Determination Of Orientational Order Parameters Research Articles

Phase transition and birefringence studies on some ferroelectric liquid crystals

In the present studies the homologous series of hydroxy phenylamino methyl phenoxy benzoate liquid crystals (n = 8,9 and m = 6) are used. The temperature variation of density is measured. Using density data the phase variants in the compounds are identified. All the compounds exhibit first-order phase transitions. By Newton rings method the birefringence is measured directly in smectic and nematic phases. The advantages of the newton ring technique are presented over the conventional methods for the determination of orientational order parameter. The orientational order parameter attained by the newton rings scheme is equated through the literature data, the results are discussed.

Optical and static dielectric study of a terminally fluorinated liquid crystalline compound

Optical and static dielectric studies on a terminally fluorinated liquid crystalline compound have been carried out. Measurements of temperature variation of refractive indices of the compound are done by using thin prism method. A four-parameter model is validated by fitting the experimentally measured values of refractive indices, birefringence and average refractive indices of the compound with the theoretical ones. Refractive index and density data are utilized for determination of orientational order parameter. The temperature variation of dielectric permittivities of the compound are measured by a LCR meter.

Considerations in the determination of orientational order parameters from X-ray scattering experiments

ABSTRACTAn assessment of the data processing and analysis methods used to obtain the second- and fourth-rank orientational order parameters of liquid crystals from X-ray scattering experiments has been carried out, using experimental data from four extensively studied alkyl-cyanobiphenyls and calculated data generated from two general types of theoretical orientational distribution function. The application of a background subtraction and two different baseline correction methods to the scattering profiles is assessed, along with three different methods to analyse the processed data. The choice of baseline correction method is shown to have a significant effect: an offset to zero overestimates the order parameters from the experimental and calculated data sets, particularly for lower order parameters arising from broad distributions, whereas an offset to a value estimated from regions of low scattering intensity provides experimental values close to those reported from other experimental techniques. By contrast, the three different analysis methods are shown generally to result in relatively small absolute differences between the order parameters. We outline a straightforward general approach to experimental X-ray scattering data processing and analysis for uniaxial phases that results in order parameters that match well with those reported using other experimental techniques.

Orientational order parameter studies in two symmetric dimeric liquid crystals – an optical study

The optical technique developed by [W. Kuczynski, B. Zywucki, and J. Malecki, Determination of orientational order parameter in various liquid-crystalline phases, Mol. Cryst. Liq. Cryst. 381 (2002), pp. 1–19; B.J. Zywucki and W. Kuczynski, IEEE transactions on optical phenomena – The orientational order in nematic liquid crystals from birefringence measurements, Dielectr. Electr. Insul. 8 (2001), pp. 512–515] is fabricated and used to determine the orientational order parameter in two dimeric liquid crystalline compounds nematic and SmA phases of α,ω-bis(4-alkylanilinebenzylidene-4′-oxy)alkane (m.OnO.m) homologous series. The compounds studied are 5.O8O.5 and 5.O10O.5 which exhibit nematic and SmA, and nematic phases, respectively. The orientational order parameter in both the phases of nematic and SmA phases of the compound one and the nematic phase of the compound two are obtained using the principle of Newton's rings which gives directly the birefringence, δn of the liquid crystal dimer. The merits of the technique used are presented over the conventional techniques for the determination of orientational order parameter. The results for the two compounds are compared with those values estimated from n e, n o and density using the two internal field models due to Vuks and Neugebauer applicable to nematic phase.

Determination of Orientational Order Parameters of Two Tri-Component Mixtures from Optical Birefringence and X-Ray Diffraction Measurements

Two tri-component mixtures as base mixtures for vertically aligned mode LCD’s were prepared. The eutectic compositions were theoretically estimated and experimentally verified from differential scanning calorimetry studies. A room temperature nematic mixture with fairly broad operating range emerged from each of the tri-component mixtures. The optical birefringence of these mixtures at the eutectic composition was measured as a function of temperature. X-ray diffraction measurements were done on these mixtures to obtain the orientational order parameters as a function of temperature. The order parameter values were also determined from birefringence measurements and the results were compared with mean field theory. Structural parameters like intermolecular distance and apparent molecular length have also been determined.

