The Smectic A-Smectic A Transition in a Homologous Series of 4-(4-Alkoxyphenoxycarbonyl)phenyl 5-Cyano-2-furancarboxylates

The thermal properties for a homologous series of 4-(4-alkoxyphenoxycarbonyl)phenyl 5-cyano-2-furancarboxylates have been examined. The smectic A phase commences from the hexyloxy homolog, and the octyloxy, nonyloxy and decyloxy homologs show two kinds of smectic A phases and experience the S_A-S_A transition. The lower phase which shows a typical fan shaped texture is assigned to a smectic A with a bilayer arrangement of the molecules (S_A2). The higher phase which shows a typical fan shaped texture on the cooling stage and a broken fan one on the heating stage, is assigned to a smectic A with a partially bilayer arrangement of the molecules (S_Ad). The S_A-S_A transitions could be easily detected by both differential scanning calorimeter and optical microscopic method.     

Reentrant nematic phase with the new pentamorphism n sa sc nre sare in a mixture

Three homologues of the new series 4 - cyanobenzylidene - 4′ - [4″ - alkoxybenzoyloxy] anilinie are presented. One of them exhibits a reentrant nematic and a reentrant smectic A phases. A diagram of state with 4 - cyanobenzylidene - 4′ - [4″ - decyloxybenzoyloxy] aniline and 4 - [4″ - decyloxybenzoyloxy] benzylidene - 4′ - cyanoaniline has been studies. In the system studies, the following new phase sequence was found with decreasing temperature N S_A S_C N S_A.     

The Existence of SA 1, SA d,SA 2 and S∼C Smectic Phases in the New Series 4-Alkoxyphenyl-4′- [4″-Cyanobenzyloxv] Benzoates

A homologous series of 4 - alkoxyphenyl-4′-[4″-cyano-benzyloxy] benzoates have been synthesized. Within this series three smectic A phases : S_{A 1}, S_{A d} and S_{A 2} were identified by mean of small angle X-Ray scattering. Furthermore three derivatives exhibit the novel smectic phase made of ribbons S～_C.     

On Packing in Smectics: X-Ray Diffraction Study on the Homologous Series 4, 4′-di-n-alkyl-azoxybenzenes

The smectic layer spacings of the homologous series 4,4′-di-n-aLkyl-azoxybenzenes with alkyl chain length from m=6 to m=10 were determined and a mean layer spacing increment of 0.109 nm per methylen group was found.

The results are discussed in relation to two different models:

(I) The diffuse cone model of A. de Vries. Differences between layer spacing and molecular length are attributed to orientational disorder leading to mean tilt angles up to 18° of the molecules.

(II) The second model differentiates between the directions of the rigid aromatic core and that of the alkyl chains. Orthogonality of the rigid core long axis to the smectic layer planes and therefore tilting of the alkyl chains results in coincidence between calculated layer thickness and measured smectic layer spacings.

X-ray film erposures of oriented samples clearly demonstrate the S_A -type of the phases and confirm a slight odd-even effect of layer spacing increments.     

Calorimetric investigations of 2-[4-n-alkylphenyl]-5-[4-n-alkyloxyphenyl]-pyrimidines

We investigated temperatures and enthalpies of the phase transitions by means of the calorimeter DSC-2. The kinetics of polymorphic transitions in the solid state, the pretransitional behaviour in the liquid crystalline phases and the transition enthalpies, ranging from 3 J/mol up to 50 kJ/mol, in relation to the occurrence of the liquid crystalline phase types S_G, S_F, S_C, S_A, and N are points of discussion.     

Structure and Packing in Smectic E and Smectic A Phases in the Series of 4-n-Alkyloxy-4′-alkanoylbiphenyls

By means of X-ray investigations the structure of the S_E and S_A-phases of the title compounds has been studied. The lattice parameters of the orthorhombic S_E structure are given. The thickness of the smectic layers is nearly the same in the S_E and S_A phases and agrees well with the length of the molecules in their most stretched form. That suggests a similar conformation of the alkyl chains in both phases. A model of the packing in the S_A phase is discussed which is based on a dense packing with antiparallel orientation of molecules with a non-symmetrical shape.     

A Chevron Model of the Electroclinic Effect across the SA*-SC* Phase Transition in a SSFLC

Abstract

We here present a thorough model of the electroclinic effect across the S_A*-S_C* phase transition, being the first to incorporate the vertical chevron structure. This structure has previously been observed in the S_C* phase of surface-stabilised FLC (SSFLC) devices, and some experimental evidence suggests that it may be present in the electroclinic effect [1–3]. Excellent agreement is achieved when comparing this model with previous experimental results of optical tilt through the A-C* transition for a SSFLC cell with homogeneous alignment.     

