N. V. S. Rao

Phase Transition and Structural Investigations in 50.5, 50.6 and 50.7

Abstract We report X-ray diffraction, density, ultrasonic velocity and refractive index studies in the N(p-n-pentyloxy benzylidene) p-n-alkylaniline compounds, viz. 50.5, 50.6 and 50.7. The nematic-smectic A (NA) transition is found to be weak first order in 50.6 while it is second order in 50.5 and 50.7. The salient features observed are cybotactic clusters in the nematic phase in all the compounds, molecular tilt which was inferred due to the end alkyl chains tilt causing orienta-tional disorder (smaller orientational order parameters ‘s’ than expected) in smectic A phase, smectic F phase and large tilt angle variation in smectic C phase in a small temperature range in 50.5. The observed results are discussed in the light of available data in other n0.m compounds.

Phase Transitions in N(p-n-Bu toxybenzylidene) p-n-Alkyl Anilines: Density and Refractive Index Studies

Abstract The density and refractive index variation with temperature in N(p-n-butoxybenzylidene)p-n-alkylanilines viz., 40.m compounds for m = 4 to 12 are presented. The density, thermal expansion coefficient and the refractive index results infer first order character for IN, AB and CB transitions and a second order character for NA and AC transitions. Pretransitional effects are discussed at IN and AB transitions. The odd-even effect is observed at both the IN and NA phase transitions. The variation of refractive index anisotropy as a function of temperature, in the nematic phase of the compounds in a homologous series has shown an interesting phenomena. The temperature dependence of nematic refractive index anisotropy yields mean field exponent v = 1 in the vicinity of NA transition. The influence of molecular chemical constitution, the nematic thermal range and the McMillan parameter values on the order of NA transition is discussed in detail.

Density Studies in Terephthalylidene-bis-p-n-dodecylaniline

Abstract The temperature dependence of density in terephthalylidene-p-n-decylaniline (TBAA10) is carried out to probe the phase transitions, associated volume jumps, order of the transitions, and to estimate the pressure dependence of transition temperatures and pretransitional effects. The compound exhibits smectic A, smectic C, smectic I, smectic F and smectic G phases. The isotropic-smectic A and smectic C-smectic I phase transitions are found to he first order and the calculated thermal expansion coefficient also supported the density results. The smectic A-smectic C and for the first time smectic I-smectic F transitions are detectable by density changes. The calculated pressure estimate of transition temperatures from the density-enthalpy data and volume changes associated with the phase transitions are compared with the reported P-T data using metabolemeter. The observed two phase coexistence of the smectic A-isotropic and smectic A-smectic C phases in the vicinity of the phase transitions is compat...

Density studies in N(p-n-heptyloxy benzylidene)p-n-butyl aniline across SG[sbnd]SC, SC[sbnd]SA, SA[sbnd]N and N[sbnd]l phases

Abstract The density p of N(p-n-heptyloxy benzylidene) p-n-butyl aniline (HYBBA) is measured as a function of temperature from the isotropic liquid to the smectic-G phase. The compound is tetramorphic exhibiting three smectic phases SG, SC, SA and a nematic phase. The density changes across the phase transformations and the calculated thermal expansion coefficient confirm the order of the SG[sbnd]SC, SA[sbnd]N and N[sbnd]I transitions as first-order, with SC[sbnd]SA transition as weakly first or second-order. The first-order SA[sbnd]N phase transformation in this compound, which is weakly polar, is in agreement with McMillans theoretical predictions, and is different from other polar compounds.

Density and ultrasonic velocity studies in N(p-n-heptyloxybenzylidene) p-n-pentylaniline

Abstract The density and ultrasonic velocity of N(p-n-heptyloxybenzylidene) p-n-pentylaniline (70.5) are measured as a function of temperature from the isotropic liquid to the smectic B phase. The compound is pentamorphic, exhibiting four smectic phases, viz. smectic G, smectic B, smectic C, smectic A, and a nematic phase. The changes in density, ultrasonic velocity and the derived parameters confirm the N-I and SB-SC transitions as first-order, the SA-N transition as weak first-order, and SC-SA transition as second-order. An estimate of the pressure dependence of transition temperatures is presented. The computed parameters: adiabatic compressibility β ad , molar sound velocity Rn and molar compressibility Aw , are discussed.

Pretransitional effects at the isotropic-mesomorphic phase transitions in the TBAA series

Abstract Thermal microscopy (TM), differential scanning calorimetry (DSC) and dilatometric p(t) experiments were carried out to characterize the phases and to study the nature of mesomorphic transitions in two higher homologues of terephthalylidene-bis-p-n-alkylanilines, TBnAs, namely terephthalylidene-bis-p-n-tetradecylaniline, TB14A and terephthalylidene-bis-p-hexadecylaniline, TB16A. Results infer the first order nature of the isotropic-smectic C (I-Sc) and smectic C-smectic I (Sc-SI) transitions, while the smectic I-smectic F (SI-SF) transition is found to be second order. Pretransitional effects are estimated through αeff in the vicinity of mesomorphic fluctuation dominated non-linear regions (FDNLR), of the isotropic to mesomorphic transitions in TBnA homologous compounds and the relative strength of growth of FDNLR is discussed at the SIN, SISA and SISC phase transitions.

Order Parameter and Molecular Polarizabilities in 60.4, 60.2 and 70.1

Abstract In some N(p-n-alkoxy benzylidene) p-n-alkylanilines the refractive indices ne, no and the order parameter are measured as a function of temperature. From the experimental data the molecular polarizabilites and its anisotropies at different wavelengths were calculated using different internal field models. The merits of each model were discussed.

Density, Refractive Index and Ultrasonic Velocity Studies Involving N(p-n-Hexyloxy Benzylidene) p-n-Octyl Aniline

Abstract The density, refractive index and ultrasonic velocity dependence on temperature for N(p-n-hexyloxy benzylidene) p-n-octyl aniline, 60.8, is presented. The compound exhibits smectic B, smectic A, nematic and monotropic smectic G phases between the solid and isotropic liquid phases. The nematic—isotropic and smectic A—smectic B phase transitions are found to be first order. The interesting nematic—smectic A transformation is confirmed to be a first order from the observed results. An estimate of the pressure dependence of the phase transition temperatures using the volume and enthalpy data are presented. A comparison of these results with other reported results in the homologous series of n0.m compounds are also discussed.

N-SATransition First or Second Order: Tricritical Point (TCP) in N(p-n-pentyloxy benzylidene)p-n-alkyl anilines?

The nature of the nematic-smectic A (NA) phase transition is discussed in the light of the available experimental data on N(p-n-pentyloxy benzylidene)p-n-alkyl anilines, 50.m series of compounds. T...

Phase transition studies in N(p-n-pentyloxy benzylidene) p-n-decyl aniline

Abstract The temperature dependences of the molar volume, the ultrasonic velocity and the refractive index in N(p-n-pentyloxy benzylidene) p-n-decyl aniline, which exhibits nematic, smectic-A and smectic-B, phases, are presented. The isotropic-nematic and smectic-A-smectic-B phase transitions are found to be of first order while the nematic-smectic-A transition is of second order. The thermal expansion coefficient (a) computed from the molar volume data agrees with a second-order N-SA transition. The computed adiabatic compressibility βad, molar sound velocity (or Rao number Rn ) and molar compressibility (or Wada constant A) are presented.