IR regulation through preferential placement of h-BN nanosheets in a polymer network liquid crystal.

Recently, there has been a great deal of interest in devices which effectively shield near-infrared light with an additional feature of external field tunability, particularly for energy-saving applications. This article demonstrates an approach for fabricating a highly efficient near-infrared regulating device based on a polymer network liquid crystal reinforced with nanosheets of hexagonal-boron nitride (BN). The device achieves ∼84% IR scattering capability over a wavelength range of 800-2300 nm, and can also be regulated by an electric field. Interestingly, the observed high IR regulation is despite individual components of the composite being IR transparent, in stark contrast to earlier attempted incorporation of IR-absorbing/scattering particles. Detailed experimental characterization methods including FESEM corroborated with EDS and Raman spectroscopy suggest that the preferential positioning of the BN nanosheets, a consequence of the photo-polymerization process, is responsible for the observed feature. The IR reflectivity/back scattering that is doubled upon incorporation of the nanosheets results in an enhanced convective/radiative heat barrier capability, as evidenced by thermal imaging and significant (2 °C) reduction in ambient temperature upon one-Sun illumination. Numerical simulation results are also found to be in good agreement with the observed enhanced reflectance values for the BN-incorporated case. The presence of BN augments the mechanical rigidity of the system by a factor of 6.8 without compromising on the device operating voltage. The protocol employed is quite general and thus advantageous with far-reaching applications in passive cooling of buildings and structures, in thermal camouflaging, and in overall energy management.

Transparent Triboelectric Nanogenerator Based on Thermoplastic Polyurethane Films.

This study aims at investigating flexible and transparent thermoplastic polyurethane (TPU) as a novel material for triboelectric nanogenerator (TENG) devices with a polyethylene terephthalate layer. Devices having TPU-either as a flat film or as electrospun micrometer-dimension fibers with varying concentrations of TPU-were tested. The best output performing device provided 21.4 V and 23 µA as open-circuit voltage and short-circuit current respectively, with the application of a small force of 0.33 N indicating the high efficiency of the device. Devices with flat films-obtained using the doctor-blade (DB) technique-have high transparency (80%) as well as high TENG output. The topography of the TPU layer, characterized by atomic force microscopy, reveals nanoscale roughness of the film surface. Finally, we demonstrate that gentle hand tapping on the TENG device can power upto 11 light-emitting diodes (LEDs). The high transparency, lightweight, simple fabrication, flexibility, and robust features of such device make it an added value for various optoelectronic applications.

Nanophase Segregation of Nanostructures: Induction of Smectic A and Re-Entrance in a Carbon Nanotube/Nematic Liquid Crystal Composite.

Although representing drastically different chemical natures and types of interactions, the similarity of shape anisotropy of the constituents has created much interest in composites of calamitic liquid crystals (LCs) and carbon nanotubes (CNTs). Despite significant volume of studies on the physical properties of such composites, influence of CNTs on the celebrated re-entrant phenomenon in LCs has not been studied. We report here that a small concentration of CNTs doped to a "nematic mesophase-only" material not only induces the layered smectic A mesophase but also leads to the nematic-smectic-nematic re-entrant sequence as well, demonstrating the delicate interplay between the two entities. To explain this unusual observation, we propose nanophase segregation of the CNT as a possible mechanism. A combination of Flory-Huggins theory and Landau-de Gennes theory has also been provided supporting the observed feature.

Transforming a C 3-Symmetrical Liquid Crystal to a π-Gelator by Alkoxy Chain Variation.

