Steepness-fragility insights from the temperature derivative analysis of dielectric data of a highly nonsymmetric liquid crystal dimer.

A comprehensive dynamic analysis of the dielectric relaxation-time data across a broad temperature range for both isotropic and nematic phases has been conducted on the CBO3O.Py liquid crystal dimer, the shorter chain-length compound within the highly nonsymmetric pyrene-based series of liquid crystal dimers (CBOnO.Py, with n ranging from 3 to 11).

DNMR measurements of an asymmetric odd liquid crystal dimer: determination of the intramolecular angle and the degree of order of the two rigid cores.

In this work, we present a Deuteron Nuclear Magnetic Resonance (DNMR) study of the non-symmetric odd liquid crystal dimer -(4-cyanobiphenyl-4'-yloxy)--(1-pyrenimine-benzylidene-4'-oxy) heptane (CBO7O.Py), formed by a pro-mesogenic cyanobiphenyl unit and a bulky pyrene-containing unit, linked alkoxy flexible chain. We have synthesized two partially deuterated samples: one with the deuterium atoms in the cyanobiphenyl moiety (dCBO7O.

Comparative dielectric and thermally stimulated-depolarization-current studies of the liquid crystal dimers 1″,9″-bis(4-cyanobiphenyl-4'-yl) nonane and heptane and a binary mixture between them, close to the glass transition.

We have performed dielectric spectroscopy and thermally stimulated-depolarization-current experiments to study the molecular dynamics of the twist-bend nematic phase close to the glass transition of two members of the 1″,7'-bis(4-cyanobiphenyl-4'-yl)alkane homologous series (CBnCB): the liquid crystal (LC) dimers CB9CB and CB7CB, as well as a binary mixture of both. By doping CB9CB with a small quantity of CB7CB, the crystallization is inhibited when cooling the sample down, while the bulk properties of CB9CB are retained and we can investigate the supercooled behavior close to the glass transition. The study reveals that the inter- and intramolecular interactions of the mixture are similar to those of pure CB9CB and confirms that there is a single glass transition in symmetric LC dimers.

Cooperative behavior of molecular motions giving rise to two glass transitions in the same supercooled mesophase of a smectogenic liquid crystal dimer.

In the present work, a detailed analysis of the glassy behavior and the relaxation dynamics of the liquid crystal dimer α-(4-cyanobiphenyl-4'-yloxy)-ω-(1-pyrenimine-benzylidene-4'-oxy) heptane (CBO7O.Py) throughout both nematic and smectic-A mesophases by means of broadband dielectric spectroscopy has been performed. CBO7O.

Influence of liquid crystalline phases on the tunability of a random laser.

In this paper, we report the temperature behavior of an optimized disordered photonic system-based liquid crystal by means of heat capacity and refractive index measurements. The scattering system is formed by a porous borosilicate glass random matrix (about 60%) infiltrated with a smectogenic liquid crystal (about 16%) and a small amount of laser dye (0.1%).

Molecular dynamics of a binary mixture of twist-bend nematic liquid crystal dimers studied by dielectric spectroscopy.

We report a comprehensive dielectric characterization of a liquid crystalline binary mixture composed of the symmetric mesogenic dimer CB7CB and the nonsymmetric mesogenic dimer FFO9OCB. In addition to the high-temperature nematic phase, such a binary mixture shows a twist-bend nematic phase at room temperature which readily vitrifies on slow cooling. Changes in the conformational distribution of the dimers are reflected in the dielectric permittivity and successfully analyzed by means of an appropriate theoretical model.

Correction: Miscibility studies of two twist-bend nematic liquid crystal dimers with different average molecular curvatures. A comparison between experimental data and predictions of a Landau mean-field theory for the NTB-N phase transition.

Correction for 'Miscibility studies of two twist-bend nematic liquid crystal dimers with different average molecular curvatures. A comparison between experimental data and predictions of a Landau mean-field theory for the NTB-N phase transition' by D. O.

Miscibility studies of two twist-bend nematic liquid crystal dimers with different average molecular curvatures. A comparison between experimental data and predictions of a Landau mean-field theory for the NTB-N phase transition.

