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.     

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.     

Creation of a nematic phase in mixtures of smectic A1 phases

Abstract

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.     

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.     

Induced smectic phases in phase diagrams of binary nematic liquid crystal mixtures

To elucidate induced smectic A and smectic B phases in binary nematic liquid crystal mixtures, a generalized thermodynamic model has been developed in the framework of a combined Flory-Huggins free energy for isotropic mixing, Maier-Saupe free energy for orientational ordering, McMillan free energy for smectic ordering, Chandrasekhar-Clark free energy for hexagonal ordering, and phase field free energy for crystal solidification. Although nematic constituents have no smectic phase, the complexation between these constituent liquid crystal molecules in their mixture resulted in a more stable ordered phase such as smectic A or B phases. Various phase transitions of crystal-smectic, smectic-nematic, and nematic-isotropic phases have been determined by minimizing the above combined free energies with respect to each order parameter of these mesophases. By changing the strengths of anisotropic interaction and hexagonal interaction parameters, the present model captures the induced smectic A or smectic B phases of the binary nematic mixtures. Of particular importance is the fact that the calculated phase diagrams show remarkable agreement with the experimental phase diagrams of binary nematic liquid crystal mixtures involving induced smectic A or induced smectic B phase.     

Induction of the smectic Ad phase in polar systems II. Role of steric effects in the smectic Ad phase induction

Binary mixtures in which one component is nCBB or its chiral analogue, and the second component is nCB, nOCB, n.CN, or one of their chiral analogues are studied by thermomicroscopy. The branched compounds induce the smectic A_d phase more strongly than unbranched compounds with the same alkyl chain length. The observed behaviour is discussed from the point of view of dimer formation. In all chiral systems, the TGB_A phase appears.     

Dielectric studies on binary mixtures of systems exhibiting intermediate nematic phase

Two binary phase diagrams of the liquid crystals (4-n-pentylphenyl-4-n-hexyloxybenzoate (PPHB) with 4-nitrophenyl-4-n-pentylbenzoate (NPPB) and 4-n-hexyloxyphenyl-4-n-decyloxybenzoate (HPDB) with NPPB) have been studied. PPHB shows only a nematic phase where as for HPDB trimorphism, with the SmC, SmA and nematic phases, was observed. Substance NPPB which has a strong polar nitro group is non-mesogenic one. Both the phase diagrams show an induction and stabilization of SmA phase and appearance of the nematic state in the high concentration range of polar component. The dielectric measurements confirm the phase transition temperatures and show changes in the short range interaction at the phase transitions.     

Induction of a chiral nematic phase in smectic polymers

A number of new photosensitive copolyacrylates of different composition were obtained by the copolymerization of chiral photochromic benzilidene-p-menthane- 3-one acrylic monomers with a smectogenic monomer containing a hexyloxyphenylbenzoate mesogenic group. The chiral, photochromic monomers differ by the length of the aliphatic spacer and the aromatic fragment. It was found that the introduction of a small number of chiral units into the copolymers (5 mol%) leads to the “degeneration” of the smectic C phase, which characterizes the hexyloxyphenylbenzoate homopolymer, and to the formation of the smectic A phase. An unusual effect of chiral nematic phase induction was observed for copolymers containing chiral side groups with two ring aromatic fragments. It should be pointed out that the chiral nematic phase does not occur in the case of the homopolymers of both initial comonomers. An explanation of this effect, based on the consideration of the chemical structure of the chiral and hexyloxyphenylbenzoate units, was suggested. The optical properties of cholesteric copolymers were investigated; the helical twisting power of the chiral groups of different structures was calculated. The possibility of using such copolymers as new photosensitive materials was demonstrated. Received: 16 December 1999/Accepted: 1 February 2000     

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.     

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.     

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.     

Model phase diagrams of binary nematic mixtures. A comparative study between linear and crosslinked polymers

Model calculations of phase diagrams of side chain liquid crystal polymers (SCLCP) and low molecular weight liquid crystals (LMWLC) are presented. The polymer is assumed to have grafted side chain units characterized by a nematic‐isotropic transition temperature T_{NI 2}, and the LMWLC presents also a similar transition at a temperature T_{NI 1} . The model calculations can accommodate for the cases where the latter two temperatures are comparable or widely different. For the sake of illustration, the case T_{NI 1} = 60°C and T_{NI 2} = 80°C is adopted here. The main point of interest here is to perform a comparative study of the equilibrium phase diagrams of SCLCP made either of linear free chains or crosslinked chains forming a single network. To our knowledge this is the first comparative study of the phase behavior of binary nematic mixtures involving linear and crosslinked polymer matrices which permits to clearly identify the effects of crosslinks present in the polymer matrix. The crosslinks attribute elasticity to the polymer constituent which induces important distortions in the phase diagram. To highlight these distortions, examples of hypothetical binary nematic mixtures are chosen involving both linear and crosslinked polymers with side chain mesogen units. The quadrupole interaction parameter between the two nematogens is related to individual parameters via a geometric average ν²₁₂ = κν₁₁ν₂₂ with a coupling parameter κ. Different values of this parameter are considered and the impact of coupling strength on the phase diagram is discussed for crosslinked and linear polymers.     

