Theoretical description of fluctuations in free-standing smectic-A films including spatial non-uniformity of their elastic characteristics

A theoretical study is reported of fluctuations in smectic layer displacements and of the correlations between them in free-standing smectic-A films formed of liquid-crystal compounds with smectic-A-isotropic liquid (Sm-A-I) and Sm-A-nematic (Sm-A-N) bulk phase transitions. The study took into account the dependence of the elastic constants for bending, K, and tension (compression), B, of smectic layers on distance to the free film surfaces. The calculations are compared with the results obtained within Hołyst’s model for spatially uniform, free-standing smectic-A films. It has been established that, below the temperature at which smectic order in the bulk of a liquid crystal disappears, taking into account the profiles of the elastic moduli K and B does not produce noticeable differences from this model. However, at temperatures considerably above the Sm-A-I and Sm-A-N phase transitions, their inclusion results in considerable discrepancies from the predictions of Hołyst’s model. Fiz. Tverd. Tela (St. Petersburg) 41, 1882–1889 (October 1999)     

Phase transition studies in polymesomorphic compounds (ii): Ultrasonic velocity studies in N-(p-n-hexyloxy benzylidene) p-n-butyl aniline

The variation of ultrasonic velocity in N-(p-n-hexyloxy benzylidene) p-n-butyl aniline with temperature in the isotropic, nematic, S_A S_B and S_G phases is measured. All the phase transitions are of first order. The adiabatic compressibility β _ad , molar sound veclocity or Rao number R_n , and molar compressibility or Wada constant A are computed. Molar sound velocity and molar compressibility of HBBA are compared with the values of other compounds. They are found to be in good agreement with theory.     

Density studies across the smectic G-nematic and nematic-lsotropic transitions

The densities of N(p-n-pentyloxy benzylidene) p-ethylaniline and N(p-n-hexyloxy benzylidene) p-ethylaniline are measured as a function of temperature from the isotropic liquid to the smectic G phase. Both the compounds exhibit enantiotropic smectic G and nematic liquid crystalline phases. The changes in density across the phase transformations and the thermal expansion coefficient confirm the order of the transitions as of first order. The particular importance of the smectic G to nematic transformation is apparent from the density jump across the transition. An estimate of the pressure dependence of the isotropic-nematic transition temperature is found to be in reasonable accord with the literature data.     

New features of a smectic-A-crystal-B phase transition in a homologous series of liquid crystals

The components L _j of the Lorentz tensor and the polarizability density of molecules G in the smectic-A and crystalline-B phases have been determined for homologues of the series of alkyl-p-(4-alkoxybenzylideneamino-)cinnamates. The quantity L _j (G) in both phases is a linear (quadratic) function of the orientational order parameter of molecules S, which is invariant (noninvariant) with respect to the A-B transition, which is manifested in the form of jumps δL _j and δG and enhancement of the G(S) dependence. An increase in the length of terminal molecular chains and weakening of interlayer correlation of molecules are accompanied by strengthening of the A-B transition of the first order and G(S) dependences in both phases together with an increase in δL _j and δG. Change δG and dependence G(S) in the B phase are related to change in the conformation (flattening) of aromatic molecular cores.     

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

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-S_A transition. The computed adiabatic compressibility β_ad, molar sound velocity (or Rao number R_n ) and molar compressibility (or Wada constant A) are presented.     

Orientational order in the vicinity of a second order smectic-A to nematic phase transition

Measurements of the quadrupolar splitting of the central N¹⁴ are reported for the liquid crystal NBOA. The splitting is a measure of the orientational order parameter. At the nematic to smectic-A transition, the orientational order parameter varies continuously as a function of temperature, but with a discontinuous slope, as expected of a second order phase transition. The results are interpreted in terms of the Kobayashi-McMillan theory.     

Nonchiral ferroelectric smectic-C films

Free-standing films of nonchiral liquid crystal compound are studied by optical reflectivity measurements. The existence of macroscopic ferroelectric properties and orientation of the tilt plane parallel to an electric field in the temperature range above the smectic-C-smectic-A bulk transition is demonstrated. Pis’ma Zh. éksp. Teor. Fiz. 67, No. 10, 808–813 (25 May 1998)     

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

The variations of molar volume M_v and ultrasonic velocity V with temperature in N(p-n-pentyloxy benzylidene) p-toluidine are presented. The molar volume jump at the nematicisotropic transition suggests it is of first order. The adiabatic compressibility β _ad, molar sound velocity or Rao number R_n , molar compressibility or Wada constant A, the thermal expansion coefficient α, and Van der Waals constant b, are computed. Molar sound velocity and molar compressibility are compared with the values of other members of the homologous series and are found to be in good agreement with Rao's and Wada's observations. The order parameter S_k, at the nematic-isotropic phase transition is 0.441 from our experimental results and the Maier-Saupe table and is found to be in good agreement with the theoretical value.     

