Segregated tunneling-percolation model for transport nonuniversality

We propose a theory of the origin of transport nonuniversality in disordered insulating-conducting compounds based on the interplay between microstructure and tunneling processes between metallic grains dispersed in the insulating host. We show that if the metallic phase is arranged in quasi-one dimensional chains of conducting grains, then the distribution function of the chain conductivities g has a power-law divergence for g -> 0 leading to nonuniversal values of the transport critical exponent t. We evaluate the critical exponent t by Monte Carlo calculations on a cubic lattice and show that our model can describe universal as well nonuniversal behavior of transport depending on the value of few microstructural parameters. Such segregated tunneling-percolation model can describe the microstructure of a quite vast class of materials known as thick-film resistors which display universal or nonuniversal values of t depending on the composition.Comment: 8 pages, 5 figures (Phys. Rev. B - 1 August 2003)(fig1 replaced

Molecular structure and the occurrence of smectic A and smectic c phases

The mesomorphic properties of various terminally alkyl and/or alkoxy substituted azobenzenes are investigated. The non-polar dialkylazobenzenes have nematic and smectic A phases. For each alkyl group that is replaced by an alkoxy group (thus introducing an outboard dipole moment) the tendency to form a smectic C phase is increased. These results can be rationalized in terms of a dipole model of the smectic C phase, and do not support the idea that this phase occurs mainly because of steric interactions between zig-zag shaped molecules. In the case of only one outboard dipole moment there are two possibilities for a model with dipole interaction, one of which has ferroelectric smectic layers. This situation could possibly provide a model for the smectic F phase.On étudie les propriétés mésomorphes de divers azobenzènes substitués en para par des chaînes n-alkyles ou n-alkoxy. Les dialkylazobenzènes non polaires présentent des phases nématiques et smectiques A. Quand on remplace une chaîne alkyl par une chaîne alkoxy (avec création d'un moment dipolaire terminal), il y a augmentation de la tendance à l'apparition d'une phase smectique C. Ces résultats peuvent être interprétés par un modèle dipolaire de la phase smectique C et ne confirment pas l'hypothèse que cette phase résulte principalement d'interactions spatiales entre des molécules en configuration zig-zag. Dans le cas d'un seul moment dipolaire terminal, l'une des deux possibilités de modèle à interaction dipolaire prévoit des couches smectiques ferroélectriques. Cette situation pourrait éventuellement fournir un modèle de la phase smectique F

A nuclear magnetic resonance study of acetone in various solvents

Applied Science

Shear-induced Smectic Ordering and Crystallisation of Isotactic Polypropylene

Shear-induced smectic ordering and crystallisation of isotactic polypropylene (iPP) has been studied with in-situ small- and wide-angle X-ray scattering. Shear-induced smectic bundles with a periodicity of about 4 nm have been observed at temperatures below as well as above the melting point. This applies to iPP of different molecular weight and from different sources. An increase in the average molecular weight leads to a larger periodicity of the smectic layers. The smectic layers assemble in a fibrillar morphology with a length and a width up to 200 and 10 µm, respectively. After crystallisation, the smectic bundles show upon heating higher melting temperatures than their crystalline counterparts. In agreement with this behaviour, the correlation length along the smectic layer normal is of the order of tens of nanometers, much larger than the crystal thickness. We present an anisotropic drop model of smectic domains forming a conserved system in which the smectic layers can rotate. On the basis of this model we can explain the relative orientation of the smectic layers, the crystalline lamellae and the long axis of the drop, as well as the reversibility of the smectic periodicity during cooling and heating. In the supercooled melt, the smectic ordering is followed by crystallisation; during this process crystals grow epitaxially on the surface of the smectic bundles. This leads to a new picture of the shish-kebab structure in which smectic bundles rather than extended-chain crystals play the role of the shish. The crystallisation process of the smectic regions themselves indicates that the mesophase previously reported in fast-quenched iPP, is a metastable state formed during the transition from the high-temperature smectic phase to a crystal. Moreover smectic domains rather than crystallites form the nuclei for crystallisation of the phase. The high-temperature smectic phase presents an ideal model system to study the coupling between density and conformational ordering under shear flow. Our results support a nucleation and growth process for polymer crystallisation, in which smectic bundles or other mesophases are the primary nuclei