We report a calorimetric study of a series of mixtures of two twist-bend liquid crystal dimers, the 1'',7''-bis(4-cyanobiphenyl)-4'-yl heptane (CB7CB) and 1''-(2',4-difluorobiphenyl-4'-yloxy)-9''-(4-cyanobiphenyl-4'-yloxy) nonane (FFO9OCB), the molecules of which have different effective molecular curvatures. High-resolution heat capacity measurements in the vicinity of the NTB-N phase transition for a selected number of binary mixtures clearly indicate a first order NTB-N phase transition for all the investigated mixtures, the strength of which decreases when the nematic range increases. Published theories predict a second order NTB-N phase transition, but we have developed a self-consistent mean field Landau model using two key order parameters: a symmetric and traceless tensor for the orientational order and a short-range vector field which is orthogonal to the helix axis and rotates around of the heliconical structure with an extremely short periodicity.

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.

Dispersion of γ-Alumina Nano-Sized Spherical Particles in a Calamitic Liquid Crystal. Study and Optimization of the Confinement Effects.

We report an experimental study on confined systems formed by butyloxybenzylidene octylaniline liquid crystal (4O.8) + γ-alumina nanoparticles. The effects of the confinement in the thermal and dielectric properties of the liquid crystal under different densities of nanoparticles is analyzed by means of high resolution Modulated Differential Scanning Calorimetry (MDSC) and broadband dielectric spectroscopy.

Disentangling molecular motions involved in the glass transition of a twist-bend nematic liquid crystal through dielectric studies.

Broadband dielectric spectroscopy spanning frequencies from 10(-2) to 1.9 × 10(9) Hz has been used to study the molecular orientational dynamics of the glass-forming liquid crystal 1",7"-bis (4-cyanobiphenyl-4'-yl)heptane (CB7CB) over a wide temperature range of the twist-bend nematic phase. In such a mesophase two different relaxation processes have been observed, as expected theoretically, to contribute to the imaginary part of the complex dielectric permittivity.

Effect of Molecular Flexibility on the Nematic-to-Isotropic Phase Transition for Highly Biaxial Molecular Non-Symmetric Liquid Crystal Dimers.

In this work, a study of the nematic (N)-isotropic (I) phase transition has been made in a series of odd non-symmetric liquid crystal dimers, the α-(4-cyanobiphenyl-4'-yloxy)-ω-(1-pyrenimine-benzylidene-4'-oxy) alkanes, by means of accurate calorimetric and dielectric measurements. These materials are potential candidates to present the elusive biaxial nematic (N) phase, as they exhibit both molecular biaxiality and flexibility. According to the theory, the uniaxial nematic (N)-isotropic (I) phase transition is first-order in nature, whereas the N-I phase transition is second-order.

Study of the critical behavior and scaling relationships at the N-to-I phase transition in hexyloxycyanobiphenyl.

An exhaustive analysis of the critical behavior of the nematic to isotropic (N-to-I) phase transition on the liquid crystal hexyloxycyanobiphenyl (6OCB) has been performed. To do so, the accurate evolution of various physical magnitudes (static dielectric permittivity data together with specific heat and volumetric determinations) around the N-to-I transition has been required. The specific heat data with the isobaric thermal expansion coefficient and the derivative of the static dielectric permittivity with temperature have been proven to be related to each other by a scaling relationship.

Thermodynamic assessment of smectic A-nematic tricritical points through the equal gibbs energy analysis.

A complete thermodynamic analysis on four two-component phase diagrams between liquid crystals belonging to the nCB and nOCB series of compounds, the so-called cyanobiphenyls, has been performed through the Oonk's equal Gibbs energy method. The binary systems dealt with in this paper show as a common feature the existence of a tricritical point at the SmA-to-N phase transition, all of which reported elsewhere. As a singular finding of the work proposed in this paper, the Oonk's method is extended in order to account for second-order phase transitions.

From the two-component system CBrCl3+CBr4 to the high-pressure properties of CBr4.

The experimental phase diagram of the CBrCl3+CBr4 system has been determined by means of X-ray powder diffraction and thermal analysis techniques from 200 K to the liquid state. Before melting, the two components have the same orientationally disordered (OD) face-centered cubic phase, and solid-liquid equilibrium is explained by simple isomorphism. The application of multiple crossed isopolymorphism formalism to the low-temperature solid-solid equilibria has enabled the inference of an OD rhombohedral metastable (at normal pressure) phase for CBr4.

