Effect of terminal polar substituents on the nature of smectic A liquid crystals of a series of 4-(4'-octyloxybenzoyloxy)benzylidene-4'-substituted anilines

The nature of smectic A liquid crystals and the phase transitions from smectic A to nematic phases are studied in the homologous series of 4-(4'-octyloxy-benzoyloxy)benzylidene-4'-substituted anilines, in which the substituents are H, F, Cl, Br, I, NO₂ and CN. Measurements have been performed for the dipole moment of the molecule, the smectic A lattice period, the temperatures and the entropies of phase transitions, and the temperature dependence of the orientational order parameter, all of which provide information on the intermolecular pair potentials. It is shown that the introduction of a terminal polar substituent brings about counteracting contributions to the stability of the smectic A state. It is suggested that the reentrant nematic phase transition can appear in the strongly polar mesogens if the extent of antiparallel association of the molecules increases on lowering the temperature.     

The kerr effect in the vicinity of the transition from the isotropic to smectic a phase

The equilibrium electrooptical effect in the isotropic phase of seven liquid crystalline substances (4-n-decyloxy-4′-cyanobiphenyl, carbosilane dendrimer of the fourth generation with cyanobiphenyl terminal fragments, and the fourth, fifth, sixth, seventh, and tenth members of the homologous series of 4-n-acylphenylene 4′-n-alkoxybenzoates) was studied. The smectic A phases were found to exhibit weaker divergence of the Kerr constant in the vicinity of the T _c isotropic melt-liquid crystal phase transition temperature compared with the nematic phases. The difference between T _c and the temperature T* of the virtual second-order phase transition varied from 3.2 to 19.0 K for the smectic A phases, which substantially exceeded the value (T _c-T*) ≤ 1 K for the nematic phases. A theory of the electrooptical properties of isotropic melts in the vicinity of the T _c temperature of the phase transition from the isotropic to smectic A phase is developed. An equation relating the T _c-T* difference to the phenomenological coefficients of the expansion of the Helmholtz energy of an isotropic melt into a series in powers of the coordination and orientational order parameters is obtained.     

Orientational ordering, conformation, and polarizability of molecules in the reentrant phase of discoid nematics

This paper presents data on orientational ordering in a reentrant discoid nematic and on changes in the polarizability of discogenic molecules in a homologous series. The phase transition of a nematic to the discotic column phase is accompanied by a jump of the orientational ordering parameter and is characterized by close relationship between the orientational and translational ordering of molecules. The lengthening of the flexible peripheral molecular chains in the reentrant nematic phase considerably changes their conformational state, decreasing the anisotropy of their polarizability. Translated fromZhumal Strukturnoi Khimii, Vol. 38, No. 1, pp. 89–97, 1997.     

Phase Transition and Structural Investigations in 50.5, 50.6 and 50.7

Abstract

We report X-ray diffraction, density, ultrasonic velocity and refractive index studies in the N(p-n-pentyloxy benzylidene) p-n-alkylaniline compounds, viz. 50.5, 50.6 and 50.7. The nematic-smectic A (NA) transition is found to be weak first order in 50.6 while it is second order in 50.5 and 50.7. The salient features observed are cybotactic clusters in the nematic phase in all the compounds, molecular tilt which was inferred due to the end alkyl chains tilt causing orienta-tional disorder (smaller orientational order parameters ‘s’ than expected) in smectic A phase, smectic F phase and large tilt angle variation in smectic C phase in a small temperature range in 50.5. The observed results are discussed in the light of available data in other n0.m compounds.     

