Discotic Nematic Research Articles

Photopolymerization and Molecular Orientational Order of the Discotic Nematic Phase in 2,3,6,7,10,11-Hexakis(4-(8-acryloyloxyoctyloxy)benzoyloxy)triphenylene

A triphenylene-based discotic liquid crystal, 2,3,6,7,10,11-hexakis(4-(8-acryloyloxyoctyloxy)benzoyloxy)triphenylene (6ABTP) was investigated on the behavior of local order parameters for the discotic nematic (N D ) phase from the viewpoint of change of the molecular orientational order by photopolymerization. The local order parameters of the aromatic C = C, C-H and two C = O bonds of the acryloyl and benzoyl C = O groups were evaluated by an infrared dichroic method of Vij et al. It was found that 6ABTP tends to show a homeotropic alignement in the N D phase on a polyimide film spin-coated on BaF 2 and the triphenylene core shows a large order parameter (ca. 0.4-1.0), while the C = O moieties have a quite lower one (< 0.3). In order to investigate the disordering of molecular alignement due to crosslinking by photopolymerization, the behavior of local order parameters in photopolymerization was also evaluated. It was found that the order parameter of the aromatic core in the N D phase remarkably decreases at the early stage of photopolymerization, but only about 20% of decrease in the order parameter could be attained at the end of polymerization.

Orientational Behavior of Discotic Nematic Phase for a Discotic Liquid Crystalline Triphenylene on Polyimide and Alkylammoniumbromide Coated Substrates

Orientational behavior of a discotic liquid crystalline triphenylene (C8OBT) on the substrates coated with a polyimide and a cetyltrimethylammoniumbromide (CTAB) was investigated by polarizing microscopy. The averaged direction and the order parameters of the triphenylene core plane and carbonyl groups of C8OBT were evaluated by an infrared dichroic method. C8OBT exhibits a homeotropic alignment on a polyimide film as for the discotic nematic (ND) phase, while it shows a tilt or homogeneous alignment on a CTAB-coated substrate. The order parameter of triphenylene ND phase is higher on the polyimide (S = 0.6) than a CTAB-coated substrate (0.2), in contrast to that of the carbonyl groups is comparable (0.2).

Alignment behaviour of the discotic nematic phase of 2,3,6,7,10,11-hexa(4-n-octyloxybenzoyloxy)triphenylene on polyimide and cetyltrimethylammonium bromide coated substrates

The orientational behaviour of the nematic discotic phase of 2,3,6,7,10,11-hexa(4-n-octyloxybenzoyloxy)triphenylene (C8OBT) on substrates coated with a polyimide or cetyltrimethylammonium bromide (CTAB) was investigated by polarizing optical microscopy. The averaged order parameters and directions of the triphenylene core and the carbonyl groups of C8OBT were evaluated by an infrared dichroic method. The discotic nematic (ND) phase of C8OBT exhibits a homeotropic alignment on a polyimide film, a typical nematic schlieren texture on a glass substrate, and a tilted or planar homogeneous alignment on a CTAB-coated substrate. The order parameter of the triphenylene core is higher on a polyimide film (S = 0.6) than on a CTAB-coated substrate (S = 0.2), whereas that of the carbonyl groups remains roughly constant at 0.2 to 0.3 independent of the substrate for the ND phase.

Liquid Crystalline and Monotropic Phase Behaviors of 2,3,6,7,10,11-Hexa(4‘-octyloxybenzoyloxy)triphenylene Discotic Molecules

Multiple phase transitions and monotropic phase behavior are observed in a discotic liquid crystal, 2,3,6,7,10,11-hexa(4‘-octyloxybenzoyloxy)triphenylene (HOBT-C8). Detailed phase structural analyses using wide-angle X-ray diffraction and electron diffraction show that, in addition to a discotic nematic (ND) phase at high temperatures, a rectangular columnar (ΦR) phase and an orthorhombic crystalline (KI) phase developing sequentially upon cooling can be identified. A crystalline (KII) phase with a higher melting temperature than that of the KI phase forms during heating. Therefore, the KI phase represents a metastable phase with respect to the KII phase, and the more stable KII phase can be bypassed during cooling at relatively fast cooling rates. The KI phase is thus monotropic with respect to the KII phase at relatively slow heating rates. The optical texture of each phase can be distinguished from polarized light microscopy. A relationship between Gibbs free energy and temperature for different phases can be constructed, which thermodynamically and kinetically represents these multiple phase transitions and the monotropic phase behavior of the KI phase.

Synthesis and Characterization of a Novel Liquid Crystalline Polymer Showing a Nematic Columnar to Nematic Discotic Phase Transition

A novel liquid crystalline polyacrylate, exhibiting a nematic columnar and a nematic discotic phase, the latter not found in any polymer previously, has been synthesized using a polymer analogous r...

