Discotic Nematic Liquid Crystals Research Articles

Self-organization of disc-like molecules: chemical aspects

The hierarchical self-assembly of disc-shaped molecules leads to the formation of discotic liquid crystals. These materials are of fundamental importance not only as models for the study of energy and charge migration in self-organized systems but also as functional materials for device applications such as, one-dimensional conductors, photoconductors, light emitting diodes, photovoltaic solar cells, field-effect transistors and gas sensors. The negative birefringence films formed by polymerized nematic discotic liquid crystals have been commercialized as compensation foils to enlarge the viewing angle of commonly used twisted nematic liquid crystal displays. To date the number of discotic liquid crystals derived from more than 50 different cores comes to about 3000. This critical review describes, after an in-depth introduction, recent advances in basic design principles and synthetic approaches towards the preparation of most frequently encountered discotic liquid crystals.

Steady state and transient rheological behavior of mesophase pitch, Part II: Theory

This paper presents a comprehensive nonlinear numerical analysis of the flow modeling of mesophase pitches performed using a previously formulated mesoscopic viscoelastic rheological theory (Singh and Rey (2000)) that takes into account short-range order elasticity, long range elasticity, and flow-induced texture transformations. A complete extra stress tensor equation is developed from first principles for liquid crystal materials under nonhomogeneous arbitrary flow. This mesoscopic viscoelastic model has been adapted to describe the rheology of flow-aligning thermotropic discotic nematic liquid crystals as models of mesophase pitches. Predictions for simple shear flow (under nonhomogeneous conditions) for the shear viscosity, first normal stress differences, and transient shear stress are presented. The accuracy of the numerical results is established by a thorough validation procedure based on [Cato et al. (2004)], which is the companion paper, and permit to validate this mesoscopic viscoelastic theory as model of liquid crystalline mesophase pitch. Very good qualitatively agreement between experiments and simulations is found for all rheological characterizations.

ABOUT THE DIFFERENCE IN THE QUADRUPOLE SPLITTING OF WATER BETWEEN CATIONIC AND ANIONIC NEMATIC LYOTROPIC LIQUID CRYSTALS: 2H-NMR AND MOLECULAR DYNAMICS STUDY

Deuterium quadrupole splittings, of deuterated water, Δv, in anionic discotic nematic lyomesophases are always much larger than in cationic mesophases. To explore the possible origins of this difference, Δv and T1 relaxation times of HDO (H2O 0.2% D2O) and decanol (DeOH 14% a-d2), in solutions of cationic and anionic discotic lyotropic nematic liquid crystals, were measured using 2H-NMR. The four component mesophases were prepared based on tetradecyltrimethylammonium bromide, (TTAB/DeOH/NaBr/H2O), and cesium N-dodecanoyl-L-alaninate, (CsDAla/DeOH/KCl/H2O), amphiphiles with cationic and anionic head-groups, respectively. For a better understanding of the experimental results, 15 ns molecular dynamics (MD) trajectories of both systems were calculated. The results suggest that the large difference observed in the quadrupole splittings of the solvent can be mainly attributed to a preferential orientation of the water molecules, induced by the strong electric field generated by the electrical bilayer formed at the interface of the anionic mesophase. Restrictions to solvent reorientational dynamics or differences in the thickness of the interface do not seem to play a significant role to explain the observed difference

Linear viscoelasticity of discotic mesophases

The small-amplitude oscillatory capillary Poiseuille flow of uniaxial incompressible discotic nematic liquid crystals, representative of discotic mesophases, is characterized using analytical, computational, and scaling methods. Linear viscoelastic material functions are calculated and discussed in terms of fundamental anisotropic viscoelastic processes. The role of orientation to generate flow and store elastic energy is discussed. Viscoelastic behavior is found only at frequencies of similar magnitude to the single orientation relaxation time. In the terminal small-frequency zone the storage modulus scale as G ′ ∼ ω 2 , and the loss modulus as G ″ ∼ ω , typical of viscous fluids. Comparisons between steady and oscillatory Poiseuille flow shows that the Cox–Merz rule is not obeyed, but that as expected the steady and complex viscosities in the terminal zone are identical. A remarkable and useful correspondence between the stored elastic energy under steady flow and the storage modulus G ′ has been discovered.

