Structural study of smectic A phases in homologous series of N-alkylpyridinium alkylsulphates

The mesomorphic properties of 27 homologues of the N-alkylpyridinium alkylsulphate series were studied by X-ray diffraction and dilatometry. All of these compounds exhibit single smectic A phases whose layer spacings are between 0.64 and 0.8 times the molecular length. Polarizing optical microscopy observation shows that these peculiar layer spacings are not due to the tilting of the molecules; a new packing model is proposed to explain the particular structure of these thermotropic ionic liquid crystal phases.     

Liquid crystalline properties of dissymmetric molecules VI. The effect of alkyl chain length on molecular arrangement in the smectic A phase in three-aromatic-ring systems with two ester groups

The effect of alkyl and alkoxy chain lengths on the layer structures of smectic A and C phases has been examined by X-ray diffraction measurements on three isomeric systems: 4-alkoxyphenyl and 4-n-alkylphenyl 4-[(4-octyloxyphenyl)carbonyloxy]benzoates (1); 4-octyloxyphenyl 4-[4-(octyloxyphenyl)carbonyloxy]benzoates (2); 4-octyloxyphenyl, 4-alkoxyphenyl and 4-n-alkylphenyl terephthalates (3); and p-phenylene 4-octyloxybenzoates, 4-alkoxybenzoate and 4-n-alkylbenzoate (4). Although all the derivatives exhibit smectic A and/or C phases having a monolayer arrangement of the molecules, the layer spacings are considerably affected by alternation of the ester linkages. The layer spacings for the homologues of 1 are a little shorter than the calculated molecular lengths, while those for 2 agree with the calculated molecular lengths. The layer spacings for 3 show a notable even-odd alternation in the higher homologues. The results are discussed in terms of a subtle change in the molecular structures due to replacement of the ester groups.     

Liquid crystalline properties of dissymmetric molecules IX. The effect of a terminal nitro group on the thermal properties of the smectic A phase in 4-nitrophenyl 4-(4-alkoxybenzoyloxyphenyl)benzoates

This paper describes the effect of alkoxy chain length on the physico-chemical properties of 4-nitrophenyl 4-(4-alkoxybenzoyloxyphenyl)benzoates. The homologous series shows a smectic A (SmA) phase commencing from the hexyloxy homologue. While the pentyloxy homologue shows only a nematic (N) phase under microscopic observation, X-ray diffraction shows that the N phase has a cybotactic domain with a layer structure of the SmA phase, where the layer spacings are longer than the calculated molecular lengths by c. Å. The layer spacings for the heptyloxy-nonyloxy homologues are longer than the calculated molecular lengths by 2.2-4.8 Å and are almost independent of temperature. The layer spacings are correlated with the calculated molecular lengths, and based on these results, a new molecular arrangement model for the SmA phase is proposed.     

Influence of structure on smectic A-smectic A phase separation

The critical temperature at the consolute point where two smectic A phases of different compositions phase separate has been measured for binary systems which combine one compound from a newly synthesized series of fluorinated mesogens with one homologue from the commercially available cyanobiphenyl family. Taking advantage of the regular evolution of the layer spacings within a series of homologues, this systematic analysis identifies a clear connection between miscibility and difference in layer spacings of the smectic A phases of the pure compounds. However, additional data obtained on significantly different systems show the non-universal character of this analysis and suggest that factors in addition to this structural difference are relevant to this phenomenon.     

Liquid crystalline properties of dissymmetric molecules VI. The effect of alkyl chain length on molecular arrangement in the smectic A phase in three-aromatic-ring systems with two ester groups

The effect of alkyl and alkoxy chain lengths on the layer structures of smectic A and C phases has been examined by X-ray diffraction measurements on three isomeric systems: 4-alkoxyphenyl and 4-n-alkylphenyl 4-[(4-octyloxyphenyl)carbonyloxy]benzoates (1); 4-octyloxyphenyl 4-[4-(octyloxyphenyl)carbonyloxy]benzoates (2); 4-octyloxyphenyl, 4-alkoxyphenyl and 4-n-alkylphenyl terephthalates (3); and p-phenylene 4-octyloxybenzoates, 4-alkoxybenzoate and 4-n-alkylbenzoate (4). Although all the derivatives exhibit smectic A and/or C phases having a monolayer arrangement of the molecules, the layer spacings are considerably affected by alternation of the ester linkages. The layer spacings for the homologues of 1 are a little shorter than the calculated molecular lengths, while those for 2 agree with the calculated molecular lengths. The layer spacings for 3 show a notable even-odd alternation in the higher homologues. The results are discussed in terms of a subtle change in the molecular structures due to replacement of the ester groups.     

