smectic-A Phase Research Articles

Influence of Moieties and Chain Length on the Abundance of Orthogonal and Tilted Phases of Linear Hydrogen-Bonded Liquid Crystals, Py16BA:nOBAs

A novel series of linear hydrogen-bonded (HB) liquid crystalline (LC) complexes (HBLCs), namely Py16BA:noBAs, is reported. The series was prepared by the complexation of a novel non-mesogenic proton acceptor moiety, namely (4-pyridyl)-benzylidene-4′-n-hexadecyl aniline (Py16BA), with a mesogenic proton donor, p-n-alkoxy benzoic acid (noBAs for n = 3, 4, 5, 6, 7, 8, 9, and 12) moieties. Formation of HB complexes is confirmed through FTIR spectra. The LC phases exhibited by Py16BA:noBAs, the transition temperatures (Tc), and the heat of transition (enthalpy) (ΔH) are determined by polarizing optical microscope (POM) and differential scanning calorimetry (DSC) techniques. A comparative study of phase abundance is presented (with respect to the HB complexes of pure noBAs). An orthogonal smectic-A phase is induced by quenching the nematic phase in the lower homologs, while a quasi-2D tilted smectic-F phase is induced in the intermediate (or higher) homologs Py16BA:noBAs. The odd–even effect is observed across the isotropic–LC phase and LC–solid phase transitions. Overall mesogenic stability is enhanced. Optimization of the tilted smectic phase stability is discussed in the wake of the configuration of hydrogen bonding and the steric intrusion due to the increase in length of the flexible end-chain. Based on the extrapolation of the odd–even effect, an ACBF multi-critical point is predicted in the phase diagram in the vicinity of intermediate homologs, possibly in a binary of Py16BA:5oBA and Py16BA:6oBA.

Rigid probe solutes in a smectic-Aliquid crystal: An unconventional route to the latter's positional order parameters

Biphenylene and pyrene were dissolved in the nematic and smectic-A phases of the liquid crystal 4,4'-di-n-heptyl-azoxybenzene and the orientational order parameters of both solutes and solvent measured via proton and deuteron nuclear-magnetic-resonance spectroscopy. This new data set was then merged with the one previously obtained, formed by 4,4'-di-chloro-benzene and naphthalene as solutes in the same solvent, and the resulting overall data set analyzed with a statistical thermodynamic density-functional theory to provide positional-orientational distribution functions of the various solutes along with the smectic solvent's positional order parameters.

Experimental Investigations on Ultrasonic Velocity and Behavior of Thermodynamic Parameters of Four Members of nO.m Series

Ultrasonic velocity measurements are carried out on fournOmcompounds viz., N-(p-n-methoxy and ethoxy benzylidene)-p-n-dodecyl and tetradecyl anilines, 1O.12, 1O.14, 2O.12 and 2O.14. The first three compounds exhibit mono variant nematic phase while the last compound exhibits nematic and smectic-A phases with variable thermal ranges. The temperature dependence of sound velocity, U shows minima at the isotropic-nematic and nematic-smectic-A transitions. In conjunction with the density data of these compounds, the thermo acoustic parameters such as adiabatic compressibility (βad), the molar sound velocity (Rao’s number) (Rn), molar compressibility (W), molecular free length (Lf) and available volume (Va) are estimated. In addition the intermolecular free length (Lf) is estimated in three different ways employing density and ultrasonic velocity data. The results are compared with the data available in literature of other compounds.

