X-ray investigation of the nematic and reentrant nematic phases of N-[(4-n-octyloxybenzoyloxy)-salicilidene]-4'-cyanoaniline

We have performed an X-ray scattering study of the nematic-smectic A and reentrant nematic-smectic A phase transitions in N-[(4-n-octyloxybenzoyloxy)-salicilidene]-4'-cyanoaniline (OOBOSCA). A diffractometer with a linear position sensitive detector was used. The results show that the smectic phase in OOBOSCA is of the A_d type with an interlayer spacing incommensurate with the molecular length L; d ∽ 1·2L. In the reentrant nematic phase two types of fluctuation modes were found. One of them corresponds to the monolayer wavevector q₁ ∽ 2π/L, and the other is due to the partial bilayer fluctuations with the wavevector q₂ ∽ 0·8q₁. The temperature dependences of the interlayer spacing, X-ray scattering intensity and longitudinal correlation length for both types of layering in the reentrant nematic phase are presented. The change of the fluctuation regime from S_Ad, to S_Cd type with decreasing temperature in the reentrant nematic phase of OOBOSCA was found. The results are discussed on the basis of models with competing order parameters. The influence of alkyl chain flexibility on the stability of a partial bilayer smectic phase is also considered.     

X-ray study of the nematic and smectic A phases of the cyano-substituted pyridines

We report an X-ray study of lamellar ordering in the nematic and smectic A phases of a homologous series of polar liquid crystals, the 2-(4-alkylphenyl)-5-cyanopyridines (nCP). Experiments were carried out using a diffractometer with a linear position sensitive detector. In the nematic and smectic A phases of the nCP and their mixtures with non-polar 4-n-butyl-4′-methoxyazoxybenzene two types of layering were found. One corresponds to the fluctuations of the smectic density wave with a monolayer wavevector q ₁, and the other is due to the partial bilayer fluctuations with the incommensurate wavevector q ₂ ≠ q ₁/2. The temperature dependences of the X-ray scattering intensity and the longitudinal correlation length for both types of layering in the nematic phase are presented. The critical behaviour in the vicinity of the smectic A-nematic phase transition occurs for a fluctuation mode, either q ₁ or q ₂, depending on the position on the liquid crystal phase diagram. The influence of the molecular structure of cyano-substituted pyridines on the formation of layered structures of different types is also discussed.     

Metastable phases in kinetics of phase transformations in the frozen liquid crystal 4-ethoxybenzylidene-4'-n-butylaniline

The nematogen 4-ethoxybenzylidene-4'-n-butylaniline gives by fast cooling a frozen phase called C₁ different from a glassy nematic state. The X-ray diffraction spectra of a non-aligned sample and a sample aligned by a magnetic field show that the C₁ phase is a monolayer smectic phase: molecules are inclined to the normal of the smectic planes by an angle of 35° ± 5°. On reheating we obtain metastable phases more and more ordered; those phases C₂ and C₃ are crystalline. The kinetics for the metastable phases correspond to a nucleation growth process of the same type (n = 2) for the two transformations C₁→C₂→C₂. If we assume a thermal process the growth is monodimensional.     

X-ray investigation of the nematic and reentrant nematic phases of N-[(4-n-octyloxybenzoyloxy)-salicilidene]-4′-cyanoaniline

We have performed an X-ray scattering study of the nematic-smectic A and reentrant nematic-smectic A phase transitions in N-[(4-n-octyloxybenzoyloxy)-salicilidene]-4′-cyanoaniline (OOBOSCA). A diffractometer with a linear position sensitive detector was used. The results show that the smectic phase in OOBOSCA is of the A_d type with an interlayer spacing incommensurate with the molecular length L; d ∽ 1·2L. In the reentrant nematic phase two types of fluctuation modes were found. One of them corresponds to the monolayer wavevector q ₁ ∽ 2π/L, and the other is due to the partial bilayer fluctuations with the wavevector q ₂ ∽ 0·8q ₁. The temperature dependences of the interlayer spacing, X-ray scattering intensity and longitudinal correlation length for both types of layering in the reentrant nematic phase are presented. The change of the fluctuation regime from S_Ad, to S_Cd type with decreasing temperature in the reentrant nematic phase of OOBOSCA was found. The results are discussed on the basis of models with competing order parameters. The influence of alkyl chain flexibility on the stability of a partial bilayer smectic phase is also considered.     

