Resumo em Inglês:

Lasing in cholesteric liquid crystals is an exciting new phenomenon. Cholesteric liquid crystals (CLC) are inherently periodic. In their ground state, they form a structure where the optic axis of the uniaxial material twists to form a helix and leads to one dimensional photonic band-gap behavior. If a fluorescent dye with emission matching the reflection band is added to these materials, the distributed feedback properties of the structure can result in laser emission when optically pumped. In this work, we show lasing in a CLC directly coupled to an optical ber. The laser emission of this device was characterized by pumping the input end of the optical fiber with a nanosecond Nd:YAG (532 nm) laser.

Resumo em Inglês:

The properties of lyotropic ferronematic liquid crystals based on new Ni, Cu and Zn ionic magnetic fluids are discussed. The efficiency of these new ferrofluids in the lyotropic nematic liquid crystal doping is verified and compared with the conventional surfacted ferro fluid lyotropic doping. It was observed that the structural characteristics of the lyotropics and ferro fluids determine the good formation of the ferronematics.

Resumo em Inglês:

In recent published papers it has been shown that the theory about the formation of magnetic walls in the neighborhoods of the Fréedericksz threshold is in profound disagreement with experiments. This finding leads to the development of a new theory for the onset of these structures in this region of magnetic fields. In this paper we present a review of these developments. The previous theory describing these unstable structures claims that the mode with the fastest initial growth will determine the observed properties of these patterns. But, just above the Fréedericksz threshold, there is a region where this leading mode vanishes and, therefore, a homogeneous bending of the director could be detected. This prediction was not confirmed by the experiment, and walls with very well defined wavelength were found. To explain these experimental facts it has been shown that the fastest growing mode can not be defined around the Fréedericksz threshold and, therefore, a new way to compute the observed periodicity must be formulated. The observed wall results from a sum of a continuum and non-sharp distribution of modes in which the null mode is at the center. This work is written in such away that the main conceptual developments can be easily generalized to systems presenting similar behavior.

Resumo em Inglês:

In this work it is analytically shown that any one-dimensional non-uniform planar configuration is unstable with relation to out-of-plane tilts. The fluctuations describing this escape into the third dimension are studied, and it is shown that the minimization of the bulk free energy makes non-null the mean values of the parameters describing these uctuations. The equations describing this phenomenon are proposed and solved in the small bending approximation.

Resumo em Inglês:

In this paper, the application of photoacoustic methods to study thermo-optical and spectroscopic properties of polymers is described. The Photoacoustic Spectroscopy, the Two-Beam Phase-Lag and also the so-called Open Photoacoustic Cell methods will be presented. The theoretical basis for quantitative measurements is discussed together with the advantages and limitations of the methods as compared with conventional measurements. Applications for spectroscopic and depth profile analysis and also for thermal properties measurements in several polymers samples are discussed.

Resumo em Inglês:

We have studied the in influence of the mean molecular length on the nature of the cholesteric (N*) – smectic A (A) liquid crystal phase transition for thermotropic compounds of cholesteryl myristate (C14), cholesteryl nonanoate (C9), and binary mixtures of C14 – C9 and cholesteryl caproate (C6) phase (dA), the mean molecular length (<img src="http:/img/fbpe/bjp/v32n2b/07eq01.gif">) in the N* domain, and the correlation length (epsilon), as well as changes along the N* – A phase transition, were determined from the X-ray diffraction data. The results show a decrease in A phase domain and an expansion of the N* – domain as the mean molecular length is diminished. Such effect induces a second order A – N* phase transition for a particular mixture of 63.1 mol% of C9 and 36.9 mol% of C6, evidenced by viscosity measurements in good agreement with previous density data.

Resumo em Inglês:

We analyze potentiometric and conductimetric measurements, simultaneously performed in several complex systems (Brönsted acid-base-type). The results show an excellent agreement between reported and obtained values of the dissociation constants of the involved equilibria. A similar analysis is proposed to account, in electric double layered magnetic fluids (EDL-MF), for the charging of the particle surface based on a proton transfer proccess at the interface with the bulk dispersion. This model allows to relate the pH dependence of the phase diagram to the variations of the nanoparticle surface charge density, which leads to a useful method to monitor the colloidal stability of EDL-MF.

