Heat Capacity and Thermodynamic Properties of p'-Substituted p-n-Hexyloxybenzylideneaniline. I. p-n-Hexyloxybenzylideneamno-p'-Benzonitrile (HBAB)

Abstract

The heat capacity of the nematogenic liquid crystal, HBAB, has been measured between 15 K and 385 K by using an adiabatic calorimeter. The crystal-crystal phase transition has been discovered at 27 K below the crystal-nematic phase transition temperature. The transition temperatures, the enthalpies and the entropies of the three phase transitions have been determined: T ₁ = 306.98 K, ΔH _t = 5.11 kJ mol^?1, ΔS _t = 16.7 JK^?l; T _m = 334.05 K, ΔH _m = 23.77 kJ mol^?1, ΔS_m = 71.2 J K^?l mol^?1; and T _c = 375.10 K, ΔH _c = 1.75 kJ mol^?1, ΔS _c = 3.2 J K^?1 mol^?1, respectively. The thermodynamic functions of HBAB from 0 K to 385 K have been determined from the heat capacity data and the enthalpies of the transitions. Two crystal modifications, one yellow and granular form and the other white and needle-like form, have been obtained during the course of the preparation of the sample. It turned out that the yellow form was the stable crystal and the white the metastable modification. The crystal-crystal phase transition has been discussed as an onset of partial melting from the entropy consideration. In this connection the total entropies of the transitions, 91.1 J K^?1 mol^?1 has been proposed to be an important measure of melting.     

Unusual Glassy Smectic-G Liquid Crystal: Heat Capacity of N-p-n-Pentyloxybenzylidene-p'-n-butylaniline between 11 and 393 K

Abstract

The heat capacities of the title compound (C₃H₁₁,O—C₆H₄,- CH=N—C₆H₄,—C₄H₉, abbreviation 5O ? 4) with a purity of 99.92 mole percent have been measured with an adiabatic-type calorimeter between 11 and 393 K. The transition temperature and the enthalpy and entropy of phase transition for stable crystal → S_G, S_G → N and N → isotropic liquid were T _c = 299.69 K/ΔH = 22.68 kJ mol^?1/ΔS = 75.70 JK^?1 mol^?1, 325.72/7.11/21.79 and 342.48/1.78/5.22, respectively. The crystal which melts at 285.5 K is a metastable modification. The S_A phase hitherto reported in between S_G and N does not exist. The glassy So state was realized by rapid cooling of the specimen from the So phase. The molar enthalpy of the glassy S_G state at 0 K was by (10.1±0.1) kJ mol^?1 higher than that of the stable crystalline state and the residual entropy of the glassy state was (9.40±0.83) JK^?1 mol^?1. The relaxational heat-capacity anomaly was observed from as low as 100 K and double glass transition phenomenon occurred around 200 K; a quite unusual phenomenon which has never been observed for the glassy states of nematic and cholesteric liquid crystals. The present results give a fair evidence that the unusual glass transition phenomenon previously found for the S_G state of 6O?4 (a homologous compound) is not exceptional at all but common to the smectic glasses; at least common to the glassy S_G states. Two possible origins responsible for the double glass transitions have been discussed.     

Chain Ordering and Sm-N Transition in Different Members of the Homologous Series 4-(4′- n-AIkoxy Benzylideneamino) Biphenyls

Our extension of Marcelja's theory of nematic-isotropic transition to the smectic A phase is used to calculate the smectic A-nematic (Sm-N) transition temperatures for the homologous series 4–(4′- n-alkoxy benzylideneamino) biphenyls. In a mean field calculation, explicit inclusion of the conformational energies of the alkyl chain of a molecule in the partition function shows an even-odd effect generally observed in experiments on Sm-N transitions. It is found that the members of this series showing the smectic phase will have first order Sm-N transitions and the transition entropies for individual members are calculated.     