Dielectric relaxation in nematic,smectic A and C phases in the MHz region

The dielectric properties of four 4-n-alkyloxyphenyl 4-n-alkyloxybenzoates with nematic, smectic C and partly additional S_A phases were investigated. In 2 substances for ϵ′ reproducible values also in the smectic phases resulted allowing to conclude on a good orientation in the samples. In all 4 substances the dielectric relaxation in the MHz region was measured and the respective activation energies were determined. The molecular mobility in the S_C phases is higher than in the S_A phases, however comparable with that of the nematic phases. In all 3 phase types a single relaxation mechanism exists, which is explained as rotation of the molecules around a short axis.     

Dielectric behaviour of the smectic I phase

The dielectric behaviour of 4-n-butyl-4′-n-dodecyloxyazobenzene was investigated in the smectic A, C and I phases from 10² to 3 ṁ 10⁶ Hz. In all phases a dielectric relaxation process could be found which is connected with the reorientation of the molecules around their short molecular axes. At the phase transition S_C/S_I a step of one decade was observed in the relaxation frequencies.     

The Liquid Crystal Properties of Some Aromatic Esters Derived From Naphthalene

Abstract The 2,6-naphthylene ring system has been used to replace each of the 1,4-phenylene rings in various 4-substituted-phenyl 4-substituted-benzoates, where the substituents were alkyl and/or alkoxy. For the dialkyl esters, the N-I temperatures were increased by 60–80° compared with the simpler, analogous phenyl benzoates, irrespective of the position of the 2,6-naphthylene ring. For the esters with one alkyl and one alkoxy substituent, the N-I temperatures were again higher than those of the corresponding phenyl benzoates (～60°); S_A properties were quite pronounced in the 6-alkyl-2-naphthyl 4-alkoxybenzoates. When both substituents were alkoxy groups, S_C phases of quite wide thermal range were exhibited, usually preceded on cooling by a nematic and a S_A phase.     

New chiral liquid crystal materials based on menthol: Synthesis and phase behavior

To study structure-mesomorphism relationships of chiral materials based on menthol, a series of chiral compounds containing menthyl group were synthesized. Their chemical structures, formula, and the purity were characterized by Fourier transform infrared (FTIR), proton nuclear magnetic resonance (¹H NMR), and elemental analyses. The corresponding phase behavior was investigated with differential scanning calorimetry (DSC) and polarizing optical microscopy (POM). The effect of the mesogenic core, flexible spacer, and terminal groups on the mesophase formation and type was discussed. The compound 4-menthyloxyacetoxybenzoyloxy-4′- (6-bromohexyloxy)biphenyl showed the smectic A (S_A) and cholesteric phases on heating cycle, and a cubic Blue Phase, chiral smectic C (S_C^*), S_A and cholesteric phases on cooling cycle. However, the compound 4-menthyloxyacetoxybenzoyloxy-4′-(4-bromobutoxy)biphenyl showed enantiotropic S_C^*, S_A and cholesteric phases, moreover, the platelet texture of a cubic Blue Phase was also observed on cooling cycle. The methacrylate monomer with six flexible methylene units and three phenyl rings as mesogenic core only showed cholesteric phase, while the methacrylate monomer with four methylene unit and three phenyl rings showed the S_C^* and cholesteric phases. However, when the terminal methacryloyloxy groups in the monomers were substituted by thiophenylacryloyloxy groups, the corresponding monomers all show no mesophase. With increasing the rigidity of mesogenic core or decreasing the flexible methylene spacer length, the corresponding melting temperature (T_m) or isotropic temperature (T_i) increased.     

New Polymorphism Variants in Binary Systems of 4,4′-BIS-(n-NONYLAMINO)-BIPHENYL

Three diagrams of state with 4,4′-bis-(n-nonylamino)-biphenyl and some cyanoethyl compounds have been studied. In the systems studied the following new phase sequences were found with decreasing temperatures: S_A N; S_A S_C N; S_A S_C N S_C; and S_A N S_C.     

The nematic reentrant phase in binary mixtures composed of a 4-(trans-4-alkylcyclohexyl)-1-(2-cyanoethenyl)-benzene and 60CB

It is shown that the smectic A phase in the 4-(trans-4-n-alkylcyclohexyl)-1-(2-cyanoethenyl)-benzene EPCH homologous series has a partially dimolecular structure of A_d type. The EPCH compounds show a virtual S_Ad → N_re transition revealed in mixtures of 60CB with members of the homologous series with five or more carbon atoms in the alkyl chain. The temperature dependences of electric conductivity and electric permittivity of the 8EPCH-60CB mixture have been tested in the three successive phases, N_re, S_Ad and N.     

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.     

