smectic-C Phase Research Articles

Measurement of electrically induced shear strain in a chiral smectic liquid-crystal elastomer

The mechanical response to electrical stimulation was investigated in a chiral smectic elastomer. The two-dimensional strain tensor in an elastomer film was precisely measured by tracking fluorescent beads dispersed on the film. Shear deformation in the film was clearly observed when an electric field was applied perpendicular to the film surface. The temperature dependence of the strain tensor was also investigated, and the origin of the electric-field-induced shear strain in the chiral smectic-C phase was mainly attributed to the Nambu-Goldstone mode.

Phase Behavior of Rod–Coil Diblock Copolymer and Homopolymer Blends from Self-Consistent Field Theory

The phase behavior of binary blends of rod-coil diblock copolymers and coil or rod homopolymers is studied by the self-consistent field theory (SCFT). The rod blocks are modeled as wormlike chains and the corresponding SCFT equations are solved using a hybrid method, in which the orientation-dependent functions are discretized on a unit sphere, while the positional space-dependent functions are treated using a spectral method. Phase diagrams of the blends are constructed as a function of the homopolymer volume fraction and phase segregation strength. It is discovered that the phase behavior of the system depends on the flexibility of the homopolymers. The addition of coil-homopolymers stabilizes the smectic phases. Low-molecular weight coil-homopolymers tend to mix with the coil-blocks, whereas high-molecular weight coil-homopolymers are mostly localized at the center of the coil-domains. On the other hand, the addition of rod-homopolymers strongly affects the orientation ordering of the system, leading to transitions between monolayer smectic-C, monolayer smectic-A and bilayer smectic-A phases.

Reentrant orthogonal smectic-Aphase below a tilted smectic-Cphase in a chiral compound

A reentrant orthogonal smectic-A (SmA) phase below the tilted smectic-C phase is established in a chiral liquid crystalline compound. The temperature evolution of the layer spacing confirms monolayer structure in both SmA phases, the upper SmA as well as the reentrant SmA phase. The reentrancy of the SmA phase is explained on the basis of the mean field free energy taking into account nonmonotoneous temperature dependence of the lowest term coefficient.

Computation of molecular free length ( Lf) in TBnA homologues series

The available volume ( V a ) and intermolecular free length ( L f ) are computed employing the density ( ρ) data for terephthalyidene-bis-p-n-alkyl aniline (TBnA's) homologues series. The variation of molecular free length ( L f ) with temperature in a liquid crystal molecule (in different phases) and in a homologous series is discussed. The increment of available volume ( V a ) per methylene unit is found to be 6.04, 6.21 and 5.64 in isotropic, smectic-C and smectic-F phases respectively. The nature of molecular free length is analyzed and discussed with respect to the different expressions used for the estimation of this parameter.

New Liquid-Crystalline Elastomers: 1. Synthesis, Structure, and Phase Behavior of a Series of Mesogenic Crosslinking Agents Containing Reactive Bifunctional Groups

The synthesis of eight mesogenic crosslinking agents (1c−8c) containing reactive bifunctional groups is described. The chemical structures were characterized by Fourier transform infrared (FTIR) and 1H nuclear magnetic resonance (NMR) spectra. The mesomorphism was investigated by differential scanning calorimetry (DSC) and polarizing optical microscopy (POM). The compounds 1c, 2c, 3c, 5c, 6c, and 8c showed an enantiotropic nematic phase. In addition, 5c exhibited a smectic A (SmA) phase on cooling; 4c exhibited a smectic C (SmC) phase, a SmA phase, and a nematic phase and a monotropic a smectic B (SmB) phase on cooling. 7c revealed an SmA phase and a nematic phase and a monotropic SmC phase on cooling.