Rational understanding of the structural features involving different noncovalent interactions is necessary to design a liquid crystal (LC) or an organogelator. Herein, we report the effect of the number and positions of alkoxy chains on the self-assembly induced physical properties of a few π-conjugated molecules. For this purpose, we designed and synthesized three C 3-symmetrical molecules based on oligo(p-phenylenevinylene), C 3 OPV1-3. The self-assembly properties of these molecules are studied in the solid and solution states. All of the three molecules follow the isodesmic self-assembly pathway. Upon cooling from isotropic melt, C 3 OPV1 having nine alkoxy chains (-OC12H25) formed a columnar phase with two-dimensional rectangular lattice and retained the LC phase even at room temperature. Interestingly, when one of the -OC12H25 groups from each of the end benzene rings is knocked out, the resultant molecule, C 3 OPV2 lost the LC property, however, transformed as a gelator in toluene and n-decane. Surprisingly, when the -OC12H25 group from the middle position is removed, the resultant molecule C 3 OPV3 failed to form either the LC or the gel phases.

Influence of chirality on the thermal and electric properties of the columnar mesophase exhibited by homomeric dipeptides.

We present the first investigation of the influence of chirality on the thermal and electric properties in a biologically important homomeric dipeptide that exhibits a hexagonal columnar liquid crystal mesophase. The peptide employed has two chiral centres, and thus the two possible enantiopures are the (R,R) and (S,S) forms having opposite chirality. The measurements reported the span of the binary phase space between these two enantiopures. Any point in the binary diagram is identified by the enantiomeric excess Xee (the excess content of the R,R enantiopure over its S,S counterpart). We observe that the magnitude of Xee plays a pivotal role in governing the properties as evidenced by X-ray diffraction (XRD), electric polarization (Ps), dielectric relaxation spectroscopy (DRS) measurements, and the isotropic-columnar transition temperature. For example, XRD shows that while other features pointing to a hexagonal columnar phase remain the same, additional short-range ordering, indicating correlated discs within the column, is present for the enantiopures (Xee = ±1) but not for the racemate (Xee = 0). Similarly, an electric-field driven switching whose profile suggests the phase structure to be antiferroelectric is seen over the entire binary space, but the magnitude is dependent on Xee; interestingly the polarization direction is axial, i.e., along the column axis. DRS studies display two dielectric modes over a limited temperature range and one mode (mode 2) connected with the antiferroelectric nature of the columnar structure covering the entire mesophase. The relaxation frequency and the thermal behaviour of mode 2 are strongly influenced by Xee. The most attractive effect of chirality is its influence on the polar order, a measure of which is the magnitude of the axial polarization. This result can be taken to be a direct evidence of the manifestation of molecular recognition and the delicate interplay between chiral perturbations and the magnitude of the polar order, a feature attractive from the viewpoint of devices based on, e.g., remnant polarization-a currently hot topic. To add further dimension to the work, the DRS measurements are also extended to elevated pressures.

Carbon Nanotube Reinforced Polymer-Stabilized Liquid Crystal Device: Lowered and Thermally Invariant Threshold with Accelerated Dynamics.

Polymer-stabilized liquid crystal (PSLC) devices comprise a polymer matrix in an otherwise continuous phase of liquid crystal. The fibrils of the polymer provide, even in the bulk, virtual surfaces with finite anchoring energy resulting in attractive electro-optic properties. Here, we describe a novel variation of the PSLC device fabricated by reinforcing the polymer matrix with polymer-capped single-walled carbon nanotubes (CNTs). The most important outcome of this strengthening of the polymer strands is that the threshold voltage associated with the electro-optic switching becomes essentially temperature independent in marked contrast to the significant thermal variation seen in the absence of the nanotubes. The reinforcement reduces the magnitude of the threshold voltage, and notably accelerates the switching dynamics and the effective splay elasticity. Each of these attributes is quite attractive from the device operation point of view, especially the circuit design of the required drivers. The amelioration is caused by the polymer decorating CNTs being structurally identical to that of the matrix. The resulting good compatibility between CNTs and the matrix prevents the CNTs from drifting away from the matrix polymer, a lacuna in previous attempts to have CNTs in PSLC systems. The difference in the morphology, perhaps the primary cause for the effects seen, is noted in the electron microscopy images of the films.

Self-Assembling and Luminescent Properties of Chiral Bisoxadiazole Derivatives in Solution and Liquid-Crystalline Phases.

