Smectic Ad Phase Research Articles

Enaminoketone mesogens having polar terminal groups

Abstract Homologous series of 4″-nitro- and 4″-cyano- derivatives of 1-(4′-alkoxyphenylamino)-3-phenyl-1-propen-3-onesandastructurally similar seriesofl-(4′-alkoxyphenylamino)-3-(4″-pyridyl-N-oxide)-1-propen-3-ones were synthesized and studied by DSC and optical methods. The compounds behave typically for strongly polar mesogens: the smectic Ad phase exists above a critical chain length and the reentrant nematic phase is observed in the vicinity of the critical double point. For pyridyl N-oxides, a second-order SmAd - SmCd phase transition line is observed.

X-ray study of the backbone conformation of a comb-shaped polyacrylate with nematic to smectic A phase transitions

Abstract The change in backbone conformation from prolate to oblate upon going from the re-entrant nematic to the smectic Ad phase is studied in a comb-shaped polyacrylate by measuring the mosaicity of the sample in the smectic phase with X-ray scattering. In combination with the possibility of aligning the director in both nematic phases, the results indicate that the conformational change takes place in the re-entrant nematic phase upon approaching the smectic Ad phase.

Difference in critical behaviour of the phase transitions nematic-smectic A1 and nematic-smectic Ad

A high resolution X-ray study is presented of the nematic to smectic Aj phase transition in BOPCBOB and of the nematic to smectic Ad Phase transition in a binary mixture of 7CB and BCB (7.6CB). The known critical exponents of the heat capacity of both these systems agree with the predictions for the 3D XY model. For BOPCBOB we find values for the susceptibility y and the parallel correlation length vi that also agree with the 3D XY model, thus confirming the predictions of de Gennes. However, the values y and vi of 7.6CB are larger than the 3D XY values. This indicates a possible difference in the critical behaviour of the nematic- smectic Ai and the nematic-smectic Ad Phase transition. In all cases vi is larger than v~, Furthermore it is shown how the values for the critical exponents depend on the specific form chosen for the structure factor,

Thermal and biphasic behaviour of a side-chain smectic polyacrylate

The thermal and biphasic behaviour is reported for a side-chain smectic polyacrylate having a number average molecular weight of 59 000 g mol −1 and a first polydispersity index of 1.9. Evidence is provided for the existence of smectic Ad and smectic A1 mesophases, the former being enantiotropic and the latter monotropic. The biphasic behaviour was studied by DSC by annealing the polymer inside the apparent biphasic gap associated with the smectic Ad-isotropic transition. A biphasic temperature range in which isotropic and smectic Ad phases coexist at equilibrium was revealed, and the thermodynamic width of this biphasic region was estimated to be about 2.0 K.

Pulsed-field-gradient stimulated-spin-echo NMR study of anisotropic self-diffusion in smectic Ad liquid crystal CBOOA

Anisotropic self-diffusion coefficient tensor was determined in the smectic Ad (S Ad) phase of a polar mesogen CBOOA. The pulsed-field-gradient stimulated-spin-echo NMR technique was applied in combination with angular displacement of a quadrupole coil, to obtain the diffusion coefficient components both parallel ( D |) and perpendicular ( D ⊥) to the director for the sample oriented at the magic angle from the large magnetic field. It was found that D &.sfnc; > D ⊥ ( D |/ D ⊥ = 2.3–3.0), and the activation energies were similar for the two components. Nematic-like diffusion characteristics have been revealed.

X-ray diffraction study of smectic A layering in terminally fluorinated liquid crystal materials

Abstract We report an X-ray study of smectic A layering for mesogenic compounds with fluorinated substituents in terminal positions. The measurements were carried out using diffractometers with one- and two-coordinate proportional chambers. It was found that in contrast to [sbnd]CN or [sbnd]NO2 terminated smectics, the polar [sbnd]OCF3 compounds form only a monolayer smectic A1 phase. The ratio of the intensity of the second harmonic to the first in smectic A phases formed by molecules with lengthy perfluorinated chains was found to be two orders of magnitude higher than is commonly reported for low molar mass thermotropic mesogens, indicating deviations of the density distribution function p(z) from a pure sinusoidal form. The layer periodicity d for these mesogens exceeds the molecular length L: d/L ≃ 1·1, which corresponds to a smectic Ad phase consisting of parallel or antiparallel dimers. We have observed that lateral fluorine substitution in the benzene ring adjacent to the perfluorinated chain lead...

