| Liquid crystal is an intermediate state of matter between liquid and crystal.It has the flexibility of liquids and the optical anisotropy of crystals.The liquid crystalline materials that have been found so far are mostly organic,including small molecules and polymers.Various liquid crystalline phases have been found,due to the different ways how molecules are arranged and the different long and short-range order resulted.The simplest ones are nematic,smectic and cholesteric.The nematic,which has only long range 1-d orientational order?e.g.for rod-like molecules all molecules tend to orient in parallel to a certain direction called director?and lacks any other long range orientational or positional order,is one of the most important member of the liquid crystal family and has been widely used in liquid crystal display technology.It was thought previously that there is only one nematic phase.However,more recently it was discovered that some liquid crystal materials?like dimers?demonstrate two nematic phases with a transition between them.While the high temperature nematic phase named N is considered “normal”,the exact structure of the low temperature nematic phase named Nx and its transition to the “normal” nematic phase is still much in debate,with many different models being proposed.This thesis focuses on the ClMBPE polymers containing 1,2-biphenylethane mesogens,and investigates their two nematic phases and the phase transition between them.In this project,liquid crystal polymers with the potential of showing two nematic phases,have been designed.To avoid the complication of crystallization,which happens easily for homopolymers,copolymerization of 1-?methylphenyl?-2-phenylethane and 1-?chlorophenyl?-2-phenylethane has been adopted.And variation of spacer lengthes between mesogens(such as —C5H10—?—C7H14— and —C9H18—)have been used.Subsequently,a series of liquid crystal polymers 1-12 were successfully synthesized.The chemical structure and the molar mass of the synthesized polymers have been investigated using nuclear magnetic resonance?NMR?and gel permeation chromatography?GPC?.Polymers 1,6,7 and 12 with high degree of polymerization and narrow polydispersity index were chosen for the study of their two nematic phases.The mesomorphic properties of the polymers have been measured by differential scanning calorimetry?DSC?,polarizing microscope?POM?,and wide-angle X-ray scattering?WAXS?.From DSC results,it was probably deduced that polymer 1 shows one nematic phase N,and polymers 6,7 and 12 show two nematic phases N and Nx;polymer 6 with —C5H10—,—C7H14— and —C9H18— chains was chosen for further investigation,because of its lower crystallization.From POM figures of polymer 6,it was known that nematic phase N is appeared at 55 ?,and nematic phase Nx exists at room temperature with different texture.WAXS experiment of polymer 6 also gives the information of two nematic phases Nx and N at 25 ? and 50 ?,respectively.Grazing incidence wide-angle X-ray diffraction experiments?GIWAXS?shows the Nx-N transition during heating and the N-Nx transition during recooling.Meanwhile,the splitting of equatorial GIWAXS scattering maximum of the polymer in the Nx phase suggests a tilt of mesogens and indicates a staggered short-range packing of mesogens.From the intensity scans of azimuthal angles,it is suggested that the orientation in the Nx phase is lower than that in the N phase.Furthermore,polarized infra-red spectroscopy has been carried out on oriented samples whose temperatures were controlled by a hot-stage.Order parameter S was calculated.There is a slight increase in the order parameter of the ClMBPE-6 mesogens at N phase when comparing with that at Nx phase,which is consistent with the GIWAXS results.Nx-N phase transition appeared from 40 ? to 45 ? shown by order parameter changes with temperature.This hopefull will shed new lights into and lead to better understanding the structures of the two nematic phases. |
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