Latent heat measurements of the smectic A-cholesteric transition on binary mixtures

New differential scanning calorimetry measurements of the smectic A-cholesteric latent heat of transition for binary mixtures of cholesteryl nonanoate (C₉) and cholesteryl heptanoate (C₇), cholesteryl nonanoate and cholesteryl caproate (C₆) are reported for cooling cycles. These measurements give evidence of a tricritical point at approximately the reduced temperature T_SACh/T_Chl ≅ 0·92 for 63·1 mol per cent C₉ in the mixture of C₆ and C₉. This tricritical point occurs at reduced temperatures and concentrations different from those already obtained for heating cycles for the same mixture.     

Effect of molecular structure on the phase behaviour of some liquid crystalline compounds and their binary mixtures V. Mixtures of bimolecular smectics and linear mesogens

Two binary mixtures, prepared from either 4-cyanophenyl 4-hexadecyloxybenzoate (Ib) or 4-nitrophenyl 4-hexadecyloxybenzoate (Ic), with 4-carboxyphenyl 4-hexadecyloxybenzoate (Ia), were thermally characterized to construct the phase diagrams for the two systems Ia/Ib and Ia/Ic. Due to differences in the smectic layering between the individual components of each system, phase separation took place either in the solid or in the mesophase. A mathematical relation was derived to calculate the composition of each phase in the solid state. Another two phase diagrams were constructed for binary mixtures of the linear dimer molecule, 4-hexadecyloxybenzoic acid (III) with either the acid Ia or its isomeric derivative, 4-hexadecyloxyphenyl 4-carboxybenzoate (II).     

Effect of molecular structure on the phase behaviour of some liquid crystalline compounds and their binary mixtures V. Mixtures of bimolecular smectics and linear mesogens

Two binary mixtures, prepared from either 4-cyanophenyl 4-hexadecyloxybenzoate (Ib) or 4-nitrophenyl 4-hexadecyloxybenzoate (Ic), with 4-carboxyphenyl 4-hexadecyloxybenzoate (Ia), were thermally characterized to construct the phase diagrams for the two systems Ia/Ib and Ia/Ic. Due to differences in the smectic layering between the individual components of each system, phase separation took place either in the solid or in the mesophase. A mathematical relation was derived to calculate the composition of each phase in the solid state. Another two phase diagrams were constructed for binary mixtures of the linear dimer molecule, 4-hexadecyloxybenzoic acid (III) with either the acid Ia or its isomeric derivative, 4-hexadecyloxyphenyl 4-carboxybenzoate (II).     

Orientational order and refractive indices in binary nematic mixtures

Mean field theory is used to calculate the temperature-composition phase diagram and component order parameters of binary nematic mixtures. Experimental values for the mixture order parameter of a binary nematic mixture close to the nematic/isotropic transition have been obtained from refractive index measurements. The experimental results qualitatively confirm the predictions of the theory.     

Order parameters and densities in the smectic C, smectic A and nematic phases of a liquid crystal mixture

Measurements of the orientational and translational order parameters for the nematic, smectic A and smectic C phases of a commercial liquid crystal material are reported. The order parameters have been obtained by analysis of the angular distribution of the intensity of X-rays scattered by a sample aligned by a magnetic field. Results are presented as a function of temperature, and it is found that the apparent orientational order parameter in the smectic C phase decreases with decrease in temperature. This is explained using a model of random tilt. The experimental order parameters are compared with those calculated from a mean field model.     

Thermodynamic properties of binary mixtures containing 1-alkanols

Densities (ρ) of pure liquids and their mixtures have been measured at 303.15 and 313.15 K and atmospheric pressure over the entire composition range for the binary mixtures of benzylalcohol with 1-propanol, 1-butanol, 1-pentanol, and 1-hexanol by using Rudolph Research Analytical digital densitometer (DDM-2911 model). Further, the ultrasonic sound velocities for the above said mixtures were also measured at 303.15 and 313.15 K. The measured density data were used to compute excess molar volumes (V ^E) and these were compared with the values obtained by Hwang equation. Isentropic compressibility (κ _S) and excess isentropic compressibilities (κ _S ^E ) were evaluated from experimental sound velocity and density data. Moreover, the experimental sound velocities were analyzed in terms of theoretic models namely, collision factor theory and free length theory. The experimental results were discussed in terms of intermolecular interactions between component molecules.     

Effect of the temperature range of the nematic phase on the induction of a twist grain boundary phase in non-chiral liquid crystals

Recent experimental results suggest that the twist grain boundary phase may be induced in non-chiral smectic liquid crystals by confining the material in a twist cell in which the nematic directors at the two surfaces are perpendicular to each other. However, the effect of the temperature range of the nematic phase on the induction of the twist grain boundary (TGB) phase has not been studied to date. We have performed experiments on non-chiral liquid crystals having the nematic-smectic A (N-SmA) phase transition sequence and confined in a twist cell. It is observed that materials with a second order N-SmA phase transition show characteristics of a TGB phase, but a material with a first order N-SmA transition does not.