Order parameter of side groups of a polysiloxane in liquid crystalline uniaxial phases

The orientational order of a liquid crystalline side chain polysiloxane has been investigated by means of polarized light spectroscopy and dielectric relaxation measurements. The order parameters (P₂) and (P₄) have been determined as a function of the temperature for the smectic and nematic phases. The orienting properties of the polysiloxane investigated have been compared with those of a polysiloxane with the same mesogenic groups, but a shorter spacer.     

Study of internal ring rotation and molecular reorientation in the liquid crystal 5O.7 by deuterium N.M.R. spectroscopy

The spectral densities of motion were determined by deuterium N.M.R. relaxation measurements in the nematic, smectic A and smectic C phases of 4-n-pentyloxybenzylidene-d₁-4'-heptylaniline and 4-n-pentyloxybenzylidene-4'-heptylaniline-2,3,5,6-d₄. By examining two atomic sites on a 5O.7 molecule, we were able to gain information on the reorientation motion and internal rotation of the aniline ring. It was also found that director fluctuations make some contribution to the spectral density J₁ (ω). We use the superimposed rotations model to account for the internal ring motion and the small step rotational diffusion model for the molecular reorientation. The derived rotational diffusion constants for the spinning and tumbling motions appear to give physically plausible activation energies in the mesophases of 5O.7.     

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

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 A_d) are presented. It is shown that the width of the nematic gap that separates the A₁ or E₁ phase region from the smectic A_d phase is related to the interaction energy of the molecules in the smectic layers and to the difference in the smectic layer spacings.     

X-ray studies of the layer thickness in smectic phases

X-ray investigations of nine smectogenic substances exhibiting the smectic A_d, A₁ and crystalline E phases were performed at various temperatures. X-ray patterns yielded the layer thickness d (A_d, A₁ phases) and orthorhombic unit cell parameters (E phase). The layer thickness of the A_d phase in 4'-n-alkyl-4-cyanobiphenyls (nCBs) has different temperature coefficients for shorter (n = 8-10) and longer (n = 12-14) members, which is explained as resulting from two competing effects: a weakening with temperature of the intermolecular association energy that favours an increase in d, and the increasing number of conformers which reduces the molecular length. A small anisotropy of the thermal expansivity in the smectic phases was found by comparing the linear quantity d(T) with the linearized bulk characteristic of the system, V^-3(T), where V = 1/ρ is the specific volume, ρ is the density. Differences between the slopes of the two quantities are less in the case of the A₁ phase of two nDBTs (5-n-alkyl-2-(4'-isothiocyanatophenyl)-1,3-dioxanes). The present X-ray data and recent results of studies of the low frequency relaxation process in these compounds (under atmospheric as well as elevated pressures) give a consistent picture of molecular reorientations around the short axes in the smectic phases.     

Sulphur containing mesogens. The mesophasic behaviour of di(4-alkyloxybenzoates) of 4,4'-dimercaptobiphenyl

A homologous series of di(4-alkyloxybenzoates) of 4,4'-dimercaptobiphenyl: CH₃(CH₂)_n-1O-C₆H₄-COS-C₆H₄-C₆H₄-SOC-C₆H₄-O(CH₂)_n-1CH₃,n=1-7, has been synthesized and the thermotropic liquid-crystalline behaviour investigated. All compounds exhibit enantiotropic mesomorphism over a remarkable temperature range. While the mesophase thermal stability is moderately higher than that found for the corresponding oxygenated analogues, the smectic stability is definitely lower. In fact, all the compounds are nematic but smectic mesomorphism (S_C) is observed for n = 7. Compounds with n = 6 or 7 exhibit enantiotropic highly ordered smectic (or disordered crystal) phases, probably S_G in type.     

