, endow the PU copolymer with good transparency and a refractive index matching with PLA matrix. Meanwhile, the physical cross-linked network constructed based on PU blocks endows the toughening phase with an excellent ability to induce matrix elastic deformation, thereby significantly improving the toughness of PLA. When the addition of PCLA-PLA-U is low (<5%), the modified PLA has good transparency (>90%) and high tensile strength (>70 MPa), while the elongation at break is significantly increased by up to 28 times compared to pure PLA. The modified PLA's impact toughness increases significantly to 52.7 kJ/m2, almost achieving supertoughness when the addition content of PCLA-PLA-U is further raised to 40%. By investigating the relationship between structure (both chemical and morphological) and properties, the mechanisms of toughening and change in transparency are discussed. This integration design of the toughening agent for PLA simultaneously takes into account the mechanical properties, optical properties, and environmental friendliness of the material, which is in line with the general trend of sustainable development and therefore largely expands the potential application of PLA.