Herein, we report the synthesis, self-assembly, and electroluminescence characteristics of a new green-emitting, pseudodiscoid chiral molecule, OXDC, containing an electron-donating stilbene core and an electron-accepting oxadiazole substituent. The helical organization and specific interaction of the chiral pseudodiscoid molecule resulted in the formation of self-assembled nanofibers with a columnar superstructure. Macroscopic chirality was observed in both the liquid-crystalline phases and the self-assembled nanofibers of OXDC, a feature which was absent in the analogous achiral oxadiazole derivative reported earlier [ Sivadas , A. P. ; Supergelation via Purely Aromatic π-π Driven Self-Assembly of Pseudodiscotic Oxadiazole Mesogens . J. Am. Chem. Soc. 2014 , 136 , 5416 - 5423 ]. A high-performance organic light-emitting device was demonstrated using OXDC as the emitting material, with a luminous intensity of 10 115 cd m-2 at 5 V and chromaticity coordinates of (0.32, 0.51).

Effect of Pressure on Dielectric and Frank Elastic Constants of a Material Exhibiting the Twist Bend Nematic Phase.

We report the first investigation on the effect of applied pressure on the now well-known dimer α,ω bis(4,4'-cyanobiphenyl)heptane (CB7CB) that exhibits two types of nematic: the regular uniaxial nematic (N) and the recently discovered twist-bend nematic (NTB) phase. At atmospheric pressure, the thermal behavior of ε⊥, the permittivity normal to the director in the N phase decreases on entering the NTB wherein the value represents permittivity orthogonal to the helical axis. Application of pressure initially decreases the magnitude of the change in ε⊥ and with further increase in pressure exhibits an increase in the value. Such a change in the feature of ε⊥ is similar to that obtained at room pressure when the monomeric heptyloxy cyanobiphenyl (7OCB) is doped to CB7CB at a high concentration of 50%. The dielectric anisotropy exhibits a trend reversal with temperature, the extent of which is affected at high pressures. Another salient feature of the study is the effect that pressure has on the Frank bend elastic constant K33. Over the pressure range studied K33 enhances by a large factor of 5. In contrast, the splay elastic constant exhibits a much smaller change of only 70%. The pressure-temperature phase boundary has a much smaller slope for the N-NTB transformation than for the isotropic-N transition. We propose that all these features can be understood in terms of the relative population of the more energetic horseshoe and lower energy extended conformer adopted by the CB7CB molecule. The extended conformer is favored at lower temperatures or at higher pressures. This argument is validated by X-ray diffraction experiments at atmospheric pressure on the binary mixture of CB7CB and 7OCB, mentioned above.

Reversible Polymorphism, Liquid Crystallinity, and Stimuli-Responsive Luminescence in a Bola-amphiphilic π-System: Structure-Property Correlations Through Nanoindentation and DFT Calculations.

We report the design, synthesis, detailed characterization, and analysis of a new multifunctional π-conjugated bola-amphiphilic chromophore: oligo-(p-phenyleneethynylene)dicarboxylic acid with dialkoxyoctadecyl side chains (OPE-C18-1). OPE-C18-1 shows two polymorphs at 123 K (OPE-C18-1') and 373 K (OPE-C18-1″), whose crystal structures were characterized via single crystal X-ray diffraction. OPE-C18-1 also exhibits thermotropic liquid crystalline property revealing a columnar phase. The inherent π-conjugation of OPE-C18-1 imparts luminescence to the system. Photoluminescence measurements on the mesophase also reveal similar luminescence as in the crystalline state. Additionally, OPE-C18-1 shows mechano-hypsochromic luminescence behavior. Density functional theory (DFT)-based calculations unravel the origins behind the simultaneous existence of all these properties. Nanoindentation experiments on the single crystal reveal its mechanical strength and accurately correlate the molecular arrangement with the liquid crystalline and mechanochromic luminescence behavior.

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.