Flow-induced mesophases in crystallizable polymers

In this chapter we review flow-induced mesophases in some crystalline polymers with rather different characteristics: polyethylene terephthalate, isotactic polypropylene and polydiethylsiloxane. Because these three polymers are representatives of flexible and semi-rigid-chain polymers, the occurrence of a mesophase suggests a similar behaviour for many other crystalline polymers. The question may be not so much whether crystalline polymers show mesophase behaviour, but rather under which conditions it can be obtained. A flow field turns out to be an effective tool to unravel structural information, even if hidden in quiescent conditions. The emergence of a mesophase affects strongly any subsequent crystallization. As a result of templating and nucleating effects, a mesophase may accelerate the crystallization rate, lead to different crystal modifications, change the morphology and guide the orientation of the crystals. On the basis of mesophase formation new insights into flow-induced polymer crystallization emerge, even though the nature of the mesophase ordering is still not fully understood. A re-evaluation of models for the polymer melt (random coil and folded-chain fringed-micellar grains) is proposed to understand the combined experimental observations on mesophase ordering and crystallization

Flow-measurements of the viscosity coefficients of two nematic liquid crystalline azoxybenzenes

To determine the Miesowicz viscosity coefficients a shear-flow set-up has been built, which allows the measurement of η1, η2, η3 and η12. These coefficients are determined by pressing the liquid crystal through a rectangular capillary which forms part of a thermostated set-up placed between the poles of a magnet in order to control the director n. In addition, the flow-alignment angle θ 0 is determined from the change in optical path difference between the ordinary and extraordinary components of polarized light, comparing the situation with n aligned along the flow-direction and with flow-alignment, respectively. Results are given for the complete set of viscosity coefficients of p-methoxy-p'-butylazoxybenzene and p, p'-dibutylazoxybenzene. The results show good agreement with visco-elastic ratios found with light-scattering techniques. Within the experimental accuracy the Onsager-Parodi relation is well fulfilled.On a construit un appareil pour mesurer les viscosités de cisaillement. η1, η2, η3 et η12 de Miesowicz. Celles-ci sont déterminées lors de l'ecoulement d'un liquide nématique dans un capillaire rectangulaire qui fait partie d'un appareil thermostatisé. L'ensemble est situé entre les pôles d'un électro-aimant pour contrôler le directeur n. De plus, on a déterminé l'angle d'alignement par écoulement en mesurant la variation de la biréfringence dans l'échantillon entre la situation où n est fixé parallèle à l'écoulement et celle où il est aligné par l'écoulement. On donne des résultats pour des ensembles complets de coefficients de viscosité de p-methoxy-p'-butylazoxybenzène et de p, p'-dibutylazoxybenzene. Les résultats sont bien en accord avec des rapports visco-élastiques obtenus par des techniques de diffusion de la lumière. La relation d'Onsager-Parodi est bien vérifiée compte tenu de la précision expérimentale

Flow-induced mesophases in crystallizable polymers

In this chapter we review flow-induced mesophases in some crystalline polymers with rather different characteristics: polyethylene terephthalate, isotactic polypropylene and polydiethylsiloxane. Because these three polymers are representatives of flexible and semi-rigid-chain polymers, the occurrence of a mesophase suggests a similar behaviour for many other crystalline polymers. The question may be not so much whether crystalline polymers show mesophase behaviour, but rather under which conditions it can be obtained. A flow field turns out to be an effective tool to unravel structural information, even if hidden in quiescent conditions. The emergence of a mesophase affects strongly any subsequent crystallization. As a result of templating and nucleating effects, a mesophase may accelerate the crystallization rate, lead to different crystal modifications, change the morphology and guide the orientation of the crystals. On the basis of mesophase formation new insights into flow-induced polymer crystallization emerge, even though the nature of the mesophase ordering is still not fully understood. A re-evaluation of models for the polymer melt (random coil and folded-chain fringed-micellar grains) is proposed to understand the combined experimental observations on mesophase ordering and crystallization