Investigations on Ferroelectric Liquid Crystal by High Resolution TEM and Solid State 13C NMR

In order to investigate the structural and dynamical properties of ferroelectric liquid crystal (FLC) in different phases a model compound [4-(3)-(S)-methyl-2-(S)-chloropentanoyloxy)]-4'-nonyloxy-biphenyl (3M2CPNOB) is synthesized. High resolution transmission electron microscopy (HR-TEM) is applied to observe the morphology of 3M2CPNOB and temperature-dependent solid state (13)C NMR to record (13)C chemical shifts at different phases. A liquid nitrogen quenching method is used to maintain the conformation of the mesophases for HR-TEM experiments. TEM images show that all the smectic A (SmA), smectic C* (SmC*) and crystalline phases have lamellar morphology. The interplanar distances in the crystalline phase are smaller than those in SmA and SmC* phases because of denser arrangement of the molecules. Both (13)C chemical shifts and line shape vary with different phases. The experimental results suggest that SmC* phase as an intermediate occurs in the anisotropy transition process from SmA to crystalline phase, the helical structure of the SmC* phase unwinds in the magnetic field and the conformations of the SmA and isotropic phase are very similar.

A single-site anisotropic soft-core model for the study of phase behavior of soft rodlike particles

A novel mesoscopic simulation model is proposed to study the liquid crystal phase behavior of the anisotropic rodlike particles with a soft repulsive interaction, which possesses a modified anisotropic conservative force type used in dissipative particle dynamics. The influences of the repulsion strength and the particle shape on the phase behavior of soft rodlike particles are examined. In the simulations, we observe the formation of the nematic phase and smectic-A phase from the initially isotropic phase. Moreover, we find that shorter soft rodlike particles with anisotropic repulsive interactions can form a stable smectic-B phase. Our results demonstrate that the soft anisotropic purely-repulsive potential between the rodlike particles can reflect the interaction nature between soft rodlike particles in a simple way and is sufficient to produce a range of ordered LC-like mesophases.

Effect of electric field on the rheological and dielectric properties of a liquid crystal exhibiting nematic-to-smectic-A phase transition

We report simultaneous measurements of shear viscosity (η) and dielectric constant (ε) of octyloxy cyanobiphenyl (8OCB) in the nematic (N) and smectic-A (SmA) phases as functions of temperature and electric field. With increasing electric field η increases in the N phase whereas it decreases in the SmA phase and saturates beyond a particular field in both the phases. The flow curves in the intermediate-field range show two Newtonian regimes in the N phase. The temperature-dependent behavior of η and ε at zero or at small electric field suggests the occurrence of several structures that results from precessional motion of the director along the neutral direction as reported in similar other system. We show that the precessional motions are gradually suppressed with increasing electric field and the effective viscosity resembles with the Miesowicz viscosity η1 at high enough electric field. In the intermediate field range the temperature-dependent η exhibits anomalous behavior across the N-SmA phase transition which is attributed to the large contribution of Leslie coefficient α1.

Separation of nematic and smectic-A potential effects on solute ordering in a series of binary 6OCB–8OCB mixtures

1H NMR and optical microscopy have been used to study four solutes dissolved in samples of a binary mixture of 4-n-hexyloxy-4′-cyanobiphenyl (6OCB) and 4-n-octyloxy-4′-cyanobiphenyl (8OCB) that are close to the region of the phase diagram where the smectic-A (SmA) and re-entrant nematic (RN) phases exist. Optical microscopy clearly indicates that one of the four studied samples shows the phase sequence of isotropic–nematic–SmA–RN. The derived solute order parameters were interpreted by means of two Maier–Saupe mechanisms in conjunction with the Kobayashi–McMillan theory in the case of the SmA phase. The novel feature of this study is that the nematic potential is extrapolated from the nematic to the SmA phase based on concentrations of the 6OCB–8OCB mixtures. The derived smectic order parameters for each of the studied solutes clearly show a maximum absolute magnitude near the centre of the SmA temperature range. The different partitioning of these solutes in the binary mixture is also discussed.