Self-assembly of luminescent N-annulated perylene tetraesters into fluid columnar phases.

A new class of N-annulated perylene tetraesters and their N-alkylated derivatives has been synthesized. N-Annulated perylene tetraesters stabilize a hexagonal columnar phase over a broad temperature range. The hexagonal columnar phase exhibited by these compounds shows good homeotropic alignment with few defects. Annulation in the bay region of the perylene tetraesters enhanced the width of the mesophase compared with the parent tetraesters. N-Alkylation of these compounds perturbed the self-assembly behaviour and the resulting compounds were non-mesomorphic. A bright green luminescence was visible under long wavelength UV light. These properties suggest that these materials may have promising applications in organic electronics.

Periodically clickable polyesters: study of intrachain self-segregation induced folding, crystallization, and mesophase formation.

A series of polyesters based on 2-propargyl-1,3-propanediol or 2,2-dipropargyl-1,3-propanediol or 2-allyl-2-propargyl-1,3-propanediol and 1,20-eicosanedioic acid were prepared by solution polycondensation using the corresponding diacid chloride; these polyesters were quantitatively "clicked" with a fluoroalkyl azide, namely CF3(CF2)7CH2CH2N3, to yield polyesters carrying long-chain alkylene segments in the backbone and either one or two perfluoroalkyl segments located at periodic intervals along the polymer chain. The immiscibility of the alkylene and fluoroalkyl segments causes the polymer chains to fold in a zigzag fashion to facilitate the segregation of these segments; the folded chains further organize in the solid state to form a lamellar structure with alternating domains of alkyl (HC) and fluoroalkyl (FC) segments. Evidence for the self-segregation is provided by DSC, SAXS, WAXS, and TEM studies; in two of the samples, the DSC thermograms showed two distinct endotherms associated with the melting of the individual domains, while the WAXS patterns confirm the existence of two separate peaks corresponding to the interchain distances within the crystalline lattices of the HC and FC domains. SAXS data, on the other hand, reveal the formation of an extended lamellar morphology with an interlamellar spacing that matches reasonably well with those estimated from TEM studies. Interestingly, a smectic-type liquid crystalline phase is observed at temperatures between the two melting transitions. These systems present a unique opportunity to develop interesting nanostructured polymeric materials with precise control over both the domain size and morphology; importantly, the domain sizes are far smaller than those typically observed in traditional block copolymers.

High-pressure investigations of a ferroelectric liquid crystal exhibiting a trend reversal in the thermal variation of polarization.

In contrast to the exhaustive measurements of various properties of ferroelectric liquid crystals at atmospheric pressure, only a few studies exist at high pressure. Here we report the isobaric thermal variation of spontaneous polarization (P(s)), coercive voltage (U(xc)), and rotational viscosity (γ(ϕ)) of a ferroelectric liquid crystal (10PPBN4) as a function of applied pressure. The material having a high value of P(s) exhibits a trend reversal: as the temperature is lowered below the transition from the smectic A to the smectic C* (ferroelectric) phase, P(s) increases to begin with but after reaching a maximum decreases with further decrease in temperature. Interestingly, the trend reversal feature becomes more dominant as the pressure is increased. Further, at a fixed reduced temperature with respect to the transition, all three parameters P(s), U(xc), and γ(ϕ) decrease with pressure. We show that the data can be well described by a model developed for materials exhibiting a sign reversal in P(s). The single characteristic parameter of the model, viz., the ratio of the inversion temperature (at which P(s) changes sign), to the transition temperature, is seen to increase with pressure, corroborating predominance of trend reversal at elevated pressures observed experimentally.

Critical behavior of three organosiloxane de Vries-type liquid crystals observed via the dielectric response.