Anomalous anisotropy of the elastic constants in the induced smectic O film

Surface tension is known to induce smectic O (S_O) films at the free surface of isotropic droplets of 1-methylheptyl terephthalylidene-bis-4-aminocinnamate (MHTAC). The S_O film has its molecules disposed in a herringbone fashion, with the layers parallel to the free surface. It constitutes a model of a 2D polar nematic liquid crystal without any contact to solid substrates. The film is oriented uniformly when applying a uniform electric field, except along disclination walls. By measuring the width of the disclination walls in the two configurations with the electric field parallel and perpendicular, we determine the anisotropy of the elastic constants in the induced S_o films. We find the ratio of the 2D bend to splay elastic constants K_b/K_s to be ≃ 0.25 which is anomalously small when compared to the corresponding 3D ratio (K₂ + K₃)/2K₁ usually found for the bulk nematic phase. This experimental result is comparable to measurements performed with 2D suspended S_c films with, however, a different physics.     

The synthesis and liquid crystal properties of some 4-n-nonylphenyl esters of 3-(4'-n-alkoxybiphenylyl)- and 3-(4'-n-alkoxybiphenylyl)-3-methyl-propanoic acids and their laterally fluorinated analogues

A number of 4-n-nonylphenyl esters and mono- and di-fluorinated 4-n-nonylphenyl esters derived from 3-(4'-n-alkoxybiphenylyl)propanoic acids (II) have been synthesized and their thermotropic liquid crystal properties assessed with a view to obtaining tilted smectic phases for possible use in ferroelectric display devices. Many of these compounds exhibit wide temperature range S_C and S_I/F phases, but none gave the ideal phase sequence S_C-S_A-N-I mainly because the structure of these compounds was not conducive to the formation of the nematic phase. To try to alleviate this problem a series of 4-n-nonylphenyl esters based on 3-(4'-n-alkoxybiphenylyl)-3-methylpropanoic acid was prepared, where a lateral methyl group was incorporated in the β-position of the -CH₂CH₂CO₂- linkage. Incorporation of the lateral methyl group encouraged the formation of a nematic phase at the expense of both the smectic A phase and tilted smectic phases.     

Synthesis and thermal properties of dimeric alkoxy-substituted tolans

Four homologous series of dimeric tolans of the general formula RC₆H₄C≏C₆H₄O(CH₂)_m OC₆H₄C'CC₆H₄R (m'6-10; R'H, OC₆H₁₃, OC₁₀H₂₁ and OC₁₄H₂₉), denoted further as TOmOT or nOTOmOTOn, according to the number of aliphatic carbon atoms, have been synthesized. Dimers with terminal chains exhibited liquid crystalline behaviour. Crystal-crystal transitions were also observed. Resulting from the optical, thermal and miscibility studies, the following mesophases have been identified: nematic, smectics A and B, and the tilted smectics C and (probably) F or I. In the 6OTOmOTO6 series, a phase with a strong tendency to self-alignment occurred between the nematic and smectic A phases. This phase resembled the nematic in its viscoelastic properties, but also showed focal-conic textures. A transition between this 'intermediate phase' and the nematic was detected only from microscopic observations. On the contrary, the 'intermediate phase'-smectic A transition was also detected by DSC (δH ∼ 1kJ mol^-1). Both tilted (S_c and S_F/I) and non-tilted (S_A and S_B) smectics were observed in the 10OTOmOTO10 series, while only tilted smectic phases were identified in the dimers with longer terminal substituents (14OTOmOTO14). Double melting behaviour was found in 14OTO8OTO14 and 14OTO10OTO14. An alternation of the transition temperatures and enthalpies with the odd-even alternation of the lengths of bridging groups was clearly observed. A correlation between total enthalpies of transition and solubilities of the dimeric tolans is stated.     