Resumo em Inglês:

In this paper we present a review of twenty years of research on cholesteric liquid crystals at the Chemistry Institute of the University of São Paulo. The results on the interaction forces, responsible for the maintenance of the helical structure, are a function of the chemical variability obtained in these systems. The last results obtained using rheological techniques are presented, and research in this field was undertaken in order to understand the cholesterization process when different systems and inductors are involved.

Resumo em Inglês:

In this work we propose a photoacoustic model based on the Rosencwaig-Gersho theory to study dyed polymer samples. The model is similar to a two layer system and it is used to obtain the thermal diffusivity and the optical absorption coefficient of one side dyed polymer foils. The tested sample was the polyethylene terephthalate dyed with disperse dye, the Blue Samarom HGS. The results showed that the dye depth penetration can be accessed from the tted data and the dyeing process can be better controlled by using the retrieved data.

Resumo em Inglês:

In this work we used the Photoacoustic Spectroscopy (PAS) to evaluate the crosslinking of the copolymer from ethylene vinyl trimethoxysilane (EVS) and the grafted vinyl trimethoxysilane (VTS) on low density polyethylene (LDPE). PAS is used for sounding the overtone bands and stretching frequencies combinations of the groupings -Si-OH, =CH2, -CH3 and -CH2 -CH3, in the near and medium infrared range. The samples were typically prepared with 3%, 5% and 7% of catalyst and crosslinked in the temperatures of 70, 80 and 90ºC. Using the overtone bands of -OH groups the PAS shows the optimum combination, pointing a better crosslinking effect for 80ºC and in the range 5% to 7% of catalyst, typically.

Resumo em Inglês:

In this work we present experimental results that show a resonance effect in the isotropic phase of a lyotropic liquid crystal. The lyotropic mixture is made up of Potassium Laurate (KL), Decanol (DeOH) and water with three different relative concentrations: 2.62, 2.79 and 2.90 (C = [KL]/[DeOH], [KL] and [DeOH] in % molar). These mixtures have an isotropic (ISO) phase between two lamellar phases as a function of the temperature. From the transmittance of the sample (I) as a function of the frequency (f) itwas possible to evaluate a characteristic time (tau) in order of magnitude as a function of the temperature (T). The existence of a maximum in tau vs. T around 30ºC indicates the possibility of employing lyotropic liquid crystals in mechanical vibration sensors. We are proposing a mechanical vibration sensor able to measure low mechanical frequencies (f <= 200 Hz).

Resumo em Inglês:

In this work the growth and the characterization of red emitting triple-layer electroluminescent organic devices using vacuum deposited (Eu(TTA)3(TPPO)2) europium complex as emitting layer are described. The observed electroluminescence (EL) is characteristic of the Eu3+ emission. In this device the hole transport layer is obtained using a thin film of 1-(3-methylphenyl)- 1,2,3,4 tetrahydroquinoline-6-carboxyaldehyde-1,1'-diphenylhydrazone (MTCD), while the tris(8- hydroxyquinoline aluminum) (Alq3) is used as electron transport layer (ETL).

Resumo em Inglês:

A comparison is made between existing theories for self-assembling systems and experimental phase diagrams of complex systems made up of amphiphile/water/additives, with emphasis on phase transitions by changes in concentration. Evidence for different types of hexagonal (H) phases, in systems with detergent and lipid amphiphiles, are reviewed. It is shown that characteristics of the H phase and of the phase sequence (cubic phases with lipids and nematic phases with detergents) are revealed through the exponent defining the variation of the hexagonal parameter with amphiphile concentration. Emphasis is also given to results obtained in the ternary system sodium dodecyl (lauryl) sulfate / water / decanol, which exhibits the phase sequence isotropic (I) - H - nematic cylindrical (Nc) { [nematic biaxial (Nb)] - nematic discotic (Nd) - lamellar (L), with co-surfactant addition. Existing theories for self-assembling systems of rigid and exible rods predict the phase sequence I - (Nc) -H asa function of increased particle volume fraction upsilonp, with a triple point separating I - H and I - Nc - H phase sequences. Possible reasons for the non-trivial experimental I -H - Nc inversion in phase sequence are discussed. A complex path through the phase diagram is able to explain the experimental results in terms of changes in micellar growth and exibility induced by the co-surfactant. The role of the surfactant parameter, that expresses the curvature of the polar-apolar interfaces, explains much of the observed behavior, including the Nc - Nd transition with increase in decanol / amphiphile molar ratio.

Resumo em Inglês:

Synthetic manipulations were carried out with the aim of obtaining an advanced linear fragment for production of thermotropic liquid crystals. The chiral fragment synthesised was introduced in a tolanbenzoate mesogenic core by coupling of Sonogashira. The final chiral tolans showed smectic A and C and nematic liquid crystalline phases. The crystallographic structure of lactone from (-) menthone is presented.

Resumo em Inglês:

Molecular motors play key roles in areas ranging from biological transport to emerging nanotechnology. They produce current as a result of transfer of energy but not of momentum from a source; many molecular motor scenarios are based on the translational Brownian ratchet mechanism. Here we consider the mechanism of photoalignment of liquid crystals both in the bulk and at the surface by a photosensitive alignment layer. We show that the photoalignment is due to an orientational ratchet mechanism, where the azo-dye molecules, functionalized into a polymer alignment layer, when irradiated by polarized light act as the rotors of Brownian motors which reorient the bulk liquid crystal against an elastic restoring torque. Results of this photoalignment experiment can be obtained directly from a remote experiment set up at the Liquid Crystal Institute, via the WWW. In addition to experimental results, we present a detailed Fokker-Planck description of this system. We discuss the implementation and the results of numerical simulations, and compare these with the experimentally observed dynamics.

Resumo em Inglês:

On the basis and with the inclusion of results described in a previous paper [2], submitted for publication almost two years ago, we add some comments on very recent developments regarding both the design and synthesis of thermotropic nematogens of low-molecular weight, supposed biaxial in character, as well as puzzles, still unsolved, with methods of their unambiguous identification. In general, this update may demonstrate the complexity and problematic nature inherent in this topical subject of research indicated by a struggle over a very relevant, but very difficult matter.

Resumo em Inglês:

The influence of electron-phonon coupling in the emission spectra of self-assembly (SA) films of poly(p-phenylene vinylene) (PPV) is discussed. PPV is an important polymeric system because it exhibits strong luminescence effect. A semi-empirical model was used to analyze the photoluminescence spectra of PPV. This model assumes that defects along the molecule give rise to a distribution of conjugated segments of different lengths. In addition, it was considered that electronic transition was essentially coupled with three effective phonon modes with correspondent energies at 62, -139 and 192 meV, which were observed experimentally.

Resumo em Inglês:

In this work thermal lens spectrometry is applied to investigate the thermo-optical properties of complex fluids as a function of the temperature. The method is applied in poly(vinyl chloride), in polycarbonate and in lyotropic liquid crystals as the testing samples. The focus of the discussion will be in the temperature range where the phase transitions occur. The perspective of future studies in this area will be discussed.

Resumo em Inglês:

The influence of the ionic adsorption on the anchoring energy of a nematic liquid crystal sample is investigated. We determine the behavior of the anchoring energy as a function of the thickness of the sample, and as a function of the adsorption energy of ions. We show that the contribution to the anchoring energy, due to ionic adsorption, can be of the same order of magnitude of the bare anchoring strength. Our analysis generalizes similar calculations previously published by incorporating the effect of adsorbed charges on the potential and field profiles in the sample.