Correlation of the molecular structure of discotic nematic liquid crystals with their orientational order and specific features of the nematic-isotropic phase transition

The orientational order parameter S of molecules in high-temperature discotic nematic liquid-crys- tal phases of triphenylene derivatives is investigated as a function of the length of side flexible molecular chains at different temperatures. It is established that the orientational order parameters S in the range of the transition from the nematic phase to the isotropic liquid phase (the N D - I transition) are smaller than those predicted from the molecular-statistical theory and computer simulation. It is shown that the N D - I transition is close to both the isolated Landau point and the tricritical point (regardless of the chemical structure of the molecules and the anisotropy of dispersion intermolecular interactions). Consistent explanations are offered for a number of experimental findings, such as the anomalously small changes in the enthalpy and entropy upon the N D - I tran- sition (as compared to those revealed upon the N - I transition in calamitic nematic liquid crystals), the anoma- lously strong response of the isotropic phase of discotic nematic liquid crystals to external fields (thermody- namically conjugate to the order parameter S ) and the long relaxation times of this response, and the formation of cybotactic discotic molecular clusters in the isotropic phase in the vicinity of the N D - I transition. © 2004 MAIK "Nauka/Interperiodica".

Poiseuille flow of discotic nematic liquid crystals’ onion textures

The Poiseuille capillary flow of discotic nematic liquid crystals (DNLCs) is analyzed using the Ericksen–Leslie theory, for three families of static patterns known as escaped and defect core onion textures, defined by the orientation at the capillary’ centerline. The escaped textures have low-elastic energy and high dissipation cores. The defect core texture has a high elastic energy-low dissipation core. The Poiseuille flow of these textures exhibits non-parabolic velocity profiles and their apparent viscosities display shear thickening, with exponents close to 1/5±0.07. Up to an Ericksen number of a thousand, core conditions have a strong effect on the non-Newtonian rheology of discotic nematics displaying the onion texture.

Computational rheology of carbonaceous mesophases

Mesophase pitches are multicomponent discotic nematic liquid crystals (DNLCs), whose characteristic molecular weight is intermediate between low molar mass and polymeric nematic liquid crystals. Flow modelling of these fluids is performed using a previously formulated mesoscopic viscoelastic rheological theory [J. Non-Newtonian Fluid Mech. 94 (2000) 87] that takes into account flow-induced texture transformations. A complete extra stress tensor equation is developed from first principles for liquid crystal materials under non-homogeneous arbitrary flow. This mesoscopic viscoelastic model has been adapted to describe the rheology of flow-aligning thermotropic DNLCs as models of mesophase pitches. We develop a fundamental understanding of the relations between rheology and flow of carbonaceous mesophases using theory and simulation by characterizing the steady and transient shear rheological material functions of flow-aligning DNLCs. Predictions for simple shear flow (under non-homogeneous conditions) for the apparent shear viscosity and first normal stress differences are presented. The predicted relations among rheological properties, shear-induced microstructure, processing conditions and material parameters of discotic mesophases are characterized and discussed.

Linear and nonlinear viscoelasticity of discotic nematics under transient Poiseuille flows

The start-up, reversal, and cessation of capillary Poiseuille flows of uniaxial isothermal incompressible discotic nematic liquid crystals are characterized using analytical, computational, and scaling methods, for low (linear regime) and high (nonlinear regime) pressure drops. In the linear regime, transient flows provide information on the main viscoelastic material properties, including the steady shear Miesowicz’ viscosity, the transient reorientation viscosity, as well as the viscosity reduction due to backflow. It is shown that cessation of weak flow provides a way to measure pressure drops. Transient flows in the nonlinear regime involve orientation-dependent material functions. The flow rate in start-up is characterized by an overshoot and strain scaling, similar to stress overshoot in simple shear. Recoil after cessation of flow is shown to be a direct function of stored Frank elastic energy. Scaling and computation show that the maximum recoil volume is shown to be close to R3, where R is the capillary radius. Scaling and computation show that flow reversal under large pressure drops is characterized by the formation of a sharp cusp at a time equal to the orientation time scale of the material.

