Modification Of Polylactic Acid By Multi-armed Polymers With Hard And Soft Blocks

Polylactic acid（PLA）is a promising biodegradable and biocompatible material.Because of its rich source,high tensile strength,and renewable properties,it has been widely used.However,its high brittleness,slow crystallization rate and poor thermal stability limit its further application.Blending PLA with other block copolymers is an economical and effective method to improve the properties of PLLA.In recent years,copolymer with soft and hard blocks is often used to improve the performance of PLA.In this method,poly（?-lactide）（PDLA）is often used as a hard segment to form stereocomplexation with poly（?-lactide）（PLLA）.However,the related mechanism of the influence of PDLA molecular weight on modification is still unclear.In this work,four-armed copolymers of poly（ε-caprolactone-co-racemic lactide）-b-poly（?-lactide）（4a-CDL-D）with various PDLA molecular weights were synthesized.Then it was blended with PLLA to improve the crystallization and mechanical properties.The contents are as follows:1.The 4a-CDL-D block copolymer was prepared by the ring-opening polymerization with pentaerythritol as initiator.The molecular weight of CDL in the middle soft segment was similar（1k）,while the PDLA block lengths were different（0.5k,1.5k and 2k）.2.The crystallization of 4a-CDL-D and PLLA blends was investigated,and the results revealed that with the change of PDLA block length,the crystallization rate of the blends varied significantly.Among them,4a-CDL-D_0.5k showed excellent crystallization promoting effect which might be due to the formation of intermolecular interaction with PLLA chain,resulting in the increase of nucleation sites.4a-CDL-D_1k、4a-CDL-D_1.5kshowed weaker acceleration effect on PLLA crystallization since low content of SC crystallites was formed.While the promoting effect of 4a-CDL-D_2k was much obvious caused by the high content of stereocomplex crystals.3.On the basis of the crystallization behavior,the mechanical and degradation properties of blends were investigated.These blends could maintain the strength of PLLA and improve its toughness.They also have certain potential applications in shape memory.This work provides a new method for PLLA modification and functionalization,which is beneficial for the application of polylactic acid.