Calorimetric study of phase transitions in nanocomposites of quantum dots and a liquid crystal

The complex specific heat is measured over a wide temperature range for the liquid crystal (LC) 4-cyano-4-octylbiphenyl (8CB) and cadmium sulfate quantum dots (QDs) composites as a function of QD concentration. The thermal scans were performed under near-equilibrium conditions for all samples having QDs weight percent (φ_w) from 0 to 3wt% over a wide range of temperature well above and below the two transitions in pure 8CB. Isotropic (I) to nematic (N) and nematic to smectic-A (SmA) phase transitions evolve in character and their transition temperatures offset by (～2.3 to 2.6 K) lower for all composite samples as compared to that in pure 8CB. The enthalpy change associated with I–N phase transitions shows slightly different behavior on heating and cooling and it also shows crossover behavior at lower and higher QD content. The enthalpy change associated with N–SmA phase transitions is independent of QD loading and thermal treatment. Given the homogeneous and random distribution of QD in these nanocomposites, we interpret that these results as arising that the nematic phase imposes self-assembly on QDs to form one-dimensional arrays leading to QDs and induces net local disordering effect in LC media.     

Surface ordering near the smectic-A-smectic-C transition in thin, free-standing, liquid-crystal films

Optical reflectivity measurements have been conducted near the smectic-A-smectic-C phase transition in free-standing films with thickness between two and eleven molecular layers. The temperature dependence of the reflectivity in thin film differs significantly from that in thick films. The optical thickness per layer increases in films with two to five layers as a result of cooling, in contrast with thick films. The average layer spacing was found to decrease with decreasing film thickness. Zh. éksp. Teor. Fiz. 111, 949–953 (March 1997) Published in English in the original Russian journal. Reproduced here with stylistic changes by the Translation Editor.     

Phase transition studies in polymesomorphic compounds

Summary The structural parameters of theS _A andS _B mesophases and the phase transitionS _G -S _B andS _B -S _A of the compound N-(p-n-octyloxybenzylidene)p-n-butyl aniline were investigated by X-ray diffraction. This work is complementary to the previous structural determination in the crystalline solid phase andS _G phase of the same compound. TheS _B hexagonal lattice parameters atT=66°C area=b=(5.03±0.01) ? andc=(27.6±0.2) ?. TheS _A structural data atT=80°C are layer thicknessd=(26.9±0.2) ? and intermolecular distanceD=(5.04±0.05) ?. The interlayer and the in-plane intermolecular distances were obtained as a function of temperature through all the mesophases. Pretransitional effects were observed at theS _G -S _B phase transition, which corresponds to a progressive reduction to zero of the molecule inclination angle with respect to the normal to the smectic layers. The layer thickness in theS _A phase appeared smaller than that in theS _B due to molecular axial fluctuations. To speed up publication, the authors of this paper have agreed to not receive the proofs for correction.     

Vibrational and reorientational dynamics,crystal structure and solid–solid phase transition studies in [Ca(H2O)6]Cl2 supported by theoretical (DFT) calculations