Orientational Order Parameter in Alkoxy Benzoic Acids—Optical Studies

Alkoxy benzoic acids are well-known liquid-crystalline compounds with nematic phase of long thermal ranges. The refractive indices measurements are carried out using a wedge-shaped cell with the help of a modified spectrometer. The birefringence (δn = ne − no) is obtained directly by using the method developed by Kuczynski et al. [10] in all the available compounds (alkoxy chain number n = 3 to 12 and 16). The higher homologues exhibit the smectic-C phase along the nematic phase. The molecular polarizabilities are calculated from the refractive indices and the density using the well-known internal field models. The molecular anisotropy is estimated from the Lippincott δ-function model and molecular vibration method. The order parameter, S, is estimated (a) using molecular polarizabilities and the molecular anisotropy for both the models, (b) from Haller's extrapolation using molecular polarizabilities, and (c) directly from the birefringence, a method developed by Kuczynski et al. [10] along with the nematic phase. Further, the order parameter from δn, without the consideration of any internal field model, to the nematic molecule is compared with S obtained from density due to Maier et al. [23]. The results are compared with one another and the advantages and disadvantages are discussed.

RECENT DEVELOPMENTS IN BIAXIAL LIQUID CRYSTALS: AN NMR PERSPECTIVE

A survey of recent studies of biaxial liquid crystals (LCs), whose nematic and/or smectic-A phases do not possess optical uniaxiality (viz., more than one optical axis exists), is given in this review. In particular, we emphasize on how Nuclear Magnetic Resonance (NMR) spectroscopy can help to advance the understanding of phase biaxiality in general, and to examine recent debates on the existence of biaxial nematic phase reported in low molecular mass bent-core or V-shaped mesogens. A general discussion of orientational order parameters is detailed, particularly in smectic-C (SmC) and biaxial nematic phases. How these orientational order parameters can be determined by various techniques such as NMR, IR absorbance and Raman scattering studies, will be mentioned. Recent X-ray observations of SmC clusters in the nematic phase of V-shaped mesogens are highlighted and contrasted with probable theory. Moreover, deuterium and carbon-13 NMR techniques are briefly reviewed, and their possible utilization to identify phase biaxiality in these biaxial LC systems is explored.

Effect of intermolecular hydrogen-bonding and terminal substituents on the mesophase behavior of binary mixtures of dissimilarly-substituted benzoic acids

Binary mixtures of two types of 4-substituted benzoic acids, covering the whole composition range, were prepared by melting both components together, stirring to give an intimate blend, and then cooling to room temperature. The mixtures prepared were characterized for their mesophase behavior by differential scanning calorimetry (DSC) and polarized-optical microscopy (POM). The first type of the acid is substituted with a small compact polar group that cover a wide range of polarity. The other type of the acid is 4-alkoxy benzoic acid bearing an alkoxy (OC n H 2 n+1 ) group with varying chain length ( n = 6–16). Binary phase diagrams were constructed whereby all phases observed were identified by POM. The smectic C (SmC) mesophase was observed in all mixtures investigated, while the nematic phase (N) was observed, together with the SmC phase, only in mixtures possessing the acid homologues with n ≤ 12 carbons. Complex formation was also supported by FT-IR spectroscopic measurements.

ChemInform Abstract: Axial Chiral Liquid Crystals - Synthesis of Trisubstituted Allenyl Ethers.

The synthesis of racemic and enantiomerical enriched axial chiral liquid crystalline compounds incorporating a trisubstituted allene unit is described. All compounds 16–28 display the smectic C-phase and mostly also a nematic mesophase. All investigated enantiomerically enriched compounds have a ferroelectrically switchable chiral smectic SC*-phase.

