Local Refractive Index Measurement on a Cholesteric Liquid Crystal Using the Surface Plasmon Technique

Abstract

The surface plasmon technique is used to measure the local refractive indices in the quasi-nematic layers of a cholesteric liquid crystal with an accuracy of about 10^?4. The local linear birefringence reflecting the orientational order of molecules in the quasi-nematic layers shows a temperature dependence characteristic of a nematic and in good agreement with the theory of nematic ordering.     

Temperature variation of effective geometry parameter, αg,refractive indices and the determination of orientational orders in three room temperature nematic liquid crystal mixtures

Orientational order parameter, S and effective geometry, α_g are calculated from the refractive index in three nematic room temperature liquid crystal mixtures, blOO3, blOO6, and blO38. The method due to Kuczynski et al. used to determine S from birefringence. The effective geometry parameter is α_g = n_o/n_e. (n_o and n_e are the ordinary and extraordinary refractive indices of the liquid crystal material.) The behavior of α_g as a function of temperature and order parameter has also been reported. The S value calculated from α_g is identically equal to the value obtained from Kuczynski et al method, i.e., from birefringence, δn. Finally, dn_e/dT and dn_o/dT are obtained and the crossover temperatures, T_CO the characteristic temperature, which is in fact the temperature of minimum in the ordinary index of refraction in three mixtures for different wavelengths are calculated from dn_o/dT.     

Multispectral correlations technique for finding phase transition temperatures in 7O.Om series

Transition temperatures in N-(p-n-heptyloxybenzylidene)-p-n-alkyloxy anilines, 7O.Om liquid crystalline compounds are computed from optical textures using Multispectral Correlations Technique in image processing. The statistical parameters like mean, standard deviation and entropy of textures as a function of temperature are computed from the image processing technique. These results show abrupt changes as a function of temperature, indicating the phase transition of the 7O.Om samples. The MATLAB simulation results are in good agreement with DSC results and the proposed methodology is one of the techniques in giving reliable and suitable technique to identify transition temperatures at low cost.     

Measurement of the An isotropic Refractive Indices of Polybenzylglutamate Liquid Crystals. Molecular Factors and Dispersion

The ordinary and extraordinary refractive indices have been measured for lyotropic liquid crystal solutions of poly-γ-benzyl-L-glutamate. The onset of the liquid crystal formation is noted as an abrupt jump in the sample birefringence in the biphasic region. The concentration dependence is otherwise unremarkable. The polymer is found to add a small, anisotropic increment to the refractivity of the supporting solvent. Both n _o and n _e can thus be measured in an ordinary Abbé refractometer for the low refractive solvents common to these liquid crystal solutions. The optical birefringence does not depend significantly on polymer molecular weight. Data is also included for the wavelength dependence (dispersion) of n _o, n _e, and Δn.     

Influence of TiO2 and ZnO nanoparticles on orientational order parameter of LC compounds – An optical study

In the present paper ZnO and TiO₂ nano particles are dispersed in Cyano-biphenyl liquid crystalline compounds (n?=?7, 12). The thermal polarizing microscopy and differential scanning calorimetry techniques are employed to measure the transition temperatures. The Nematic transition temperatures are decreased by 2.34?°C and 1.53?°C in Heptylbiphenyl, 1.07?°C and 1.12?°C in dodecylbiphenyl compounds due to the dispersion of nanoparticles. The refractive indices and Newton’s rings methods are exploited to measure birefringence at different temperature in nematic phase. The orientational order parameters are estimated by finding the birefringence in perfect order. The orientational order parameter is increased in nanoparticles dispersed liquid crystals when compare to pure liquid crystalline compounds.     

Birefringence and Order Parameter of Some Alkyl and Alkoxycyanobiphenyl Liquid Crystals

The refractive indices and densities of six liquid crystals—two alkylcyanobiphenyls, three alkoxycyanobiphenyls and one mixture of two alkylcyanobiphenyls have been reported as a function of temperature. The effective polarizabilities α_eand α_o in the nematic phase calculated using the methods due to Neugebauer, Vuks and Saupe and Maier, are found to be appreciably different, though the order parameter S evaluated with those values of (α_e - α_o) are in good agreement. The preference of one method over the other two is discussed.     

