X-ray and Raman scattering study of orientational order in nematic and heliconical nematic liquid crystals (original) (raw)
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Raman Scattering Study of Phase Biaxiality in a Thermotropic Bent-Core Nematic Liquid Crystal
Physical Review Letters, 2010
Polarized Raman spectroscopy was used to investigate the development of orientational order and the degree of phase biaxiality in a bent-core mesogenic system. The values of the uniaxial order parameters hP 200 i and hP 400 i, and biaxial order parameters hP 220 i, hP 420 i, and hP 440 i, and their evolution with temperature were determined. The temperature dependence of almost all order parameters reveals a second order transition from the uniaxial to biaxial nematic phase with hP 220 i increasing to $0:22 before a first order transition to the smectic-C phase, upon cooling.
Physical Review E, 2010
The orientational order parameters in liquid crystals have been a subject of interest during the last decades since they quantify the degree of intrinsic long range orientational order, which is characteristic of all liquid crystal phases. In the earliest attempts to explain the anisotropic properties of nematic liquid crystal several assumptions had to be made, for instance, rigid rod-like molecular structure and uniaxiality of the nematic phase . The theoretical efforts for modeling a nematic phase led to an anisotropic orientational distribution function (ODF), f(T). The ODF is expanded in terms of Legendre polynomials and their expansion coefficients may be viewed as the orientational order parameters. The first two non-trivial orientational order parameters, <P 2 > and <P 4 >, are of particular interest since they can be measured experimentally. As a matter of fact, the molecular field theories such as Maier-Saupe (MS) or Humphries-James-Luckhurst (HJL) impose conditions in the possible behavior of these order parameters . Several experimental techniques have been developed for obtaining reliable values of <P 2 >. However, only few of them can also determine the value of <P 4 > [4-14], Raman spectroscopy and X-ray diffraction are among these methods. Measurements of vibrational Raman depolarization ratio in different orthogonal linear polarization conditions is a traditional method to find the order parameters <P 2 > and <P 4 > in liquid crystals . In their pioneering work, Jen et al. laid the foundations to determine simultaneously <P 2 > and <P 4 > by Raman spectroscopy in a backscattering configuration . Even though the values for <P 2 > with this method were in agreement with previous results derived by several other methods , the values for <P 4 > were rather in disagreement with those results and beyond experimental uncertainty. In fact, <P 4 > was unexpectedly negative in many cases . Several attempts were made trying to explain these results [9]; however, there could not be found a convincing argument. Other more reasonable ideas regarding internal field correction were suggested . Although these corrections improved the results of <P 4 >, they were not enough to fit the values in any of the molecular field theories. Moreover, the values for <P 4 > were still lower than those predicted by the Maier-Saupe model, suggesting a theoretical overestimation in this model. Therefore, there was a deep mystery concerning the value of <P 4 > and its sign anytime it was determined by the Raman technique. In recent years, Jones et al. extended the experimental work done by Jen et al and developed a theoretical method to determine <P 2 > and <P 4 > by a fitting procedure . Basically, a detailed intensity profile over a particular angular distribution of the scattered Raman signal and under two different polarization conditions is measured. Thus, the order parameters and the differential polarizability ratio r can be obtained from a simultaneous fitting procedure of such intensities profiles, I ZZ and I YZ . The main advantage of this method is that only one planar cell is required unlike the method by Jen et al., where three different alignment geometries are required. On the other hand, X-ray as mentioned above is another conventional method to obtain <P 2 > and <P 4 > . However, X-ray and Raman experiments probe features of the sample differently. On one hand, the X-ray diffraction pattern arises because of the interaction between the X-ray beam and the electron density distribution inside the sample; on the other
Thermotropic biaxial nematic order parameters and phase transitions deduced by Raman scattering
EPL (Europhysics Letters), 2008
PACS 61.30.-v-Liquid crystals PACS 61.30.Gd-Orientational order of liquid crystals; electric and magnetic field effects on order PACS 78.30.-j-Infrared and Raman spectra Abstract-Raman Scattering was used to investigate biaxiality in the nematic phase formed by the bent-core material, C5-Ph-ODBP-Ph-OC12. Linearly polarised light was normally incident on a homogeneously aligned sample, and the depolarisation ratio was measured over a 360 • rotation of the incident polarisation for the Raman-active phenyl stretching mode. By modeling the bent-core structure and fitting to the depolarisation data, both the uniaxial (P200 and P400) and biaxial (P220 , P420 and P440) order parameters, are deduced. We show unequivocally the presence of a uniaxial to biaxial nematic phase transition approximately 30 • C above the underlying smectic phase. Further, we report the temperature evolution of the biaxial and uniaxial order parameters, which increase in magnitude continuously with reducing temperature, reaching values of 0.1, −0.15 and −0.18 for P220 , P420 and P440 , respectively.