Morphological phase behaviour of a homologous series of thermotropic cubic mesogens under pressure

The phase behaviour of a homologous series of thermotropic cubic mesogens of 1,2-bis(4'-n-alkoxybenzoyl)hydrazines (BABH-n, where n is the number of carbon atoms in both terminal alkoxy chains, ranging from n = 11 to n = 18) was studied under hydrostatic pressure using a polarising optical microscope (POM) equipped with a high-pressure optical cell. Pressure-temperature phase diagrams were constructed for all the samples. All of the samples showed the crystal (Cr)–cubic (Cub)–isotropic liquid (I) phase sequence under atmospheric and lower pressures, but the Cub phase was replaced completely by a high-pressure smectic C (SmC) phase; SmC(hp), under higher pressures. There is an intermediate-pressure region as a boundary in which the Cr–SmC(hp)–Cub–I phase sequence was recognised. The SmC(hp)–Cub transition lines showed positive slopes with pressure and shifted to higher pressures with increasing alkoxy chain length in BABH-n. What should noted here is the inversion of the Cub and SmC phases in the phase sequence, in comparison to those (Cr–Cub–SmC–I) for BABH-8 and -10. In BABH-14, -16 and -18, a solid state with a featureless texture and stronger intensities of transmitted POM light appeared just below the SmC(hp) phase on cooling under intermediate and high pressures. This phenomenon is not due to the formation of a monotropic Cub phase but is an induction period for spherulitic crystallisation from the SmC(hp) phase.

Liquid-crystalline phases formed by symmetrical azines with different terminal chains: Thermal, optical and electrical study

One series of symmetrical azine-type liquid crystals were synthesized and their chemical structure identified by FTIR, NMR, high resolution mass spectrometry-electro spray ionization (MS-ESI) and elemental analysis. The absorption (UV–vis), photoluminescence (PL) and thermoluminescence (TL) features of the compounds are documented. X-ray diffraction was employed to evaluate their phase transitional behaviour. Their mesomorphic properties were investigated by polarizing optical microscopy (POM) and differential scanning calorimetry (DSC). Terminal groups of the liquid crystals had an effect on their mesomorphic properties. Azine with alkoxysemiperfluorinated end-groups ( A1) exhibited smectic C (SmC) phase, whereas azine with octadecyloxy end chains ( A3) showed SmC and SmA phases. Azine with 5″-octyl-2,2′:5′,2″-terthiophene groups ( A2) exhibited only nematic phase. Different types of N and SmC textures were found and investigated by POM technique. Azine with thiophene groups ( A2) has an absorption band about 138 nm batochromically shifted compared to the analogous band of other compounds. The lowest optical band gap value (2.34 eV) was detected for A2. Azines emitted violet or green light. The thermoluminescence emission of azines occurred at about 390–440 nm wavelengths. Current–voltage measurements were performed on ITO/compound/Alq 3/Al and ITO/TiO 2/compound/Alq 3/Al devices before and after light irradiation. To prepare TiO 2 layer, sol–gel technique was applied.

Microphase separation and liquid-crystalline ordering of rod-coil copolymers

Microphase separation and liquid-crystalline ordering in diblock and triblock rod-coil copolymers (with rod-to-coil fraction f=0.5) were investigated using the dissipative particle dynamics method. When the isotropic disordered phases of these systems were cooled down below their order-disorder transition temperatures T(ODT), lamellar structures were observed. For rod-coil diblock copolymers, the lamellar layers were obtained below T=2.0. This temperature was found to be higher than the T(ODT) for normal coil-coil diblock copolymers. Significant ordering of the rods was observed only below T=0.9 which is the isotropic-nematic transition temperature for rodlike fluids. For the triblock rod-coil copolymers, both microphase separation and rod ordering occurred at T=0.9. Normal coil-coil triblock copolymers were found to undergo microphase separation at T=0.8, which is about half the T(ODT) of the normal diblock copolymers. Investigations of the mean square displacement and the parallel and the perpendicular components of the spatial distribution function revealed that at low temperatures, the rod-coil diblock copolymers exhibit smectic-A and crystalline phases, while the triblock copolymers show smectic-C and crystalline phases. No nematic phases were observed at the density and interaction parameters used in this study.