Rheological Properties of a Chiral Liquid Crystal Exhibiting Type-II Character

We measured rheological properties of cholesteryl nonanoate and its mixture with 4′-octyloxy-4-cyanobiphenyl. In both systems, wider temperature range of blue phase under shear with anomalous increase in viscosity associated with non- Newtonian flow behavior has been observed. Shear thinning followed by Newtonian flow is found in cholesteric phase and non-Newtonian flow behavior at high shear rates is seen in smectic-A phase. In cholesteryl nonanoate a small peak in the viscosity at cholesteric to smectic-A transition along with a shear rate dependent viscosity at higher shear rate is attributed to the type-II character of the compound. We find that the type-II character of the compound is further enhanced by mixing a small amount (20 wt%) of 4′-octyloxy-4-cyanobiphenyl in cholesteryl nonanoate. No significant change in the temperature dependent rheological behavior is observed at the twist-grain-boundary-A to smectic-A transition. It is suggested that the small anomalous peak in the viscosity just above the twist-grain-boundary-A phase is due the existence of chiral line liquid phase.

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.

Estimation of effective Debye temperature from sound velocity of N-( p-n-alkoxybenzylidene)- p-n-alkylanilines, (nO.m) liquid crystalline compounds

The ratio of specific heats ( γ), Poisson's ratio ( σ), effective Debye temperature ( θ D ) collision factor ( S m ), etc., are estimated in different LC phases of N-( p-n-alkoxybenzylidene)- p-n-alkylanilines, (nO.m's) liquid crystalline compounds using the density and sound velocity data. The variation of these parameters in different mesomorphic phases with the chain length is studied. The effective Debye temperature ( θ D ), the collision factor ( S m ) exhibits a similar variation with the alkyl chain length. Further, the space filling factor ( r fm ) estimated using Kittel's formula is constant for all the materials irrespective of the phase with a value of 0.416. The molecular radius shows an increment of 0.074, 0.072 and 0.068 Ǻ per CH 2 group in isotropic, nematic and smectic-A phases respectively in 5O.m series. Further, the specific heat ratio ( γ) value is found to be lower in isotropic phase than in liquid crystalline phases. The data is compared with the body of the data in the literature.

A helix inversion from the temperature-dependent intramolecular chiral conflict

A new liquid crystal, cholesteryl (S)-4′-(2-methylbutyl)biphenyl-4-carboxylate (CB15-COOChol*), has been designed, synthesised and characterised. CB15-COOChol* exhibits multiple liquid crystalline phases: smectic A (SmA), nematic (N) and chiral nematic (N*). When CB15-COOChol* is heated, two N* phases exhibiting opposite handedness (NR* and NL*) and opposite temperature dependence of the helical pitch separated by an N phase are observed above the SmA phase. The entire rare phase transition sequence can be described as crystalline Cr→SmA→NR*→N→NL*→I isotropic. The helix inversion behaviour occurs mainly due to the temperature-dependent intramolecular chiral conflict between the (S)-4′-2-methylbutyl and cholesteryl ester moieties with opposite handedness and opposite temperature dependence of the helical twisting power value.

Nematic electroclinic effect in a carbon-nanotube-doped achiral liquid crystal

A small quantity of carbon nanotubes (CNTs) dispersed in an achiral liquid crystal (LC) matrix transmits chirality a short distance into the LC, and the LC+CNT mixture is found to exhibit a bulklike electroclinic effect in the nematic phase. The magnitude of the effect increases rapidly on cooling, showing significant pretransitional behavior on approaching the nematic-smectic-A transition temperature (T(NA)) from above. The variation of the electroclinic coefficient is negligible over the frequency range 100 Hz to 100 kHz in the in the nematic phase well above T(NA) and in the smectic-A phase, whereas the electroclinic coefficient falls off significantly with increasing frequency just above T(NA).