[Ca(H₂O)₆]Cl₂ between 93 and 300 K possesses two solid phases. One phase transition (PT) of the first‐order type at = 218.0 K (on heating) and = 208.0 K (on cooling) was determined by differential scanning calorimetry. Thermal hysteresis of this PT (10 K), as well as the heat flow anomaly sharpness, suggests that the detected PT is a first‐order one. The entropy change value [ΔS ≈ 8.5 J mol⁻¹ K⁻¹ ≈ Rln(2.8)] associated with the observed PT suggests a moderate degree of molecular dynamical disorder of the high‐temperature phase. The temperature dependencies of the full width at half maximum values of the infrared band are due to ρ(H₂O)A₂ mode (at 205 cm⁻¹), and two Raman bands are arising from τ(H₂O)E and τ(H₂O)A₁ modes (at ca. 410 and 682 cm⁻¹, respectively), suggesting that the observed PT is associated with a sudden change of speed of the H₂O reorientational motions. The estimated mean value of activation energy for the reorientation of the H₂O ligands in the high‐temperature phase is ca. 11.4 kJ mol⁻¹ from Raman spectroscopy and 11.9 kJ mol⁻¹ from infrared spectroscopy. X‐ray single‐crystal diffraction measurement and spectroscopic studies (infrared, Raman and inelastic neutron scattering) also confirm that [Ca(H₂O)₆]Cl₂ includes two sets of differently bonded H₂O molecules. Ab initio calculations of the complete unit cell of one molecule of calcium chloride with a different number of water molecules (2, 4 and 6) have also been carried out. A comparison of Fourier Transform Infrared (FT‐IR), Fourier Transform Raman Scattering (FT‐RS) and inelastic neutron scattering spectroscopies results with periodic density functional theory calculations was used to provide a complete assignment of the vibrational spectra of [Ca(H₂O)₆]Cl₂. Copyright © 2016 John Wiley & Sons, Ltd.     

Dielectric investigation on some binary mixtures of hockey-stick-shaped liquid crystal and octyloxy-cyanobiphenyl

We report the measurement of temperature-dependent dielectric parameters in some binary liquid crystal mixtures comprising of a hockey-stick-shaped mesogen 4-(3-decyloxyphenyliminomethyl) phenyl-4-decyloxycinnamate (SF7) and calamitic compound 4′-octyloxy-4-cyanobiphenyl (8OCB). All the investigated mixtures possess a large positive dielectric anisotropy (Δε), although a noticeable reduction has been found by increasing the diverse-shaped dopant concentration. Investigation on the pretransitional behavior in the vicinity of isotropic to nematic (I–N) phase transition suggesting a tricritical character for all the studied mixtures. Parameterization of dielectric permittivity close to the nematic to smectic-A (N–Sm-A) phase transition exhibits non-universal values of the critical exponents describing a second-order nature of the transition. Systematic variation of critical exponents against dopant concentration and McMillan ratio reveals a well consistency with those obtained from the high-resolution optical birefringence measurements.     

Optical studies of the ferroelastic phase transitions in KFe(MoO4)2

ABSTRACT

The ferroelastic phase transitions in KFe(MoO₄)₂ have been studied by means of polarized light microscopy. The crystal undergoes a sequence of ferroelastic phase transitions. It has been found that the second transition consists of two transitions separated by the temperature interval of about 0.4 K. Both these transitions are of the first order and are evidenced through a phase front passing, without the domain structure rebuilding. The disposition of optical indicatrix axes n_g, n_m has been established, and the birefringence has been measured in the plane (0001) in the temperature range covering all ferroelastic phases. From temperature studies of the morphic birefringence, a critical exponent of the order parameter has been estimated.     

Structures and orientational transitions in thin smectic films of tilted hexatic

We present detailed systematic studies of structural transformations in thin liquid crystal films with the transition from the smectic-C (SmC) to hexatic (HSmB) phase. For the first time, all possible structures reported in the literature are observed for one material (5O.6) with varying temperature and thickness. In unusual modulated structures, the equilibrium period of stripes is twice the domain size. We interpret these patterns in the framework of a phenomenological Landau-type theory, as equilibrium phenomena produced by a natural geometric frustration in a system having spontaneous splay distortion. The text was submitted by the authors in English.     

Phase transitions in polymesomorphic compounds

Summary The results of a X-ray diffraction study, performed on N(p-n-pentyloxy benzylidene)p′-n-octylaniline (50.8) and N(p-n-pentyloxy benzylidene)p′-n-decylaniline (50.10) are presented. The investigated compounds exhibit smecticB, smecticA and nematic phases in between solid and isotropic liquid phases. In addition, after a thermal cycle the 50.8 compound exhibits a smecticG phase at room temperature. The interlayer and the in-plane intermolecular distances were obtained as functions of temperature through all the mesophases. A phase coexistence has been observed during the solid to smecticB and the smecticB to smecticA phase transitions of the 50.8 compound. The nematic to smecticA phase transitions are found to be of second order for both compounds. Pretransitional effects due to cybotactic-cluster formation in the nematic region are observed.

