Theoretical Calculation On Synthesis Of Polylactide And Studies On Miscibility Of Its Blends

The synthesis mechanism of polylactide from lactic acid by indirect method was studied with density functional theory. And the mixed performances of biomaterial polylactides were investigated with blending and plasticizing.Theoretical studies on synthesis process of lactide were carried out with the density functional theory. The geometries of compounds which maybe appear in reactive system were optimized. Two new kinds of lactide were firstly obtained, which we named WL-lactideαandβ, and they are isomer. On the basis of frequency calculation, meso-lactide is unstable. According to IRC (intrinsic reaction coordinate) analysis, meso-lactide is the transition state which connects the two kinds of WL-lactide, and the potential barrier is about 8.11kJ/mol. The changes in enthalpy and Gibbs free energy were computed at B3LYP/6-31G* and B3LYP/6-311+G* levels of theory, suggesting that the selected model reactions are endothermal and approximately reversible. The results indicated that control on water in the reaction system is very important to the formation of lactide. Lactide was also synthesized experimentally and infrared spectra were taken in different periods of reactive system. Combined the results of quantum computation with infrared spectra, the possible synthesis mechanism was proposed.The ring opening polymerization of meso-lactide and L-lactide with Al(OH)3 were studied by density functional theory. There were three reaction intermediate and two kind transition state in meso-lactide with Al(OH)3 and L-lactide with Al(OH)3 system each. All geometries were optimized, and their frequencies, intensities and dipole moment were computed. The data of thermal energy, thermal enthalpy and Gibbs free energy were obtained from energy calculation. The thermal energy and thermal enthalpy change tendency of meso-lactide with Al(OH)3 system was similar with L-lactide with Al(OH)3 system. But ring opening polymerization of meso-lactide would be more difficult than L-lactide by judging from Ea. Removing the influence of free molecule in reaction system, the way of polymerization could consider as a circulation process. First lactide was inserted coordinate with metal, new circle was formed by ester exchange, and then circle was separate to chain, finally the metal in chain action with other lactide.Blends of poly(D,L-lactide)/poly(vinyl alcohol) (PDLLA/PVA), poly(D,L-lactide) /poly(ethylene glycol)300 (PDLLA/PEG300) and poly(D,L-lactide)/poly(ε-caprolactone) (PDLLA/PCL), obtained by solvent-casting, have been studied by differential scanning calorimetry (DSC) and Fourier transform infrared spectroscopy (FT-IR). DSC results showed that PDLLA/PVA and PDLLA/PEG300 were miscible in whole composition range and PDLLA/PCL blends were poor miscible. The influences of hydrogen bonding and mainly polymer chain were studied, the mixed temperature and solvent of the blends were also discussed in this study. With the increase of PEG300, the plasticizing agent role became obvious in PDLLA/PEG300. Compared with PEG300, the plasticization of ionic liquid using in polylactides was investigated. TGA results show that thermostability of PLA blends with PEG300 changed to disappointing due to heating decomposition of PEG300 first to PLA. And the thermostability of PLAs with ionic liquid were enhanced because heating decomposition temperature of ionic liquid was higher than PLA. Tg of PLAs with ionic liquid reduced effectively, and the results of PLLA and PDLLA were quite. The materials with plasticization were studied with DSC and polarizing microscope. The segment of PLA was more active due to plasticizing, and the crystallization property was enhanced with increasing of plasticization.