Molecularengineering of discotic nematic liquid crystals

Connecting two columnar phase forming discotic mesogens via a short rigid spacer leads to the formation of a π-conjugated discotic dimer snowing discotic nematic (N D) phase. Attaching branched-alkyl chains directly to the core in hexaalkynylbenzene resulted in the stabilisation of ND phase at ambient temperature. Pentalkynylbenzene derivatives possessing a combination of normal- and branched-alkoxy chains display a very broad ND, phase which is stable well below and above the room temperature.

Poiseuille flow of Leslie–Ericksen discotic liquid crystals: solution multiplicity, multistability, and non-Newtonian rheology

Computational modeling of the steady capillary Poiseuille flow of flow-aligning discotic nematic liquid crystals (DNLCs) using the Leslie–Ericksen (LE) equations predicts solution multiplicity and multistability. The phenomena are independent of boundary conditions. The steady state solutions are classified into: (a) primary, (b) secondary, and (c) hybrid. Primary solutions exist for all orientation boundary conditions and all flow rates, and are characterized by a flow-alignment angle that is closest to the anchoring angle at the bounding surface. Secondary solutions exist for all orientation boundary conditions and flow rates above a certain critical value. The secondary solutions are characterized by a flow-alignment angle which can be either the nearest neighbor below the primary solution or any multiple of π above. Hybrid solutions interpolate between the primary and the nearest secondary solutions, and hence exhibit two alignment angles. All solutions are stable to planar finite amplitude perturbations. Hybrid solutions are unstable to front propagation and lead to primary or secondary solutions. The non-Newtonian rheology of the primary and secondary solutions is characterized by non-classical shear thinning and thickening apparent viscosity behavior. Well-aligned monodomains can lead to shear thickening, thinning, or a sequence of both. The degree of rheological uncertainty is present for planar and homeotropic anchoring conditions. The non-Newtonian rheology of non-aligned samples leads to shear thinning and lack the uncertainty of well-aligned samples, since the apparent viscosity becomes insensitive to orientation.

Theoretical and Computational Rheology for Discotic Nematic Liquid Crystals

This paper presents an analysis of the role of orientation on the rheology of discotic nematic liquid crystals. The shear rheological properties exhibited by flow-aligning discotic mesophases are calculated by using a complete generalized nonlinear second-order tensor Landau-de Gennes model that takes into account short-range order elasticity, long-range elasticity, and viscous effects. A unified expression for the extra stress tensor is given. The second-order tensor Landau-de Gennes model was reduced to the uniaxial Leslie-Ericksen to obtain limiting rheological material functions valid at low and high shear rates. Analytical results are able to predict the material functions computed by the full Landau-de Gennes model for nonhomogeneous flow-aligning discotic nematic liquid crystals. Experimentally reported changes in the sign of the first normal stress differences with shear rate are captured by the model. A new Carreau-Yasuda liquid crystal model has been used to characterize the shear rheology for characteristic boundary conditions, and a viscosity power law exponent of 0.5 and a normal stress coefficient power law exponent of 0.44 have been obtained.

Room-Temperature Discotic Nematic Liquid Crystals

The use of discotic nematic liquid crystals instead of calamitic nematic liquid crystals to improve the viewing angle of a liquid crystal display device has recently been reported. Compared to the large number of calamitic molecules showing nematic phase at room temperature, the number of disk-shaped molecules showing subambient discotic nematic phase N D are rare. In this paper we present the design, synthesis, mesomorphic behaviour and structure-property relationship of discotic nematic liquid crystals based on benzene multiyne core.

Modeling elastic and viscous effects on the texture of ribbon-shaped carbonaceous mesophase fibers

Carbonaceous mesophases are discotic nematic liquid crystals that are spun into high performance carbon fibers using the melt spinning process. The spinning process produces a wide range of different fiber textures and cross-sectional shapes. Circular planar polar (PP), circular planar radial (PR), ribbon planar radial (RPR), and ribbon planar line (RPL) textures are ubiquitous ones. This paper presents, solves, and validates a model of mesophase fiber texture formation based on the classical Landau-de Gennes theory of liquid crystals, adapted here to carbonaceous mesophases. The effects of fiber cross-sectional shape and elongational flow on texture formation are characterized. Emphasis is on qualitative model validation using existing experimental data [1]. The role of elasticity and flow-induced orientation on texture selection mechanism on ribbon-shaped mesophase fibers is characterized. The model is able to predict the formation of the commonly observed line texture, and the fine structure of the line is reproduced and explained in terms of classical liquid crystal defect physics. The results provide additional knowledge on how to optimize and control mesophase fiber textures.

