Viscoelasticity of ambient-temperature nematic binary mixtures of bent-core and rodlike molecules

Viscoelastic and dielectric properties of 5CB nematic liquid crystal doped by magnetic and nonmagnetic nanoparticles

In this article we show how spherical nanoparticles (NPs) imposing planar anchoring can strongly impact the viscoelastic, dielectric, and electro-optical properties of a nematic liquid crystal when they are not aggregated. We also demonstrate that when the NPs are magnetic, most nematic properties are more impacted than when they are nonmagnetic. With magnetic NPs a molecular disorder is induced that decreases the nematic order parameter, this decrease impacting the values of elastic constants, viscosity, and response time. The impact on 5CB liquid crystal (LC) has been investigated with spherical nanoparticles (NPs) of identical size around 6 nm, magnetic (γFe_{2}O_{3}), and nonmagnetic (CeO_{2}) ones that are both surface functionalized by poly(aminopropylmethylsiloxane-b-dimethylsiloxane) (PAPMS-b-PDMS) block copolymer ligands to promote planar anchoring. In the presence of nonmagnetic NPs, despite an almost constant nematic order parameter, a significant decrease of elastic constants (25.4%), viscosity (22%), and response time (23%) is measured. It suggests a dilution effect for the intermolecular interactions in the presence of NPs. This hypothesis is supported by the observation of an enhanced decrease of the same nematic parameters in the presence of magnetic NPs that can be fully explained by the corresponding order parameter decrease. This finally leads to a remarkable decrease of the splay elastic constant by 51% in the presence of magnetic NPs. The decrease of the nematic order parameter by 18% in the presence of magnetic NPs demonstrates that the NP magnetic moments are only weakly coupled to the nematic director and consequently only induce a disorder in the composite system. A significant influence of the expected large LC structural modifications in the presence of magnetic NPs is, however, shown by a particularly large increase of the diffusion coefficient 43% and large decrease of the dielectric anisotropy (43%). We believe that the observed impact of NPs with planar anchoring on nematic properties could be extended to most spherical NPs if their aggregation can be avoided. In particular, the difference between magnetic and nonmagnetic NPs could be extended to ferroelectric and nonferroelectric NPs.

Microrheology to probe smectic clusters in bent-core nematic liquid crystals

Many bent-core nematic liquid crystals exhibit unusual physical properties due to the presence of smectic clusters, known as "cybotactic" clusters, in the nematic phase. Here, we investigate the effect of these clusters on the complex shear modulus (G*(ω)) of two asymmetric bent-core liquid crystals using a microrheological technique. The compound with a shorter hydrocarbon chain (8OCH3) exhibits only a nematic (N) phase whereas the compound with a longer chain (16OCH3) exhibits both nematic (N) and smectic-A (SmA) phases. The rheological results are correlated with the measurements of curvature elastic constants. Our results show that the directional shear modulus of 16OCH3, just above the SmA to N phase transition temperature, is strikingly different than that of 8OCH3, owing to the smectic clusters. An approximate size of the clusters is estimated using a simple model. Therefore, microrheological studies on bent-core nematic liquid crystals are very useful in extracting information about underlying smectic clusters.

Whispering gallery mode lasing in mesomorphic liquid crystal microdroplets

In recent years, investigation on the non-display applications of liquid crystals has increased considerably. One of the emerging applications is whispering gallery mode (WGM) lasing. Here, we report experimental studies on the morphology and WGM lasing in nematic (N), smectic-A (SmA) and smectic-C (SmC) microdroplets dispersed in a highly transparent and low refractive index perfluopolymer. The mesomorphic microdroplets, obtained by varying the temperature, exhibit radial director configuration. The SmA microdroplets are found to be highly stable and robust against mechanical stress compared to the N and SmC microdroplets. We study lasing properties such as intensity, threshold pump energy and linewidth, and show that overall the SmA microdroplets are superior to the N and SmC microdroplets. The experimental results are discussed based on the orientation of the dye molecules, director fluctuations and tilting at the interface.

