[Show abstract][Hide abstract] ABSTRACT: ChemInform is a weekly Abstracting Service, delivering concise information at a glance that was extracted from about 100 leading journals. To access a ChemInform Abstract of an article which was published elsewhere, please select a “Full Text” option. The original article is trackable via the “References” option.
No preview · Article · Jan 2010 · Journal of the Chemical Society Perkin Transactions 2
[Show abstract][Hide abstract] ABSTRACT: We report in this paper detailed measurements of the linear and nonlinear optical properties of thin films of P(VDF-TrFE) copolymers (with composition 70/30 mol%). The polymer films are cast from solutions using the spin coating technique and poled at saturation using very low-frequency a.c. fields. The refractive index (n = 1.42) is determined from measurements of the Brewster incidence, and the thickness from measurements of the optical path close to the normal incidence. Recording of the Maker fringes and calibration of the set-up with a quartz plate allow us to determine the second-harmonic coefficients: d33 = 4.3, d31 = 0.22 10−12 m/V and the optical dispersion n(2ω) − n(ω) = 0.0077 (ω is the frequency of the incident laser beam at 1060 nm). The spin-coated copolymer films are optically unaxial due to a preferential orientation of the chain axes parallel to the plane of the film, and measurement of their birefringence gives ne − no = −0.0083. Therefore, this feature allows us to find a direction of phase matching along which the first- and second-harmonic waves have an infinite coherence length. It is observed using an index-matching liquid.
[Show abstract][Hide abstract] ABSTRACT: Without application of an external electric field, the temperature dependence of the shear angle uxy, which is proportional to the spontaneous polarisation and therefore to the order parameter, has been measured by means of Bragg scattering of 412 keV gamma -radiation in KH2PO4 and RbH2PO4. Simultaneously, optical observations of the domain structure were performed. The shape of the rocking curve of KDP and RbDP are compared and discussed. The ferroelectric transition in RbDP is of second order in contrast to KDP which exhibits a first-order one. The modified Slater's theory provides for RbDP a good fit to the experimental data uxy(T) in the whole temperature range investigated. The fact that beta =0.5 near Tc can be understood as an effect of the dipolar interaction; so this system exhibits a cross over from a Slater behaviour to a dipolar behaviour.
No preview · Article · Feb 2001 · Journal of Physics C Solid State Physics
[Show abstract][Hide abstract] ABSTRACT: A phenomenological theory of the incommensurate phase transition which occurs close to the alpha - beta transition in quartz is presented. This theory assumes that the coupling between the soft optic mode of the alpha - beta transition and the acoustic modes play an important role in the formation of the modulated structure. According to the values of the phenomenological parameters, different kinds of incommensurate structures can form, in particular single-k or triple-k structures. The available experimental data concerning the quartz transition are discussed.
No preview · Article · Nov 2000 · Journal of Physics C Solid State Physics
[Show abstract][Hide abstract] ABSTRACT: Monolayers of short chiral alcohols at the water surface provide a simple model for chiral interactions between molecules.
This paper is focused on alcohols with two particular chain lengths, 2-tridecanol (2C13) and 2-tetradecanol (2C14). Thermodynamic
and structural parameters were measured, varying the ratio of left and right enantiomers within monolayers. The evolution
of melting temperatures and entropies is not consistent with the formation of a racemate. Grazing X-ray diffraction, revealing
the molecular stacking, shows clearly that there is no chiral separation. In these two compounds molecules spontaneously self-assemble
at the water surface as a solid solution. The chiral polar head-group is partially screened by the chain thermal disorder
of the rotator phase. 2-tridecanol exhibits a hexagonal rotator phase stable in time and independent of temperature. Experiments
performed close to the melting point show pre-transitional effects for mixtures different from 50/50. This broadening of the
peak can be related to defects in 2D crystals, phenomena already observed for the melting of some 3D systems.
Full-text · Article · Aug 2000 · The European Physical Journal E
[Show abstract][Hide abstract] ABSTRACT: We formulate a simple Landau type model describing macroscopic behavior recently discovered in new smectic phases composed of achiral bent-shaped molecules. Films of such smectics exhibit three types of ordering related to dipole polarization, molecular tilt, and chirality. However, due to specific third-order coupling of the order parameters, these three symmetry-breaking types are not independent and this fact leads to specific domain structures that are observed in experiments. (C) 2000 MAIK "Nauka/Interperiodica".
No preview · Article · Apr 2000 · Journal of Experimental and Theoretical Physics
[Show abstract][Hide abstract] ABSTRACT: We formulate a simple Landau type model describing macroscopic behaviour recently discovered new smectic phases composed of achiral bent-shaped molecules. Films of such smectics exhibit three types of ordering related to dipole polarization, molecular tilt, and chirality. However due to specific third order coupling of the order parameters these three types of symmetry-breaking are not independent ones, and this fact leads to specific domain structures really observed in experiments.
[Show abstract][Hide abstract] ABSTRACT: Structural relaxations near surfaces and interfaces are analysed in a simple, generic model with first-, second- and third-layer interactions. The relaxations have exponential envelopes with three types of structural distortion: (i) ferrodistortive, (ii) antiferrodistortive, and (iii) modulated (incommensurable). Their stability conditions in the field of control parameters and their relationship with structural phase transitions is derived. A tricritical point is found for three-layer interactions and a transition to an incommensurate phase. All phase transitions to the ferrodistortive phase are first-order in the model. The theory is applied to the analysis of lattice relations near internal interfaces in ionic polytypic materials (e.g. PbI2). Diffuse X-ray scattering and the shift of diffraction angles are the typical fingerprints for such relaxations. The relevant structure factors are calculated. There is tentative agreement between the calculated and observed diffraction profiles.
