A study of the nematic-smectic A-smectic C point in a binary liquid-crystalline mixture by adiabatic scanning calorimetry

An adiabatic scanning calorimetric technique has been used to investigate the nematic-smectic A-smectic C (NAC) multicritical point in mixtures of 4-n-pentyl-oxyphenyl-4′-n-octyloxybenzoate and 4-n-hexyloxyphenyl-4′-n-octyloxybenzoate. The NC transitions are weakly first order and the latent heat becomes zero at or very near the NAC point. Near the NAC point no thermal feature associated with the NA transitions could be observed. The heat capacity anomalies along the second order AC transition line and along the first order NC line indicate the presence of a tricritical point at or very near the NAC multicritical point. No thermal indications for a biaxial nematic phase were observed.     

X-ray diffraction studies of the liquid crystal phases formed by certain 4'-n-alkylbiphenyl-4-yl 5-n-alkylthiophene-2-carboxylates

Twenty esters selected from the first ten homologues of each of the 4'-n-octylbiphenyl-, 4'-n-nonylbiphenyl- and 4'-n-decylbiphenyl-4-yl 5-n-alkylthiophene-2-carboxylates have been examined by X-ray diffraction techniques. These esters show extensive smectic polymorphism, but only tilted phases are observed, all with tilt angles of around 45°     

FTIR of liquid crystals. Organization and phase transitions in binary mixtures of nonylcyanobiphenyl and phenyl-cyclohexanecarboxylate

The FTIR spectra of the isotropic and mesomorphic N, S_Ad, S_A1, S_B and S_G phases of 4-n-nonyl-4'-cyanobiphenyl (9CB), 4-n-pentylphenyl-trans-4'-n-pentylcyclohexane-1-carboxylate (5H5) and their binary mixtures have been recorded and analysed. Changes in absorbance related to spontaneous reorientation of the molecules at the phase transitions have been observed and assigned to the orientation of the transition dipoles of typical vibrations. The results are discussed in terms of the phase diagram obtained by D.S.C. and by optical microscopy.     

Properties of the liquid crystals formed by certain 4'-n-alkylbiphenyl-4-yl 5-n-alkylthiophene-2-carboxylates

Thirty compounds, ten members of each of three homologous series of esters derived from 4-hydroxy-4'-n-octyl-, -nonyl-, and -decyl-biphenyl and the 5-n-alkylthiophene-2-carboxylic acids (methyl through to decyl homologues) have been prepared. The liquid crystal properties of these esters, which show extensive smectic polymorphism, have been investigated by thermal optical microscopy, differential scanning calorimetry, and miscibility studies. Four members of the corresponding 4-n-alkylbenzoates have also been prepared in order to compare their liquid crystal behaviour with the 5-n-alkylthiophene-2-carboxylate counterparts: both smectic and nematic thermal stability are higher for the 4-n-alkylbenzoates, by 30.6° and 36.8° C, respectively.     

Behaviour of the pitch of the cholesteric and chiral smectic C helix near and at the cholesteric-smectic A-chiral smectic C multicritical point

A cholesteric-smectic A-chiral smectic C multicritical point was established in binary mixtures of 4-n-hexyloxyphenyl-4'-(2'-methylbutyl)biphenyl-4-carboxylate with cholesteryl myristate or cholesteryl benzoate. At this point the cholesteric phase, smectic A phase, and chiral smectic C phase become indistinguishable. Whereas the pitch of the cholesteric helix at the S_A-Ch phase transition is infinite already in the vicinity of the multicritical point, the pitch of the cholesteric helix at the S*_C-Ch transition becomes infinite only at this point. In accord with the theory of Beresnev the pitch of the chiral smectic C helix remains finite at the multicritical point. Additional high pressure experiments support the results obtained at atmospheric pressure.     

Induction of smectic mesomorphism by mixing a non-liquid-crystalline methacrylate with a nematic liquid-crystalline compound having a cyano-containing mesogen

The structure and phase diagram of a binary mixture of a non-liquid-crystalline, mono-functional (meth)acrylate monomer and a liquid-crystalline compound having a cyano group-containing mesogen were examined. The monomers had mesogenic units such as biphenylene and phenyl benzoate, but did not show liquid-crystallinity. The liquid-crystalline compounds possessed a cyanobiphenyl or a cyanophenyl benzoate mesogenic unit. The liquid-crystalline 4-cyanophenyl 4'-n-octyloxybenzoate did not show a smectic A phase, while the binary mixture of the 4-butoxy-4'-(ω-methacryloyloxyhexyloxy)biphenyl monomer with 4-cyanophenyl 4'-n-octyloxybenzoate showed the induction of a smectic A phase in a nearly equimolar composition range. However, the binary mixture of the monomer, containing a phenyl benzoate group, and the liquid-crystalline compound did not induce a smectic phase.     

