Calamitic Nematic Research Articles

Electro-optic characterization of a nematic phase formed by bent core mesogens

The purpose of this paper is the demonstration that bent core nematic phases behave quantitatively and qualitatively very different from ordinary calamitic nematics in their electro-optical characteristics. We present measurements of the elastic properties from the analysis of Brochard-Leger walls that are formed during the splay Fréedericksz transition in sandwich cells. These walls possess an unusually large shape anisotropy as compared to similar structures in calamitic nematics. The wall shapes can be explained when one assumes that the bend elastic constant K(33) is one order of magnitude larger than the twist constant K(22) of the material, supposing that flexoelectricity in the description of the elastic deformations can be neglected. Further we report periodic structures above the splay Fréedericksz transition with a wave vector perpendicular to the director easy axis. They represent either a static instability or an unconventional type of electrically driven convection.

Critical exponents at a reentrant isotropic–calamitic nematic phase transition

The critical exponents of the reentrant isotropic (I RE)–calamitic nematic (N C) and the N C–isotropic (I) phase transitions found in a lyotropic mixture of potassium laurate, decanol, and water are experimentally determined. Using optical birefringence measurements, along the entire range of the nematic phase, it was found that while the usual high temperature N C–I phase transition presents the well-known discontinuity, typical of a first order phase transition, with a critical exponent in the same class of universality of the three-dimensional Ising model, within the interval ( 0.36 ± 0.05 ), the low temperature N C–I RE phase transition presents strong indications of a second order phase transition, with a critical exponent in the interval ( 0.16 ± 0.01 ). This experimental achievement indicates that the high temperature and the low temperature (reentrant) nematic–isotropic phase transitions do not belongs to the same universal class.

Shear alignment behavior of nematic solutions induced by ultralong side-group liquid crystal polymers.

Addition of a low concentration of a very long (430 kg/mol) side group liquid crystal polymer is shown to produce dramatic changes in the flow characteristics of a calamitic nematic liquid crystal. This polymer causes a typical flow-aligning nematic liquid crystal to align near the velocity gradient direction rather than near the velocity direction, corresponding to having a tumbling parameter lambda<-1, for concentrations greater than 7.5% polymer. Such flow-aligning behavior has not been reported previously in a calamitic nematic. The large molecular weight of the present polymer relative to those examined in the prior literature is responsible for these new phenomena.

Refractive index measurements in a reentrant isotropic-calamitic nematic phase transition.

In this work the temperature dependence of the extraordinary/ordinary refractive indices and optical birefringence are determined near the reentrant isotropic (I(RE))-calamitic nematic (N(C)) and N(C)-isotropic (I) phase transitions in a lyotropic mixture of potassium laurate, decanol, and water. This reentrant isotropic phase is verified by our experimental data and discussed in terms of thermal variation of the micellar shape anisotropy. The results showed also, with basis in the Vuks's equation, that near the maximum optical birefringence in the range of N(C) phase the extraordinary coefficient of electronic polarizability (phi(parallel)) is equal to the ordinary coefficient (phi( perpendicular)).

Coefficient of Electronic Polarizability in a Lyotropic Nematic Phase

The coefficient of electronic polarizability (φ i ) is determined by means of the density and extraordinary/ordinary refractive indexes measurements near the reentrant isotropic (I RE ) - calamitic nematic (N C ) and N C - isotropic (I) phase transitions in a lyotropic mixture of potassium laurate, decanol and water. These parameters were related to the extraordinary/ordinary refractive indexes via the temperature derivative of the Vuks's equation. The results showed that near the I RE - N C - I phase transitions the measured value of φ i was found to be about two order of magnitude smaller than that obtained for nematic MBBA system. Sign inversion of this coefficient near these phase transitions is also discussed.

Nature of the liquid crystalline phase transitions in the cesium pentadecafluorooctanoate (CsPFO)-water system: the nematic-to-isotropic transition.

Deuterium NMR spectroscopy of 2H2O has been used to monitor the magnetic-field-induced order on approaching a transition to a nematic phase in isotropic solutions of disklike micelles of cesium pentadecafluorooctanoate. Highly accurate data on the phase boundaries and spinodals have been obtained for solutions with volume fraction concentration straight phi between 0.078 and 0.201. The quantity TIN-T*, where T* is the spinodal limit of the isotropic phase and TIN is the temperature at which the nematic phase first appears on cooling, decreases linearly with decreasing concentration, extrapolating to zero only at zero concentration. Thus, there is no evidence to support the presence of a Landau point along the transition line as has previously been conjectured. The values for (TIN-T*)/TIN are in the range 10(-5)-10(-4), up to two orders of magnitude smaller than corresponding values reported for calamitic thermotropic nematics. The transition gap (phiNI-phiIN)/phiIN approximately 0.33% for phi<0.20 is also very small, although finite as required for a first-order phase transition. These data, when combined with previously measured properties, present an intriguing picture of the isotropic-to-nematic phase transition in a paradigmatic system of self-assembled diskotic particles. However, it is not completely clear, within the context of current theoretical understanding, whether the behavior of this system is explicable by hard-particle models, or if the self-assembly plays a crucial role in weakening the phase transition.

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.

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.