Random disorder and the smectic-nematic transition in liquid-crystalline elastomers

We report effects of disorder due to random cross-linking on the nematic to smectic-A phase transition in smectic elastomers. Thermoelastic data, stress-strain relations and high-resolution x-ray scattering profiles have been analyzed for two related compounds with a small and a larger nematic range, respectively, each for 5% as well as 10% cross-links. At 5% cross-link density the algebraic decay of the positional correlations of the smectic layers survives in finite-size domains, providing a sharp smectic-nematic transition. At an increased cross-link concentration of 10% the smectic order disappears and gives way to extended short-range layer correlations. In this situation neither a smectic-nematic nor a nematic-isotropic transition is observed anymore. The occurrence of disorder at a relatively large cross-link concentration only, indicates that smectic elastomers are rather resistant to a random field. The temperature dependence of the correlation lengths and thermoelastic behavior suggest a shift to a "parasmectic" regime of a first-order smectic-isotropic transition.

Effect of carbon nanotubes on the isotropic to nematic and the nematic to smectic- A phase transitions in liquid crystal and carbon nanotubes composites

A high-resolution ac-calorimetric study on the weakly first-order isotropic to nematic (I -N) and the continuous nematic to smectic-A (N -SmA) phase transitions of the liquid crystal octyl-cyanobiphenyl (8CB) doped with well-dispersed multiwall carbon nanotubes (CNTs) as a function of CNT concentrations is reported. Thermal scans were performed for all samples having CNT weight fraction from Φ(w) = 0.0005 to 0.0060 over a wide temperature range well above and below the two transitions in pure 8CB. Both the I -N and the N -SmA transitions evolve in character and have their transition temperatures qualitatively offset by ∼ 1.10 K lower as compared to that in pure 8CB for all 8CB+CNTs samples. The enthalpy change associated with each phase transition is essentially the same as that of pure 8CB and remains unchanged with increasing Φ(w). However, there is an evidence that the thermal transport properties of the composites differ from the pure LC upon cooling below a Φ(w)-dependent temperature within the nematic phase. In addition, a new C(p) feature is resolved for intermediate Φ(w) samples that appears to be correlated to this onset temperature.

Undulation Instabilities in the Meniscus of Smectic Membranes

Using optical microscopy, phase shifting interferometry, and atomic force microscopy, we characterize the undulated structures which appear in the meniscus of freestanding ferroelectric smectic-C* films. We demonstrate that these periodic structures correspond to undulations of the smectic-air interface. The resulting striped pattern disappears in the untilted smectic-A phase. The modulation amplitude and wavelength of the instability both depend on meniscus thickness. We study the temperature evolution and propose a model that qualitatively accounts for the observations.

Critical behavior of three organosiloxane de Vries-type liquid crystals observed via thedielectric response

Dielectric measurements have been made on three organosiloxane liquid crystal compoundsexhibiting a smectic A (SmA) to smectic C* (SmC*) transition, the SmA phase being ofthe de Vries type. The electroclinic response of the molecules in the de Vries phase of thesecompounds exhibits a double-peak profile, and is thus different from the conventional chiralSmA phase, a feature explained on the basis of an antiferroelectric (AF) block model(Krishna Prasad et al 2009 Phys. Rev. Lett. 102 147802). The differential interactionsarising from the different molecular ends of these siloxane-based compounds, whichare the basis for the AF block model, can also be expected to enhance the layertranslational order. We present x-ray integrated intensity data that show a high (∼0.9) translational order in the SmA phase. Dielectric relaxation spectra bring out the fact thatthe magnitude of the soft mode relaxation parameters is dependent on the number ofsiloxane groups in the terminal part of the molecule. A range-shrinking analysis of thetemperature-dependent dielectric relaxation strength has been carried out, using apower-law expression. The characteristic exponent shows a systematic growth with rangeshrinking and reaches limiting values comparable to that predicted for the 2D Isinguniversality class.

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.