The preparation and properties of low molar mass liquid crystals possessing lateral alkyl chains

The first twelve members of three new mesogenic homologous series have been characterized, each of which are composed of molecules possessing lateral alkyl chains. The n-alkyl 2,5-bis-(4-n-hexyloxy-benzoyloxy)-benzoates are purely nematic whereas both nematic and smectic behaviour is observed for the 4-cyano-4'-biphenylyl 3'-n-alkyloxybenzoates and the 4-cyano-4'-biphenylyl 3',4'-n-alkyloxybenzoates. The transitional properties of these series are similar to those of the analogous conventional mesogens and hence, can be rationalized without making any special assumptions concerning the conformational distribution of the lateral alkyl chain.     

High pressure phase studies of some liquid crystalline cyclohexane derivatives by differential thermal analysis

T(p) phase diagrams for the mesomorphic compounds trans-4-n-butylcyclohexane-1-carboxylic acid 4-cyanophenyl ester (D4N), trans4-n-pentylcyclohexane-1-carboxylic acid 4-n-pentylphenyl ester (D55), trans-4,4'-di-n-propyl-1,1-bicyclohexyl-cis-4-carbonitrile (33CCN), trans-4-methoxy-4'-propyl-1,1'-bicyclohexane (3O1CCH) and trans-4-methoxy-4'-n-butyl-1,1'-bicyclohexane (4O1CCH) are presented. The experiments were performed using high pressure microcomputer-assisted differential thermal analysis equipment in the temperature range 300 to 500 K up to a maximum pressure of 8 kbar. Some special high pressure effects for liquid crystals, such as pressure-induced or pressure-limited phases and changes from monotropic to enantiotropic polymorphism were observed. For the correlation of the experimental results, an extension of the Simon equation was used; one of the regression parameters of the clearing curve equation was found to be correlated with the molecular structure of the liquid crystal substances under test.     

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.     

Influence of the order parameter and of the director orientation on the surface tension of free nematic films

A radial hydrodynamic flow in the nematic phase of free, suspended cylindrical films of 4-n-heptyl- and decyloxybenzoic acid and in 4,4'-di-n-heptyl-oxyazoxybenzene has been observed. The flow starts about 7°C before the phase transition into the smectic C phase. Under the same experimental conditions such a hydrodynamic flow is not established in free nematic films of 4-n-heptyl-and octyloxy-4'-cyanobiphenyl, 4,4'-dimethoxyazoxybenzene and N-(4-ethoxybenzylidene)-4'-n-butylaniline after the completion of the transition from the isotropic liquid to the nematic phase. The observed hydrodynamic flow is explained by a non-linear temperature dependence of the surface tension.     

Phase-transition studies of 5O.5 and 9O.4

As part of our detailed comparative studies of groups of liquid-crystalline compounds that belong to a homologous series, we present phase-transition studies of the compounds N-(4-n-pentyloxybenzylidene)4'-n-pentylaniline (5O.5) and N-(4-n-nonyloxybenzylidene)4'-n-butylaniline (9O.4) using different experimental techniques. The compound 5O.5 is reported to exhibit a phase sequence N, S_A, S_C, S_B and S_G, while 90.4 shows the sequence S_A, S_F and S_G. The salient features of our work on 5O.5 are (i) a new smectic F phase is found in place of the reported smectic B phase, which is confirmed by both miscibility and X-ray studies; (ii) the formation of smectic-C-like short-range order in the nematic phase well above the S_A-N transition; and (iii) a large tilt-angle variation in the smectic C phase (0-23·5°C) in a small temperature range (4·1°C). The phase changes across the S_A-I transition, and for the first time across S_F-S_A transition, are carried out by volumetric studies. The detailed inferences of these are also presented.     

The synthesis and liquid crystal properties of certain 1-(4'-n-alkoxy-2,2',3,3',5,5',6,6'-octafluorobiphenyl-4-yl)- 2-(trans-4-n-pentylcyclohexyl)-ethanes and -ethenes

The synthesis and liquid crystal properties of eight members of the homologous series of 1-(4'-n-alkoxy-2,2',3,3',5,5',6,6'-octafluorobiphenyl-4-yl)-2-(trans-4-n-pentylcyclohexyl)-ethanes and eight members of the corresponding -ethanes, which have the E-configuration, are reported. These compounds are nematic, the ethenes having a considerably mesophase higher thermal stability than the analogous ethanes.     