The orientational order parameters of a dendritic liquid crystal organo-siloxane tetrapode oligomer, determined using polarized infrared spectroscopy

The observed macroscopic anisotropic properties such as the components of infrared (IR) absorbances of liquid crystals are expressed in terms of the order parameters of the long molecular axis, molecular, and phase biaxiality. The order parameters of the organo-siloxane tetrapode liquid crystal of zero dendritic order (G0) in its nematic and smectic phases have been determined using results of the polarized IR spectroscopic measurements on a planar homogenously and hometropic aligned cells. The spatial components of the absorbances for the vibrational bands (in the mesogenic unit, terminal chains, and spacer) have been measured and analyzed. For the laboratory reference system, the apparent orientational order parameter S of the mesogen unit shows a significant drop in the transition from the nematic to the smectic phase while the phase biaxiality order parameter P increases to almost 0.4 in the smectic phase. This result shows that the director is tilted out of the sample plane in the smectic phase. The molecular biaxiality parameter D is found to be positive both for the nematic and smectic phases. This suggests that the carbonyl dipoles are oriented close to the tilt plane. For the vibrational bands in the chains, low values of S and D indicative of their low orientational order are obtained. As a result of the interaction among the molecules in the tilted smectic phases, the transition dipoles show positive correlations for the transversal and negative for the longitudinal dipoles.     

Surface ordering at the air–nematic interface. Part 2. A spectroscopic ellipsometry study of orientational order

Spectroscopic ellipsometry has been used to measure enhanced orientational ordering at the nematic–air interface of 8CB as the smectic A phase was approached by cooling from the isotropic phase. The depth profile of the orientational order has been estimated by calculating the ellipsometric parameters for a homeotropic uniaxial surface film on a uniaxial sub‐phase using the Abelès matrix method. This showed that the depth of the enhanced orientationally ordered region was ～10 nm at 0.5°C above the nematic–smectic A transition. This is substantially less than the thickness of the region with surface enhanced smectic order as determined by neutron reflection and a model of the surface structure consistent with both sets of results is proposed.     

NMR determination of smectic ordering of probe molecules

The NMR spectra of the three solutes ortho-, meta-, and para-dichlorobenzene in the nematic and smectic A phases of the liquid crystals 8CB and 8OCB are analyzed to yield two orientational order parameters for each solute. Extrapolation of the asymmetry in the energy parameters that describe the orientational ordering in the nematic phase are used to provide estimates of the strength of the nematic potential in the smectic A phase. The experimentally determined asymmetry of the orientational order parameters in the smectic A phase is then used in conjunction with Kobayashi-McMillan theory applied to solutes to give information about the smectic A layering and the nematic/smectic A coupling. In both smectic A solvents, the solute smectic coupling constant, tau, is negative (with the origin fixed at the center of the smectic layer) for all solutes. The signs and relative values of tau indicate that the ortho and para solutes favor the interlayer region while the meta solute is more evenly distributed throughout the layers.     

Creation of nematic and reentrant nematic phases in bi- and multi-component smectic mixtures

Abstract

Experimental results referring to the transformation of smectic phases, mainly smectic A, into nematic and reentrant nematic phases are reviewed. A new explanation of some experimental results is proposed. Factors which are responsible for the depression of smectic phases in mixtures of polar mesogens are discussed and the possibility of forming mixtures with a broad temperature range of nematic phase from smectic compounds, which can be useful for liquid crystal displays (LCDs), is shown. A nematic gap observed in some cases between monolayer (S_A1) or monolayer and partially bilayer (S_Ad) smectics results from the differences in the organization of the molecules in the smectic layers. It is concluded that polar phase from smectic A₁ phases can be divided into two groups: (a) the first one is characteristic for compounds with the -NCS, -F, -CI, -I or ?COC _m H_{2m + 1} terminal group. The spacing of the smectic layer slowly expands with the increase in alkyl chain length and the structure of the smectic A₁ phase slowly changes to be more like the smectic A_d phase (d/1 > 1). It is proposed that such a smectic is called an enhanced monolayer smectic (S_A1e (b) the second one is typical for compounds with the -CN terminal group. This kind of smectic A₁ phase is rapidly transformed into the smectic A_d phase with increasing alkyl chain length. These latter monolayer mesogens easily form the reentrant nematic phase when they are mixed with other polar smectic mesogens.     