Magnetic Field Effect on the Alignment of a Discotic Liquid Crystal

The influence of a magnetic field on the alignment was investigated in the discotic nematic (ND) phase of a triphenylene derivative. The uniform alignment was achieved when a magnetic field of 5T was applied parallel to the cell surface during the cooling process. The director was perpendicular to the surface when the cell was coated with polyimide alignment layers and parallel when the surface was uncoated.

Transient Periodic Dissipative Structures in Nematics Confined between Coaxial Cylinders

The onset of transient periodic dissipative structures (TPDS) is studied using the linear perturbation approximation when a sufficiently strong magnetic field (B) is impressed on the uniformly aligened director (n o) of a nematic confined to the annular space between two coaxial cylinders of radii R 2 and R 1 (R 2>R 1). Only TPDS with axial or azimuthal periodicity is considered. An approximate expression for the magnetic body force density is employed. The solutions mainly concern rigid anchoring. When n o is along the common axis, a radial field or an azimuthal field causes TPDS with periodicity along the axis when the calamitic nematic has positive diamagnetic anisotropy (χA>0); an axial field has a similar effect when χ A<0. The dependence of the rise time as well as the periodicity wavevector are studied as functions of the applied field strength and the ratio of the radii (R 21 = R 2/R 1). A weakening of director anchoring has a deletcrious effect on the formation of TPDS. When n o is azimuthal and χA0, a radial or an axial field may induce TPDS having an azimuthal periodicity wavevector which is an integer and related to the number of domains; continuous changes in control variables cause discontinuous changes in the wavevector. Using hypothetical material parameters, it is shown that an axial (or an azimuthal) field may induce TPDS with a continuously varying (or discontinuously varying) wavevector in a radially aligned discotic nematic having χA>0. If χA<0. a radial field may induce TPDS with modulation along both the axial and the azimuthal directions in a discotic nematic. When R 21 is close to unity, the results of this work go over to those obtained for a flat sample.

Theoretical analysis of texture dependent extensional viscosity of discotic mesophases

Rheological functions for uniaxial extensional flows predicted by a previously selected and validated constitutive equation (CE) for discotic mesophases are presented. The predicted relations between extensional viscosities, flow-induced microstructure, processing conditions, and material parameters of discotic mesophases are characterized and discussed. It is found that, in contrast to rod-like nematics, two distinct uniaxial extensional viscosities need to be defined to characterize the extensional rheological functions of discotic mesophases completely. The model predicts non-Troutonian extensional viscosities of discotic nematics, such as strain thinning and strain thickening, depending on the process temperature, and the ratio of viscous to elastic stress contributions. The uniaxial extensional viscosities are also found to depend strongly on the flow-induced microstructure. The rheological analysis is then used to characterize the relations between extensional flow viscosities and the classical microstructures that arise during the industrial fiber spinning of discotic mesophase pitches.

Suspension of spherical particles in nematic solutions of disks and rods

We report on the behavior of colloidal spheres suspended in nematic solutions of rodlike (calamitic phase) and disklike (discotic phase) micelles. The particles form anisotropic structures that depend on the shape of the surrounding micelles. An analysis of these structures indicates that dipolar and quadrupolar elastic interactions are, respectively, observed in discotic and calamitic nematics. This shape dependency can be understood by considering the entropy driven alignment of the micelles at the surface of the particles.

Computer simulation studies of anisotropic systems. XXVII. The direct pair correlation function of the Gay–Berne discotic nematic and estimates of its elastic constants

Monte Carlo simulations have been performed for a discotic liquid crystal composed of Gay–Berne particles. On the basis of these simulations for the nematic phase, a subset of the spherical harmonic expansion coefficients of the direct pair correlation function (DPCF) were determined from the pair distribution function (PDF) by solving the Ornstein–Zernike (OZ) equation. This was achieved by generalizing the Wiener–Hopf factorization scheme for the numerical solution of the OZ equation. Only the expansion coefficients gl1,l2,l(r) (lα⩽4) of the PDF in the laboratory frame were used when solving the OZ equation; this means that the DPCF so obtained is equivalent to that for a nematic in which the director is randomly distributed. From the DPCF, the scaled Oseen–Zöcher–Frank elastic constants K11*, K22*, and K33*, as well as the surface constant K13*, have been calculated from the subset of expansion coefficients. Generally, we find that K33*&amp;lt;K11*&amp;lt;K22*, in agreement with what is expected and found for discotic nematics. These results are quantitatively but not qualitatively different from those calculated with the help of analytical approximations for the same spherical harmonic expansion coefficients of the direct pair correlation function. For example, the values of the bulk elastic constants determined via the OZ equation are about three times larger than the bulk elasticity obtained with the low density approximation.