New fluorescent mesogens with a chalcone central linkage

Mesogens with a chalcone central linkage are rare. It is believed that the chalcone linkage is not conducive to mesomorphism. In the present study two homologous series were synthesized having the chalcone as a central linkage. 4-n-alkoxy-4'-hydroxy chalcones do not exhibit mesomorphism, however, higher members of 4-n-alkoxy-4'(4'-n-octyloxybenzoyl) chalcones, exhibit mesomorphism. The fluorescent behaviour of the chalcones is evaluated. All the compounds are characterized by standard methods.     

High precision density studies near the smectic A-nematic tricritical point

We have carried out very accurate density measurements (with a precision of ±5 × 10^-5g cm^-3) near the smectic A-nematic transition in binary mixtures of 4-n-nonyl-4'-cyanobiphenyl (9CB) and 4-n-decyl-4'-cyanobiphenyl (10CB). The transition crosses over from second to first order as the temperature range of the nematic phase decreases. For mixtures with the shortest nematic range the data deviate noticeably from a single power law behaviour. Such a deviation is an indication of the first order nature of the transition. Very good fits to a single power law have been obtained for pure 9CB and the x = 0·04 mixture where x is the mole fraction of 10CB in 9CB. The critical exponent obtained from the power law fitting has enabled us to locate the tricritical point to be very close to x=0·04, which is in agreement with the results obtained previously by high resolution calorimetric [1] and X-ray scattering studies [2].     

Creation of a nematic phase in mixtures of smectic A1 phases

Phase diagrams were determined for binary mixtures consisting of two 5-n-alkyl-2-(4'-isothiocyanatophenyl)-1,3-dioxane compounds (n-DBT) or 4'-isothio-cyanatophenyl 4-(trans-4'-n-decylcyclohexyl)benzoate and n-DBT. All compounds investigated have monolayer smectic A phases. A nematic phase in the upper temperature range and a nematic gap between two smectic regions also were observed, with the smectic layer spacing ratio, d/d', of 1.23 and 1.87 respectively. The variation of the enthalpy of transition with mixture composition in relation to changes of layer spacing ratio are also discussed for these systems.     

Analysis of phase transitions by photothermal probe beam deflection

A recent photothermal technique has been used to study the smectic A-nematic and nematic-isotropic phase transitions in 4-n-octyl-4prime;-cyanobiphenyl and in binary mixtures of 4-n-octyl- and 4-n-octyloxy-4prime;-cyanobiphenyl. A numerical analysis shows that both amplitude and phase of the photothermal signal are sensitive to the change of certain thermal parameters for the sample.     

Synthesis and mesomorphic properties of some platinum(II) and oxovanadium(IV) complexes

The synthesis and mesomorphic properties of a homologous series of N-(2-hydroxy-4-n-alkoxybenzylidene)-4'-n-decylphenylanilines and their platinum(II) and oxovanadium(IV) complexes are reported. All the ligands and their metal chelates exhibit enantiotropic mesophases, predominantly smectic A and smectic C phases. The transition temperatures and enthalpies have been determined for most of the compounds. The platinum(II) complexes have higher melting points and mesophase thermal stabilities. However, the oxovanadium(IV) complexes have a wider thermal range for the mesophase. Both platinum(II) and oxovanadium(IV) complexes containing only a chain on the biphenyl moiety exhibit a nematic phase.     

Landau theory for isotropic,nematic, smectic-A,and smectic-C phases

ABSTRACT

We use the Landau theory of phase transitions to describe the phase diagram of a liquid crystal displaying the isotropic (i), nematic (N), smectic-A and smectic-C phases. The order parameter of the smectic-C phase is defined as the projection of the director on the plane of the smectic layers, vanishing in the smectic-A phase. We present a detailed phase diagram that shows transition between any two of these phases, containing a triple point INA, a Lifshitz point NAC, a tricritical at the NA line, and a critical end point IAC. As one approaches the NC line from the smectic-C phase, the tilt angle approaches a nonzero value, but if the AC line is approached, the tilt angle vanishes according to the distance to the AC line to the power 1/2.     

Electrohydrodynamic instability in some nematic cyanobiphenyls in an a.c. electric field

Electrohydrodynamic instability in homeotropically oriented nematic samples of 4'-n-octyl-4-cyano-biphenyl and 4'n-alkyloxy-4-cyanobiphenyl, (n = 8.9) have been studied in an a.c. electric field. The domain patterns during the instability in these compounds in a very low frequency a.c. field are very similar to those in a d.c. field. The domain patterns observed at higher frequencies have been identified as 'maltese crosses' or 'crossed isogyres'. The electro-convective 'isotropic' flows near the electrode play an important role in the observed instability.