Synthesis and properties of new materials with banana-shaped achiral cores and chiral end groups

New chiral bent-core mesogens, derivatives of 1,3-phenylene bis[4-(alkanyloxyphenyliminomethyl)benzoate], were synthesized with variation of a substituent (X=F, Cl); their antiferroelectric properties are described. The mesomorphic and switching properties were characterized by differential scanning calorimetry, polarizing optical microscopy, triangular wave method, and X-ray diffractometry in the small and wide angle regions. The presence of chiral tails at the terminals of side wings in the bent-core molecules induced a decrease in transition temperature and formation of the switchable SmC* phase in the melt. In addition, the introduction of a lateral halogen substituent in the Schiff's base moiety prevented the regular stacking of the molecules, resulting in the formation of very complex optical textures. The smectic phase with F-substituted PBFDOB and Cl-substituted PBCDOB showed layer spacings of 39 and 38.5?Å, respectively, corresponding to the end-to-end distance of molecules with a bent conformation. Significantly, the smectic phases of PBFDOB and PBCDOB exhibited a period of 179.5 and 131?Å, respectively, compatible with a helical structure with periodicity about 4.6 and 3.4 times the layer spacings.     

Small-angle X-ray scattering from binary liquid-crystalline mixtures of polar and non-polar compounds

The phase diagrams of several binary mixtures consisting of cyanobiphenyls and different terminally non-polar mesogenic compounds exhibit induced smectic A phases. The smectic A layer spacing, d, of these induced phases have been investigated by small-angle X-ray scattering as a function of the molar fraction x_i . Large deviations of the layer spacings from the additivity rule d _mix = σ x_id_i , have been observed in all mixed systems. Assuming a dimerization of the terminally polar compounds, the degree of association β(x) was calculated as a function of composition for different dimerization constants. The theoretical relations d _mix(x) obtained from the calculated β(x) values do not fit the experimental curves for d(x). This fact can be interpreted by the formation of weak complexes between the polar and the terminally non-polar molecules of the binary mixtures. This kind of complexing seems to be responsible for the formation of induced smectic phases of the S_Ad-type and might disturb the monomer-dimer equilibrium 2M _p D _p of the polar compounds.     

Liquid crystals of 4-octyloxy-N-(benzylidene)aniline derivatives bearing trifluoromethyl or trifluoromethoxy end groups

Abstract

Seven new derivatives of 4-octyloxy-N-(4-substituted benzylidene) aniline have been synthesized. 4-Trifluoromethyl and 4-trifluoromethoxy derivatives exhibit stable smectic B and A phases, respectively, while both the 4-methyl and 4-methoxy derivatives have monotropic nematic phases. Fundamental liquid crystalline properties such as entropies of the phase transitions, microscopic textures, smectic layer spacings, orientational order parameters, and molecular dipole moments were determined. It has been revealed that moderately polar nature of these mesogens act to stabilizing monolayer smectic states. The smectic A phase of 4-trifluoromethoxy derivative exhibit very high orientational order. None of the disubstituted compounds, 3,5-bis(trifluoromethyl), 3,5-dimethyl, and 3,5-dimethoxy derivatives were mesogenic. The effect of terminal trifluoromethylation on the liquid crystalline properties is discussed.     

Phase studies of long-chain alkanol complexes with alkylammonium layer silicates

Measurements of basal spacings ofn-tetradecylammonium-n-tetradecanol complexes of beidellite and montmorillonite in a wide range of tetradecanol concentrations have been carried out. Three different intercalate phases have been established at room temperature. In definite concentration ranges two of the phases coexist. These miscibility gaps disappear when the temperature is increased above 80°C. They appear again at cooling. This reversibility with temperature confirms the equilibrium nature of the phases observed. Additional information about the structure of the phases has been obtained by basal spacing measurements at constant temperature and alkanol concentration with all nine members of then-alkyl homologs with 10 to 18 carbon atoms in the chains. The structures of the interlayers in the different phases have been discussed.     

