Preparation of toughened poly(lactic acid)-poly(ε-caprolactone)-lignin nanocomposites with good heat- and UV-resistance

Poly(lactic acid) (PLA) is an ideal biodegradable polymer material, widely applied in some disposable areas, such as packaging, biomedical and health care. However, its inherent brittleness, poor heat resistance and mono-functionality limit its application to a certain extent. In this study, PLLA-PCL-lignin nanocomposites (lignin nanoparticles (LNP) grafted with PLLA and poly(ε-caprolactone) (PCL) copolymer) produced by melt processing, were used as an interfacial compatibilizer. Meanwhile, partial replacement of PLLA by PDLA, in combination with an annealing treatment, were considered to enhance mechanical and heat-resistance of neat matrix. Mechanical thermal,optical properties, and heat resistance of the nanocomposites were comprehensively investigated. Interestingly, the notched impact strength was significantly increased from 2.3 (neat PLLA) to 15.3 and 12.1 kJ/m2 for L8-C2 and LD8-C2 samples (PLLA, PDLA and PCL shorted as L, D and C, respectively), respectively. The lignin containing nanocomposite films also showed excellent UV-B resistance, due to the presence of LNP. Furthermore, a remarkable improvement of storage modulus up to 355 and 221 MPa at 90 °C was obtained for h-LD9-C1 and h-LD8-C2 (heat treatment shortened as h) via annealing treatment, enhancement ascribed to the transformation from homocrystallites to stereocomplex crystallites. As a consequence, a toughened PLA-PCL-lignin nanocomposite with good heat- and UV-resistance was successfully prepared, enlarging its possible use as multi-functional PLA based engineering plastic.