| A series of poly(L-lactide acid)(PLLA)and poly(D-lactide acid)(PDLA)prepolymers with different molecular weights were synthesized via direct melt polycondensation.To the preparation of polylactic acid stereocomplex(sc-PLA),those prepolymers with similar relative molecular weights were blended through solution in different mass compositions.Finally,solid-state polycondensation(SSP)was conducted to increase molecular weights to a higher level.The main contents were as follows:(1)Studied the molecular weight on the effect of the basic thermodynamic behavior,crystallization behavior and crystal structure of PLLA.The results showed that,with the molecular weight increasing,the glass transition temperature(Tg)and melting temperature(Tm)of PLLA shifted to higher temperature,and emerged melting doublet;during the 130 ?isothermal crystallization process,with the molecular weight decreasing,the crystallization rate of PLLA was faster and the grain size was larger;the concentric cracks and banded spherulites can be observed in the PLLA spherulite morphology maps,its crystal structure steadily existed in a crystal form.(2)Studied thermodynamic behavior,crystal structure,crystallization morphology and thermal stability of the prepared sc-PLA with different blending ratios.The Tm of sc-PLA was 50? higher than PLLA or PDLA,and the Tm was strongly dependent on the contents of sc structure.Sc crystal as the nucleating agent can promote the crystallization of PLA.However,the presence of sc structure cannot retard the thermal degradation of PLA obviously.(3)Explored process conditions of SSP which start from sc-PLA preliminary,it was found that the SSP can increase the molecular weight of sc-PLA,which was realized mainly through the the chain growth of PLA homopolymer parts.The diffraction peak intensity and Tm of the SSP products were lower than before,and with the proportions of PLLA increasing,the initial thermal degradation temperature(Td,5%)of sc-PLA was higher,up to 270?. |
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