Small angle X-ray scattering study of chiral side chain liquid crystalline polymers in 5CB and 8CB solvents

Small angle X-ray scattering was used to examine the new chiral side chain liquid crystalline polyacrylates (P_4M and P_11M) and their mixtures (2 wt %) in the low molar mass nematogenics 4'-n-pentyl-4-cyanobiphenyl (5CB) and 4'-octyl-4-cyanobiphenyl (8CB). Complementary data were obtained by polarizing optical microscopy. In agreement with previous studies, the mesophases of the bulk polymers show a dependence on the aliphatic spacers linking the mesogenic units to the polymer backbone. Chiral nematic and smectic A₁ phases were observed for the polyacrylates with four (P_4M) and eleven (P_11M) methylene units as spacers, respectively. In solution with 5CB and 8CB, P_4M exhibits an injected smectic phase, whereas P_11M maintains the smectic arrangement already observed in the bulk, with swollen smectic layers. In all the mixtures, layer stability was found to depend on the liquid crystal used as solvent, as well as on the temperature. At temperatures corresponding to the nematic 5CB and 8CB, the coexistence of two mesophases was observed in the mixtures. Moreover, with the liquid crystal solvents in the isotropic phase, microstructures suspended in the solvent matrix containing the liquid crystalline polymer in the smectic arrangement were detected.     

Synthesis and characterization of a series of side chain chiral smectic liquid crystalline elastomers

A series of new chiral smectic liquid crystalline elastomers was prepared by graft polymerization of a nematic monomer with a chiral and non-mesogenic crosslinking agent, using polymethylhydrosiloxane as backbone. The chemical structures of the monomers and polymers obtained were confirmed by FTIR and ¹H NMR. The mesomorphic properties were investigated by differential scanning calorimetry, thermogravimetric analysis, polarizing optical microscopy and X-ray diffraction. Monomer M₁ showed a nematic phase during heating and cooling. Polymer P₀ exhibited a smectic B phase; elastomers P₁-P₃ showed the smectic A phase, P₄-P₆ showed a chiral smectic C(SmC*), and P₇ displayed stress-induced birefringence. Elastomers containing less than 15 mol % M₂ displayed elasticity, reversible phase transitions with wide mesophase temperature ranges, and high thermal stability. With increasing content of the crosslinking unit, glass transition temperatures first increased, then fell, then increased again; isotropization temperatures and mesophase temperature ranges steadily decreased.     

Synthesis and mesomorphic properties of 6-n-decyloxy-2-[4'-N-alkoxyphenylimino)methyl]quinolines (IV)

The synthesis and mesomorphic properties of a new series of liquid crystals derived from quinoline with an imine central bond is described. All compounds exhibit mesophase ranges greater than 50°C. The lower homologues (n = 4-6) show trimorphism S_c-S_a-N (for n = 4, the S_c phase is monotropic). The higher homologues (n = 7-10) show nematic and smectic C phases. Compared to the styrylquinoline analogues the imine bond gives rise to similar liquid crystal phase ranges but lower melting points.     

Structure of the liquid crystalline state in hexakis (4-(4'-alkyloxy)biphenoxy)cyclotriphosphazenes

X-ray measurements on hexakis(4-(4'-alkyloxy)biphenoxy)cyclotriphosphazenes [PN-(OC₆H₄C₆H₄OC_nH_{2n + 1})₂]₃ (HACP, n = 7-9), confirm the previous mesophase identification. The apparent molecular length measured in the mesophase compares to twice the length of an alkyloxybiphenyl side group. Specific features are added to the usual features of the nematic and smectic diffraction patterns which show that the molecular arrays in directions parallel and perpendicular to the director both reflect the peculiar shape of the cyclotriphosphazenes.     

