Melt and nucleation reinforcement for stereocomplex crystallites in poly (L-lactide)/lignin-grafted-poly(D-lactide) blend

Lignin -grafted-poly (o-lactide) (LGPD) with two different poly (o-lactide) (PDLA) block lengths (1100 and 2000 Da), denoted as LGPDL and LGPDH respectively, were synthesized by ring opening polymerization using dodecylated lignin as initiator. They were then blended with poly(L-lactide) (PLLA) to improve its crystallization, mechanical properties and UV-absorbance properties. It was found that stereocomplex (SC) crystallites could be formed in both PLLA/LGPD blends, and the crystallization rate of PLLA/LGPD(H) was faster than that of PLLA/LGPD(L) because of the longer PDLA block length. Both in the non-isothermal and isothermal crystallization process, the crystallization of PLLA in the PLLA/LGPD(H) blend was the fastest when the ratio of LGPDH was 3%. SC crystallites in the PLLA/LGPD(H) blends also showed broader melting ranges and higher final melting temperatures comparing to the blends of PLLA and 4-armed PDLA (4a-PDLA), whose number of PDLA arms and molecular weight was similar to LGPDH. These results proved that the lignin structure grafted by the PDLA arms could affect the state of SC crystallites and their acceleration effect on the crystallization of PLLA in the blend. The degradation behavior showed that the degradation rate of the LGPDH-5% blend in the final stage of degradation (about 24 h) was the slowest comparing to other blends. Besides, the PLLA/LGPD blends had better tensile strength and UV-absorbance properties than neat PLLA.