Complex layering observed in high internal phase emulsions at a silicon surface by neutron reflectometry

The neutron reflectivity profiles from the interface between silicon and aqueous phase-in-oil high internal phase emulsions of steadily increasing surfactant hydrophilicity, are reported for two isotopic contrasts for each surfactant. Layered models are required to fit the structured reflectivity profiles that demonstrate that the oxidised top layer of the silicon is always covered by a surfactant monolayer. Interposed between the surfactant monolayer and the bulk emulsion is a layer of oil--a geometric effect caused by reorganisation of the aqueous droplets. As the surfactant hydrophilicity increases, alternating aqueous and oil+surfactant layers are inserted between this topmost oil layer and the oxide attached surfactant monolayer. The resulting structures have compositions and layer spacings suggestive of sections from lamellar phases. This increase in layer ordering with increasing surfactant hydrophilicity is expected. The bulk emulsions are observed to exhibit lamellar or sponge phases increasingly as surfactant hydrophilicity increases.     

Rod-like mesogens with antipathetic fluorocarbon and hydrocarbon tails

A perfluorinated (or semi-perfluorinated) chain is affixed to various classical mesogenic moieties in lieu of the usual hydrocarbon tail. The consequences of this modification on the mesogenic behaviour are discussed by comparison with the perhydrogenated homologues. Smectic A phases are highly favoured. Their layer spacings show a trend to bilayer stacking in many cases likely to result from microsegregation among incompatible portions of the molecules.     

Liquid crystal assemblies in biologically inspired systems

In this paper, which is part of a collection in honour of Noel Clark’s remarkable career on liquid crystal (LC) and soft matter research, we present examples of biologically inspired systems, which form LC phases with their LC nature impacting biological function in cells or being important in biomedical applications. One area focuses on understanding network and bundle formation of cytoskeletal polyampholytes (filamentous actin, microtubules and neurofilaments (NFs)). Here, we describe studies on NFs, the intermediate filaments of neurons, which form open network nematic LC hydrogels in axons. Synchrotron small-angle-X-ray scattering studies of NF protein dilution experiments and NF hydrogels subjected to osmotic stress show that NF networks are stabilised by competing long-range repulsion and attractions mediated by the NF’s polyampholytic sidearms. The attractions are present both at very large inter-filament spacings, in the weak sidearm-interpenetrating regime, and at smaller inter-filament spacings, in the strong sidearm-interpenetrating regime. A second series of experiments will describe the structure and properties of cationic liposomes (CLs) complexed with nucleic acids (NAs). CL-NA complexes form liquid crystalline phases, which interact in a structure-dependent manner with cellular membranes enabling the design of complexes for efficient delivery of NA (DNA and RNA) in therapeutic applications.     

Smectic A phases with strings of interdigitated molecules in swallow tailed compounds

X-ray diffraction studies are reported of smectic A phases exhibited by swallow-tailed compounds. The special molecular shape consisting of regions having differing space filling requirements results in antiparallel packing of the molecules; this influences the conformations of the alkyl chains. The layer spacings show unusually good agreement with the molecular lengths and this is the case for molecules with long and short aliphatic chains. We have observed also an additional incommensurable structural element which we believe to be unique for non-polar, low molar mass compounds. The X-ray diffraction patterns provide evidence for the existence of strings of interdigitated molecules.     

A Structural Investigation of Some Terminally Cyano-Substituted Side Chain Liquid-Crystalline Polysiloxanes

Abstract

A series of terminally cyano substituted side chain polysiloxanes have been characterized using optical microscopy, D.S.C., and X-ray diffraction. Values for the lamellar spacings, d, of the interdigitated smectic A phases were obtained, and the variations in d with temperature and length of the flexible aliphatic spacer are discussed. The spacing for m = 4, 5, and 6 increased regularly with m whereas the short m = 3 spacer gave an anomalously high value.     

Phase behaviour and structural properties of two members of biphenylyl benzoate chiral mesogenic series

ABSTRACT

Phase behaviour and structural properties of two members of biphenylyl benzoate chiral mesogenic series (2F3R and 3F3R) have been investigated. While both the compounds exhibit SmC* phase over a wide temperature range, 2F3R forms orthogonal SmA* from tilted SmC* on heating but 3F3R melts directly to the isotropic phase. The SmA* phase of 2F3R is found to have de vries characteristics with small effective layer contraction. Both the samples on cooling form hexagonal SmF* phase below SmC* phase. On further cooling soft crystal like hexagonal SmJ* phase is formed in 2F3R, undergoing a change in the tilt direction, but in 3F3R, SmG* phase is formed without any change in the tilt direction. A coexistence phase of (SmC*+SmF*) is also observed in a certain temperature range. Slight differences in the dipole moment and molecular conformation of the two molecules give rise to a subtle change in the intermolecular interaction and play an important role in the appearance of different phases in the two compounds. Cell parameters of SmF*, SmG*, SmJ* phases have also been determined. Layer spacings, tilt angles, average intermolecular spacings and correlation lengths have been measured. How some of these properties compare with other members of the series has been discussed.     

