| Global awareness of material sustainability has increased the demand for bio-based polymers like poly(lactic acid),which are seen as a desirable alternative to fossil-based polymers because they have less environmental impact.Poly(lactic acid)(PLA)has emerged as a green and environmentally friendly polymer material for applications in daily life on account of its properties such as renewable raw materials,biodegradability and good biocompatibility.However,in practical applications,PLA's application is limited due to its high processing temperature,it would cause hydrolytic degradation,oxidative degradation,and poor viscosity characteristics.Therefore,reducing the processing temperature of PLA is of great significance.The aim of this work is to deveop PLA with reduced cost and low processing temperauter.To realize this aim,new processes to synthesize and purify lactide were developed to increase the yield of lactide.The copolymerization of meso-lactide(meso-LA)and rac-lactide(D,L-LA)was further investigated and the thermal processability and mechanical properties o f the formed new copolymer were examined.The main research contents and conclusions are as follows:(1)Ethylene glycol(EG)as molecular weight control agent was added into D,L-lactic acid to synthesize lactide.The effects of the amount of EG on the molecular weight of lactic acid oligomers and on the yield of crude lactide were investigated.The results from ~1H NMR spectra showed that EG can control the molecular weight of lactic acid oligomers during the polycondensation stage of lactic acid,with the molecular weight increasing first and then decreasing with the increase of EG dosage.Addition of EG can significantly reduces the carbonization of oligomers and leaves less residuals in the reactor,thereby the yield of crude lactide increases.The molar ratio of D,L-lactic acid to EG as 40:1 gives the highest yield of lactide(95.01%)and the lowest ratio o f residuals(4.0%).(2)Two methods including vacuum distillation and isopropanol-based recrystallization were investigated to purify lactide.The results from ~1H NMR spectra showed these two technologies can both retain meso-LA,a kind of lactide generally accompanied with D,L-LA formation.The vacuum distillation method can recover lactide of 76.2%yet the lactide contains residual lactic acid,while isopropanol-based recrystallization method gives a lactide yield of 68.6%with no residual lactic acid.The resulting yields of lactide are both significantly higher that from traditional ethyl acetate-based recrystallization,the latter of which cannot recover meso-LA.(3)A melt polymeration technology conducted at a lower temperature first and then a higher temperature was investigated to copolymerize meso-LA and D,L-LA.An optimal polymerization process was identified as:70°C for 7 h and then 140°C for 14 h.The effects of meso-LA content on the chemical structure of the resulting copolymer(meso-co-D,L-PLA)was further examined by using ~1H NMR.The results indicate that the copolymer is a diblock copolymer,with one block as meso-PLA and the other as pure PDLLA when meso-LA content is 3%?42%or as PDLLA containing meso-LA units when meso-LA content is 52%?74%.(4)To demonstrate the thermal processability of meso-co-D,L-PLA,the melt flow rate(MFR)was detected.The MFR of meso-co-D,L-PLA was significantly higher than that of PDLLA.The finding confirmed that the copolymerization of meso-LA and D,L-LA improves the melt fluidity of the obtained PLA.Further,meso-co-D,L-PLA was found to be be well extruded and injected at 100°C?105°C,which is?50°C lower than that for PDLLA.Moreover,meso-co-D,L-PLA has comparable tensile mechanical properties to PDLLA.All together,this work has successfully increased the yield of crude lactide and the recovery ratio of the purified lactide.The prepared meso-co-D,L-PLA copolymer integrates advantages of low-temperature proccessability and good mechanical properties.These outcomes definitely help to reduce the cost of PLA and promote and broaden the applications of PLA. |
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