Phenyl-cyclohexyl enaminoketone ligands and their Cu(II) complexes

Abstract The liquid crystalline properties of 1-(alkylamino)-3-[(4″-hexyl-trans-cyclohexyl-4′-phenyl]-prop-1-en-3-one-s, from methyl to octadecyl, and their copper (II) complexes have been examined by optical, DSC, X-ray and EPR methods. The compounds are enantiotropic nematogens except those having the shortest and the longest terminal chains. Short chains promote the SA phase in both ligands and complexes, whereas long chains promote SC and crystal H phases for the ligands or SA and SC phases for the complexes. A partly bilayer smectic Ad phase is observed from ligands terminated with short non-polar substituents. Direct isotropisation from the crystal H phase for some of the compounds, as well as other phase transitions have been studied. The molecular shape of the complexes and the organization of their mesomorphic phases are discussed, based on the refractive indices and X-ray data.

Carbohydrate liquid crystals

Abstract There has been a marked increase during the past 5 years in the number of mesogenic compounds synthesized from the common, naturally occurring carbohydrates, particularly from glucose and glucitol. The structure of the mesophases depends on the shape of the molecules. Cyclic and acyclic carbohydrates with single n-alkyl or acyl chains of more than six carbon atoms are calamitic amphiphiles which form thermotropic smectic Ad phases. The more soluble of these mesogens are used as non-ionic surfactants for solubilizing and crystallizing membrane proteins. They form gels and lyotropic phases with water. The lamellar phase is most common, but some carbohydrate amphiphiles exhibit all of the classical lyotropic phases associated with the ionic surfactants, i.e. lamellar, cubic and hexagonal. Both the smectic Ad and the lamellar Lα phases are believed to be bilayers with interdigitized alkyl chains in the interior of the molecular clusters. The crystal structures of the alkylated cyclic sugars hitherto determined have head-to-head bilayers with interdigitizing alkyl chains. With the acyclic sugars, head-to-tail monolayer crystal structures are also observed. It is believed that these convert to bilayer structures prior to forming the bilayer smectic phases. Carbohydrates substituted with more than one hydrocarbon chain form hexagonal mesophases. Both cyclic and acyclic sugars, when monosubstituted with double alkyl or acyl chains, are wedge-shaped molecules which aggregate into discs that stack in hexagonally arranged columns. When full substituted, the flatter carbohydrate molecules, such as scyllo-inositol, form a large class of discotic mesophases. Those that are only partially substituted, leaving two hydroxyl groups free, are reported to form multilayer discotic mesophases which combine stacking forces with hydrogen bonding. The amphiphilic carbohydrates that form hexagonal phases thermotropically may form hexagonal, cubic or lamellar phases lyotropically.

NMR study of molecular dynamics in a mixture of two polar liquid crystals (CBOOA and DOBCA)

Abstract A mixture containing 70 per cent 4-cyanobenzylidene-4′-n-octylaniline (CBOOA) and 30 per cent 4-n-dodeciloxybenzylidene-4′-cyanoaniline (DOBCA) has been studied by optical microscopy, differential scanning calorimetry, X-ray diffraction and, primarily, by NMR spectroscopy. The smectic phase of the mixture is a partial bilayer smectic A phase (SAd), in which about one half of the molecules are associated in pairs through their polar cyano end groups. The molecular dynamics in the smectic Ad phase of the mixture was studied by proton spin-lattice relaxation. Self-diffusion and rotations/reorientations were found to be essential relaxation mechanisms while the contribution of the order director fluctuations seems to be very small in all frequency regions. The correlation times associated with the molecular rotational motion around the short molecular axis, τS, are of the order 10−10 s and the ratio τS/τL ≈6.2. The translational diffusion coefficients D∥ are of the order 10−11m2s−1. The possible cont...

