V.G.K.M. Pisipati

Orientational order parameter estimated from molecular polarizabilities – an optical study

An optical study of N-(p-n-alkyloxybenzylidene)-p-n-butyloxyanilines, nO.O4 compounds with the alkoxy chain number n = 1, 3, 6, 7, and 10 has been carried out by measuring the refractive indices using modified spectrometer and direct measurement of birefringence employing the Newtons rings method. Further, the molecular polarizability anisotropies are evaluated using Lippincott δ-function model, the molecular vibration method, Hallers extrapolation method, and scaling factor method. The molecular polarizabilities α e and α 0 are calculated using Vuks isotropic and Neugebauer anisotropic local field models. The order parameter S is estimated by employing the molecular polarizability values determined from experimental refractive indices and density data and the polarizability anisotropy values. Further, the order parameter S is also obtained directly from the birefringence data. A comparison has been carried out among the order parameter obtained from different ways and the results are compared with the body of the data available in the literature.

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 Newtons 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.

Cross over temperature (T CO) and temperature gradient of refractive indices of a number of homologous series of liquid crystal materials

The temperature gradient of refractive indices, dn e/dT and dn o/dT of a number of liquid crystals of different homologous series are analysed. It was found that the materials studied possess high clearing temperatures and the cross over temperatures estimated are well above room temperature and they show an odd–even effect with the chain length in a homologous series.

Histogram equalisation technique to analyse induced cholesteric phase in nanodoped liquid crystalline compound

The 1% Fe3O4 nanodoped in dodecyloxy benzoic acid induces cholesteric phase quenching the nematic phase that the pure compound exhibits. Further, the solid–solid transition also occurs when compared to the pure compound and the transition temperatures are reduced along with the cholesteric thermal ranges of LC phases. The histogram is the operation by which the occurrences of intensity value in the image is shown. Further, histogram equalisation is the technique through which the dynamic range of the histogram of an image can be increased. Histogram equalisation method is used in this article to analyse the 12Oba + 1% Fe3O4 nanodoped liquid crystalline textures by considering global and local image information and for the adjustment of image intensities to enhance the contrast. The basic idea of histogram equalisation is to remap the grey levels of the image, and this method is used to identify the uniform regions and detect the defects that are not clearly observed from the textures that are recorded by polarising microscope.

Image enhancement using virtual contrast image fusion on Fe3O4 and ZnO nanodispersed decyloxy benzoic acid

Low contrast and noisy photographic pictures can be considerably improved by image-processing techniques. Techniques like histogram equalisation produce high-contrast images but often fail to preserve the colour texture information. To overcome this deficiency, a contrast-enhancement approach has been devised, using virtual contrast image fusion (in Haar wavelet domain). This technique has been evaluated by a study of the optical textures of nano-dispersed decyloxybenzoic acid with small quantities of Fe3O4 and ZnO added.

Phase Transition Studies of 6.O12O.6 and 7.O6O.7 Using Density Measurements

The symmetric liquid crystal dimer homologous series, α,ω-bis-(4-n-alkylaniline benzylidene-4′-oxy) alkanes, show a rich variety of smectic polymorphism. We report here the nature of different transitions exhibited by two of these dimers using density measurements as a function of temperature. The compound 6.O12O.6 exhibits only nematic phase; while 7.O6O.7 exhibits smectic A and smectic F phases. All the phase transitions studied viz., isotropic – nematic, isotropic – smectic A, and smectic A – smectic F are confirmed to be of first order nature. The calculated density jumps, thermal expansion coefficient data and pressure dependence of transition temperatures are analysed in the light of available literature data.

Computation of Molecular Free Length (L f ) and Molecular Radius (M r ) of p-n-Alkoxy Benzoic Acids

Dilatometric (density, ρ) and optics (refractive index, n) studies are utilized for the computation of thermoacoustic parameters, molecular radius, and intermolecular free length of p-n-alkoxy benzoic acids (nbas). For the estimation of Lf , the parameters like V 0, molar volume at absolute zero, and (Va ), the available volume is calculated from thermodynamic parameters. Further, V 0 can be obtained by the extrapolation of temperature variation of molar volume in isotropic phases to absolute zero. The molecular radius (Mr ) is obtained from the density and refractive index. The computed values using different methods are compared with one another as well as with the body of the data available. The temperature variation of different thermoacoustic parameters like Moelwyn-Hughes (C 1), reduced molar volume (V ∼), isochoric temperature coefficient of internal pressure (X), etc., calculated using different expressions are also presented. The increment in the molecular radius per methylene group in this homologous series is found to be 0.084 Å from density data and 0.054 Å from optics data. Further, it is observed that the Mr values obtained from optics data are found to be less compared to those obtained from density results. The Lf values estimated from different methods are found to show a similar type of variation with the chain length. However, the values differ slightly from one another and refinement is needed in the expressions to get concurrent results from different experimental data.

Homomorphic filtering textural analysis technique to reduce multiplicative noise in the 11Oba nano-doped liquid crystalline compounds

The compound undecyloxy benzoic acid (11Oba) exhibits nematic and smectic-C phases while a nano-doped undecyloxy benzoic acid with ZnO exhibits the same nematic and smectic-C phases with reduced clearing temperature as expected. The doping is done with 0.5% and 1% ZnO molecules. The clearing temperatures are reduced by approximately 4 ° and 6 °, respectively (differential scanning calorimeter data). While collecting the images from a polarizing microscope connected with hot stage and camera, the illumination and reflectance combined multiplicatively and the image quality was reduced to identify the exact phase in the compound. A novel technique of homomorphic filtering is used in this manuscript through which multiplicative noise components of the image are separated linearly in the frequency domain. This technique provides a frequency domain procedure to improve the appearance of an image by gray level range compression and contrast enhancement.

Crossover temperature (T CO) and the temperature gradient of refractive indices of a homologous mixture of 4O.6 and 6O.4

Temperature gradient of refractive indices, dn e /dT and dn o /dT, of the isotropic mixtures of the liquid crystals of nO.m compounds, namely N-(p-n-butyloxybenzylidene)-p-n-hexyl aniline, 4O.6, and N-(p-n-hexyloxybenzylidene)-p-n-butyl aniline, 6O.4, are estimated using the refractive index data. The former one exhibits a second-order nematic-to-smectic-A (N-SmA) phase transition, while the later compound shows a first-order (N-SmA) phase transition. These two compounds exhibit high clearing temperatures, i.e. well above the room temperature. It is found that the two materials and the mixtures exhibiting second-order N-SmA transition show the crossover temperatures (T CO’s) well below the room temperature while the mixtures showing first-order transition exhibiting T CO’s well above the room temperature.

Induced smectic-B and smectic-G phases in hydrogen-bonded complexes (nOBA:PFOA): thermal and phase behaviour studies

A new series of intermolecular hydrogen-bonded liquid crystals are synthesised. The hydrogen bond formed between the mesogenic p-n-alkoxy benzoic acids (nOBAs) and non-mesogenic perfluorooctanoic acid (PFOA) (where n represents the alkoxy carbon numbers from 4 to 12 and 16). The IR spectral investigation confirms the hydrogen bond. The thermal and phase behaviour of these materials were studied by the use of polarising thermal microscope attached with a hot stage and the differential scanning calorimeter. From the experimental observations, both smectic-B and smectic-G phases are induced in the complexes having the chain number n = 7–12 by quenching smectic-C phase.