Continuously adjustable period optical grating based on flexoelectric effect of a bent-core nematic liquid crystal in planar cells

The structures of flexodomains, which are similar to optical gratings and can be controlled by the amplitude of applied voltage and temperature, were verified through polarizing microscopy and light diffraction techniques. The properties of the optical grating induced by a bent-core nematic liquid crystal in planar cells with varied cell gaps and pretilt angles were studied. The period of optical grating decreases with the increase in the amplitude of the applied voltage and pretilt angle. In addition, the period increases with the increase in cell gap and temperature. The period of optical grating has a linear relationship with temperature. The continuously adjustable period has the potential to become an important and extended application of optical grating.

Elastic and viscous properties of the nematic dimer CB7CB

We present a comprehensive set of measurements of optical, dielectric, diamagnetic, elastic, and viscous properties in the nematic (N) phase formed by a liquid crystalline dimer. The studied dimer, 1,7-bis-4-(4'-cyanobiphenyl) heptane (CB7CB), is composed of two rigid rodlike cyanobiphenyl segments connected by a flexible aliphatic link with seven methyl groups. CB7CB and other nematic dimers are of interest due to their tendency to adopt bent configurations and to form two states possessing a modulated nematic director structure, namely, the twist-bend nematic, N_{TB}, and the oblique helicoidal cholesteric, Ch_{OH}, which occurs when the achiral dimer is doped with a chiral additive and exposed to an external electric or magnetic field. We characterize the material parameters as functions of temperature in the entire temperature range of the N phase, including the pretransitional regions near the N-N_{TB} and N-to-isotropic (I) transitions. The splay constant K_{11} is determined by two direct and independent techniques, namely, detection of the Frederiks transition and measurement of director fluctuation amplitudes by dynamic light scattering (DLS). The bend K_{33} and twist K_{22} constants are measured by DLS. K_{33}, being the smallest of the three constants, shows a strong nonmonotonous temperature dependence with a negative slope in both N-I and N-N_{TB} pretransitional regions. The measured ratio K_{11}/K_{22} is larger than 2 in the entire nematic temperature range. The orientational viscosities associated with splay, twist, and bend fluctuations in the N phase are comparable to those of nematics formed by rodlike molecules. All three show strong temperature dependence, increasing sharply near the N-N_{TB} transition.

Room-Temperature Oligomeric Discotic Nematic Liquid Crystals over a Wide Temperature Range: Structure-Property Relationships

The design and synthesis of three room-temperature discotic nematic (N_D ) liquid crystals (LCs) is presented. The LC consists of an azobenzene core attached to which are four pentaalkynylbenzene (PA) units through flexible alkyl spacers. The presence of a short azo linking group provides more disorder in the system, thereby reducing the packing efficiency among the PA units and resulting into the formation of a room-temperature N_D phase over a wide temperature range. Dielectric constant and birefringence studies were performed to gain further insights into the physical properties of the mesophase.

Binary System Exhibiting the Nematic to Twist-Bend Nematic Transition: Behavior of Permittivity and Elastic Constants

We describe measurements of the permittivity and Frank elastic constant in the nematic phase of a binary system displaying a transition between the nematic (N) and the recently discovered twist-bend nematic (NTB) phase. Among the salient features observed are (i) the existence of the NTB phase even when the system is loaded with a high concentration (∼64 mol %) of a rodlike component; (ii) a clear signature in permittivity of the N-NTB transition; and (iii) a lower value of the bend elastic constant compared to the splay over a large phase space, with the difference between the two becoming a maximum for an intermediate mixture. These studies further support the surprising idea that the elastic features associated with bent molecules can be further augmented by suitable rodlike additives.

Unusual Electro-Optic Kerr Response in a Self-Stabilized Amorphous Blue Phase with Nanoscale Smectic Clusters

We investigated the electro-optic response in the "foggy" amorphous blue phase (BPIII) as well as in the isotropic phase. To the best of our knowledge, such a study has not yet been performed due to the very limited thermal range of BPIII. In this study, we used a single-component chiral bent-core liquid crystal with a self-stabilized BPIII, which is stable over a wide temperature range. The results show that the response time is on the order of hundreds of microseconds in the isotropic phase and increases to 1-2 ms in the BPIII (at TI-BP -T <1), then drastically increases up to a few tens of milliseconds upon further cooling in BPIII. Such an unusual behavior was explained on the basis of the high rotational viscosity and/or the existence of nanoscale smectic (Sm) clusters. The Kerr constant was also measured and found to be ∼500 pm V(-2) , which is the largest among bent-core BP systems reported so far and comparable with that of polymer-stabilized BPs.