Macroscopic DNA helices.

Presently there is great interest in the construction of nanostructures from DNA fragments. Here we report the preparation of much larger helical textures with different shapes from pure DNA fragments. We have observed them while preparing crystals of the dodecamer d(AAAAAATTTTTT), which only contains adenine and thymine.

Multiple crossed isopolymorphism: two-component systems CCl4 +CBr2Cl2 and CBrCl3 + CBr2Cl2; inference of a metastable rhombohedral phase of CBr2Cl2.

The phases diagrams of the two-component systems CCl4 +CBr2Cl2 and CBrCl3 + CBr2Cl2 have been determined by means of X-ray powder diffraction and thermal analysis techniques from the low-temperature ordered phase to the liquid state. The isomorphism relationship between the stable orientationally disordered (OD) face-centered cubic (FCC) phases of CBrCl3 and CBr2Cl2 and the metastable OD FCC phase (monotropic behavior with respect to the OD rhombohedral stable phase) of CCl4 has been put into evidence throughout the continuous evolution of the lattice parameters and the existence of the two-phase equilibrium [FCC + L] for the whole range of composition in both two-component systems. This equilibrium interferes, for the CCl4 +CBr2Cl2 system, with a rhombohedral (R) plus liquid ([R + L]) equilibrium giving rise to a peritectic invariant.

Re-entrant nematic behavior in the 7OCB+9OCB mixtures: evidence for multiple nematic-smectic tricritical points.

The metastable phase diagram of the two-component system heptyloxycyanobiphenyl (7OCB)+nonyloxycyanobiphenyl (9OCB) was determined by means of modulated differential scanning calorimetry (MDSC) and optical microscopy measurements. It was experimentally established that the 7OCB+9OCB two-component system exhibits a monotropic re-entrant nematic behavior. A complete quantitative thermodynamic analysis, through Oonk's equal G analysis, was performed, allowing for the calculation of the monotropic re-entrant behavior and the prediction of two tricritical points, one of them experimentally accessible for the SmAd-to-N transition and the other non-experimentally accessible for the RN-to-SmAd transition.

Two-component system CCl4 + (CH3)3CBr: extrema in equilibria involving orientationally disordered phases.

Phase equilibria involving orientationally disordered (OD) and liquid phases of the two-component system between carbon tetrachloride (CCl4) and 2-methyl-2-bromomethane ((CH3)3CBr) have been determined by means of X-ray powder diffraction and thermal analysis techniques from 210 K up to the liquid state. The isomorphism relation between the OD stable face-centered cubic (FCC) phase of (CH3)3CBr and the metastable FCC phase of CCl4 has been demonstrated throughout the continuous evolution of the lattice parameters and the existence of the two-phase equilibrium [FCC + L] for the whole range of composition, despite the monotropy of the FCC phase for the CCl4 component with respect to its OD rhombohedral (R) stable phase. A continuous series of OD R mixed crystals is found, which confirms the R lattice symmetry of the OD phase II of (CH3)3CBr, for which the crystallographic results have been long-time misinterpreted.

Poly(gamma-glutamic acid) in aqueous solution: molecular dynamics simulations of 10- and 20-residue chains at different temperatures.

A series of molecular dynamics simulations in aqueous solution have been carried out in order to investigate the effects of the length of the peptide chain and the temperature on the helical conformation of un-ionized poly(gamma-D-glutamic acid). The results reveal that the helix is not stable for a small number of residues independent of the temperature. On the other hand, the temperature induces a conformational transition from the helical state to the random coil.

Dissociation constants of organophosphinic acid compounds.

The dissociation equilibria of di(2,4,4-trimethylpentyi) phosphinic acid, mono(2,4,4-trimethylpentyl) phosphinic acid, di(n-octyl)phosphinic acid and mono(n-octyl)phosphinic acid have been studied in ethanol-water mixtures by potentiometric titration at 25 degrees C. These data have been analysed both graphically numerically using the program LETAGROP-ZETA. The obtained pK(a) values have been correlated with the corresponding values in water, determined both indirectly by means of extraction measurements and by estimation using the suitable Hammett equation.