Optical and Dielectric Studies of a Mesogenic Mixture: Determination of Orientational Order Parameter and Angle of Inclination

Optical and dielectric studies have been conducted on a liquid crystalline mixture (code name: ZLI 1275) containing phenyl cyclohexane, cyclohexyl phenyl carboxylate, and cyclohexyl benzoate. From optical studies, the orientational order parameter has been determined as a function of temperature and compared with theoretical Maier–Saupe values. The thermal variation of dielectric permittivity has been determined for different operating frequencies and the thermal dependence of the angle of inclination of the effective molecular dipole moment with the director direction has been investigated. Comparison of the optical and dielectric properties of ZLI 1275 has been made with two similar mixtures (ZLI 1701) and (ZLI 1800-000) exhibiting similar nematic temperature ranges and each containing three moieties of which one (phenyl cyclohexane) is common with ZLI 1275.

Spatially resolved electron diffraction and the determination of orientational order parameters in thermotropic liquid crystalline polymer

A low-dose, high-resolution, electron-diffraction technique has been used to calculate local orientational order parameters from thermotropic liquid crystal polymer (TLCP) fibers. Diffracted intensities are extracted from digital electron diffraction patterns for the orientational order parameter calculation, in a manner similar to that used with X-ray diffraction data. The resolution of local orientation is made possible by electron diffraction as opposed to other methods because of the ability to sample regions as small as 100nm in diameter. Working within the critical radiative dose for structural damage constrains the ultimate spatial resolution. The signal-to-noise ratio (SNR) of the diffraction data collected at high spatial resolution is low due to the small volume sampled. This work demonstrates the dependence of the orientational order parameter on signal-to-noise effects and the convergence of the incoming electron beam.

On the molecular orientation and viscoelastic behaviour of liquid crystalline polymers: the influence of macromolecular architecture

The influence of macromolecular architecture on the flow‐induced orientation of main‐chain thermotropic liquid crystalline polymers (LCPs) is investigated using in situ wide‐angle X‐ray scattering. In order to get more insight into the interrelationship between microscopic and rheological behaviour, the viscoelastic properties of the nematic melts were also studied. The LCPs studied are wholly aromatics , composed only of mesogenic units, and semiflexibles , which consist of mesogenic units separated by alkyl spacers. Rheo‐X‐ray scattering evidenced the detrimental influence of the flexible spacers on the orientation process. The wholly aromatic LCPs readily orient along the flow direction, even under modest shear flows ( γ = 0.1 s‐1). The semiflexible LCPs, however, display lower levels of orientation than the wholly aromatics, and the quality of the orientation worsens as the length of the flexible spacer increases. The rheological characterization shows that, like conventional flexible‐chain polymers, the thermotropic LCPs exhibit a linear viscoelastic (LVE) regime. Moreover, dynamic measurements, within the LVE regime, suggest a level of elasticity in the nematic melts. As the length of the alkyl spacer increases, the rheological behaviour is more akin to that displayed by common flexible molecular chains. The poor shear orientation exhibited by the semiflexible LCPs is then associated with a molecular network formed by the less‐than‐rigid molecular chains. Furthermore, the steady shear viscosity is always smaller than the dynamic viscosity, i.e. the Cox‐Merz rule does not hold. This is due to the fact that steady shear induces molecular orientation, whereas oscillatory shear does not. The relaxation of orientation after steady shear showed that the molecular orientation relaxes after hundreds of seconds. The shear stress, however, relaxes within only a few seconds. Strikingly, the rate of molecular orientation relaxation was slower for the semiflexible than for the wholly aromatic LCPs.

Determination of Orientational Order Parameters from 2H NMR Spectra of Magnetically Partially Oriented Lipid Bilayers

The partial orientation of multilamellar vesicles (MLVs) in high magnetic fields is known to affect the shape of 2H NMR spectra. There are numerical methods for extracting either the orientational order parameters of lipid molecules for a random distribution of domain orientations in the sample, or the distribution of orientations for a known set of spectral anisotropies. A first attempt at determining the orientational order parameters in the presence of an unknown nonrandom distribution of orientations is presented. The numerical method is based on the Tikhonov regularization algorithm. It is tested using simulated partially oriented spectra. An experimental spectrum of a phospholipid-ether mixture in water is analyzed as an example. The experimental spectrum is consistent with an ellipsoidal shape of MLVs with a ratio of semiaxes of ∼3.4.

Determination of DPH order parameters in unoriented vesicles

This article reviews the determination of orientational order parameters in non-macroscopically oriented membranes from the data obtained with the fluorescent probe all-trans-1,6-diphenyl-1,3,5-hexatriene (DPH). Special attention is paid to the effect of microheterogeneity in the probe environment on the recovered values of the order parameters. An effort is made to accommodate new findings in the existing picture of orientational order in membranes.