“In-Situ Photopolymerization” of Discotic Liquid Crystalline Acrylates in the Discotic Nematic Phase

In order to stabilize the anisotropic properties of films made from discotic liquid crystals (DLCs), we applied the “in-situ photopolymerization” process to reactive DLCs. We synthesized triphenylene benzoate derivatives with 1 to 6 acrylate end groups and studied the influence of the number of acrylates on their discotic nematic (ND) phase and on their photo-initiated polymerization behavior. The discotic acrylates align spontaneously between substrates in the ND phase, leading to thin films with a negative birefringence. We used the Haller extrapolation method to evaluate the order parameter of these materials. Oriented thin films of monomer were then “frozen in” using photo-initiated polymerization. We studied the optical properties of the resulting polymer films and in particular the dependence of the birefringence on the polymerization temperature. X-ray diffraction analyses of polymer films made from the diacrylate show a decrease of order with higher polymerization temperatures.

On the Signs of the Leslie Viscosities α2 and α3 for Nematics and Discotic Nematics

The signs of the Leslie viscosities α2 and α3 for rod-like and disc-like molecules in the uniaxial nematic phase are discussed using a dynamic molecular mean field theory based upon a Fokker-Planck equation. The viscosities are shown to depend on the particle geometry and alignment order parameters. In particular, for positive degree of alignment S 2, the calculations tend to confirm Carlsson's conjecture relating the signs of the two viscosities to the particle geometry. Furthermore, a transition in the signs of α2 for discotics and α3 for nematics is predicted with increasing order parameter S 2 in accord with recent experiments. For small degree of alignment S 2, the results reproduce previous calculations obtained by Hess in a truncation approximation. The qualitative flow behaviour depends upon the signs of these viscosities.

Multiple light scattering in anisotropic random media

In the last decade diffusing wave spectroscopy (DWS) has emerged as a powerful tool to study turbid media. In this article we develop the formalism to describe light diffusion in general anisotropic turbid media. We give explicit formulas to calculate the diffusion tensor and the dynamic absorption coefficient, measured in DWS experiments. We apply our theory to uniaxial systems, namely, nematic liquid crystals, where light is scattered from thermal fluctuations of the local optical axis, called director. We perform a detailed analysis of the two essential diffusion constants, parallel and perpendicular to the director, in terms of Frank elastic constants, dielectric anisotropy, and applied magnetic field. We also point out the relevance of our results to different liquid crystalline systems, such as discotic nematics, smectic-A phases, and polymer liquid crystals. Finally, we show that the dynamic absorption coefficient is the angular average over the inverse viscosity, which governs the dynamics of director fluctuations.

Flow Alignment in Biaxial and Discotic Nematics

Continuum theory for nematic liquid crystals predicts two distinct types of flow alignment, one that observed in thermotropic rod-like nematics, and the second not so far observed but thought to be relevant to discotic nematics. This paper examines this latter possibility by considering recent flow alignment predictions for biaxial nematics, particularly for such materials where the molecular structure is close to discotic, and obtains results that support this claim. In addition it shows that in the disc-like limit biaxial theory reduces to that for a uniaxial nematic with viscosities consistent with this conjecture.

Spectroscopic properties of nematic discotic phenylethynylbenzene derivatives: Symmetry effects

The spectroscopic properties of two phenylethynylbenzene derivatives capable of forming nematic discotic (N D ) liquid crystals are studied in solution and in their N D phase. Their electronic absorption spectra are analysed with the help of fluorescence anisotropy measurements and on the basis of a previous theoretical study. The results presented here illustrate the effects of a symmetry lowering of a radial-symmetric chromophore, arising from the substitution of an ethynylbenzene group by an alkoxy chain, upon the positions and the intensities of the electronic transitions. The absorption and fluorescence spectra of the nematic discotic phases are compared to those obtained in solution. The observed spectral shifts are discussed in terms of both exciton coupling and environment effects on the excitation energy.

On the effects of doping calamitic and discotic nematics with materials of the opposite aspect ratio

Abstract In this paper the inter-mixing of calamitic and discotic materials is discussed. The systems are found to be immiscible near the middle of the phase diagram, as was predicted theoretically. However, a degree of miscibility is seen at each end of the phase diagram, allowing some interesting effects to be explored. A negative permittivity anisotropy is induced in a discotic material by dissolution of a polar calamitic dopant. The relaxation behaviour of the dopant in the material is studied and found to be similar to that in the pure calamitic, as far as can be determined in this experiment. The elastic constants of a calamitic mixture are enhanced by addition of a discotic dopant. This is believed to be due to the packing of the discotic molecules in such a way as to hinder director distortions, and may be useful for changing the values of calamitic elastic constants.

