Dielectric Relaxation in Low Temperature Smectic Phases of 4-n-Alkyloxy-benzylidene-amino-4′-cyanobiphenyls

The dielectric behaviour of five homologous 4-n-alkyloxy-benzylidene-amino-4′-cyanobiphenyls was investigated. Especially, we were interested in the dielectric relaxation frequencies of the smectic low temperature phases S_B and S_E as well as the dielectric behaviour in the solid state. We have found in all liquid crystalline phases a low frequency relaxation region explained by a hindered rotation of the molecules around their short axes. By supercooling of the hexyl and heptyl derivatives a new liquid crystalline phase can be obtained. A dielectric relaxation in the solid phase could not be found.     

Studies on induced smectic phases (III). Fourier transform infrared and laser-raman spectral differences between isotropic and induced smectic phase of binary mixtures of nematogens with contrasting dielectric anisotropies

The S_E, S_B, S_A induced phase sequence was detected in binary mixtures of two nematogens, 4,4′-di-n-heptyl azobenzene (7AB) with 4-n-pentyl-4′-cyano biphenyl (5CB). The mixtures as well as the single components were studied by Fourier transform IR and laser-Raman spectroscopy at different temperatures. Charge redistribution of 5CB is demonstrated by the spectral differences found by the ratio-recorded IR spectra and also by integrating the intensities of some Raman lines. The spectra of the different phases of the 7AB—5CB mixture, built up from components of different length, indicate a population increase of the gauche conformers of 7AB in the smectic A phase.     

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.     

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.     

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.     

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.     

Ab initio calculation of vibrational frequencies of GexPxS1−2x glass

The vibrational frequencies of GeS₄, GeP₄, Ge₂S₆, GeP₃, Ge₃P, Ge₂P₂, P₂S₂, P₃S, P₄S₃, α-P₄S₄, β-P₄S₄, α-P₄S₅, β-P₄S₅, P₄S₇, P₄S₉ and P₄S₁₀ are theoretically computed from the first principles. The Raman frequencies of Ge_xP_xS_1−2x glass are obtained for x varying from 0.05 to 0.019. The computed fundamental frequencies of clusters are compared with those experimentally found. In this way, we are able to identify the vibrating clusters in the real glass. The clusters identified in the real glass are found to be Ge₂P₂, P₄S₃, α-P₄S₄, β-P₄S₄, β-P₄S₅, P₄S₇, P₄S₉, β-P₄S₅, Ge₂S₆, Ge₃P.     

2H-NMR Spectral Patterns in Magneto-Aligned Smectic E and G Phases

Unique ²H-NMR spectral patterns can be obtained from biaxially ordered smectic E and G phases, where only one director of the phase is aligned by the magnetic field. Two effects on the shape of the deuterium spectral pattern are demonstrated: (1) The effect of a motionally induced asymmetry parameter in the time averaged deuterium quadrupole interaction, and (2) the effect when the principal z-axis of the time averaged interaction is not parallel to the aligned director. Spectral patterns for the S_G phase of the compound 4-n-pentoxybenzylidene-4-n'-heptylaniline selectively deuterated on one of the aromatic rings (50.7-d ₄) were obtained for both unaligned and magneto-aligned samples. Both of these spectral patterns were consistent with that expected for an asymmetry parameter, n < 0.2. Spectral patterns in the S_G phase of the compound 50.6 (24.7 wt.%) + 90.4-αd ₂-γd ₂ (75.3 wt.%) were studied showing a different behavior with respect to the spectral patterns obtained in 50.7. We believe that a shift in the orientation of the z principal axis relative to the sample c axis has occurred in this S_G phase. Spectral patterns were also recorded for the S_E phase in magneto-aligned samples of the mixture of compounds 4-cyano-4'-octyloxybiphenyl (80CB) with 50 wt.% 40.8-d ₄. These data also show a small asymmetry parameter but, in addition, a distribution in the orientation of the principal z-axis indicating that the time scale of the motion of the long molecular axis is in a regime where it is of the order of the ²H-NMR measurement. Models for the smectic E and G phases based on these observations are discussed.     