Influence of Oxygen on Phase Behavior of N(p-n-Octyloxy Benzylidene) p-n-Alkoxy Anilines: A Comparative Study

The N (p-n-octyloxy benzylidene) p-n-alkoxy anilines (8O.Om) compounds were synthesized, and the mesomorphic properties were characterized by using thermal microscopy (TM) and differential scanning calorimetry (DSC) for m = 3 to 7. The nematic (N), smectic-C (SC), and smectic-G (SG) phases are the more commonly observed in these compounds. The smectic-A (SA) and smectic-B (SB) phases are observed only in the 8O.O5 compound. The influence of oxygen on phase behavior of the 8O.Om compounds is compared with the data available on nO.m compounds.

Smectic C to Cubic Phase Transition of 4′-n-Docosyloxy-3′-nitrobiphenyl-4-carboxylic Acid (ANBC-22) and Alternating-Current Electric Field Effect

Smectic C (SmC) to cubic (Cub) phase transition behavior of 4'-n-docosyloxy-3'-nitrobiphenyl-4-carboxylic acid (ANBC-22) and alternating-current (AC) electric field effect on the SmC phase were examined. The most important and unexpected finding is that even at a temperature 8 K below the zero-field SmC to Cub phase transition temperature (T(SmC-Cub) approximately 408 K) determined previously for the compound and without field, the Cub phase growth occurs, after a very long induction period of several hours. The X-ray diffraction measurements revealed the formation of an Im3m-type Cub phase at the temperature. It is suggested that the "true" transition temperature, which is difficult to determine precisely, exists around 396 K. The time-dependence of the Cub phase growth both without field and under field was analyzed using the well-known Avrami equation, implying the nucleation and growth mechanism mainly operating in the SmC to Cub phase transformation. The fact uncovered is that between 408 and 396 K, the SmC to Cub transformation is virtually prohibited by the strongly limited nucleation. It is concluded that the effect of the electric field on the transition is to promote the nucleation of the Cub phase in the temperature region where the Cub phase is potentially more stable than the precursory SmC phase.

Influence of alkoxy tail length and unbalanced mesogenic core on phase behavior of mesogen‐jacketed liquid crystalline polymers

AbstractWhen the flexible terminal substituent changes from butoxy to hexyloxy or longer, smectic C (SC) liquid crystalline phase was firstly reported to develop from a kind of mesogen‐jacketed liquid crystalline polymer (MJLCP) whose mesogenic side groups are unbalancedly bonded to the main chain without spacers. A series of MJLCPs, poly[4,4′‐bis(4‐alkoxyphenyl)‐2‐vinylbiphenyl(carboxide)] (nC2Vp, n is the number of the carbons in the alkoxy groups, n = 2, 4, 6, 8, 10, and 12) were designed and synthesized successfully via free radical polymerization. The molecular weights of the polymers were characterized with gel permeation chromatography, and the liquid crystalline properties were investigated by differential scanning calorimetry, polarized light microscopy experiments, and 1D, 2D wide‐angle X‐ray diffraction. Comparing with the butoxy analog, the polymer with unbalanced mesogenic core and shorter flexible substituents (n = 2, 4) keeps the same smectic A (SA) phase, but other polymers with longer terminal flexible substituents (n = 6, 8, 10, and 12) can develop into a well‐defined SC phase instead of SA phase. © 2008 Wiley Periodicals, Inc. J Polym Sci Part A: Polym Chem 47: 505–514, 2009

New ionic liquid crystals based on azobenzene moiety with two symmetric imidazolium ion group substituents

Imidazolium‐substituted azobenzene liquid crystal derivatives were synthesised and their chemical structures were determined by 1H NMR, 13C NMR and UV spectroscopy and elemental analysis. The liquid crystalline properties pf the new compounds were investigated by differential scanning calorimetry, polarising optical microscopy and powder X‐ray diffraction. The monomer DC10 was found to exhibit a monolayer smectic C (SmC) phase with schlieren texture on both heating and cooling. The effects of ionic interactions as well as the length of the alkyl chains on the mesophase are discussed. The results indicate that ionic interactions between imidazolium groups are the driving force for the formation of the monolayer SmC phase, the thermal stability of which is enhanced.