Dielectric measurements have been made on three organosiloxane liquid crystal compounds exhibiting a smectic A (SmA) to smectic C* (SmC*) transition, the SmA phase being of the de Vries type. The electroclinic response of the molecules in the de Vries phase of these compounds exhibits a double-peak profile, and is thus different from the conventional chiral SmA 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 interactions arising from the different molecular ends of these siloxane-based compounds, which are the basis for the AF block model, can also be expected to enhance the layer translational 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 that the magnitude of the soft mode relaxation parameters is dependent on the number of siloxane groups in the terminal part of the molecule. A range-shrinking analysis of the temperature-dependent dielectric relaxation strength has been carried out, using a power-law expression. The characteristic exponent shows a systematic growth with range shrinking and reaches limiting values comparable to that predicted for the 2D Ising universality class.

Confinement-driven weakening of the rotator phase transitions in an alkane through a possible tricritical point.

We report calorimetric and X-ray measurements in the R1, R2, and R5 rotator phases of a long chain alkane (n-tetracosane) in bulk and confined to porous matrices (PTFE and Anopore) of two different length scales. Probing the order within and normal to the layers, in the Anopore case having a mesoscopic length scale (200 nm), drastic weakening of the R2-R1 and R1-R5 transitions is seen. The effect on the latter is to such an extent that it results in the first observation of a confinement-driven second order transition in these systems. A significant reduction of the temperature range of the R1 phase is also seen in the Anopore case, a feature argued to cause the change in the order of the transition. Comparisons are also made on the recent prediction of such a point in a Landau model. These findings, while paving a new means of realizing a tricritical point, will lead to better understanding of finite size effects in alkanes.

High-pressure dielectric investigations of nanocolloidal Aerosil-nematic liquid crystal composites.

We report the effect of applied pressure on the dielectric properties of composites of a nematic liquid crystal with hydrophilic Aerosil particles. The study, which is the first of its kind on the nanocolloidal Aerosil systems, also has the novelty that a weakly polar nematogen is used as the host material. In conjunction with differential scanning calorimetric data, these measurements show that not only T(N-Iso), the nematic-isotropic transition at room pressure, but several other parameters exhibit a nonmonotonic variation with increasing Aerosil concentration. The dependence of the relaxation frequency of the mode connected with the reorientations of the molecules around their short axis is probed along isobaric as well as isothermal paths, and the determined activation parameters are discussed in terms of the chemical structure of the host molecule. The temperature dependence of the relaxation frequency at atmospheric pressure, exhibiting non-Arrhenius behavior, has been analyzed using the Vogel-Fulcher-Tamann expression, yielding information regarding the influence of the Aerosil concentration on the fragility strength as well as the glass transition temperature. The calorimetric data is also discussed in terms of the concept of a crossover from the random-dilution to the random-field limits.

Soft glass rheology in liquid crystalline gels formed by a monodisperse dipeptide.

Thermal and extensive rheological characterization of a nematic liquid crystal gelated with a novel monodisperse dipeptide, also a liquid crystal, has been carried out. For certain concentrations, the calorimetric scans display a two-peak profile across the chiral nematic-isotropic (N*-I) transition, a feature reminiscent of the random-dilution to random-field crossover observed in liquid crystal gels formed with aerosil particles. All samples show shear thinning behavior without a Newtonian plateau region at lower shear rates. Small deformation oscillatory data at lower frequencies exhibit a frequency dependence of the storage (G') and loss (G'') moduli that can be described by a weak power-law, characteristic of soft glassy rheological systems. At higher frequencies, while lower concentration composites have a strong frequency dependence with a trend for possible crossover from viscoelastic solid to viscoelastic liquid behavior, the higher-concentration gels show frequency-independent rheograms of entirely elastic nature G' > G''. The plateau modulus of G' is described by a power-law with an exponent again common to soft materials, such as foams, slurries, etc. Other features which are a hallmark of such materials observed in the present study are: (i) above a critical strain, a strain softening of the moduli with a peak in the loss modulus, (ii) power-law variation of the storage modulus in the nonlinear viscoelastic regime, and (iii) absence of Cox-Merz superposition for the complex viscosity. An attractive feature of these gels is the fast recovery upon removal of large strain and qualitatively different temporal behavior of the recovery between the low and high concentration composites, with the latter indicating the presence of two characteristic time scales.