Determination of the Molecular Arrangement in Liquid Crystals by Polarization of Fluorescence

Polarized luminescence intensity reading and the polarized absorbance spectra of 4-diethyl-amino-4′-nitrostilbene “impurity” in the n-butyl-p/p-hexyloxyphenyloxycarbonyl/-phenyl-carbonate liquid crystal were measured. Degrees of orientational order, S₁ and S₂, of the nematic liquid were calculated from relative values of the absorbance and luminescence intensity components. A function, which describes the angular molecular distribution with respect to a preferred direction in the liquid crystal, is plotted.     

Molecular Relaxation in hi Ghly Ordered Orthogonal Smectic SA,SB and SE Phases

Dielectric relaxation and calorimetric investigations were perforemed on different smectic phases. Discountinuities of the critical frequency were found at both S_A-S_B and S_B-S_E phase transitions. S_A and S_E structures can be regarded as two-dimensional liquid and two-dimensional crystal, respectively. The low frequency dielectric dispersion presented here demonstrates the difference btween the S_E and a three-dimensional cyrstal where this phenomena is absent.     

New Mesogenic Compounds Exhibiting a Thermotropic Optically Isotropic Phase

Abstract

The only two series of carboxylic acids, 3-nitro- or 3-cyano-4-alkoxybiphenyl-4′-carboxylic acid, are known to exhibit the optically isotropic phase (S_D). Now we synthesised 4-(branched alkoxy)-anilinebenzylidene-4′-carboxylic acid series, all being racemic, and investigated their mesophase properties by means of DSC, optical microscopy and X-ray measurement. As a result, some branched azomethine derivatives were found to form a S_D phase. We also synthesised cinnamic acid derivatives with branched alkoxy tail that were turned out to exhibit either S_D phase or hexagonal columnar phase. We tried biphenyl or azobenzene core analogues which were found to produce a highly ordered smectic phase or no mesophase, respectively. At this stage, we suppose it plays an important role that the branch is introduced into the terminal chain and a certain dipole moment is located within the mesogenic core. These factors may allow the peculiar molecular packing that causes an optically isotropic property.     

X-ray Studies on the Liquid Crystalline Phases of the 4-[4-n-Heptyloxy-Benzylidene-Amino]4'-cyanobiphenyl

By means of X-ray diffraction studies of oriented and non-oriented samples the lattice parameters of the orthorhombic cell of the S_E phase as well as of the hexagonal cell of the S_B phase were determined. By packing considerations a bimolecular building unit with oppositely arranged cyano-groups was assumed in the case of the S_E phase. The S_B phase is characterized by an ABAB sequence of the smectic layers in which the molecules are hexagonal arranged. The increasing mobility of the molecules at increasing temperature offers the possibility for a part of the molecules to slide in a more stable dimerized form. In the N_R and S_A phase the existence of monomers as well as of dimers is assumed. In this way a conception of the phase transitions S_E → S_B → N_R → S_A is given on the base of a decrease by steps of the dipole-dipole repulsion.     

Densities,refractive indices,and order parameters of a binary liquid-crystalline system with an induced smectic A phase

For the binary mixture of 4′-Nitrophenyl 4-n-octyloxybenzoate and 4-n-Octyloxyphenyl 4′-pentyloxybenzoate with an induced smectic A phase the refractive indices (n_c and n_o) and the density were measured in dependence on the temperature. The orientational order parameter and the density were calculated using a mean field fitting method. In the induced S_A phase the refractive indices and the calculated orientational order parameters are showing only a weak dependence on concentration. The calculated and measured densities are in the same order of magnitude.     