Resumo em Inglês:

The correlation between chemical structure and mesomorphic properties is one of the most important problems in liquid crystals science. 3,5-Disubstituted 1,2,4-oxadiazoles are very convenient model-compounds for studying the dependence of the LC properties on the molecular design. The transition temperatures and dielectric properties of 1,2,4-oxadiazoles depend significantly both on the position of the substituents with respect to the heterocycle and on their donor or acceptor features.

Resumo em Inglês:

In the last decade it has been experimentally found a periodic domain pattern arising in smectic C* liquid crystals in surface stabilized bookshelf geometry. Such a periodic texture appears after switching-off an external electric field, even in strong anchoring conditions. It has a static character and can be bidimensional, being dependent on both directions normal to the smectic planes and normal to the cell plates. In the present work an explanation to this phenomenon is proposed. According to our model in the antiferroelectric phase the biperiodic texture is a threshold phenomenon, appearing for values of the spontaneous polarization greater than a critical value, whereas in the ferroelectric phase this type of bidimensional instability is hindered.

Resumo em Inglês:

The physical meaning and the geometrical interpretation of causality implementation in classical field theories are discussed. Causality in field theory are kinematical constraints dynamically implemented via solutions of the field equations, but in a limit of zero-distance from the field sources part of these constraints carries a dynamical content that explains old problems of classical electro-dynamics with deep implications to the nature of physical interactions.

Resumo em Inglês:

A method for obtaining critical properties of physical systems is presented. Based on a recursive relation involving a physical parameter of the system, it drives the system spontaneously to the critical point, providing an efficient way to estimate critical properties. The method is illustrated for several ferromagnetic Ising systems on well-known Bravais lattices. A finite-size scaling approach is performed, by applying the method on lattices of different sizes. The efficiency of the method is confirmed by evaluating critical temperatures, as well as critical exponents, that turn up to be in good agreement with those available in the literature, with a relatively small computational effort.

Resumo em Inglês:

The consequences of the March 1991 storm on the Earth's radiation environment are now well known through many in-situ magnetospheric observations. Measurement from the CRRES spacecraft show the sudden formation of a new Helium ion structure deep within the magnetosphere. In that event, two additional new radiation belts were created, one containing large fluxes of 10 MeV electrons, and the other forming a structure containing 30 MeV protons. These sudden space weather phenomena appear to be due the conjunction of the two occurrences: (1) a solar energetic particle (SEP) event arriving in the vicinity of the Earth, and (2) a very intense magnetic storm within the Earth's magnetosphere. Attempts to reproduce the appearance of storm time helium belt by computational modeling are made using a three-dimensional charged particle code (Salammbo code) incorporating Helium ion charge exchange and post-event classical diffusion theory. We have extended the capabilities of the code to Helium ions, and we show that these physical phenomena can also lead to a build-up of a new Helium belt in the 30 MeV range. Detailed modeling predictions of the location of this belt and computed flux levels will be presented and discussed in the context of space weather phenomena in the Earth's magnetosphere.

Resumo em Inglês:

The enhancement of the resonant instability of right-hand polarized electromagnetic ion cyclotron waves by alpha particles for physical parameters corresponding to coronal mass ejections is studied. We focus on the effects of alpha thermal anisotropy and relative He++/H+ abundance on growth and absorption rates. The first parameter governs directly wave emission, while the second modifies also the wave speed and indirectly enhances the wave excitation.

Resumo em Inglês:

In a previous paper, we found the most general boundary conditions for the Aharonov -Bohm scattering of a Dirac particle. We found the resulting wave functions but we did not worry about delta normalizing them. As is well know, in practice, it is not easy to evaluate the diverging integrals occurring in the process. The purpose of this paper is to evaluate those integrals and present the resulting delta normalized eigenfunctions.

Resumo em Inglês:

A short summary of the theory of symmetric group and symmetric functions needed to follow the theory of Schur functions and plethysms is presented. One then defines plethysm, gives its properties and presents a procedure for its calculation. Finally, some aplications in atomic physics and nuclear structure are given.