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Riassunto Si presentano i risultati di uno studio effettuato mediante la diffrazione dei raggi X sul N(p-n-pentilossi benzilidene)p′-n-ottilanilina (50.8) e sul N(p-n-pentilossi benzilidene)p′-n-decilanilina (50.10). Tra le fasi solida ed isotropica i composti esaminati presentano le mesofasiS _B ,S _A e nematica. Inoltre, dopo un ciclo termico il composto 50.8 presenta una faseS _G a temperatura ambiente. Sono state misurate le distanze tra i piani smettici e le distanze laterali tra le molecole in tutte le mesofasi in funzione della temperatura. Una coesistenza di fase è stata osservata durante le transizioni solido →S _B eS _B →S _A del composto 50.8. Le transizioni nematico-S _A sono risultate del second'ordine per entrambi i composti. Nelle fasi nematiche sono stati osservati effetti pretransizionali dovuti alla formazione di gruppi cibotattici.

     

Spectroscopy properties of the amide group in valpromide and some derivatives with antiepileptic activity

Infrared spectra at 300 and 77 K and Raman spectra at 300 K of the valpromide (Vpd), N‐substituted derivatives, N‐ethylvalpromide (Etvpd), N‐isopropylvalpromide (Ipvpd) and the N,N‐disubstituted derivative, N,N‐dimethylvalpromide (Dmvpd) with antiepileptic activity, have been measured and analyzed with results derived from computational chemistry calculation. In agreement with theoretical predictions, experimental data indicate that while in Etvpd, Dmvpd and Ipvpd there are four different conformational co‐existing components (Etvpd: TTCG⁺, TCCG⁻, TTTC, G⁺G⁺C G⁺; Dmvpd: TTCC, G⁻TTA⁺, G⁺A⁻TC, G⁺A⁻C A⁺; Ipvpd: TTCT, TCCT, TCCC, G⁻ TTT) in the Vpd there are only three distinct stable conformations of C₁ symmetry group: TTC, TCT, G⁺G⁺T. Based on the accuracy of the B3LYP calculation, with the 6‐31 + G** basis set estimated by comparison between the predicted values of the vibrational modes and the available experimental data, we performed a structural and vibrational study of the amide group in the Vpd and their derivatives. We found that small nonplanarity deviations of C(O)N backbone induce significant changes on the structural and spectroscopic properties. These are not compatible with the decreasing of the resonance effect as it is produced when the twisting around the C(O) N increases. From the Natural Bond Orbital (NBO) analysis the existence of stabilizing electrostatic interactions of type C H···O/N and C H···H N/C, which induce significant structural changes and a complex electronic redistribution of charge on the π‐system in those structures becomes evident. We view this as a consequence of the filled electron density change Lewis‐type NBOs type lp_{O1, 2}, lp_N1, σ_{(C H)N acyl} and empty non‐Lewis NBOs type σ*_{(C H)N acyl}, σ*_{N H}. Copyright © 2009 John Wiley & Sons, Ltd.     

The effect of DNA phase structure on DNA walks

We have performed several kinds of DNA walks which often are the first steps for further analysis of DNA structure and long range correlations. The DNA walks analysing frequency of G+C versus A+T cannot indicate the coding strand while purine versus pyrimidine DNA walks or two-dimensional (A-T,G-C) DNA walks in some instances can indicate the coding strand but cannot resolve the coding frame. The modified two-dimensional (A-T,G-C) DNA walks respecting the three-nucleotide codon structure show very high correlation in nucleotide composition of DNA coding sequences. They can distinguish between coding and non-coding sequences and indicate the strand and the phase in which DNA is coding. Received: 8 January 1998 / Received in final form and Accepted: 28 January 1998     

Cerussite,PbCO3 – a new Stimulated Raman Scattering (SRS)‐active crystal with high‐order Stokes and anti‐Stokes lasing

Orthorhombic PbCO₃, known as natural crystal cerussite, is presented as a new Stimulated Raman Scattering (SRS)‐active crystal. With picosecond laser pumping high‐order Raman‐induced χ⁽³⁾ generation is observed. All registered Stokes and anti‐Stokes sidebands in the visible and near‐IR are identified and attributed to the SRS‐promoting phonon mode A_1g of the carbonate group, with ω_SRS ≈ 1054 cm⁻¹. The first Stokes steady‐state Raman gain coefficient in the visible spectral range is estimated as well to a value not less than 4.6 cm·GW⁻¹.