Etude par Calorimetrie,Diffraction des Rayons X et Diffusion Raman d'une Transition de Phase dans (C4 H4P) Mn (CO)3

Abstract

Heat capacity measurements (95-300K), X-ray diffraction (78-300K) and low frequency Raman spectroscopy (10-350K) have evidenced an order-disorder phase transition in phosphacymantrene, (C₄ H₄P) Mn (CO)₃. This transition has been characterized by a monoclinic ←→ triclinic structural change at about 110 K and by a pretransitional phenomenon. The measured transition enthalpy and entropy are 480 ± 10J.mil^?1 and 4.17 ± 0.08J.K^?1 mol^?1, at 115 K, respectively.

A complete assignment of the observed Raman bands in h₄ and d₄ derivatives is proposed. From the temperature dependence of frequencies, intensities and half-widths of some Raman bands we have discussed the order, the nature and the mechanism of the phase transition: intermolecular interactions appear to be mainly involved in the mechanism and an activation energy roughly equal to 2100 ± 840 J. mol^?1 has been determined.     

The Heat Capacity of 1,2,4,5-Tetracyanobenzene and of its 1 : 1 Charge-Transfer Complex with Pyrene from 10 K to 300 K

Abstract

The heat capacity of 1,2,4,5-tetracyanobenzene and of its 1 : 1 complex with pyrene has been measured form ～ 10 K to ～ 300 K. There is a transition in the complex which appears as an approximately symmetrical anomaly in the heat capacity-temperature curve between ～ 220 K and 250 K, but the heat capacity is not reproducible in this region even if the sample is subjected to the same preliminary thermal treatment. Moreover, the room-temperature form can be undercooled at least down to 150 K. In the two runs in which the heat capacity anomaly was most prominent. C_p reached its maximum at 232 ± 1 K. The largest measured value of the entropy of transition was 9.2 JK^?1 mol^?1. This value is compatible with but does not prove, the exisstence of static disorder above the transition, with one of the components (pyrene, in the light of the structural information) having at least two distinguishable orientations. At all temperatures in the range covered (apart from the transition region), the heat capacity of the complex is less than the sum of that of the pure components, and possible reasons for this are brifly discussed.     

The Metabolemeter II-Detection of Mesogenic Polymorphism under Pressure Using Small Quantities

Every first order phase transition for mesogens can be detected by the continuous recording, as a function of temperature, of the pressure of the sample enclosed in a metallic cell. Studies of a compound (octylcyanobiphenyl) exibiting several transformations (crystal-smectic A, smectic A-nematic, nematic-liquid) show that this barometric method is convenient to detect polymorphism from one thermobarogram, and to draw the Clapeyron phase diagram; the smectic A-nematic transition involving small enthalpy and volume changes is clearly observable and this method is very sensitive. Moreover, the enthalpies and volumes of transition can be deduced from the pressure changes. Lastly, pressure and temperature are intensive data, and very little sample (1.3 mm³) may be used.     

Smectic C-to-Nematic Transition in p-n-Heptyloxybenzylidene-p-aminobenzoic Acid

The results on the phase transitions in p-n-heptyloxybenzylidene-p-aminobenzoic acid are reported. The microscopic and X-ray studies show that the compound goes over from smectic C to nematic phase directly. The transition entropies and specific heat are determined using differential scanning calorimetry. The translational order parameter is determined using low angle X-ray intensity measurements. The tilt angle in smectic C phase is found to be 56° and is independent of temperature. The X-ray data also lead to the conclusion that the compound exists as a dimer.     

Density Measurements in cholesteryl Myristate

Abstract

The densities of cholesteryl myristate were measured in the smectic, cholesteric, blue and isotropic phase. At the transitions smectic/cholesteric and bluelisotropic within the temperature interval of 0.01 to 0.02°K discontinuities in the density curves occured. which indicate phase transitions of first order. At the transition cholesteric/blue only a change in the expansion coefficients was observed. which points to a phase transition of second order.