Short-range smectic fluctuations and the flexoelectric model of modulated nematic liquid crystals

We show that the flexoelectric model of chiral and achiral modulated nematics predicts the compression modulus that is by orders of magnitude lower than the measured values. The discrepancy is much larger in the chiral modulated nematic phase, in which the measured value of the compression modulus is of the same order of magnitude as in achiral modulated nematics, even though the heliconical pitch is by an order of magnitude larger. The relaxation of a one-constant approximation in the biaxial elastic model used for chiral modulated nematics does not solve the problem. Therefore, we propose a structural model of the modulated nematic phase, which is consistent with the current experimental evidence and can also explain large compression modulus: the structure consists of short-range smectic clusters with a fourfold symmetry and periodicity of two molecular distances. In chiral systems, chiral interactions lead to a helicoidal structure of such clusters.

Twist, tilt, and orientational order at the nematic to twist-bend nematic phase transition of 1″,9″-bis(4-cyanobiphenyl-4'-yl) nonane: A dielectric, (2)H NMR, and calorimetric study

The nature of the nematic-nematic phase transition in the liquid crystal dimer 1″,9″-bis(4-cyanobiphenyl-4'-yl) nonane (CB9CB) has been investigated using techniques of calorimetry, dynamic dielectric response measurements, and (2)H NMR spectroscopy. The experimental results for CB9CB show that, like the shorter homologue CB7CB, the studied material exhibits a normal nematic phase, which on cooling undergoes a transition to the twist-bend nematic phase (N(TB)), a uniaxial nematic phase, promoted by the average bent molecular shape, in which the director tilts and precesses describing a conical helix. Modulated differential scanning calorimetry has been used to analyze the nature of the N(TB)-N phase transition, which is found to be weakly first order, but close to tricritical. Additionally broadband dielectric spectroscopy and (2)H magnetic resonance studies have revealed information on the structural characteristics of the recently discovered twist-bend nematic phase. Analysis of the dynamic dielectric response in both nematic phases has provided an estimate of the conical angle of the heliconical structure for the N(TB) phase. Capacitance measurements of the electric-field realignment of the director in initially planar aligned cells have yielded values for the splay and bend elastic constants in the high temperature nematic phase. The bend elastic constant is small and decreases with decreasing temperature as the twist-bend phase is approached. This behavior is expected theoretically and has been observed in materials that form the twist-bend nematic phase. (2)H NMR measurements characterize the chiral helical twist identified in the twist-bend nematic phase and also allow the determination of the temperature dependence of the conical angle and the orientational order parameter with respect to the director.

Viscoelastic behavior of a binary system of strongly polar bent-core and rodlike nematic liquid crystals

We have investigated the permittivity and viscoelastic behavior of a binary system comprising bent-core and calamitic compounds, both of which are polar, the calamitic being more strongly so, and exhibiting only the nematic mesophase. The permittivity data in the nematic as well as the isotropic phase indicate strong polar interactions between the molecules, even for mixtures with a significant content of the bent-core compound. The thermal dependence of both the splay and bend elastic constants exhibit features different from the literature. The splay constant displays a large increase with increasing concentration of bent-core material, before undergoing a precipitous drop for small calamitic content materials. Upon lowering the temperature, certain mixtures exhibit a convex-shaped feature for the bend elastic constant; that is, the value of the elastic constant is maximum at a specific temperature in the nematic phase, diminishing when the temperature is either increased or decreased. Surprisingly, the pure compounds, especially the bent-core one, show only a monotonically increasing trend for the bend elastic constant. We present two arguments to explain these features: one of these is based on coupling between the molecular shape and director distortion presented in the literature. Then we put forth a new concept of frustration in the packing between the two types of molecules and the polar interactions as an alternative.

Rheology of nematic liquid crystals with highly polar molecules

We report experimental studies on the rheology of a few nematic liquid crystals with highly polar molecules (CCH-7, PCH-7, CB-7). The selected molecules have the same alkyl chain (-C(7)H(15)) and cyano (-CN) end group. In the core part of the molecule, CCH-7 has two cyclohexane rings, PCH-7 has one cyclohexane and one aromatic ring, and CB-7 has two aromatic rings. Two viscosities were measured as a function of temperature, namely, η(2) (director parallel to the shear direction) and η(1) (director perpendicular to the shear direction). The orientation of the director was studied using small angle light scattering techniques. η(2) was measured in presheared sample, whereas the electrorheological technique was used to measure η(1). We show that both viscosities of the liquid crystals depend on the number of aromatic rings and Kirkwood correlation factor. The temperature dependent viscosities can be understood based on the intramolecular π-electron conjugation and intermolecular association of highly polar molecules.