NMR study of diffusive processes in novel liquid crystalline phases

Various deuterium one- and two-dimensional (2D) NMR techniques are used in the present study to characterize molecular self-diffusive processes in some novel mesophases. Both aligned and powder samples of liquid crystals (LC) are chosen to investigate rotational and/or translational jump diffusions. It is demonstrated that rotation of aligned LC samples in a NMR goniometer probe can be helpful in obtaining dynamic and structural information. Both spectral simulations and/or pulse separation ( τ) dependences of the quadrupole echo intensity are successfully employed. Deuteron spin-lattice relaxation times and angle dependent spectra are measured in a columnar phase of a monomeric discotogen to illustrate both the fast and intermediate motion regimes. Jump diffusions of chiral rodlike molecules in various smectic C⁎ (SmC⁎) phases and twist grain boundary A⁎ (TGBA⁎) phase are studied. In the LC 10B1M7, the jump rate (or diffusion constant) is determined as a function of temperature in the ferro-, ferri- and antiferro-electric chiral C⁎ phases. 2D deuterium exchange NMR is used to monitor the dynamic processes (self-diffusion) in the ferroelectric SmC⁎ and TGBA⁎ phases. Moreover, packing information of the phase can also be obtained in the two ferrielectric subphases and TGBA⁎ phase, which can be compared with results obtained by X-rays measurements.

Mesophases of hockey stick mesogens

Enantiotropic anticlinic smectic C (SmCa), smectic C (SmC), smectic A (SmA) and nematic (N) phases of hockey stick molecules were obtained by tuning the length of hydrocarbon chain located at the meta‐position to the carbon–carbon double bond of a stilbenyl moiety of calamitic mesogenic skeletons. The thermodynamic properties and some aspects of optical appearance of these phases are indiscernible from those of calamitic mesogens. Distinctions are noted such as the appearance of two‐brush defects and two types of domains in the tilted smectic phases. X‐ray results indicate the existence of clusters in the nematic phase and simple layer structure in the smectic phases. These phases exhibit electric field driven colour switching, and all the results imply that all three molecular axes are simultaneously aligned in the N and SmC phases, a behaviour unprecedented and different from those of conventional calamitic mesogens. Doping with a chiral calamitic mesogen results in formation of corresponding chiral N and ferroelectric SmC phases, but the effect for the SmCa phase is not clear. Small and comparable values of spontaneous polarisation are obtained for the doped and non‐doped phases. Molecular organisations of the SmC and SmCa phases are proposed and discussed.

Tricritical behavior of the smectic-A to smectic-C phase transition in a liquid crystal mixture

A phenomenological approach to the description of the smectic-A to smectic-C phase transition in a liquid crystal mixture is proposed. The problem of the first or second order transition is explored by means of the variation of the concentration. The influence of the concentration on this transition is discussed by varying the coupling between the concentration variable and the order parameters. We show that a tricritical point on the smectic-A to smectic-C phase transition line can be achieved under certain conditions. The theoretical prediction is found to be in good qualitative agreement with experimental results.

Chirality and Molecular Dynamics of Smectic Compound Studied by 13C-NMR Spin-Lattice Relaxation Time Observation

The temperature dependence of the spin-lattice relaxation time T 1 is investigated in the racemic mixture of a smectic liquid crystal, 4-(1-methylhexyloxycarbonyl)phenyl 4′-octyloxybiphenyl-4-carboxylate ((±)-MH(6)POBC) by means of solid-state 13 C-NMR spectroscopy. T 1 of almost all of aromatic carbons has a minimum at the smectic C (SmC) temperature region, because the frequency of the molecular rotational motion corresponds to the Larmor frequency of 100 MHz in the present measurements. In addition, analysis of the temperature dependence of T 1 reveals that the activation energy of the molecular rotation in the SmC phase is higher than that in the SmA phase.