Die Dichten von Cholesterylmyristinat wurden in der smektischen, cholesterinischen, blauen und isotropen Phase gemessen. An den Umwandlungen smektisch/cholesterinisch und blau/isotrop treten innerhalb eines Temperdturintervah von 0,O1 bis 0.02°K Diskontinuitäten in den Dichtekurven auf. welche Phasenumwandlungen 1. Ordnung anzeigen. An der Umwandlung cholesterinisch/blau ist nur eine änderung in den Ausdehnungskoeffizienten vorhanden, welche auf eine Phasenumwandlung 2. Ordnung hinweist.     

Mesophase design: I. Molecular structure and crystal packing motifs of 4-alkylcyanobiphenyls

A comparative study of the crystal and molecular structures of 4-alkylcyanobiphenyls (ACB-n) C _n H_{2n + 1}-C₆H₄-C₆H₄-CN (n = 2, 4–12) belonging to liquid-crystal compounds revealed no direct relationship between the molecular geometry of these compounds and their liquid-crystalline properties. Mesogenic properties are determined by the alternation of the loosely packed aliphatic and closely packed aromatic regions in the crystals. Graphs describing the crystals and mesophases were designed for ACB-n. The graph for nematic mesogens of ACB-n (n = 5−7) has one structure-forming element. The graph for smectic-nematic (n = 8 and 9) and smectic (n = 10−12) mesogens have more than one structure-forming element. If different types of secondary bonds in the smectic mesophase have equal energies, the disruption of these interactions caused by the temperature rise occurs simultaneously and the system undergoes a transition from the smectic phase to the isotropic liquid. If the energies of different types of secondary bonds are different, the destruction of the mesophase occurs in steps and the phase transitions are more complicated (smectic-nematic-isotropic transitions).     

Dielectric Anisotropy and the Order Parameter of HXBBA

Abstract

Measurements of dielectric anisotropy (Δ?), refractive indices (n _e , n _o), birefringence (Δn) and density (ρ) have been made in the nematic and smectic phases of N(-p-hexyloxybenzylidene)-p-butylaniline (HXBBA). The results indicate that the various transitions are of the first order type except smectic B-smectic G, which may be a second order transition. The order parameter S has been determined using the isotropic internal field model (Vuks approach) and the anisotropic internal field model (Neugebauer's approach) and both the values agree fairly well. The dielectric anisotropy (Δ?) increases strongly in the smectic phases whle S increases only slowly. It is interpreted by an increase of dipole-dipole correlations.     

Magnetic Field Dependence of Laser Light Scattering through CBOOA in Nematic Phase

In the nematic phase of homeotropically aligned liquid crystals which have the smectic A phase at lower temperature, interference rings have been observed above some threshold magnetic field H_c which nearly coincides with the ocurrence of Freedericksz transition, only for the light polarized parallel to the direction of magnetic field. This interference ring disappears with increasing magnetic field above second threshold H_s . The origin of these interference rings is explained tentatively by a periodic deformation of bulk directors.

The bend elastic constant of CBOOA estimated from the measurements of the interference ring as a function of temperature, diverges with the critical exponent (0.5 ± 0.02) near the smectic A-nematic transition temperature and agrees well with a recent suggestion of McMillan based on the mean field theory.     

Phase Transition and Phase Diagram of Anion Radical Salts of [(C6H5)3PCH3]+ [(C6H5)3AsCH3]+ ×(TCNO)− 2 (0 ≤×≤ 1)