Unusual temperature dependence of the splay elastic constant of a rodlike nematic liquid crystal doped with a highly kinked bent-core molecule

We report an unusual temperature dependence of the elastic constants of a rodlike nematic liquid crystal (RLC) mixed with a highly kinked bent-core liquid crystal (BLC). On cooling through the nematic phase, the splay elastic constant (K(11)) of the RLC-BLC mixture increased below the nematic-isotropic phase transition temperature, but started to decrease midway through the nematic phase. The decrease of K(11) was more prominent with a greater concentration of BLC. On the other hand, the bend elastic constant (K(33)) of the RLC-BLC mixture monotonically increased through the nematic phase with decreasing temperature.

Effect of cybotactic clusters on the elastic and flexoelectric properties of bent-core liquid crystals belonging to the same homologous series

We report results of the splay (K_{11}) and bend (K_{33}) elastic constants and the effective flexoelectric coefficient of three bent-core liquid crystals belonging to a homologous series of 4-cyanoresorcinol bisbenzoates with varying chain lengths. Based on the results of x-ray scattering studies, one of the three compounds with a shorter chain length (C4) has few, if any, clusters present in its nematic phase and behaves quite normally, whereas the others two with longer chain lengths (C6 and C7) show the presence of cybotactic nematic phase with smectic C type clusters. These grow in size with a reduction in temperature. K_{33} is found to be the least for C7, whereas it is weakly dependent on temperature. K_{33} is somewhat higher for C4 and C6 and is almost independent of temperature. K_{11} for C6 and C7 is higher by 20% to 50% than C4 depending on the temperature. K_{11} increases linearly with a reduction in temperature for the three compounds. For C6 K_{11}>K_{33} by a factor up to ∼2 depending on the temperature, for C4 it is greater by a factor up to 1.3, and for C7 it is greater by a factor of ∼2.5. These results suggest that the clusters do not have any effect on K_{11}. The magnitude of the effective flexoelectric coefficient e=(|e_{1}-e_{3}|) is measured by creating a uniform lying helix (ULH) configuration in a planar cell. By doping the bent-core system with a small wt% of a chiral dopant, the ULH is obtained by cooling planar cells to the cholesteric phase under weak electric field. The effective flexoelectric coefficient is greater for the bent-core systems than for calamatics but it is much lower than would otherwise have been expected for such systems. |e_{1}-e_{3}| for C4 > C6 ≈ C7 is greater by 20% to 25% than C6 and C7 at the same reduced temperature. These differences in the effective flexoelectric coefficient can easily arise from a difference in the chain lengths among the members of the series but if the presence of clusters were to have an influence on |e_{1}-e_{3}|, then these would reduce it, contrary to the expectations for the bent-core systems.

Understanding the distinctive elastic constants in an oxadiazole bent-core nematic liquid crystal

The splay and bend elastic constants of the bent-core oxadiazole material [C5-Ph-ODBP-Ph-OC12] have been investigated as a function of temperature across the nematic phase. The bend constant K(33) is found to take values of ~3.0 pN and to be almost temperature independent, whereas, the splay constant K(11) increases monotonically from ~3.5 pN close to the isotropic phase transition to values of ~9 pN deep in the nematic phase. No pretransitional divergence is observed in either K(11) or K(33) at temperatures approaching the underlying phase. This behavior of the elastic constants is distinct from that observed in rodlike liquid crystal systems but appears to share characteristics with the few other bent-core nematic systems studied to date. We discuss the interdependence of the elastic constants, the birefringence, and the order parameter to allow a comparison of the observed behavior with theory. We show that calculations of the elastic constants via molecular-field theory and atomistic modeling are in excellent qualitative as well as good quantitative (within 2 pN) agreement with the measurements across the temperature range, offering a deeper understanding of the elasticity in bent-core nematic materials than has been, hitherto, available.