Fabrication of UV- and Heat-Resistant PDLA/PLLA-g-Nanolignin Composite Films by Constructing Interfacial Stereocomplex Crystallites

In the present work, a lignin nanoparticle grafted on a PLLA copolymer (LNP-g-PLLA) was prepared via ring-open polymerization with L-lactide as the raw material and -OH of lignin as the initiator. Then, PDLA/PLLA-g-nanolignin composite films with various quantities of LNP-g-PLLA were fabricated by a solvent casting method. Results from differential scanning calorimetry and wide-angle X-ray diffraction evidenced the formation of SC at the interface between PDLA and the LNP-g-PLLA copolymer, which was helpful to improve the heat resistance. Dynamic mechanical analysis showed that the storage moduli of the PDLA/15g-LNP composite sample are 547 and 264 MPa at 80 and 140 degrees C, respectively, while they are only 12 and 4 MPa for pure PDLA. The tensile test results illustrated a significant improvement in the mechanical performance, which was supposed to be attributed to the interfacial compatibility between PDLA and the LNP-g-PLLA copolymer, resulting in the enhancement of the HC crystallization of PDLA. Furthermore, PDLA/PLLA-g-nanolignin composite films were found to have excellent UV resistance due to the inherent UV shielding property of lignin nanoparticles. When the LNP-g-PLLA amount was 9.0 wt %, the PDLA/9g-LNP film shielded nearly 100 wt % of UV-B irradiation, while only quite limited UV-A irradiation was transmitted. The excellent heat resistance and UV shielding properties endow the composite films with great potential application in some heatable and UV shielding packaging.