| Poly(lactic acid)or polylactide (PLA) was widely used in the fields of tissue engineering, carrier materials, drug controlled release and environment protection due to its biocompatibility and biodegradability. Lactide, which was synthesized through lactic acid condensation, was polymerized to form PLA by ring-opened of lactide. However, the yield of lactide wasn't satisfical, and hydrophilicity of PLA was so bad that it leaded to low degradation rate, these shortcomings limited its use in some biomedical fields. To extend appliaction of PLA, attention must be paid to improve the yield of lactide and the hydrophilicity of PLA.To improve the yield of D,L-lactide, inorganic salt as diluents was added to the reaction system in the time of depolymerization of oligomerization formed by lactic acid condensation, and several kinds of inorganic salts that might improve yield of D,L-lactide were tested. The optimization were performed with inorganic salt as diluents,and the optimizd process of D,L-lactide synthesis was obtained. In addition, the synergistic effect of catalysts were studied by incorporating catalysts available, and the structure and purity of the D,L-lactide were characterized and assayed, especially residues of diluent used (potassium dihydrogen phosphate, PDP) was qualitatively analyzed. Finally, the mixture of PDLLA and ethyle cellulose(EC)was prepared and characterized with IR and DSC.The results in the experiments showed that the yield of D,L-lactide was significantly improved from 25.56±0.91% to 36.48±1.08% with the help of inorganic PDP diluents,which suggested that diluents was an effective way to enhance yield of D,L-lactide. The optimized process of D,L-lactide synthesis was obtained after optimization experiments and variance analysis were perfomed, most importantly, the inorganic salt diluent of PDP was the most significant factor to yield of lactide. A variety of catalysts and their incorporation was tested under the optimization process of D,L-lactide synthesis, the highest yield of D,L-lactide was up to 42% ,while Sn(Oct)2 incorporatin with CdO was used as catalyst in the reaction system. There was no difference to the structure of D,L-lactide identified by IR, and the purity of D,L-lactide was 99.47%, there was only0.0013% of residual PDP in final product, implied that the PDP didn't influence on the purity as well as the structure of D,L-lactide. The PLA, which was identified by IR, was polymerized by ring-opened of lactide and the molecular weight was bout 76,000, it meet the requirements of carrier materials for drug controlled release. Modification of PDLLA was conducted by mixing with EC, the results showed that the degradation rate of mixture was the most fast when the content of EC was 70%-80%, suggested that the hydrophilicity of PDLLA was improved after PDLLA was modified by EC. The IR assay of PDLLA/EC mixture showed the position and strength of peaks of PDLLA/EC mixture have been changed in comparison with EC, those changes might result from the hydrogen bondings between two materials. The Tg of the mixture was 180.29℃when DSC assay was performed, indicated that a macro homogenous phase was form in the mixture of PDLLA and EC, and the compatibility between PDLLA and EC is good. Dispersion peaks in XRD results showed that the integrality of crystal of PDLLA or EC in mixture decreased, amorphous structure might be formed in PDLLA/EC mixture.In general, the yield of D,L-lactide had been improved by salt diluent and the synergestic effect of catalysts. There were compatibility between PDLLA and EC, and PDLLA properties and structure might be changed by mixing with EC. The mixture might have potential application in amphiphilic drug controlled release. |
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