Abstract

Much attention has been paid to the solid anion radical salts of 7,7,8,8-tetracyanoquinodimethane (TCNQ), because of their prominent electronic properties.^1–4 In particular, the salts containing mixed cations represented by [(C₆H₅)₃PCH₃]⁺ _1–x [(C₆H₅)₃AsCH₃]⁺ _× (TCNQ)^? ₂, (0 ≤×≤ 1), are known to undergo phase transitions at 1 atm pressure in the solid state.^1–4 The phase transition of pure methyltriphenylphosphonium salt, (x = 0.00), takes place at 315.7 K. Heat-capacity measurements of this phase transition have been made by Kosaki et al. ³ The transition has thus been found to be of the first order. The enthalpy and the total entropy change associated with the phase transition were experimentally determined to be 485.18 cal/mol and 1.7206 cal/deg.mol, respectively. For the solid solutions, it was found that the transition temperature (T_c ) is increased, while the magnitude of the heat of transition (δH) is decreased, progressively with an increase in the composition parameter (x) and that pure methyltriphenylarsonium salt, (x = 1.00), has no such phase transition up to the decomposition temperature of about 480 K at 1 atm pressure.^1–3 Figure 1 shows the experimental relation between T _c and x, together with the relation between δH and x.⁴ In the present paper, we attempted to explain thermodynamically the phase diagram of Figure 1 for the solid solutions of those TCNQ anion radical salts.     

Glass transition in the stable crystalline state of 2,5-dichlorothiophene

Heat capacities of 2,5-dichlorothiophene were measured between 13 and 300 K using an adiabatic calorimeter. Fusion temperature at the triple point was found to be 232.72 K and the enthalpy and entropy of fusion were determined to be 11.87 kJ mol⁻¹ and 51.0 J K⁻¹ mol⁻¹, respectively. The glass transition temperature and the associated heat capacity jump in the stable crystalline phase were determined to be 138 K and 0.53 J K⁻¹ mol⁻¹, respectively. The activation energy was estimated to be 45 kJ mol⁻¹, and a linear relation between the activation energy and the glass transition temperature was confirmed to hold.     

Density and Ultrasonic Velocity Measurements in Hexyloxybenzylidene Phenylazoaniline

The temperature variation of density and ultrasonic velocity of the liquid crystal hexyloxybenzylidene phenylazoaniline are reported. The density across the smectic A—smectic B transition is more predominant than the other transitions. The density variation with temperature and the calculated thermal expansion coefficients suggest that the transitions isotropic liquid—nematic, nematic—smectic A and smectic A—smectic B are of first order. Anomalous behaviour of ultrasonic velocity is observed across the isotropic liquid—nematic transition and prominent dips in velocity are observed at the nematic—smectic A and smectic A—smectic B transitions. The adiabatic compressibility (β _ad ) Rao number (R _a ) and molar compressibility (B) are estimated using the experimental density and ultrasonic velocity.     

Raman Scattering From TMTSF-Salts

Abstract

Experimental results for Raman scattering from TSeF, TMTSF-D12, (TMTSF)₂PF₆ and (TMTSF)₂AsF₆ excited with blue-green laser light at liquid nitrogen temperature are reported. A strong resonance effect for excitation of TMTSF with 4579 Å was observed and an assignment for the a_g modes was obtained by comparison with results from TTF. Methyl group modes were identified at 328 cm^?1, 472 cm^?1 and 916 cm^?1, respectively. Charging the TMTSF molecule resulted in a strong frequency shift of -122 cm^?1 and -76 cm^?1 for the a_gu₂ mode, respectively. A study of the temperature dependence of the scattering from molecular modes between 2 K and 300 K did not reveal any correlation with the phase transitions in the conducting compounds.     

The Smectic Phase of p. p'-di-n-Heptylazoybenzene: A 1H NMR study of Its Anisotropy of Diamagnetic Susceptibility and of Its Use for Measuring Proton Chemical Shift Anisotropies

Abstract

The anisotropy of the diamagnetic susceptibility of the smectic liquid crystal p,p'-di-nheptylazoxybenzene (HAB) has been measured by use of ¹H NMR. The value of (X‖ - X⊥) is found as e.g. 1.418. 10^?6 at 318 K. Furthermore, we have observed local solvent effects of HAB on dissolved globular molecules, which are of the order of 0.10 to 0.19 ppm for methane, 0.37 to 0.44 ppm for tetramethylsilane, 0.35 to 0.40 ppm for tetramethylstannane, and 0.39 to 0.41 ppm for neopentane. These values are dependent on temperature and may lead to large uncertainties in the determination of proton chemical shift anisotropics. A theoretical treatment of the magnetic fields in the annulus of a double-wall tube and inside the inner tube filled with a smectic liquid crystal and variable angle between the applied field and the optic axis is also given.     

