Synthesis And Characterization Of Monodisperse Lactic Acid

Because of its excellent biocompatibility, mechanical properties, processing properties and biodegradability, poly(lactic acid)(PLA) has been widely used as biomedical, packaging, clothing, agriculture, forestry and food industry etc. However, With the further research of PLA, we are no longer confined to the modification of the basic structure, basic properties and physical chemistry, but is gradually turning to the structure, property, function and internal relationship.PLA is an aliphatic polyester which contains chiral molecules, studies show that lactic acid oligomers can form stable helical structure makes the rotation become very powerful. Due to the strong interaction between the helical chain, High crystallinity of the pure PLA makes it difficult to self-organization. Liquid crystal polymer own self-organization and the stability properties of polymer, so that we can obtained orientation PLA through liquid crystal’s self-organization and lower crystallinity of PLA.In this paper, biphenyl mesogens were synthesized, then L-lactic acid oligomers segments were obtained by ring opening polymerization, and then Monodisperse lactic acid oligomers (DP1-14) were obtained by preparative HPLC.Then the poly lactic acid segment was terminated by esterification. Terminated Monodisperse lactic acid oligomers (DP1-13) were obtained by HPLC agin.Characterizing the structure and molecular weight of monodisperse lactic acid by nuclear magnetic resonance spectroscopy (1HNMR), liquid chromatography-mass spectrometry (LC-MS), indicate that are target products.The differential scanning calorimetry (DSC), polarized optical microscopy (POM), X ray diffraction (XRD) and so on analysis the thermal properties, liquid crystal, phase structure of monodisperse poly lactic acid. The results show the monodisperse poly lactic acid is a thermotropic liquid crystal, thermal properties change with polymerization degree, n≤10monodisperse poly lactic acid melting temperature (Tm) decrease with degree of polymerization, n>11, poly lactic acid appear crystallization peak.The circular dichroism (CD) was used to measure the circular dichroism of monodisperse poly lactic acid. In acetonitrile solution, CD spectra shows a positive single circular dichroism absorptions band around210nm. Monodisperse poly lactic acid rotation capacity change with the degree of polymerization. n<2, monodisperse poly lactic acid optical capability is extremely strong,2<n<5-8, poly lactic acid rotatory power increase with polymerization degree, Less after all, it shows that n=5-8poly lactic acid form stable helical structures.