Lattice cluster theory for phase behavior of rectangular mesogens. II. Nearest-neighbor interactions, phase diagrams, and competitive nematic orderings

The lattice cluster theory is extended to study all the possible nematic orderings of rectangular mesogens, namely, rodlike, discotic, and biaxial nematics for oblong mesogens, and rodlike and discotic nematics for square mesogens. The theory describes both the anisotropic hard core excluded volume and the anisotropic attractive van der Waals interactions on an equal footing. The partition function is expanded in terms of cluster contributions from the packing induced correlations and van der Waals interactions, with a Flory-type approximation as the zeroth-order treatment. The correlation contributions to the free energies for the isotropic liquid and the various homogeneous liquid crystalline phases are evaluated to second order in mesogen density and to first order in interaction energy. This theory therefore describes the anisotropic character of both the attractive and repulsive portions of the mesogen–mesogen interactions. Models with greater dimensionality provide greater orientational freedom and are shown to produce qualitatively different phase diagrams. As expected, the phase diagram is a strong function of the mesogen dimensions, and the placement of the biaxial phase with respect to the other phases is strongly dependent on mesogen size and the degrees of orientational freedom, in general accord with the nonuniversal behavior exhibited by experiments and computer simulations.

Micelle size and orientational order across the nematic-isotropic transition: A field-dependent nuclear-spin-relaxation study.

Magnetic field-dependent counterion $^{23}\mathrm{Na}$ spin relaxation and quadrupole splitting measurements are reported from the discotic nematic (${\mathit{N}}_{\mathit{D}}$) and isotropic phases of the system sodium dodecylsulphate--decanol--water. Carefully designed relaxation experiments were used to determine the individual field-dependent motional spectral densities. A quantitative analysis of the splitting and spectral density data allowed a simultaneous determination of the nematic order parameter S and the axial ratio of the disklike micelles as functions of temperature throughout the extension of the ${\mathit{N}}_{\mathit{D}}$ phase and into the isotropic phase. The axial ratio varies only weakly with temperature in the ${\mathit{N}}_{\mathit{D}}$ phase, from 6 at 10 \ifmmode^\circ\else\textdegree\fi{}C below the nematic-isotropic transition temperature ${\mathit{T}}_{\mathit{N}\mathit{I}}$ to 4 just below ${\mathit{T}}_{\mathit{N}\mathit{I}}$, but then increases to 8 in the isotropic phase just above ${\mathit{T}}_{\mathit{N}\mathit{I}}$. This behavior is contrary to the existing theoretical predictions. The nematic order parameter decreases from 0.9 at 10 \ifmmode^\circ\else\textdegree\fi{}C below ${\mathit{T}}_{\mathit{N}\mathit{I}}$ to 0.4 just below ${\mathit{T}}_{\mathit{N}\mathit{I}}$. While the latter value is close to that predicted by Maier-Saupe theory, the temperature dependence of S is much stronger than that predicted theoretically.

Extension dynamics of discotic nematics of variable order : geodesic flow and viscoelastic relaxation

Variational methods are used to develop the governing equations that describe the flow of spatially invariant uniaxial discotic nematic liquid crystals of variable order; since the equations are based on a phenomenological truncated expansion of the entropy production, the equations are approximations. Restrictions in the phenomenological parameters appearing in the governing equations are imposed taking into account the ordering of the discotic phase. Numerical and analytical solutions of the director n and alignment S are presented for a given uniaxial extensional start-up flow. The unit sphere description of the director is used to discuss and analyze the sensitivity of the director trajectories and the coupled alignment relaxation to the initial conditions (n 0 , S 0 ) and to the alignment Deborah number (De). The numerical results are used to characterize the relaxation of the tensor order parameter Q and to compute the steady flow birefringence. When the poles of the unit sphere are along the extension axis and the equator lies in the compression plane of the flow, it is found that the director trajectories belong to the meridians (great circles through the poles) and the dynamics follows a geodesic flow; when subjected to flow the director follows the shortest path that connects the initial orientation n 0 and the equator (compression plane). As typical of geodesic flows, there is a strong sensitivity to initial conditions: when n 0 lies on the poles no predictions on the eventually steady director orientation are possible. If the prior to flow orientation is close to the poles the coupled alignment relaxation along the geodesics is nonmonotonic and for large De the discotic may become temporarily isotropic. The couplings between n and S are captured by the tensor order parameter relaxation. At steady state, the director lies on the equator, and the alignment and birefringence increase with increasing De

Molecular dynamics simulations of discotic liquid crystals using a hybrid Gay–Berne Luckhurst–Romano potential

Abstract We report the results of the computer simulation of a collection of particles interacting via an anisotropic potential proposed by Luckhurst and Romano and modified by scaling with part of the anisotropic well depth formalism employed by Gay–Berne. Using the molecular dynamics technique for 256 particles in the NVE ensemble, the system is shown to exhibit a variety of mesophases, as the temperature is lowered, and these are provisionally identified as isotropic, discotic nematic, a highly ordered fluid phase with some columnar features and a crystal.