Orientational diffusivities measured by Rayleigh scattering in a lyotropic calamitic nematic ( ) liquid crystal phase: the backflow problem revisited

Using a light-beating technique we have measured the damping time of thermal fluctuations of the nematic director for the so called cylindrical or calamitic nematic (NC) phase of the lyotropic system K-laurate/decanol/ . By varying the scattering angle in suitable geometries, we have been able to estimate the orientational diffusivities associated to the three pure deformations of splay, twist and bend. A former measurement made in the disk-like ND phase of the same system yielded a large deviation between the splay and twist diffusivities. The effect was then attributed to induced flows, or backflow, which could be responsible for the reduction of the splay viscosity. In fact, this is the analogous effect, for disks, to the one recognized since long time ago arriving for rod-like molecules in a classical nematic, though in this case it is associated with bend deformations. The analogy comes about thanks to the interchange of the role played by disks and cylinders for, respectively, splay and bend fluctuations.The measurements reported here provide a new test on the applicability of the backflow model to a nematic system composed of micelles, that is, aggregates made of amphiphilic (surfactant) molecules, in its cylindrical-like variant, i.e. the NC phase. In addition, the comparative study made here with the previous results existing in the literature for the ND phase, allows us to conjecture on structural issues concerning lyotropic nematics.

Spectroscopic manifestations of the difference of the local symmetry of calamitic and discoidal nematics

It is shown that the difference in the local symmetry of nematics consisting of rod-shaped and disk-shaped molecules is manifested as an observable qualitative difference in their spectroscopic characteristics. The well-known controversial experimental data are explained.

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.

Understanding the dependence of the transitional properties of liquid crystal dimers on their molecular geometry

Abstract The characteristic feature of liquid crystal dimers, in which two mesogenic groups are linked by a flexible spacer, is often thought to be the strong odd-even effect exhibited by their transitional properties. That is, the nematic-isotropic transition temperature and the entropy of transition are large for dimers with an even number of groups in the spacer in comparison with those for neighbouring dimers with an odd number of groups. However, the magnitude of the odd-even effect along a homologous series of dimers is found to depend strongly on the nature of the link between the mesogenic group and the spacer. This dependence is thought to originate in the variation of the molecular geometry with the linking group, a view which is supported by detailed molecular field theory calculations involving all of the conformational states. Here we are concerned with developing a more transparent understanding of this geometrical effect using a simple model of the dimers in which all of the conformational states are replaced by just two, a linear and a bent conformer. The model has been found to exhibit a strong odd-even effect as well as a nematic-nematic transition when the bond angle is tetrahedral. We have used this model to explore the dependence of the transitional properties of liquid crystal dimers on their geometry by varying the bond angle of the bent conformer. The behaviour predicted by the model for the nematic-isotropic transition is found to be in qualitative agreement with experiment. In addition, the nematic-nematic transition is observed to exhibit a critical behaviour as the bond angle is increased. At the other extreme, when the bond angle is reduced to its limiting value of 90° there is a very strong first order transition between a discotic and a calamitic nematic.

First order transitions to a lyotropic biaxial nematic

Abstract The phase diagram of the sodium dodecylsulphate/decanol/water system is studied by2H NMR spectroscopy in the range between the calamitic nematic (N+ C) and discotic nematic (N− D) phases. In this narrow range a nematic biaxial phase (NBX) is observed. The phase transitions between the nematic phases are all of first order. The shape of the surfactant aggregates in the nematic phases varies with composition and temperature.

Micellar fluctuations in a calamitic nematic phase of a lyotropic liquid crystal studied by photon correlation

Abstract Using a Rayleigh scattering light beating technique, we have made new measurements on the slow polarized fluctuations, previously identified as being of micellar origin, for a lyotropic system consisting of K-laurate, decanol, D2O and presenting only the uniaxial calamitic nematic (Nc) phase (from among the three known nematic types: uniaxial disk-like (ND), calamitic (Nc) and biaxial (NB) phases). An elaborated analysis of these data is made in order to clarify the relationship between the light scattering signals coming from the micellar fluctuations and those from (classical) orientational fluctuations. The main conclusions of this analysis are that the micellar mode is independent of the temperature and wave-vector, and that its relaxation frequency is substantially lower than that found previously for the same lyotropic components, but presenting the biaxial nematic phase. Finally, some considerations are made of the possible relationships between the micellar fluctuations and the structural properties of lyotropic nematics.

On the influence of short range order upon the physical properties of triphenylene nematic discogens

Abstract The dielectric and elastic properties of two novel triphenylene nematic discogens are measured and compared with those of calamitic nematics and the known material H7OBT. The new compounds have nematic phases at much lower temperatures than H7OBT. All three compounds have a small positive permittivity anisotropy, indicating the presence of a permanent dipole contribution parallel to the director. Measurement of the molecular moment dipole provides a value consistent with nearly-free rotation of the substituent groups. The temperature dependence of ∊ and the concentration dependence of the dipolar strength Δ show evidence of antiparallel dipole correlations in the ND phase. The strong temperature dependence of |k 33/Δχ|, the large value of k11/k 33 and the rise in k 11/k 33 as the temperature approaches the clearing point are believed to be due to short range columnar order. Theoretical study suggests that dipole correlations favour local columnar order, and we predict that antiparallel dipole correlations require that the molecules be tilted within the columns.