Densities,refractive indices,and order parameters of a binary liquid-crystalline system with an induced smectic A phase

For the binary mixture of 4′-Nitrophenyl 4-n-octyloxybenzoate and 4-n-Octyloxyphenyl 4′-pentyloxybenzoate with an induced smectic A phase the refractive indices (n_c and n_o) and the density were measured in dependence on the temperature. The orientational order parameter and the density were calculated using a mean field fitting method. In the induced S_A phase the refractive indices and the calculated orientational order parameters are showing only a weak dependence on concentration. The calculated and measured densities are in the same order of magnitude.     

Liquid Crystal Auto-Induction and Amplification Function for Circularly Polarized Luminescence (CPL) with High gem-Value,and Dynamically Controllable CPL Devices

Optically active phenylenevinylene derivatives with fluorescence are employed for preparation of a cholesteric liquid crystal based circularly polarized light (CPL) emission device. The device shows intense CPL with a quite large value of the g_em-factor (= 0.6) and quick CPL light switching driven by cholesteric-nematic transition with a homeotropic alignment. This research develops the auto-induction of fluorescent chiral inducers in host liquid crystal for formation of a helical structure with CPL amplification. The present research applies a classical LC light-scattering system using chiral technology as a new method for obtaining CPL dynamic control with an intense g_em-factor.     

Crossover temperature (TCO) and temperature gradient of refractive indices and length to breadth ratio of 4O.m mesomorphic compounds – an optical study

Refractive indices as a function of temperature are measured in a number of liquid crystals which belong to the N-(p-n-butoxy benzylidene)-p-n alkyl anilines, 4O.m compounds of the famous Schiff’s base nO.m liquid crystal compounds with m = 4 to 10 and 12. The temperature gradient of refractive indices, dn_e/dT and dn_o/dT, of these compounds are estimated. It is well known that in the case of dn_o/dT, there exists a crossover temperature, T_CO, which exhibits an odd–even effect with the alkyl chain length as expected. Further, using the birefringence data, the length to breadth ratio, k, for these compounds is estimated, which shows not exactly even–odd effect but irregular change with the chain number. The results are discussed with the body of the data.     

Thermal conductivity and refractive indices of Nd:GdVO4 crystals

The thermal conductivities of Nd:YAG, M(Y,Gd)VO₄ crystals were measured at 298 K. The value of Nd:GdVO₄ crystal along <001> direction was 11.4 W/mK, which was higher than that of YAG crystal measured to be 10.7 W/mK. The principal refractive indices of Nd:GdVO₄ crystal in the temperature range from 20 °C to 170 °C were determined by auto‐collimation method. Based on the measured values of refractive indices, the Sellmeier equation and expression of temperature dependence of refractive indices have been obtained. The measured results show that the birefringence Δn is 0.22007 at 20 °C and temperature coefficient of birefringence is 4.33 × 10⁻⁶/°C for 1.064 μm. These results prove that the GdVO₄ crystal is a laser crystal with excellent thermal and birefringence properties. (© 2003 WILEY‐VCH Verlag GmbH & Co. KGaA, Weinheim)     

Polarization of the Scattered Light in the Isotropic Phase of a Cholesteric Liquid Crystal

In the only measurement to date of the ratio R_d of depolarized to polarized light scattering intensities in the isotropic phase of a cholesteric liquid crystal, Harada and Crooker reported that the value of ¾ predicted by the order parameter fluctuation theory of de Gennes was not observed. We present here measurements of R_d in cholesteryl olevl carbonate (COC) using a more precise rotating polarizer technique. R_d is observed to have a strong temperature dependence which we can explain by the de Gennes theory plus an additional fluctuating component as described by Courtens.     

Cholesteric Pitch and Blue Phases in a Chiral-Racemic Mixture

The cholesteric and “blue” phases (BP) of a mixture of the chiral and racemic forms of the same compound were studied as a function of composition and temperature. It was found that the clearing point and the cholesteric-S_A transition temperature was independent of composition. The pitch of the cholesteric at a given temperature was inversely proportional to the fraction of chiral material, as was the lattice parameter of BPII. The lattice parameter of BPI did not follow the above rule. No second-order BPI-BPIIb transition was seen.