A Revised Model for the Molecular Arrangement in the Smectic AdPhase of Carbohydrate Derived Amphiphiles with One Alkyl Chain†

Abstract We propose a different molecular arrangement in the bilayers of the smectic Ad phase of mesogenic Carbohydrate amphiphiles from that previously postulated. The molecular arrangement proposed in this report is consistent with all experimental data currently available. In this new model, the smectic bilayers are identical (except for the absence of water) to the fluid lamellar (Lα) lyotropic phase, i.e., the molecules within the bilayers are arranged tail-to-tail, with the carbohydrate moieties on the outside of the bilaycrs and partially interdigitized alkyl chains in the core of each layer.

A nematic phase created in mixtures of polar smectics The effect of smectic layer properties

Abstract Phase diagrams of binary mixtures composed of compounds with the NCS terminal group (n-DBT, n-PBT, n-TPB (smectic A1) or n-BT (smectic E1)) and n-OCB, n-CB (smectics Ad) are presented. It is shown that the width of the nematic gap that separates the A1 or E1 phase region from the smectic Ad phase is related to the interaction energy of the molecules in the smectic layers and to the difference in the smectic layer spacings.

Effect of properties of the smectic Adphase on the induction of a nematic phase in binary mixtures of smectics A1and Ad

Abstract The phase diagrams of two series of binary mixtures composed of 5-trans-n-butyl-2-(4-isothiocyanatophenyl)-1,3-dioxane (smectic A1) and 4-(trans-4′-n-alkyl-cyclohexyl)-1-(2-cyano-ethenyl)benzene or 4-cyanophenyl-4′-n-alkoxybenzoates (both smectic Ad) are determined. It is shown that the smectic layer spacing ratio and smectic phase transition enthalpy are the factors influencing the width and the position of the nematic gap created between the two smectic regions. It is found that their influence on this ability is in the opposite direction.

X-Ray Studies in a Reentrant Liquid Crystal Phase

X-ray diffraction and EXAFS measurements have been carried out in a mixture of two kinds of liquid crystals; 4-(4-heptyloxy-benzoyloxy) benzylidene-4-cyanoaniline 45.5 mol% and 4-cyanophenyl 4-(4-nonyloxy-3-bromobenzoyloxy) benzoate 54.5 mol%, where the phase transitions occur as following sequence with decreasing temperature; isotropic liquid, nematic, smectic-Ad(1), reentrant nematic, smectic-Ad(2) and solid phases. It is found that in the reentrant nematic phase, the smectic-Ad(1) phase and the smectic-Ad(2) phase coexist with the short-range ordering. The local structures obtained from the EXAFS spectra in the directions parallel and perpendicular to the director (long axis of molecule) of the liquid crystal are discussed.

Two Kinds of State in the Smectic-Ad Phase of Octyloxycyanobiphenyl (80CB) and Octylcyanobiphenyl (8CB)

Abstract From both ac calorimetry and picosecond time-resolved fluorescence experiments, new phases are observed such that on cooling 4-octyloxy-4′-cyanobiphenyl undergoes the phase transitions, I, N, SmAd and K in order and on the other hand, on heating, gives the phase transitions, K, new K', new SmAd', N and I in order. The SmAd’ phase is metastable, since the heat capacity of the SmAd’ phase changes to that of the SmAd phase just below the SmAd-to-N transition and hereafter the heat capacity has a large value corresponding to the SmAd phase. A clear difference between the K and the K’ phases appears in the fluorescence experiment and it takes more than 30 min to transform from the K phase to the K’ phase, unlike the other transitions. A similar behavior of the phase diagram is obtained from ac calorimetry of octylcyanobiphenyl.