Physical Review E, 2011
The liquid-crystal dimer 1 ,7-bis(4-cyanobiphenyl-4-yl)heptane (CB7CB) exhibits two liquid-crystalline mesophases on cooling from the isotropic phase. The high-temperature phase is nematic; the identification and characterization of the other liquid-crystal phase is reported in this paper. It is concluded that the low-temperature mesophase of CB7CB is a new type of uniaxial nematic phase having a nonuniform director distribution composed of twist-bend deformations. The techniques of small-angle x-ray scattering, modulated differential scanning calorimetry, and dielectric spectroscopy have been applied to establish the nature of the nematic-nematic phase transition and the structural features of the twist-bend nematic phase. In addition, magnetic resonance studies (electron-spin resonance and 2 H nuclear magnetic resonance) have been used to investigate the orientational order and director distribution in the liquid-crystalline phases of CB7CB. The synthesis of a specifically deuterated sample of CB7CB is reported, and measurements showed a bifurcation of the quadrupolar splitting on entering the low-temperature mesophase from the high-temperature nematic phase. This splitting could be interpreted in terms of the chirality of the twist-bend structure of the director. Calculations using an atomistic model and the surface interaction potential with Monte Carlo sampling have been carried out to determine the conformational distribution and predict dielectric and elastic properties in the nematic phase. The former are in agreement with experimental measurements, while the latter are consistent with the formation of a twist-bend nematic phase.
Liquid Crystals, 2010
The observation and quantification of the biaxiality in the nematic phase of thermotropic liquid crystals is one of the topical problems in the science of liquid crystals. The biaxiality in the refractive index and/or relative permittivity, both constituting tensors of the second rank, are expressed in terms of the four scalar order parameters S, D, P and C and the relevant molecular quantities. In this paper we review the theory of determining these order parameters using polarised infrared spectroscopy and present the method that we have developed in obtaining results for the order parameters of tetrapodes (with symmetrical and asymmetrical mesogens) and tripodes with symmetrical mesogens. In the tetrapodes, four mesogens are linked through the siloxane chains to the central Si atom. In the tripode, three mesogens are linked to a benzene ring via the oxygen atom through the siloxane spacers. In the first case a platelet-like structure is formed where the major director corresponds to the highest refractive index. A disc-like structure is formed for a tetrapode with symmetrical mesogens and for a tripode. The order parameters are calculated in the framework of the quasi-flat platelets. For the discotic-like structure, the major director corresponds to the lowest of the three refractive indices. For the three cases, the compounds are shown to exhibit a biaxial nematic phase. Biaxiality in the refractive indices is expressed in terms of the biaxial order parameters. A comparison of the results on the IN U and N U-N B transitions for tetrapodes compare favourably with the predictions from the mean field models of de Gennes, Virga and his co-workers.
Orientational ordering of small molecules in nematic liquid crystals
2004
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Enhancement of the orientational order parameter of nematic liquid crystals in thin cells
The European Physical Journal E, 2004
We report measurements of birefringence (∆n) of several nematic liquid crystals having transverse as well as longitudinal dipole moments in thin (1.4 to 2.3 µm) and thick (7 to 16 µm) cells. Rubbed polyimide-coated glass plates are used to get planar alignment of the nematic director in these cells. We find significant enhancement (6 to 18%) of ∆n (∝ S, where S is the orientational order parameter) in thin cells in all compounds with aromatic cores even at temperatures far (∼ 20 • C) below the nematic-isotropic transition point. The enhancement is larger in compounds having several phenyl rings and lower if the number of phenyl rings is reduced. In a compound that does not have an aromatic core no significant enhancement is observed, implying that the strength of the surface potential depends on the aromaticity of the cores. Assuming a perfect orientational order at the surface, calculations based on the Landau-de Gennes theory show that the thickness averaged enhancement of S is sharply reduced as the temperature is lowered in the nematic phase. The measured order parameter S is further enhanced in thin cells because of the stiffening of the elastic constant which reduces the thermal fluctuations of the nematic director. The combined effect is however too small at low temperatures to account for the experimental data.