It is concluded that this mixture is thermodynamically ideal^(6,7) in that the intermolecular interactions between two molecules are independent of their relative chirality. Thus, this system comes close to the theorist's ideal of one in which only the chirality is varied.     

High Pressure Studies of the Phase Transition Enthalpies of the Liquid Crystal EBBA up to 2.5 kbar

The liquid crystal p-Ethoxy-benzylidee-p-n-butylaniline(EBBA) exhibits one nematic-liquid isotropic and two solid-nematic coexistence lines.

With a high pressure micro calorimeter based on differential scanning calorimetry (DSC) previously described the phase transition enthalpies of these three phase transitions have been measured in the pressure range 1 bar to 2.5 kbar. Numerical data are presented for the transition temperatures, the enthalpy changes, the entropy changes, and the volume changes for both solid-nematic transitions as well as for the nematic-liquid isotropic transition.

Additionally the effect of impurities on the nematic-liquid isotropic transition enthalpy at normal pressure is discussed.     

A neutron diffraction study of the phase transition of fully deuterated triglycine sulphate (ND2CD2COOD)3.D2SO4

Using neutron single crystal and powder diffraction, the first thorough investigation of the structure of fully deuterated triglycine sulphate, (ND₂CD₂COOD)₃.D₂SO₄ is presented, including its evolution with T, through its structural phase transition. This includes new precise structural parameters determined at several key temperatures above and below T_C using single crystal diffraction, and for the first time a parametric study has been undertaken over a wide temperature range — from 4 to 500 K in 2 K steps. It was found that fully deuterated TGS shows a structure consistent with hydrogenous TGS and partially deuterated TGS. The evolution of several key hydrogen bond lengths suggests that weakening of the H‐bond network with T is crucial in decoupling the polarising glycine molecules from the other glycines and allowing the long‐range ferroelectric order to break down. A new parameterisation of the phase transition is demonstrated. Contrary to results of physical properties measurements, there is no evidence of a second low temperature phase transition in TGS – no low temperature anomalies were observed in the crystal structure.     

Behaviour of Ultrasonic Velocity in Cholesteric Liquid Crystals

Ultrasonic velocities (V) have been determined, employing a fixed path double crystal interferometer, in three cholesteric liquid crystals, namely cholesteryl propionate, cholesteryl laurate and cholesteryl myristate in their isotropic and anisotropic phases including the region of the phase transition. The variation of specific volume (v) is also studied in the same temperature range by a special dilatometer constructed for this purpose. Anomalous behaviour of ultrasonic velocity is observed near the isotropic-cholesteric phase transition in all three compounds. In the two polymesomorphic liquid crystals, namely cholesteryl laurate and cholesteryl myristate, contrary to the ultrasonic behaviour of cholesteryl stearate, prominent velocity dips are observed at cholesteric-smectic transition temperatures. The parameters adiabatic compressibility (β_ad) and molar sound velocity (R) are estimated and they are found to exhibit sudden jumps at cholesteric-smectic and isotropic-cholesteric transitions. The thermal expansion and temperature co-efficient of compressibility are found to show abnormal increase near the phase transition indicating the existence of large-magnitude pre-transitional effects near the phase transition. A comparative study of the ultrasonic behaviour of six aliphatic esters of cholesterol has shown that the magnitude of the velocity dip observed at the isotropic-cholesteric transition increased with increase of molecular weight and only cholesteryl acetate shows deviation.     

Role of dielectric medium on benzylidene aniline: A computational analysis

A computational analysis of ordering in N-(p-n-ethoxy benzylidene)-p-n-butyl aniline (2O.4) was performed based on quantum mechanics and intermolecular forces. The atomic charge and dipole moment at atomic centre were evaluated using the all valance electron CNDO/2 method. The modified Rayleigh-Schrodinger perturbation theory and multicentre—multipole expansion method were employed to evaluate long-range intermolecular interaction, while a 6-exp potential function was assumed for short-range interactions. The total interaction energy values obtained in these computations were used as input for calculating the probability of each configuration in a noninteracting and nonmesogenic solvent (i.e., benzene) at room temperature (300 K) using the Maxwell-Boltzmann formula. The molecular parameter of 2O.4, including the total energy, binding energy, and total dipole moment were compared with N (p-n-butoxy benzylidene)-p-n-ethyl aniline (4O.2). The present article offer theoretical support to the experimental “observations, as well as a new and interesting way of looking at liquid crystalline molecule in a dielectric medium.     

