Investigation Of Crystallization Behavior Of Poly(Ethylene Glycol)-Poly(Lactide) Block Copolymers And Blends

Poly(ethylene glycol)-poly(L-lactide)block copolymers(PEG-PLLA)have received extensive attention for their potential applications in the biomedical materials.It is found that many properties of the block copolymers,such as drug permeability,biodegradation,and mechanical properties,are significantly affected by their crystallization behavior and crystal structure.However,the study of crystallization and melting behaviors of PEG-PLLA copolymers and their blends with different compositions have not systemic yet.In this paper,a series of block copolymers were synthesized by ring-opening polymerization of L-or D-Lactide with methoxy polyethylene glycol(MPEG)and different molecular weight polyethylene glycol(PEG),including MPEG-PLLA,PEG-PLLA and MPEG-PDLA,PEG-PDLA block copolymers.The crystallization behavior of PEG-PLLA block copolymers and the PEG-PLLA/PEG-PDLA blends was systematically studied by differential scanning calorimetry(DSC)and wide-angle X-ray diffraction(WAXD).And the effects of the structure and composition of block copolymers on the crystallization behavior of PEG and PLA were discussed.The results are as follows:(1)The variation of structure and composition of PEG-PLLA block copolymers significantly affected the melting and crystallization behavior of PEG and PLLA.The crystallization of PEG block was confined by the existence of PLLA block,the more fraction of PLLA,the greater restriction on PEG,and the restriction of PEG within MPEG-PLLA diblock copolymer was less than that of PEG-PLLA triblock copolymer.(2)For the PLLA block,the PEG acted as a solvent,which reduced the viscosity and enhanced the chain mobility of PLLA.The melting temperature of PLLA block decreased with increasing the length of PEG or reducing the molecular weight of PLLA.With the increase of the molecular weight of PLLA block,the crystallinity of PLLA firstly increased and then decreased.For the copolymers with different structures and molecular weights of PEG blocks,the crystallinity and melting point of PLLA block exhibited different unique component dependence.(3)PEG and PLLA crystals compete to form during solution crystallization.The confinement of PEG crystallization increased with the increase of PLLA component.Conversely,the crystallization of PLLA was also restricted by PEG block.However,PLA stereocomplex(PLA SC)would be formed preferentially during solution evaporation,which created a restricted environment for the crystallization of PEG.And PLA homogenous(PLA HC)appeared in the blends of higher molecular weight.The formation of PLA SC and PLA HC was affected by the length of the PEG block,the increase of PEG block length was beneficial to the formation of PLA SC.(4)During the melting crystallization,the crystallisation of PEG block was mainly affected by the hard confinement of PLA crystals,which was not related to the PLA HC or SC.The"solvent"effect of PEG enhanced the mobility of PLLA or PDLA molecular chains.With the increase of"solvent"content,the T_g moved to low temperature and the cold crystallization peak decreased.