Viscoelastic behaviour of the SmC* phase

Using a light scattering technique at a fixed temperature, we have investigated viscoelastic behaviour exhibited by the ferroelectric smectic phase (SmC*) of C₈tolane in a homeotropic orientation. Experiments were performed in backward and forward scattering geometries that allowed us to deduce separately orientational diffusivities k₃/η and k₊/η corresponding to the Goldstone mode. The k₃/η value measured in the SmC* phase is about 100 times higher than in the SmC_A* phase exhibited by the same liquid crystal compound. The factor 100 may be attributed in great part to the molecular arrangement mode in adjacent smectic layers. However k₊/η measured in the SmC* phase is in the same order of magnitude as those measured previously in SmC_A* phases.     

Binary mixtures of smectic A1S with abnormal behaviour

Binary mixtures are composed of compounds belonging to the 4'cyanobiphenyl-4-yl 4'-n -alkylbiphenyl-4 carboxylate homologous series and halogenophenyl 4-(trans-4-n-decylcyclohexyl)benzoate (10XPCHB) have been studied. Compounds CBnAB are smectic A₁ for n ≤ 7 and smectic A_d for n ≥ 8; compounds 10XPCHB are smectic A, only. It was found that the S_Ad phase of compounds CBnAB exhibits ideal miscibility with the S_A1 phase of compounds 10XTCHB, and that the S_A1 phases of both compounds show abnormal behaviour. The S_A1 phase of compounds CBnAB is destabilized and the range of the S_A1 phase is enhanced on the side of 10XPCHB. The reasons for this behaviour are discussed.     

Novel bent-shaped liquid crystalline compounds II. Synthesis and properties of the 1,3-bis4-[4-(4-alkyloxybenzoyloxy)benzylidene]aminophenoxypropan-2-ols

A series of new symmetric dimer compounds was synthesized, constaining 2-hydroxy-1,3-dioxypropylene as the central linkage and terminal alkyl chains with different lengths. The chemical structures of the liquid crystal dimers (2ES-n) were examined by FTIR and ¹H NMR spectroscopy. Their mesomorphism, thermodynamic properties and optical textures were investigated by differential scanning calorimetry, polarizing optical microscope and X-ray diffraction. For homologues with terminal propyloxy and butyloxy chains, no liquid crystalline phase was observed. Homologues with pentyloxy and hexyloxy terminal chains showed nematic phases, while those with heptyloxy, octyloxy, nonyloxy and decyloxy terminal chains displayed nematic phases and smectic phases. The results confirmed that the liquid crystalline phase changes from nematic to smectic as the terminal chain length increases.     

Phase transfer Pd(0)/Cu(I) catalysed polymerization reactions 7. Synthesis and thermotropic behaviour of 1,4-bis[2-(3',3'-difluoro-4',4'-di-n-alkyloxyphenyl)-ethynyl]benzene dimers

The symmetrically difluorinated aryl-acetylene dimers, 1,4-bis[2-(3',3'-difluoro-4',4'-di-n-alkyloxyphenyl)ethynyl]benzenes (n = 7-12), were prepared by a one pot phase transfer. Pd(0)/Cu(I) catalysed, three step coupling of 1,4-diiodobenzene with 2-methyl-3-butyn-2-ol and the appropriate 1-bromo-3-fluoro-(4-n-alkyloxy)benzene. All members of the series display enantiotropic nematic and smectic C phases as well as an additional smectic mesophase in the sequence C-S-S_C-N-I. The textures observed by polarized optical microscopy strongly support identification of the second smectic phase as a S_E mesophase. The S_C temperature window increases as the length of the n-alkyloxy substituent increases, whereas the stability of the nematic and S_E temperature windows decrease. In these fluorinated dimers, destabilization of mesomorphic behaviour by lateral fluorine substitution is compensated by stabilization due to the large length to breadth ratio of the mesogen, such that melting and crystallization are depressed more than isotropization when compared to the non-fluorinated dimers.