Influence of the smectic A-nematic transitional order on the formation of the smectic phases of 4-n-hexyloxybenzylidene-4′-n-butyIaniline and 4-n-butyloxybenzylidene-4′-n-octyIaniline from an electrically deformed nematic phase

Abstract

The crucial role of the smectic A-nematic transitional order for the formation of the smectic A, B and G phases from an electrically deformed nematic phase of the liquid crystal 4-n-hexyloxy-benzylidene-4′-n-butylaniline (6O.4) with a typical smectic A-nematic first order transition and the formation of the smectic A and B phases from an electrically deformed nematic phase of the liquid crystal (4-n-butyloxy-benzylidene-4′-n-octylaniline (40.8) with a smectic A-nematic second order transition has been demonstrated. The nematic phase was deformed by an AC voltage of 2U,_th 5U _th and 10U _th, where U _th is the threshold voltage which causes the appearance of the Fréedericksz transition in the homeotropic nematic layer. The smectic textures have been observed on free cooling of the nematic phase or after the use of an oven. The smectic A phase of the liquid crystal 60.4 was observed with the formation of a clear smectic A-nematic phase boundary while the smectic A phase of the liquid crystal 40.8 has been formed from intermediate pretransitional stripes, observed by Cladis and Torza [1]. The homeotropic anchoring of the direction was crucial for the formation of the smectic phases of the liquid crystal 40.8 but not significant for the liquid crystal 60.4.     

A molecular theory including hard rod interactions of liquid crystalline phases exhibited by highly polar compounds

We extend our molecular mean field model of highly polar compounds to include hard rod interactions to develop a hybrid model. The latter interactions are restricted to second virial terms, following a method developed by Koda and Kimura. This allows us to calculate pressure-temperature phase diagrams. Depending on the model parameters used, the phase diagrams exhibit the following features: nematic-nematic phase transition, bounded SmA_d region, double reentrance, reentrant nematic lake surrounded by the smectic A phase (the reentrant nematic lake merging with the main nematic sea) as well as the possibility of quadruple reentrance. We compare the calculated phase diagrams with the available experimental data.     

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.     

Glass formation in the Gay-Berne nematic liquid crystal

We present the results of molecular dynamics simulations of the Gay-Berne model of liquid crystals, supercooled from the nematic phase at constant pressure. We find a glass transition to a metastable phase with nematic order and frozen translational and orientational degrees of freedom. For fast quench rates the local structure is nematic-like, while for slower quench rates smectic order is present as well.     

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.     

Determination of elastic constants of a binary system (7CPB+9.CN) showing nematic,induced smectic Ad and re-entrant nematic phases

The temperature variation of the splay and bend elastic constants of a binary system exhibiting nematic-induced smectic A_d and re-entrant nematic phases measured by electric field-induced Freedericksz transition method has been reported. As bend deformation is not permitted in the smectic A phase, bend elastic constant (K₃₃) could only be determined in the nematic and re-entrant nematic phases. In both the nematic phases, the splay elastic constant has the same order of magnitude and does not show any pretransitional effect. However, in the induced smectic A_d phase, the value of K₁₁ is about one to two orders higher than that in the nematic phases. The bend elastic constant shows a strong pretransitional effect near the nematic–smectic and smectic–re-entrant nematic phase transitions. The influence of the presence of the induced smectic phase is observed even in those mixtures which have no induced smectic phases. As the smectic phase is approached, the ratio K₃₃/K₁₁ increases rapidly and diverges to infinity.     

Side chain functionalized liquid crystalline polymers and blends, 1. Reentrant nematic phase formation in hydrogen-bonded blends of liquid crystalline functionalized copolymers with a low molecular weight non-mesogeic dopant

Hydrogen-bonded blends based on smectic comb-shaped functionalized LC copolymers containing alkyloxy-4-hydroxybenzoic acid fragments (proton donor) and the low molecular weight dopant 4-(4-pyridyloyl)cyanobenzoate (proton acceptor) were obtained. It was observed that blends containing 10–25 mol-% of low molecular weight dopants form a reentrant nematic phase (SmA-RN-SmA-I). The blend behavior in the magnetic field was studied, and the orientational elastic constants of the RN phase were determined.     

