Nonlinear dielectric spectroscopy was used for studies of the reorientational dynamics of the pseudonematic domains in the isotropic phase of the mesogenic substance in the vicinity of the isotropic to nematic phase transition. The results were interpreted in the frame of the Landau-de Gennes theory.

International audienceA series of new bipyrimidine-based chromophores have been prepared presenting alkoxystyryl donor groups carrying aliphatic chains in the 3,4, 3,5 or 3,4,5 positions, connected to electron-accepting 2,2′-bipyrimidine cores. Their linear and nonlinear optical properties were investigated as well as their mesomorphic properties by various techniques (light-transmission measurements, polarized-light optical microscopy, and differential scanning calorimetry measurements). Only two derivatives, BPM-3,4-C12 and BPM-3,4-C16, were found to exhibit liquid-crystalline behavior with the formation of lamella-columnar phases and/or hexagonal columnar phases over large temperature ranges. Small-angle X-ray scattering analysis allowed proposing a stacking model inside the mesophase in which the molecules are interdigitated alternatively along their long axis and their short axis to form columns. Dielectric measurements were performed as a function of the temperature, showing the centrosymmetric nature of the mesophases. Large quadratic hyperpolarizabilities have been measured for the individual mesogens in solution by using hyper-Rayleigh scattering. These chromophores exhibit also cubic nonlinear optical properties, revealing relatively large two-photon absorption cross sections. The nonlinear optical properties in the liquid crystalline state of compounds BPM-3,4-C12 and BPM-3,4-C16 have been studied by wide-field second-harmonic generation and two-photon fluorescence microscopy, confirming centrosymmetry for these achiral mesogens and the excellent third-order nonlinearity for multiphoton imaging

Nonlinear dielectric spectroscopy is used to study the self-association of 2-pyrrolidinone in benzene solutions up to 0.9 mol/L at 25 °C. The nonlinear dielectric spectra below 100 MHz are recorded using an LC-resonant circuit, while above 100 MHz a partial coaxial resonant cavity is used. Both the kinetic and thermodynamic data are very well described by a dimerization mechanism with an equilibrium constant for dimerization K(ε) = 2676 exp[−17.1(ε − 1)/(2ε + 1)] M-1, depending on the permittivity of the solution according to the reaction field theory. At infinite dilution in benzene, K = 53 M-1 and the rate of association is diffusion-controlled with k 1 = 0.87 × 1010 M-1 s-1. The description proposed in the present work is also confirmed by the analysis of (static) linear dielectric data up to 0.9 mol/L and qualitatively predicts the behavior at higher concentrations. The present study has shown that even for high concentrations there is no need to introduce oligomers such as trimers or tetramers in the association model. Both linear and nonlinear dielectric data lead to a dipole moment value of 3.96 D for the monomer and 2.30 D for the dimer. The latter is believed to undergo a rapid interconversion between a closed and a (partially) open form explaining the relatively high dipole moment. As an overall conclusion, it can be stated that the dynamic measurements have proven to be a much more sensitive tool to discriminate between different association models than thermodynamic results.

The nonlinear dielectric spectra of 4-(trans-4'-n-hexylcyclohexyl)isothiocyanatobenzene (C6H13CyHx Ph NCS, 6CHBT) in benzene solutions were recorded in the whole concentration range up to an appearance of the two-phase isotropic and nematic region (0.82 mole fraction of 6CHBT, at 25 degrees C). Two electric fields: (i) the field of high strength (110 kV/m) and low frequency (85 Hz) causing the dielectric nonlinearity, and (ii) the probing field of low strength and high frequency (100 kHz-3 GHz), were used. At about of 0.2 mole fraction of 6CHBT the nonlinear dielectric increment becomes positive. With increasing of 6CHBT concentration the dynamics of the increment shows a critical-like behavior, which can be interpreted in terms of the reorientation of the growing pseudonematic domains in the solutions studied.

No abstract

No abstract

Negative Poisson’s ratio materials (called auxetics) reshape our centuries-long understanding of the elastic properties of materials. Their vast set of potential applications drives us to search for auxetic properties in real systems and to create new materials with those properties. One of the ways to achieve the latter is to modify the elastic properties of existing materials. Studying the impact of inclusions in a crystalline lattice on macroscopic elastic properties is one of such possibilities. This article presents computer studies of elastic properties of f.c.c. hard sphere crystals with structural modifications. The studies were performed with numerical methods, using Monte Carlo simulations. Inclusions take the form of periodic arrays of nanochannels filled by hard spheres of another diameter. The resulting system is made up of two types of particles that differ in size. Two different layouts of mutually orthogonal nanochannels are considered. It is shown that with careful choice of inclusions, not only can one impact elastic properties by eliminating auxetic properties while maintaining the effective cubic symmetry, but also one can control the anisotropy of the cubic system.

No abstract