| Biodegradable materials have become the focus of basic research and industrial production in recent years.PLA is considered as the most potential biodegradable material among many biodegradable materials.However,due to its poor heat resistance,PLA materials are limited in production and application.The study found that in the Poly(L-lactic acid)/ Poly(D-lactic acid)(PLLA/PDLA)blend system can form a stereocomplex crystals(SC)with melting point higher than that of homogeneous crystal(HC)about 50 ℃,so how to obtain high content SC in high molecular weight blend is the key to improve the heat resistant performance of PLA.In this paper,the evolution of SC and HC in PLLA/PDLA blends with high molecular weight was investigated from the perspectives of hydrogen bond,preformed order structure and different constraints of molecular chain,so as to reveal the mechanism of their competitive growth and provide theoretical reference for the preparation of polylactic acid materials with high molecular weight and high SC content.Firstly,the temperature regional distribution of SC and HC in sequence after the rapid cooling of symmetrical PLLA/PDLA blends from a completely random melt to a set temperature was studied,and the role of hydrogen bonds in the crystallization process of PLLA/PDLA was discussed.As a result of combining micro-FTIR and Raman,hydrogen bond is formed when the melt cools at 2.5 K /s,but not formed at 3000 K /s.Annealing the melt without hydrogen bonds at 100 ℃ led to HC emerging first,while annealing the melt with hydrogen bonds resulted in SC emerging first.Secondly,the evolution process of different crystal forms formed in the symmetrical PLLA/PDLA blends with preordered structures under different melting conditions was studied.Compared with the traditional isothermal crystallization kinetics,the existence of preordered structure does promote the crystallization rate of the system.When the partial melting temperature is lower,the system is heterogeneous melt and the crystallization is faster,but with the further increase of partial melting temperature,the system tends to be homogeneous melt and the crystallization slows down.In addition,within a certain range of cooling,the stereocomplexation ordered structure can be used as nucleating agent,and beyond a certain range,it will hinder the further growth of crystals.Finally,a crosslinked system was introduced into PLLA/PDLA blends,focusing on the evolution of different crystal types in PLLA/PDLA blends when the degree of curing of the crosslinked system regulates the limitation of the molecular chain diffusion of PLA.The results showed that the addition of Epoxy resin(EP)to the PLLA/PDLA blend could promote SC crystallization of the blend.When the ratio of EP to the blend was 20%,the SC crystallization content in the blend was the highest.Secondly,when PLLA/PDLA/EP/ Diethylenetriamine(DETA)composite is added with DETA: EP =10%,phase separation will occur in the blend system.When DETA: EP= 5%,the best crosslinked structure appears in the blend system,which maximally limits the movement and diffusion ability of PLLA and PDLA molecular chain and promotes the formation of SC crystallization of the blend.In addition,the evenly dispersed EP cross-linked structure can promote the formation of SC crystals in the subsequent PLLA/PDLA/EP/DETA composites,namely the existence of "memory effect". |
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