Determination of Orientational Order Parameter from Optical Studies for a Homologous Series of Mesomorphic Compounds

Abstract We have measured the refractive indices (no , ne ) and densities of four members (nonyl, decyl, undecyl and dodecyl) of a homologous series cyanobiphenyl alkyl ether over their entire mesomorphic range and in their isotropic phases. The molecular polarizabilities have been calculated from refractive indices using both Vuks' and Neugebauer's relations. The additive rule of bond polarizabilities has also been employed to calculate the molecular polarizabilities of the four compounds assuming two different configurations of the alkyl chains. The orientational order parameter values at different temperatures for each compound have been calculated from these polarizability data. Since all the four compounds possess smectic A phases, we have compared our experimental order parameters with those calculated from the McMillan's model. Our experimental (P 2)values for these four compounds show a pronounced odd-even effect.

The Study of Birefringence and Orientational Order in Plastic Tert-Butyl Bromide

The method of measuring birefringence in uniaxial crystals with weak anisotropy has been developed. The results are presented for plastic tert-butyl bromide investigation and determination of orientational order parameter in noncubic phase of this crystal.

Physical Principles Underlying the Experimental Methods for Studying the Orientational Order of Liquid Crystals

The basic physical principles underlying different experimental methods frequently used for the determination of orientational order parameters of liquid crystals are reviewed. The methods that are dealt with here include the anisotropy of the diamagnetic susceptibility, birefringence, linear dichroism, Raman scattering, fluorescence depolarization, electron paramagnetic resonance (EPR), and nuclear magnetic resonance (NMR). The fundamental assertions that can be obtained by the different methods as well as their advantages, drawbacks and limitations are inspected. Typical sources of uncertainties and inaccuracies are discussed. To quantitatively evaluate the experimental data with reference to the orientational order the general tensor formalism developed by Schmiedel (Wiss. Z. KMU Leipzig, Math.-Naturwiss. R. 33 (1984) 441) was employed throughout according to which the order matrix comprises 25 real elements yet. Within this context the interplay of orientational ordering and molecular conformation is scrutinized.

IR and Raman bandshape analysis of rotational relaxation in ordered fluids

Fourier analysis of IR and Raman bands is used to obtain the reorientational correlation functions of aligned anisotropic fluids. New equations for the simultaneous determination of orientational order parameters, the rotational diffusion coefficients and the reorientational relaxation times in anisotropic ordered systems are suggested.

Determination of orientational order parameters in oriented lipid membrane systems by angle-resolved fluorescence depolarization experiments

Angle-resolved fluorescence depolarization experiments have been performed on some lipid membrane systems in order to test the validity of a recently developed theory describing this type of experiments. Good agreement between theory and experiment was found. The experiments yield the order parameters 〈 P 2〉 and 〈 P 4〉 and a rotational correlation time τ c. On applying an information-theoretic form of the orientational distribution function. 〈 P 2〉 and 〈 P 4〉 were used to estimate the degree of order in the systems studied. Knowledge of 〈 P 4〉 proved useful to get more detailed information on the orientational order. Another finding was that the angle between the absorption and emission moment of the used fluorescent probe, diphenyl-hexatriene, depends on the membrane system in which it is incorporated. Finally, the experimental results point to the fact that for some membrane systems the molecular motion is inadequately described by the simple strong-collision model.

Influence of a.c. Electric Field on Infrared Absorption Spectra of Liquid Crystals and Determination of Orientational Order Parameter by Infrared Dichroism. I. Molecules with Strong Positive Dielectric Anisotropy

Influence of a.c. Electric Field on Infrared Absorption Spectra of Liquid Crystals and Determination of Orientational Order Parameter by Infrared Dichroism. I. Molecules with Strong Positive Dielectric Anisotropy

Influence of a.c. Electric Field on Infrared Absorption Spectra of Liquid Crystals and Determination of Orientational Order Parameter by Infrared Dichroism. II. Molecules with Weak Negative Dielectric Anisotropy

Abstract The influence of a.c. electric field (10 kHz) on the IR spectra of the nematic phase of six p-alkoxybenzylidene-p,n-butylanilines with weak negative dielectric anisotropy was studied and the temperature dependence of the orientational order parameter and threshold voltage have been investigated. The values of band elastic constants K33 were calculated for MBBA and EBBA.