| Poly(L-lactic acid)(PLLA)is a thermoplastic aliphatic polyester that is prepared from renewable resources and is therefore increasing in importance in terms of applications,with the potential to replace traditional petroleum-based polymer materials in the future.Polylactic acid is also a compostable and biocompatible/biodegradable polymer that can be used in single-use products,biomedicine,packaging films,automotive industry,electronic equipment components and other fields.So among the various polymers produced from renewable resources,PLLA has received the most attention.In this study,a series of polylactic acids were prepared by direct polycondensation method by synthesizing a new titanium-based catalyst N6TS,and a series of polylactic acid were prepared by direct polycondensation method,and the effects of different polymerization reaction conditions on the molecular weight,yield and color of polylactic acid products were studied,and the crystallization behavior of synthetic products was explored.Then,polylactic acid enantiomeric composites were prepared by solution blending,and the effects of different optical purity,molecular weight and blending ratio of PLLA on the crystallization properties of stereotactic composites were investigated.The conclusions of the study are as follows:(1)Research on the process of synthesizing polylactic acid from two-component catalysts:In this paper,it was found for the first time that the two-component catalytic system composed of N6TS catalyst and stannous chloride could successfully catalyze the condensation and polymerization of lactic acid to polylactic acid.The effects of different reaction conditions(temperature,time and catalyst dosage)in the prepolymerization and polycondensation stages on the molecular weight,yield and color of polylactic acid were investigated.The experimental results show that the prepolymerization temperature is 165℃,the addition of catalyst N6TS 0.5 wt%should be 4 h,and then the input amount of tin dichloride dihydrate of the compound catalyst is 0.3 wt%of the mass fraction of lactic acid raw material,and the reaction is 8 h at 185℃,which is the best synthesis condition.Under this experimental parameter,a target product with a molecular weight of more than 30,000 was synthesized.Compared with the traditional tin dichloride catalyst with high catalytic activity,although the molecular weight improvement effect of the titanium-tin dual catalytic system is not significant,the yellowing and blackening phenomenon of the product is significantly improved,and the color is purer.The product structure was characterized by FIIR,1H NMR,and 13C NMR.(2)Optimization of polymerization process of three-component titanium-tin compound catalyst and study on crystallization properties of polylactic acid:Using the improved melt polycondensation process,the N6TS/stannous chloride/stannous octanoate three-way catalytic system was constructed for the first time,and the molecular weight of more than 40,000,the yield was as high as 73%,and the polylactic acid with slight discoloration was successfully synthesized.The effects of polycondensation time,temperature and catalyst dosage on polymerization were studied,and the results showed that the addition of stannous octanoate could effectively inhibit the side reactions and increase the molecular weight of poly lactic acid.The optimal synthesis parameters were determined:the mass fraction ratio of tin catalysts was 1:1,the addition amount was 0.3 wt of lactic acid raw materials,the polycondensation temperature was 185℃,and the polycondensation time was 8 h.The crystallization behavior of the synthetic products was characterized by differential scanning calorimetry(DSC)and polarizing light microscopy(POM),and the results showed that the polylactic acid prepared by melt polycondensation method was semi-crystalline polylactic acid,and long-term hightemperature polymerization would cause the polylactic acid to racemic into an amorphous product.During isothermal cold crystallization,the crystallization peak was obvious and the crystallization rate was fast,and double melting peaks appeared during the heating sweep.The crystallization process and morphology of polylactic acid were observed by polarizing microscopy,and it was found that polylactic acid was spherical in morphology,with the fastest crystallization rate at 90℃,which was consistent with the results of DSC analysis.(3)Study on crystallization properties of stereoscopic composites:In this paper,PLLA/PDLA binary blends with different blended mass ratios were prepared by solution blending,and the effects of different optical purity and molecular weight on the crystallization properties of stereoscopic composites were systematically studied.The optical rotation of PL A raw materials was qualitatively and quantitatively analyzed by 1H NMR,13C NMR and polarimeter.DSC and XRD characterization methods were used to study the changes of crystallization properties of pure polylactic acid and PLLA/PDLA blends.The results show that for pure polylactic acid,the larger the molecular weight of the product,the higher the melting point.The greater the optical purity,the higher the melting point,the greater the crystallinity,and the sharper the characteristic absorption peak in the XRD diagram.In different PLLA/PDLA blends,the optical purity of polylactic acid raw materials has a direct impact on the crystallization performance of the blend,and the addition of PDLA with high optical purity can effectively improve the crystallinity,and the melting point is increased by about 50℃ compared with the homopolymer.The greater the optical purity of the raw material,the higher the melting point of the prepared stereoscopic composite,and it is easier to form stereoscopic composite crystals(SC)with high crystallinity.Compared with high molecular weight PLLA,PLLA/PDLA blends prepared with low molecular weight PLLA are easier to form SC,and their cold crystallization peaks are obvious and SC crystallization is more perfect.PLLA/PDLA blends with similar molecular weight and high optical purity have high melting points,and PLLA/PDLA blends with a blending ratio of 5:5 have the highest crystallinity. |
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