ChemInform Abstract: Design and Synthesis of Discotic Nematic Liquid Crystals.

AbstractChemInform is a weekly Abstracting Service, delivering concise information at a glance that was extracted from about 100 leading journals. To access a ChemInform Abstract of an article which was published elsewhere, please select a “Full Text” option. The original article is trackable via the “References” option.

Texture formation in carbonaceous mesophase fibers.

Carbonaceous mesophases are discotic nematic liquid crystals that are spun into high performance carbon fibers using the melt spinning process. The spinning process produces a wide range of different fiber textures. Planar polar (PP) and planar radial (PR) textures are two ubiquitous ones. This paper presents theory and simulation of the texture formation process using the Landau-de Gennes mesoscopic theory for discotic liquid crystals. The computed PP and PR textures phase diagram, given in terms of temperature and fiber radius, is presented to establish the processing conditions and geometric factors that lead to the selection of these textures. Thin fibers adopt the PR texture, while thicker fibers and higher temperatures adopt the PP texture. The influence of elastic anisotropy to the formation of textures and structure is thoroughly characterized.

Design and synthesis of discotic nematic liquid crystals.

[structure: see text] A new principle for the design of discotic nematic liquid crystals is presented. The first examples of compounds which contain two triphenylene discotic mesogens connected via a rigid pi-conjugated diacetylene spacer are reported. These discotic twins form a discotic nematic mesophase over a wide temperature range.

Star-shaped discotic nematic liquid crystal containing 1,3,5-triethynylbenzene and oxadiazole-based rigid arms

A novel three-armed discotic liquid crystal based on 1,3,5-triethynylbenzene as a core and 2,5-diphenyloxadiazole as rigid arms has been synthesized, which is the first star-shaped molecule exhibiting a discotic nematic phase.

Theory and Simulation of Fiber Texture Formation and Rheology of Carbonaceous Mesophase Fibers

ABSTRACTCarbonaceous mesophases are discotic nematic liquid crystals that are spun into high performance carbon fibers using the melt spinning process. The spinning process produces a wide range of different fiber textures and cross-sectional shapes. Circular planar polar (PP), circular planar radial (PR) textures, ribbon planar radial (RPR), and ribbon planar line (RPL) textures are ubiquitous ones. This paper presents, solves, and validates a model of mesophase fiber texture formation based on the classical Landau-de Gennes theory of liquid crystals, adapted here to carbonaceous mesophases. The effects of fiber cross-sectional shape and elongational flow on texture formation are characterized. Emphasis is on qualitative model validation using existing experimental data [1, 2]. The results provide additional knowledge on how to optimize and control mesophase fiber textures.

Properties of the discotic-nematic to isotropic transition: influence of short range orientational order

The present investigation concerns the analysis of the influence of short range orientational correlation on the thermodynamic properties of discotic-nematic liquid crystals. Two-site cluster approximation is applied to the orientational molecular coordinates to include the short range orientational correlation. The role of short range orientational order, dispersion interaction, molecular length-to-width ratio and pressure on the thermodynamic and orientational behaviour of discotic nematogens close to the discotic-nematic to isotropic transition are analysed. It is observed that the short range orientational order has a strong influence on the thermodynamic properties and that the transition properties of both the calamitic and discotic mesogens exhibit quite similar behaviour.

Modelling surface structure in carbonaceous mesophase thinfilms

This paper presents a macroscopic model of free-surface pattern formation incarbonaceous mesophase thin films. Carbonaceous mesophases are discoticnematic liquid crystals and the model presented here is based on the equationsof classical continuum physics of liquid crystals, including bulk andinterfacial force and torque balance equations. The model predicts that forsufficiently thin films, of the order of tens of microns, differentorientations at the substrate and at the free surface produce elasticdistortions, which can be less costly by setting up a free-surface pointdefect lattice. The free-surface point defect lattice produces a periodicsurface relief, such that at a defect location the free surface has a cusp ora dip. The elevation (depression) of the cusps (dips) is proportional to theorientation elastic modulus and inversely proportional to the surface tension,and their magnitude lies in the micron range. The free-surface lattice ofcusps and dips is a way to produce ordered surface reliefs in carbon-basedthin films.