Laterally aryl-substituted bent-core mesogens

Laterally aryl-substituted bent-core mesogens, prepared from 2-methyl-isophthalic acid, were studied. Optical polarising microscopy gave hints for nematic and smectic A phases which were supported by X-ray measurements. Two-dimensional patterns of samples aligned in the magnetic field prove clusters of the smectic C type within the nematic phases. The layer spacings point to intercalated structures because they are significantly shorter than the molecular lengths derived from molecular models. The shape of the molecules seems to be neither rod-like, nor really bent, nor discotic due to the extension of the bending angle caused by the lateral methyl group in a bay position and the different cross-sections of different molecular parts.     

X-ray scattering from the long spacing of completely crystalline oligoethylene glycol di-n-alkyl ethers

Completely crystalline samples of oligoethylene glycol di-n-alkyl ethers, H(CH₂)_nO-(CH₂CH₂O)_m(CH₂)_nH, where m is 9 or 15 and n is in the range 12-18, were orientated in capillaries by slow crystallization in a temperature gradient. X-ray scattering from the long spacings of the oriented samples, rotated continuously through 360°, was concentrated on the equator of a Debye–Scherrer photograph and many orders of reflection could be seen (e.g., up to order 30). The intensities of these reflections were analyzed by use of a model electron density distribution through the layer structure. Thus it was shown that the central oxyethylene block, in helical conformation, is oriented normal to the layer end plane, while the end alkyl bolcks, in planar zigzag conformation, are tilted relative to the layer end plane at an angle in the range 70°–64°.     

Liquid crystalline behaviour of hydrogen-bonded complexes of alkoxycinnamic acids with octyloxystilbazole

Hydrogen-bonded liquid crystalline complexes have been obtained through 1:1 (molar ratio) complexation of 4-n-alkoxycinnamic acids (nCNA: n = 4, 8, 10, 12, where n is the number of carbons in the alkyloxy chain) and trans-4-octyloxystilbazole (8Sz). These hydrogen-bonded complexes (nCNA8Sz) form stable mesophases. The mesomorphic range was extended by the mixing of complexes. Hexatic modification of smectic B (SmB_h), smectic C (SmC), smectic A (SmA), and nematic mesophases of these complexes were determined by a combination of X-ray diffraction and polarizing optical microscopy. Transitions between the various smectic phases were deduced from the temperature-dependent layer spacing of nCNA8Sz. The layer spacing of these complexes in the SmB_h and SmA phases gradually increased with increasing alkoxy chain length. The favouring of smectic phases in these complexes is believed to originate from the increment of polarity of the mesogen by intermolecular H-bonding.     

Shear flow and magnetic field effects on smectic C,C*, CM and C*M liquid crystals

Abstract

The continuum equations of Leslie et al. [1] for smectic C, and the extension of this theory for chiral smectic C* [2], are applied to problems involving simple planar layer configurations which accommodate uniform layer thickness constraints. The chiral smectic C*_M and non-chiral smectic C_M [3] are considered as either biaxial smectic A phases or antiferroelectric smectic C phases and are therefore included as interesting degenerate cases of the smectic C* and C phases, respectively. The effects of static and time dependent magnetic fields on these materials are compared with related deformations occurring in nematics [4] and cholesterics [5,6]. Their reaction to applied shears is also investigated yielding examples of flow alignment, induced secondary flows and unwinding of the chiral helix and testing the validity of enforcing a constant layer thickness.     

Solubility of liquid crystalline side-chain polymers in smectic A solutions

We describe and analyse in this paper the behaviour of solutions of side-chain liquid crystalline polymers (LCP) in molecular compounds in their smectic A phases. We give evidence for a straight correlation between phase separation and the difference in smectic layer spacings of the components of the binary system. In addition, making use of materials containing fluorinated segments, we show that fluorocarbon moieties greatly enhance the incompatibility of the mixture.