Visco-elastic properties of reentrant nematic liquid crystalline mixtures

The visco-elastic properties for binary mixtures of 4-n -hexyloxy-4′-cyanobiphenyl (6OCB) and 4-n-octyloxy-4′-cyanobiphenyl (8OCB) are investigated in detail by a light scattering technique. The mixtures exhibit a reentrant nematic (RN) phase between the smectic A (SmA) and crystal phases in the range 22.0–29.5 wt % of 6OCB. The viscosity and the elastic constant increase with cooling in both the nematic and reentrant nematic phases. It is also found that the mixtures exhibit anomalously large values of viscosity and elastic constant near the phase transition, i.e. a pretransitional phenomenon can be observed. The activation energy for viscosity is smaller in the RN phase than in the ordinary nematic phase appearing at higher temperatures, while the normalized elastic constant (defined as the ratio of the elastic constant to the square of the dielectric anisotropy) is larger. Moreover, the visco-elastic behaviour depends on the specific time during which the sample has been kept in the SmA phase. These results confirm that the SmA phase has a strong influence on the formation of molecular aggregates, which plays an important role in the reentrant phenomenon of these systems.     

Phase Transition and Structural Investigations in 50.5, 50.6 and 50.7

We report X-ray diffraction, density, ultrasonic velocity and refractive index studies in the N(p-n-pentyloxy benzylidene) p-n-alkylaniline compounds, viz. 50.5, 50.6 and 50.7. The nematic-smectic A (NA) transition is found to be weak first order in 50.6 while it is second order in 50.5 and 50.7. The salient features observed are cybotactic clusters in the nematic phase in all the compounds, molecular tilt which was inferred due to the end alkyl chains tilt causing orienta-tional disorder (smaller orientational order parameters 's' than expected) in smectic A phase, smectic F phase and large tilt angle variation in smectic C phase in a small temperature range in 50.5. The observed results are discussed in the light of available data in other n0.m compounds.     

A molecular theory including hard rod interactions of liquid crystalline phases exhibited by highly polar compounds

We extend our molecular mean field model of highly polar compounds to include hard rod interactions to develop a hybrid model. The latter interactions are restricted to second virial terms, following a method developed by Koda and Kimura. This allows us to calculate pressure-temperature phase diagrams. Depending on the model parameters used, the phase diagrams exhibit the following features: nematic-nematic phase transition, bounded SmA_d region, double reentrance, reentrant nematic lake surrounded by the smectic A phase (the reentrant nematic lake merging with the main nematic sea) as well as the possibility of quadruple reentrance. We compare the calculated phase diagrams with the available experimental data.     

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.     

Effect of polydispersity and soft interactions on the nematic versus smectic phase stability in platelet suspensions

We theoretically discuss, using density-functional theory, the phase stability of nematic and smectic ordering in a suspension of platelets of the same thickness but with a high polydispersity in diameter, and study the influence of polydispersity on this stability. The platelets are assumed to interact like hard objects, but additional soft attractive and repulsive interactions, meant to represent the effect of depletion interactions due to the addition of nonabsorbing polymer, or of screened Coulomb interactions between charged platelets in an aqueous solvent, respectively, are also considered. The aspect (diameter-to-thickness) ratio is taken to be very high, in order to model solutions of mineral platelets recently explored experimentally. In this regime a high degree of orientational ordering occurs; therefore, the model platelets can be taken as completely parallel and are amenable to analysis via a fundamental-measure theory. Our focus is on the nematic versus smectic phase interplay, since a high degree of polydispersity in diameter suppresses the formation of the columnar phase. When interactions are purely hard, the theory predicts a continuous nematic-to-smectic transition, regardless of the degree of diameter polydispersity. However, polydispersity enhances the stability of the smectic phase against the nematic phase. Predictions for the case where an additional soft interaction is added are obtained using mean-field perturbation theory. In the case of the one-component fluid, the transition remains continuous for repulsive forces, and the smectic phase becomes more stable as the range of the interaction is decreased. The opposite behavior with respect to the range is observed for attractive forces, and in fact the transition becomes of first order below a tricritical point. Also, for attractive interactions, nematic demixing appears, with an associated critical point. When platelet polydispersity is introduced the tricritical temperature shifts to very high values.