Synthesis And Properties Of Liquid-Crystal Helical Polylactic Acid Copolymers With Huge Optical Rotatory

In recent years, biodegradable polymers have become an important research topic in the field of polymer. Among all of the biodegradable materials, polylactic acid and copolymers, due to their unique properties and excellent machinability, have attracted much attention.Chirality is of great significance in life science and material science, and organic chiral molecules are important components of the optical function materials. Polylactic acid can form chiral helical structure with the thermodynamic and kinetic stability because of large numbers of chiral carbon atoms in main chain. While helical structure is one of the basic chiral structures of polymer, which can not only give the polymer huge optical rotatory power along the helix axis but also affect the stability of polymer optical activity closely related to the stable conformation of polymer helical structure. Moreover, spiral electronic track can be modulated by modulating polymer helical structure with applying electric field, so that helical polymer is expected to play an important role in electro-optical modulation as optical elements and optical switching devices.Liquid crystalline polymer is unique in terms of the combination of molecular self-organization of the liquid crystal and the stability of polymer. It assumes better mechanical and physical properties than other structural materials. Therefore, liquid crystalline polymer has enormous prospects in the field of high technology as a new category of high-performance materials. High crystallinity of polylactide reduces its self-organization ability, so it is diffcult to orient the molecules to prepare excellent optical devices with excellent molecular orientation. Hence by attaching with liquid crystal groups polylactic acid molecules can be help to be more ordered, and this could achieve its potential perfect optical properties.Herein, a series of oligo polylactic acid with different repeat units and narrow polydispersity were obtained. First, a series of rigid mesogens containing active hydroxyl were synthesized, then reacted with lactide as simply alcohols directly as coinitiators and Sn(Oct)₂ as initiator. The characterization of the molecular structure of synthesized copolymers by ¹H NMR and Uv-Vis spectroscopy testified the molecular structure of the objectives. And results also showed coinitiators have direct covalent link with polylactide segments.Gel permeation chromatography (GPC), ¹H NMR and Uv-vis spectroscopy were used to characterize the molecular weight and molecular weight distribution. Results indicated that molecular weight increased with the increase of feed ratio and molecular weight distribution was narrow around 1.2. All the abovementioned methods had good reproducibility. GPC results also showed active alcoholic hydroxyl group exhibited better initiating activity than phenolic hydroxyl group, and the polymerization reached an equilibrium in 10 hours at about 100°C with the constancy of the molecular weight of PLA.Polarized optical microscope (POM), differential scanning calorimetry (DSC), small angle X-ray diffraction (SAXD) and wide angle X-ray diffraction (WAXD) were used to study the optical texture of liquid crystal, thermodynamic properties and molecular ordering on the micro-level of mesogens and polylactic acid copolymers. And the liquid crystal phase behavior of the compounds was studied.POM and DSC results indicated that mesogens containing cyclohexyl-phenyl form smetic liquid crystals, while mesogens containing azebenzene, such as 4OPNPO6OH, 6OPNPO6OH, 8OPNPO6OH, 4OPNPO3OH, were nematic liquid crystals. But mesogens with biphenyl or azebenzene (6OPNPO3OH, 8OPNPO3OH, HO3PNP3OH, HO6PNP6OH) could not display thermotropic liquid crystallinities.All the polylactide copolymers showed thermotropic liquid crystalline properties. The copolymers initiated with several aliphatic alcohols formed thermotropic nematic liquid crystals, which was one of the important discoveries. The copolymers containing mesogens including cholesterol, cyclohexyl-phenyl, biphenyl and azebenzene present thermotropic semetic liquid crystalline behaviors, and the mesogens made polylactide molecules orient ordered. The liquid crystallinity of copolymers was affected by the mesogens and the polylactide fragments. Humidity may have influence in some extent on the self-assembly of polylactic copolymers.DSC study on copolymers indicated, with increasing number of repeat units of polylactide, the glass transition temperature, melting temperature and clearing temperature increased. But liquid crystal temperature range also decreased gradually.Circular dichroism spectroscopy (CD) was used to measure the circular dichroism of polylactic acid solution, whose optical rotatory was related to degree of polymerization (n). The helical structure of polylactic acid had been formed, and its mean molar residual ellipticity reached to the maximum at n=25-40.The optical rotatory of polylactic acid thin solid films was investigated by circular dichroism spectroscopy. Polylactic acid solid films were transparent, and were also homogeneous under microscope. The CD spectra of these films had a single circular dichroism absorption band at 226 nm. Compared with polylactide solution (210nm), the maximal absorption wavelength of CD spectra was about 16 nm red shift. And optical rotatory power of their casting films poled and oriented with high voltage electric field increased rapidly, indicating that the molecular alignment was more ordered. More detailed research is still going on.