Enthalpies of Lyotropic Liquid Crystalline Phases. : 2. The Partial Molar Enthalpies of the Lamellar and Hexagonal Phases in the System N-Pentanol-Sodium N-Octanoate-Water

The relative partial molar enthalpies of the components in the system N-pentanol-sodium N-octanoate-water (C₅OH-NaC₈-H₂O) have been determined in the lamellar (D-) and hexagonal (E-) liquid crystalline phases formed in this system at 25°C. The enthalpies are correlated with Bragg spacings and earlier nmr studies. They indicate that (a) the minimum amount of water needed to hydrate the polar groups limits the region of existence of both D- and E-phase towards low water contents, (b) the interaction between -OH and -COO is especially important in stabilizing the D-phase containing large amounts of water, (c) the Bragg spacings and the enthalpies both change their concentration dependence markedly when the maximum amount of water that can be bound by primary hydration of the polar groups and the counter ions in the D-phase is exceeded, (d) enthalpies of transition from one phase to another are small compared to enthalpy changes with composition within the phases; the same holds for Bragg spacings. Hence, the composition of the sample (above all, the mole fraction of NaC₈) is more important than the phase structure in determining these properties. It is obvious that further studies of enthalpies in similar systems can give important information on the factors governing phase stabilities and structural changes within the phases.     

Structure and Correlations in Smectic B,F and I Phases

The question of which of the known smetic phases exhibitis long range 3 dimensional order is first discussed briefly, The major theme of the paper is the nature of the structure and correlations in S_B, S_F, and S₁ phases. Results are presented about the temperture dependence of the interlayer stacking arrangement and the existence of strong layer undulations in crystalline S_B phases and about the existence of a S_B phase with very weak positional correlations between layers and short range order within the layers.

The raltion between the S₁ and S_F structures is established for the first time: both consist of uncorrelated layers in which the molecules are tilted relative to the layer normal and have limited positional correlations within layers. Both have 3-d long range bond orientational order of the C-centered monoclinic lattices. They differ in the direction of tilt of the molecules relative to the pseudo-hexagonal packing in the plane normal to the long axes: in S_F the tilet is directed towards an edge of the hexagon, while in S₁ it is directed towards an apex. These results enable previously described phases to be classified (or reclassified) as S₁ phases and emphasise the existence of more ordered phases with similar tilt to the S₁ phase but otherwise analagous to S_G and S_H—we call these S_G and S_H. Finally a preliminary repor is given of X-ray structural work which shows that the recently postulated S₁ and S_K phases do not exist.     

The Metabolemeter III: Phase Behaviour Under Pressure of bis-(4-4′-n-Heptyloxybenzylidene)- 1, 4-Phenylenediamine and Terephthalylidene-bis-(4-n-Decylaniline)

By using a metabolemeter, the phase transitions of bis-[4, 4′-n-heptyloxybenzylidene]-1, 4 phenylenediamine (HBPD) and terephthalylidene-bis-[4-n-decylaniline] (TBDA) have been studied. The weakly first order phase transitions (S_G-S_F and S_F-S_I) and the narrowly separated phase transitions (S_C-S_A and S_A-I) of TBDA are detectable. For HBPD, both transitions S_G-S_I and S_I-S_C are individually observed and the enthalpy changes are deduced from pressure increments at the tranformations: two triple points are detected, one directly by a change of slope in the drawing of the equilibrium curves. The pressure-temperature phase diagrams are given; for each transiton, the volume increases on increasing the temperture.     

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.     

The influence of the boundary conditions on the character of the phase transition nematic-smectic C

The possibility is investigated for inducing the intermediate smectic A structure in substances with nematic and smectic C phases by the boundary forces. Different geometries are used: homeotropic, homogeneous and tilted under a different angle toward the supporting substrates of the liquid crystal cell. At homeotropic and at tilted orientation where the molecules tilt toward the supporting substrates is different from the molecule tilt in the smectic layer, an intermediate smectic A structure is observed at the transition nematicsmectic C in a narrow temperature interval. In this case the phase transition nematicsmectic C extends and transforms into phase transition of second order. At homogeneous orientation and at tilted orientation of an angle equal to the molecule tilt in the smectic layer, the transition nematic-smetic C carried out in two stages, as partially it begins on the surface still in the nematic phase and terminates at the temperature of the phase transition N-Sc. The influence of the boundary forces depends essentially on the energy of the phase transition N-S_C of the substances.     

Induced SA phases and reentrant nematic phases in binary systems of homologous terminal-nonpolar compounds

The isobaric phase diagrams of binary systems are presented in which one component is represented by members of the homologous series of swallow tailed compounds. The occurrence of induced S_A phases and reentrant nematic phases was studied in dependence on the length of the alkyl chain of the swallow tailed compound. In some selected binary systems X-ray investigations and calorimetric measurements were performed. An attempt has been made to interprete the results by a simple packing model.