Vibrational entropy near glass transition in a chalcogenide glass and supercooled liquid

Raman spectroscopic measurements have been performed on Ge₂₀Se₈₀ glass and supercooled liquid at temperatures ranging between 298 and 500 K. Temperature dependent softening of vibrational mode frequencies has been used in conjunction with the available vibrational density of states data at ambient temperature to estimate the relative contributions of vibrational and configurational entropies across glass transition. Nearly 20% of the additional entropy above glass transition is estimated to be vibrational. Thermal expansion effect on vibrational mode softening is found to be insufficient to account for the anharmonic component of vibrational entropy implying possible coupling between the vibrational and configurational entropies at temperatures above T_g. These results may have important consequences in shaping our understanding of various aspects of glass transition.     

Liquid-Crystalline Solvents as Mechanistic Probes. 10. Dynamics of Intramolecular Quenching of Pyrenyl Fluorescence in the Liquid-Crystalline and Isotropic Phases of a Cholesteric Solvent1

Abstract

The effect of cholesteric order in a 59.5/15.6/24.9 (w/w/w) mixture of cholesteryl oleate/cholesteryl nonanoate/cholesteryl chloride (CM) on the intramolecular fluorescence quenching of l,3-bis(l-pyrenyl) propane (P3P) has been explored. A comparison with fluorescence quenching of N,N-dimethyl-4-[3-(l-pyrenyl)propyl]aniline (P3D) in CM is made. From the Arrhenius activation parameters for quenching in the cholesteric and isotropic phases, it is concluded that the motions which take the ground state conformers of P3P to their quenching transition state are nearly impervious to macroscopic CM mesophase order: in the cholesteric phase, E'_a = 10.5±0.4 kcal mol^?1 and ΔS^? = 1 ± 1 eu; in the isotropic phase, E'_a = 10.0 ± 0.2 kcal mol^?1 and ΔS^? = 0 ± 0.5 eu. An explanation of these results is advanced.     

Study of the Phase Situation in 4-n-pentylphenyl-4′-n-heptyloxythiobenzoate (7S5)

Results of measurements of the specific heat of 4-n-pentylphenyl-4′-n-heptyloxythiobenzoate in the temperature range from about 90 K to about 370 K are presented. They were carried out with an adiabatic calorimeter, a differential scanning calorimeter and a polarizing microscope. Adiabatic calorimetry revealed four anomalies in the specific heat vs. temperature curve viz., two associated with transitions in the solid phase, one with melting and one with the liquid-crystalline nematic-to-isotropic phase transition. Also, two modifications of the solid, stable and metastable, were found to exist. In addition, the DSC method applied in the 285 K to 370 K range revealed a smectic C phase, but only when the sample is being cooled. The polarizing microscope showed the structural polymorphism of the solid phase in the 297 K to 327 K range to be more complex than would stem from adiabatic calorimetry and DSC measurements alone. It also confirmed the occurence of the S_c phase on cooling as ascertained by the DSC method.     

Proton Spin-Lattice Relaxation Dispersion Study of the Ferroelectric Smectic Phases in Dobambc

The frequency dispersion of the proton spin-lattice relaxation rates T^?1 ₁ in the ferroelectric smectic phases of DOBAMBC has been determined in the MHz region and analyzed in terms of the order fluctuations, self-diffusion, and rotational contributions to T^?1 ₁. In the smectic C* phase the main rate determining contribution are order fluctuations with a T^?1 ₁ ∝ ν^?1/2 _L frequency dependence, whereas in the smectic H* phase the translational diffusion, which is slower than in the smectic H* phase, determines the dispersion of T^?1 ₁.