Study On Melt Structure Evolution And Crystallization Behavior Regulation Of Poly(Lactic Acid) Racemic Blends

In recent years,the environmental problems caused by"white pollution"are attracting the attention of the whole society.The biodegradable polymer materials represented by poly（lactic acid）have become the focus of research and application.In 1987,Ikada et al.found that when L-poly（lactic acid）and D-poly（lactic acid）formed racemic blends,stereocomplex crystals were generated in the system,and their melting point was about50℃higher than that of homo-crystals.This result will greatly expand the application range of poly（lactic acid）（PLA）materials.However,the existing research results show that under the condition of high molecular weight,the poly（lactic acid）racemic blends will form the stereocomplex crystals and the homo-crystals at the same time,and the two show a competitive growth relationship.In order to further clarify the competitive growth mechanism of stereocomplex crystals and homo-crystals,and to master the preparation method of poly（lactic acid）material with high molecular weight and high stereocomplex crystals content,the melt structure evolution process of poly（lactic acid）racemic blends was studied,and the crystallization behavior of the racemic blends was regulated.Firstly,the interaction between enantiomer chains and the conformational transition behavior of poly（lactic acid）racemic blends melt were studied by means of Flash DSC and infrared microscopy.The results show that the helical conformation of 10₃is always formed first in the racemic melt in the temperature range investigated.With the increase of isothermal time,the characteristic infrared absorption peaks of C-H symmetric vibration,asymmetric vibration and C_α-H stretching vibration on methyl group shift to the low-frequency direction,and the interaction of C=O···H-C between enantiomer chains gradually forms.Subsequently,a transition from 10₃to 3₁helical conformation will occur in the melt.Comparison of the sequence between enantiomer chain interactions and conformational transitions shows that C=O···H-C interactions trigger the helical conformational transitions described above.Secondly,in view of the important role of interchain interaction in the formation of stereocomplex crystals,a small amide oligomer was added into the racemic blends of poly（lactic acid）,and the effects of the latter on the crystallization behavior and thermo-mechanical properties of poly（lactic acid）were investigated.The results showed that the addition of amide oligomer could achieve the whole stereocomplex crystallization of 390 kg/mol poly（lactic acid）racemic blends.The introduction of amide oligomer greatly increased the crystallization rate of racemic blends in the intermediate temperature region（110～130℃）.During the cooling process of poly（lactic acid）racemic blends,the temperature of crystal nucleus increased first and then decreased with the content of amide oligomer,indicating that amide oligomer enhanced the interaction between enantiomer chains,which was conducive to the nucleation of melt at high temperature,and also hindered the relative slip between enantiomer chains.In addition,the increase of amide oligomer content has a certain plasticizing effect on reducing the glass transition temperature and energy storage mode of poly（lactic acid）racemic blends.Finally,a degradable silk fibroin protein was introduced into the racemic blends of poly（lactic acid）,and the crystallization behavior of the composite system was studied.The results showed that the introduction of silk fibroin protein could achieve the total stereocomplex crystallization of 860 kg/mol poly（lactic acid）racemic blends,and the crystallization rate of the latter was increased by 2 orders of magnitude.