Electrical Conductivity of Liquid Crystal Mixtures with Induced Smectic Phases

The electrical conductivity of the two liquid crystal mixtures N-(4-ethoxybenzylidene)-4′-amino benzonitrile (PEBAB)/4,4′-di-n-hexyloxy-azoxybenze (HEXOAB) and 4-n-heptyloxy-4′-cyanobiphenyl (7 CBP)/HEXOAB, which exhibit induced smectic A phases is investigated. In the smectic phases, the conductivity anisotropy of the PEBAB/HEXOAB mixtures is negative at the lowest PEBAB concentrations; this behaviour is usually expected for a smectic layer structure. With increasing PEBAB concentration the anisotropy increases and becomes positive. Possibly, this is an indication for a growing double layer structure, which was observed in polar smectic phases. In the nematic phases the conductivity anisotropy of the pure HEXOAB is considerably reduced by adding a relatively small amount of the polar component. An addition of 10 mole% 7 CBP reduces the anisotropy ratio of the electrical conductivity to V = K_∥/K ₁ ≈ 0.2, which probably is the lowest value observed in a nematic phase so far. Besides the negative conductivity anisotropy, these mixtures also exhibit a positive anisotropy of the dielectric constant. They thus fulfil the conditions for inverse dynamic scattering.     

Structure of pulsed-laser deposited arsenic-rich As–S amorphous thin films,and effect of light and temperature

Thin amorphous films from As–S system (∼As₅₀S₅₀) were prepared by the pulsed laser deposition technique. Light- and thermally-induced changes of structure of studied films have been investigated using Raman scattering spectroscopy results and interpreted in terms of chemical reactions and/or phase transitions between individual structural units. The irradiation of as-deposited thin films causes light-induced reactions, in which As₄S₃, α- and β-As₄S₄, and pararealgar/χ-As₄S₄ molecules are formed. Thermal-annealing of exposed thin films leads to the formation of β-As₄S₄ molecules.     

Sattructural changes during post-ageing of an Al?Zn (15 at.%) alloy at 100 °C studied by positron annihilation,small angele X-ray scattering and microhardness measurements

The positron lifetime τ, the shape parameter of the Doppler-broadened annihilation line S, the microhardness H_V and the Guinier radii r_G are studied in dependence on the ageing time during isothermal ageing of an Al Zn (15 at.%) alloy at 100 °C following a pre-ageing at R.T. for 1 h after the quench. Weak effects in H_V and r_G are interpreted by a transformation of spherical Guinier-Preston zones into ellipsoidal ones. At an ageing time of 10⁴s H_V decreases strongly while r_G starts to increase more rapidly. This behaviour is explained by the transformation of coherent GP zones into semicoherent α′_R-particles. Incoherent part of the α′_R-particle-matrix interface are able to localize positrons resulting in a strong increase of τ and S. The characteristic parameters τ and S of the coherent defect-free GP zones and of the semicoherent α′_R-particles are estimated. It is concluded that positrons can be used as a very sensitive tool in detecting the transformation of coherent particles into semicoherent or incoherent ones.     

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.     

Ab initio calculation of vibrational frequencies of Ge0.25S0.75−yIy glass

The vibrational frequencies of Ge₄, S₄, I₄, SI₃, Ge₂I₂, S₂I₂, Ge₂S₂, Ge₂SI, GeS₃, GeI₂S, Ge₃S and GeI₃ are computed using first principles. These frequencies are compared with those found experimentally in the Raman spectra of Ge_0.25S_0.75−xI_x glass. In this way, it is found that Ge₄, GeI₂S and GeI₃ tetrahedra occur in the glass. These tetrahedra are of different sizes so they stack to form the floppy phase of the glass.     

A Molecular Mean Field Theory of Reentrant Phases

The observed phase transition sequence N-S_A-N_re S_re with a lowering in temperature in pure compounds of molecules with a strongly polar end group is reproduced in a mean field model which is taken as an extension of the McMillan potential. This model shows how the dimers in presence of space filling monomers can account for the high temperature S_A phase. As the temperature is lowered, more and more monomers form dimers leaving empty spaces and this initiates an intercalation of dimers belonging to the neighbouring layers. This intercalation is responsible for the lowering of the strength of layering interaction in a McMillan type potential and thus a reentrant nematic phase occurs. This model for the first time successfully accounts for the thickness of the low temperature smectic phase.     

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.