Syntheses And Properties Of Two Nematic Liquid Crystalline Molecules

The nematic-nematic phase transition was first confirmed in a series of main chain liquid crystal（LC）“RBPE”copolymers,featuring semi-flexible diphenylethane based mesogens.Originally,the partial flexibility of the diphenylethane segments was thought to be the key property for observing the transition,but since then it has been witnessed frequently in systems of LC dimers with rigid aromatic mesogens.The common factor between these two sample types is the inclusion of flexible oligomethylene spacers,which pair the mesogens at an angle to each other（bent conformation）and comprise an odd number of CH₂ linkers.For the polymer’s low-temperature nematic phase,it has been confirmed that its phase is the same as that of the liquid crystal dimer,both of which are twist-bend nematic phases(ie,N_tb).In order to further study the N_tb-N phase transition of polymers,a novel main chain liquid crystal polymer with triphenylbisethane as mesogen was designed,and the influence of flexible oligomethylene spacers on its liquid crystal behavior was explored.And the objective is to explore what happens when the different substituents of monomeric benzene on their crystallization properties.Also,bistolane-based and triphenylethane dimers and trimers liquid crystal were designed,and the effect of the carbon chain length at different positions on the liquid crystal behavior was explored.In the polymer part of this project,a series of polymer liquid crystals were prepared by copolymerization of triphenylbisethane as the mesogens with dibromine alk.They were MMTPO,MClTPO,MMTPE and MClTPO/MMTPO.Differential scanning calorimetry（DSC）,polarizing microscopy（POM）,and grazing incidence X-ray scattering（GIXS）was revealed that the polymer MMTPO,MClTPO and MClTPO/MMTPO with odd number of carbon spacers show the N_tb-N phase trasition under heating or cooling.The polymer MMTPE with even number of carbon spacers only shows the nematic phase N to isotropic liquid（Iso）phase transition,and its enthalpy change is higher than the odd number of spacer MMTPO polymer.So the based-triphenylbisethane liquid crystal polymers as mesogen also have odd-even effect.When DSC was used to test the stability of the N_tb phase,it was found that about 60%of the MMTPO sample was crystallized after 2 days at room temperature.The MClTPO sample can be kept at room temperature for more than 5 days without crystallizing.While MClTPO/MMTPO was kept stable at room temperature for two days,it had an obvious crystallization.Therefore,the MClTPO was selected to test the modulated differential scanning calorimetry（MDSC）.Compared with the based-diphenylethane polymer ClMBPE,when the number of conformations of the mesogen increased,the N_tb-N phase progress may be changed from a first-order transition to a second-order transition.In the small molecule part of this project,bistolane-based liquid crystal dimers were prepared,which were C1DC7,C1DC8,C5DC7,C5DC8 and C1DC7R.And the trimers TBT and TTT which are mixed with semi-flexible rod-like molecules and rigid rod-like molecules have also been synthesized.The DSC and POM tests showed that the five dimers have only one normal nematic phase N,and the number of CH₂ linkers could affect the phase transition temperature of the nematic phase.For the trimeric TTT,there is only one nematic phase.The trimeric TBT has double peaks on the DSC trace.The temperature of the range is about 10°C.It is presumed that there may be two liquid crystalline phases.The above studies lay a foundation for understanding the effects of carbon-oxygen bonds and carbon-carbon bonds on the conformation of liquid crystal molecules.The third part is concerned the dynamic calorimetry study of the two nematic phase transition.Based on MDSC experiments for the dimers DTC5Cn,the phase transition of N_tb-N was changed from first-order transition to second-order transition,when the spacer length of the compound increases.In the N-Iso phase transition,there are two doublets of C5,C7,and C9 that differ by about 1.5°C.