New chiral liquid crystal materials based on menthol: Synthesis and phase behavior

To study structure-mesomorphism relationships of chiral materials based on menthol, a series of chiral compounds containing menthyl group were synthesized. Their chemical structures, formula, and the purity were characterized by Fourier transform infrared (FTIR), proton nuclear magnetic resonance (¹H NMR), and elemental analyses. The corresponding phase behavior was investigated with differential scanning calorimetry (DSC) and polarizing optical microscopy (POM). The effect of the mesogenic core, flexible spacer, and terminal groups on the mesophase formation and type was discussed. The compound 4-menthyloxyacetoxybenzoyloxy-4′- (6-bromohexyloxy)biphenyl showed the smectic A (S_A) and cholesteric phases on heating cycle, and a cubic Blue Phase, chiral smectic C (S_C^*), S_A and cholesteric phases on cooling cycle. However, the compound 4-menthyloxyacetoxybenzoyloxy-4′-(4-bromobutoxy)biphenyl showed enantiotropic S_C^*, S_A and cholesteric phases, moreover, the platelet texture of a cubic Blue Phase was also observed on cooling cycle. The methacrylate monomer with six flexible methylene units and three phenyl rings as mesogenic core only showed cholesteric phase, while the methacrylate monomer with four methylene unit and three phenyl rings showed the S_C^* and cholesteric phases. However, when the terminal methacryloyloxy groups in the monomers were substituted by thiophenylacryloyloxy groups, the corresponding monomers all show no mesophase. With increasing the rigidity of mesogenic core or decreasing the flexible methylene spacer length, the corresponding melting temperature (T_m) or isotropic temperature (T_i) increased.     

Phase transitions from density and order parameter from polarisabilities in symmetric dimeric liquid crystals

As a part of our systematic studies on liquid crystal dimers, we present in this article the nature of phase transitions across isotropic–nematic and nematic–smectic-A exhibited by DLCs, α,ω-bis-(4-n-alkylaniline benzylidene-4?-oxy) alkanes. Further, the orientational order parameter in the nematic phase of these DLCs are estimated from the molecular polarisabilities calculated using the experimental refractive indices and density results. The molecular polarisabilities α_e and α_o are obtained for the compounds using the above results for both Vuks and Neugebauer local field models applicable to nematic liquid crystal. α_e and α_o calculated in this way are used to obtain Δα. The polarisability anisotropy in the perfect order (absolute K) is calculated semi-empirically using the δ-function model developed by Lippincott et al. and molecular vibration method. The values of polarisability anisotropy for both local electric field models differ significantly. No criterion is known to decide which value is correct. To avoid the determination of uncertain α and Δα values considering different local field models, a simple procedure developed by Kuczynski et al. was used for evaluation of S, based solely on birefringence δn = (n_e-n_o) and this value of S is compared with those obtained from field models.     

Investigation of the Cholesteryl Oleate Mesophases by X-Ray Diffraction

The following phase transitions occurring in the cholesteryl oleate were investigated by X-ray diffraction: solid-isotropic liquid, isotropic liquid-cholesteric, cholesteric-smectic.

The sample purity was 98%. Strong pretransitional effects were observed at the solid-isotropic liquid phase transition and at the cholesteric-smectic phase transition.

At this last transition the longitudinal coherence length ξ_∥ appears to diverge as the critical temperature is approached in the cholesteric phase, whereas the transversal coherence length ξ_τ increases at the critical temperature in the smectic phase in an abrupt way. It seems from the temperature dependence analysis of the angular position of the low angle peak that the smectic phase is a smectic A phase and that a progressive melting of chains occurs at the temperature increases.