Investigations of molecular orientational orderings in ferro-, antiferro-, and ferri-electric liquid crystals by polarized Raman scattering Investigations of Molecular Orientational Orderings in Ferro-,Antiferro-,and Ferri-Electric Liquid Crystals by Polarized Raman Scattering
Investigations of molecular orientational orderings in ferro-, antiferro-, and ferri-electric liquid crystals by polarized Raman scattering
Investigations of Molecular Orientational Orderings in Ferro-,Antiferro-,and Ferri-Electric Liquid Crystals by Polarized Raman Scattering
The polarized vibrational Raman scattering measurement has some distinguished advantages. First, The vibrational Raman scattering line is accompanied by the vibrational normal coordinate of a molecule so the polarized signal of the specific Raman line gives the orientational information about the local structure of a molecule. Secondly, The Raman scattering gives not only the second-order parameter ＜P<SUB>2</SUB>(cosβ)＞ but also the fourth-order one ＜P<SUB>4</SUB>(cosβ)＞. The ＜P<SUB>4</SUB>(cosβ)＞ exhibits a larger sensitivity than ＜P<SUB>2</SUB>(cosβ)＞, so the polarized Raman scattering measurement is much effective in investigating the subtle change or difference in the molecular orientational distribution of the liquid crystal than the other method. The ferro-, ferri-, and antiferro-electricity are found in Smectic C (SmC) and the variant phases. These phases have layered structures. The molecule tilts in a layer with respect to the layer. The c-director is defined as the unit vector pointing the direction that is parallel to the projection of the molecular long axis on the smectic layer. When the c-directors in the adjacent layers are oriented to the same direction, this molecular arrangement is called a synclinic molecular arrangement. Meanwhile, when the c-directors in the adjacent layers are oriented to the opposite direction, this molecular arrangement is called an anticlinic molecular arrangement. The combination of synclinic and anticlinic arrangements characterizes the SmC's variant phase. The thermal agitation triggers the change of the combination and leads the phase transition within the SmC's variant phase. The change of the combination caused by the distribution of the c-director alters the biaxiality in the phase. The observation of the biaxiality by the vibrational polarized Raman scattering can trace the mechanism of the phase transition. The hindered molecular rotation about its long axis increases the interlayer molecular interaction. The interlayer molecular interaction causes the ferroelectric synclinic and antiferroelectric anticlinic molecular orderings. The frustration and the competition between the synclinic and anticlinic molecular orderings bring about many interesting phenomena in the ferro-, antiferro-, and ferri-electric phase. This thesis contains six chapters. In Chapter 1, the basic concept of this thesis is described. The procedure how the orientational order parameters are obtained from the polarized Raman intensities is described in Chapter 2. The improved analysis of the polarized Raman scattering provides more precise order parameters than those ever reported. The order parameters can be obtained even from the small Raman intensities in the time-resolved measurement during the electro-optic response of a thin sample cell (～1μm). In Chapter 3, the analysis is applied to the system of MHPOBC in the thick homogeneous alignment cell [published in Phys. Rev. E, 63, 02176 (2001)]. The close relation of a biaxial molecular ordering to the successive transition between the SmC* variant phases is investigated. An unusual change of the orientational order parameters was observed with decrease in temperature. It was concluded that this irregular variation of the order parameter stemmed from the biaxiality of the molecular orientational distribution, which was attributed to the hindrance of the molecular rotation about its long axis. This result suggests that the growth of a degree of the hindrance as the temperature decreases is closely related to the appearance and the transitions of the phases because the hindered molecular rotation incrceases the interlayer molecular interactions. The mcolecular reorienation induced by an external electric filed in SmC<SUB>A</SUB>* phase is studied in Chapter 4 [Jpn. J. Appl. Phys., in press (2002)]. The reorientation can be described with the electric coherence length which relates to the interlayer molecular interaction, the spontaneous polarization, and the field strength. The theoretical model proposed before is verified. The electric coherence length estimated from the experimental results was much larger than the theoretical prediction. This suggests the necessities of the more precise theoretical description of the interlayer interachion and the dependence of the effective spontaneous polarization on the phase. ln Chapter 5, the molecular orientational distributions of two types of the liquid crystal showing V-shaped switching are examined [published in Phys. Rev. Lett., 87, 015701 (2001) and Phys. Rev. E, 65, 041714 (2002)]. The 'random' and 'collective' models have a difference in the molecular distribution during the switching as described before. The difference can be detected by the improved analysis of the polarized Raman scattering. The process of the molecular reorientation in the V-shaped switching is discussed in terms of the interlayer molecular interaction. The interlayer molecular interaction is evalualed by the dependence of the c-director distribution on the dc electric field strength. The obtained distribution of the c-director at the tip of the V was considerably broad for one liquid crystal, while it is narrow in the other liquid crystal. These differences have been explained, mainly, by the barrier between the ferroelectric synclinic and antiferroelectric anticlinic orderings. The summary is given in Chapter 6.