Fast responding robust nematic liquid crystalline gels formed by a monodisperse dipeptide: electro-optic and rheological studies.

Realization of mechanically robust electrically fast responding liquid crystal devices with low operating voltage is one of the current research interests. Here we report a gel system comprising a commercially available nematic liquid crystal material and a new monodisperse dipeptide liquid crystalline organogelator that results in these properties. The gels exhibit nearly 2 orders of magnitude faster switching response than the pure nematic liquid crystal while having 3 orders of magnitude higher zero shear rate viscosity, and with the attractive feature that the switching threshold voltage is hardly altered. Electro-optic and rheological studies of this system are described here.

Pretransitional behaviour in the vicinity of the isotropic-nematic transition of strongly polar compounds.

The isotropic-nematic transition, being weakly first order, exhibits pretransitional effects signifying the appearance of the nematic-like regions in the isotropic phase. In the isotropic phase, strongly polar liquid crystals, such as the popular alkyl and alkoxy cyano biphenyl behave in a non-standard fashion: whereas far away from the transition the dielectric constant ε(iso) has a 1/T dependence (a feature also commonly seen in polar liquids), on approaching the nematic phase the trend reverses resulting in a maximum in ε(iso), at a temperature slightly above the transition, an effect explained on the basis of short-range correlations with an antiparallel association of the neighbouring molecules. Recently, there has been a revival in studies on this behaviour to possibly associate it with the order of transition. Here we report dielectric measurements carried in the vicinity of this transition for a number of compounds having different molecular structures including a bent core system, but with a common feature that the molecules possess a strong terminal polar group, nitro in one case and cyano in the rest. Surprisingly, the convex shape of the thermal variation of ε(iso) was more an exception than the rule. In materials that exhibit such an anomaly we find a linear correlation between δε = (ε(peak)-ε(IN))/ε(IN) and δT = T(peak)-T(IN), where ε(peak) is the maximum value of the dielectric constant in the isotropic phase, ε(IN) the value at the transition, and T(peak) and T(IN) the corresponding temperatures.

X-ray and dielectric measurements of the smectic- A -hexatic- B transitionin bulk and confined geometries.

We report x-ray and dielectric measurements on a liquid crystal exhibiting a smectic- A -hexatic- B (Sm- A -Hex- B ) transition in bulk and confined geometries. The confined geometry experiments have been performed by filling the compound, n -butyl 4(') - n -hexyloxybiphenyl-4-carboxylate, into Anopore membranes with 20 nm pore size. The surprising result seen in the x-ray studies on both the bulk and the Anopore samples is the presence of unusually strong second harmonic reflection arising from the smectic layers in both the Sm- A and Hex- B phases. In fact, the third harmonic reflection is also observed in the entire temperature range of the Hex- B phase and close to the transition in the Sm- A phase of the bulk sample, with the ratio of I(2qo) / I(qo), and I(3qo) / I(qo) being approximately 1/40 and 1/470, respectively (here I(qo), I(2qo), and I(3qo) correspond to the peak intensities of the first, second, and third harmonic reflections). These features point to the fact that the layering is much better defined in this system, unlike the usual description of pure sinusoidal mass density wave with a typical value of 10(-4) for I(2qo) / I(qo). Another notable feature observed is that whereas the extent of positional correlations of the molecules within the smectic layer, is comparable in the Sm- A phase of the bulk and Anopore samples, the correlations are reduced in the Hex- B phase. In the bulk as well as the Anopore case, the critical behavior of the positional correlations is found to be well described by a power-law expression with the exponent alpha being slightly larger for the Anopore sample. It is also seen that the weak first order nature of the transition seen in the bulk, gets weakened further, perhaps becoming second order. Despite such significant differences between the bulk and Anopore samples, the dynamics associated with the director fluctuations is only slightly influenced by confinement.