Computer simulation of charged hard spherocylinders at low temperatures

In this work we report the stability of liquid crystalline phases of charged hard spherocylinders (CHSC) of aspect ratio L/σ=5 at low temperatures using NPT Monte Carlo computer simulations. Following the methodology used in previous work [C. Avendaño, A. Gil-Villegas, E. González-Tovar, J. Chem. Phys. 128, 044506 (2008); Chem. Phys. Lett. 470, 67 (2009)], long-range coulombic interactions are handled using the Wolf method. The supramolecular organization of CHSC is obtained by compression of a low-density isotropic state. The system under consideration exhibits the expected isotropic, nematic, smectic-A, and crystal phases. However two important phenomena emerge at low temperatures, namely the existence of an isotropic–nematic–smectic triple point, with the ending of the nematic phase for lower temperatures, and the apparent hexatic arrangement of the layers in the smectic phases. Assuming that the smectic-layers behave as quasi-bidimensional systems, lowering the temperatures is possible to observe the formation of hexatic phases, which are detected analysing the structure factor, order parameters and distribution functions. This hexatic ordering indicates that the CHSC phase diagram presents a smectic-B phase at low temperatures.

Synthesis of a novel smectic liquid crystalline glass and characterization of its charge carrier transport properties

A new organic semiconductor material with both a self-organized molecular alignment in smectic liquid crystals and a solid nature in amorphous materials, i.e., a glassy liquid crystalline organic semiconductor has been proposed, and its model compound, 1,3,5-benzenetricarboxylic acid, tris{12-[6′-(4′′-octylphenyl)-2′-naphthyloxy]-1-dodecyl ester} (LCG-triester 8-PNP-O12), was synthesized, and we characterized its phase transition behaviors in slow cooling and rapid cooling. It exhibited smectic A (SmA) and smectic B (SmBhex) phases in slow cooling, and a SmBhex glassy phase with a high Tg close to 60 °C when cooled rapidly from the SmA and SmBhex phases. Its charge carrier transport properties in the smectic mesophases, including the liquid crystal glassy phase, were first studied by the time-of-flight (TOF) technique, which revealed that the hole mobility in the SmBhex glassy phase was about 3.5 × 10−4 cm2 v−1s−1, comparable to that in the SmBhex liquid crystal phase. We concluded that the high charge transport properties of the smectic mesophase are successfully maintained in the solid glassy state.

Liquid crystalline behavior of a coarse-grained model of shape-persistent macrocycles with flexible attractive chains

Shape-Persistent Macrocycles (SPMs) are synthetic organic macromolecules characterized by a rigid open hoop or ring with flexible side groups pointing either towards the inside or outside of the structure. In a previous work, we have studied the liquid crystalline behavior of a Coarse-Grained model of SPMs using Molecular Dynamics simulations [C. Avendano and E. A. Muller, Phys. Rev. E, 2009, 80, 061702]. In the aforementioned model, the system tends to organize in a rather unusual smectic-A phase in which the particles are aligned perpendicularly to the layers, i.e. the system presents antinematic order. In this work, this model is extended to study the effect of flexible chains attached to the macrocycles on the self-assembly of SPMs. Considering flexible chains linked to the SPM backbone allows a qualitative comparison with real systems where the chains play an important role in the supramolecular organization of the system. In our model particles are composed of a rigid hexagonal arrangement of 24 soft-repulsive spheres with chains of attractive spheres grafted into the cycle. Two different cases (isomers) have been considered in this work depending whether the attached chains are pointing inward (intraannular substituents) or outward (extraannular substituents) from the rigid structure. The isomers self-assemble into contrasting liquid crystalline phases upon temperature quenching from an isotropic state. It is observed how systems with extraannular substituents tend to order at high densities into a lamellar phase with the chains laying in between the layers. The orientation of the rings in the layers shows antinematic order. For SPMs with intraannular substituents, the systems exhibit a transition from an isotropic phase to an hexagonal-disordered columnar phase, where the attractive chains fill the inner voids of the ring and help to stabilize the columns. The liquid crystalline phases in both models are influenced by the amphiphilic nature of the different components of the model.