No preview · Article · Dec 1998 · Journal of Physics Condensed Matter
[Show abstract][Hide abstract] ABSTRACT: Biquadratic coupling between two order parameters Q1 Q2 can lead to transitions between a phase with only one of the two order parameters active (phase I or II) and a mixed phase III in which both order parameters are symmetry breaking. Domain walls for positive coupling energies are chiral under thermodynamic conditions close to the transition points I-III and II-III. Annihilation of chirality in the wall leads to wall widening and wall bifurcations. The widths of the walls are different for the two order parameters leading to greater lattice distortions on either side of the wall than in the centre of the wall.
No preview · Article · Dec 1998 · Journal of Physics Condensed Matter
[Show abstract][Hide abstract] ABSTRACT: The electrostatic quadrupolar interaction between spherical nematic droplets in an isotropic (and nonconducting) liquid is
calculated. It is found to have an anisotropic form U
5, where R is the distance between droplets, with repulsion for droplets having parallel orientation of the quadrupole moments and attraction
at oblique angles around the orthogonal orientation. In an external magnetic field aligning the orientations of the quadrupole
moments, a competition of the quadrupolar repulsion and van der Waals attraction (U
6) leads to a specific spatial organization of droplets which is in fact often reported in experimental observations (see the
monograph by P. Drzaic, Liquid Crystal Dispersions, World Scientific, Singapore (1995) and references cited therein).
[Show abstract][Hide abstract] ABSTRACT: In the lactam structure, C6H7NO, molecules are linked by hydrogen bonds between N-H and C=O [N ... O 2.885(2)Angstrom] to form ribbons parallel to the  2(1) axis. The shortest intermolecular distances outside the ribbons are relatively large [C ... O 3.535 (2) and C ... C 3.600 (2) Angstrom].
No preview · Article · Dec 1997 · Acta Crystallographica Section C Crystal Structure Communications
[Show abstract][Hide abstract] ABSTRACT: The structure factor of a crystalline monolayer on the surface of a liquid is calculated. It is shown that in the first approximation the finite rigidity of the monolayer maintains the power-law profile of quasi-Bragg peaks typical for two-dimensional systems but leads to a reduction of the X-ray scattering intensity. It is found that this effective Debye-Waller factor has very strong dependence on the length h of the chains composing the monolayer. Namely, it is proportional to exp(-h3/ξa2), where ξ is a chain persistence length and a is a characteristic atomic scale. We have also studied the spectrum of modes where the motion of the liquid is localised near the film. It turns out that for thick films there are three propagating sound-like modes: the transverse sound with a velocity which becomes zero if the wave vector q is directed along the normal to the film (z-axis); the conventional longitudinal sound with the almost isotropic velocity (determined mainly by the compressibility of the film); and the mode analogous to the second sound in smectics. For thin films the last mode transforms into the fast pure relaxational one. For tilted or hexatic layers there is also an orientational diffusion mode. This mode strongly influences the viscosity of the system. The fluctuation contribution in the viscosity coefficients is proportional to (T/Γ)In(qa), where Γ is the orientational diffusion coefficient. Transverse and longitudinal sound are transformed into overdamped modes with the following dispersion law ω ∝ (√3 - i)q4/3.
[Show abstract][Hide abstract] ABSTRACT: From surface tension measurements we extract the melting entropy Deltas2D of fatty-alcohol monolayers on water. Deltas2D is found to be 4kB/mol lower than in the bulk. Because of the role of the conformational entropy, the melting transition is discontinuous for long chains, but tends to be continuous for molecules shorter than 1-nonanol. For 1-decanol Deltas2D is sufficiently small to allow observation of critical fluctuations; the diffraction peak, measured with a high resolution synchrotron experiment, is described by a power-law singularity which broadens as the temperature approaches the melting point.
No preview · Article · Oct 1994 · Physical Review Letters
[Show abstract][Hide abstract] ABSTRACT: Owing to macroscopic electric fields arising from thermal fluctuations of a ferroelectric domain wall, the attraction energy of two parallel walls decays at large distances between the walls (h) as h-2 with a prefactor that is proportional to T and to a ratio of dielectric constants that is equal to unity for the isotropic medium. For proper ferroelectrics, and improper ones with a quadratic dependence of the spontaneous polarization on the order parameter, this law is valid for all h greater than the domain wall thickness (which is the minimum length of the problem). For improper ferroelectrics with cubic (or higher-order) dependence of the polarization on the order parameter, there exists a region with a logarithmic dependence of the interaction energy on h. This attraction, which proves to prevail over the Van der Waals one, leads to a discontinuity of the ferroelectric-incommensurate transition. Estimation of the period of the incommensurate phase at the transition provides a reasonable order of magnitude and shows that this period does not depend much on the crystal parameters other than the width of the domain wall. The interaction of the ferromagnetic and ferroelastic domain walls is qualitatively analogous to that of the ferroelectric ones. The role of quantum fluctuations becomes important at not very low temperatures. For nonferroelectric, nonferroelastic, nonferromagnetic domain walls the power-law fluctuation-induced attraction is shown to arise as well.
[Show abstract][Hide abstract] ABSTRACT: We argue that any commensurate-incommensurate phase transition in a crystalline media is discontinuous due to power-law attraction of the domain walls. The main origin of this attraction are the thermal fluctuations of the walls which are always accompanied, in a crystal, by at least one of the long-range fields: electric, magnetic, elastic.
[Show abstract][Hide abstract] ABSTRACT: Some analogies exist between parametrically driven dynamical system and anisotropic XY model. We discuss one aspect of this analogy : the existence of domain walls, their different structures and the existence of “wetting” like transitions. The work is done in a Landau-Ginzburg framework.