Exact-Exchange Kohn-Sham formalism applied to one-dimensional periodic electronic systems

The Exact-Exchange (EXX) Kohn-Sham formalism, which treats exchange interactions exactly within density-functional theory, is applied to one-dimensional periodic systems. The underlying implementation does not rely on specific symmetries of the considered system and can be applied to any kind of periodic structure in one to three dimensions. As a test system, $trans$-polyacetylene, both in form of an isolated chain and in the bulk geometry has been investigated. Within the EXX scheme, bandstructures and independent particle response functions are calculated and compared to experimental data as well as to data calculated by several other methods. Compared to results from the local-density approximation, the EXX method leads to an increased value for the band gap, in line with similar observations for three-dimensional semiconductors. An inclusion of correlation potentials within the local density approximation or generalized gradient approximations leads to only negligible effects in the bandstructure. The EXX band gaps are in good agreement with experimental data for bulk $trans$-polyacetylene. Packing effects of the chains in bulk $trans$-polyacetylene are found to lower the band gap by about 0.5 eV

Fibras químicas. Innovación y nuevos productos.

Se hace referencia a la producción mundial de fibras químicas en 1996 y a hs previsiones para el año 2005, así como a las necesidades de materias primas para fabricarlas. También son objeto de estudio aspectos como el reciclado, nuevos procesos de producción (polimero, hilatura-extrusión, procesado) y nuevas fibras de polímeros naturales (lyocel, quitina, quitosano) y de polímeros sintéticos (politrimetiltereftalato, polihidroxiácidos, poliamida 4.6, melamina-formaldehido, PBO, polímeros superabsorbentes).Reference is made to world production of chemical fibres in 1995, predictions for the year 2005 and also the taw materials necessary for this level of production. The study also examinates such aspects as recycling, new production proceses (polymers, extrusion, spinning, processing) and synthetic polymer fibres (polymethyl enterephthalate, polyhydroxy acids, polyamida 4.6, melamine-formaldehyde, PBO, superabsorbent polymers).Il s'agit de la production mondiale de fibres chimiques en 1996 et des prévisions pour l'an 2005 ainsi que des besoins en matieres premieres pour les fabriquer. L'étude porte aussi sur des aspects tels que: le recyclage, les nouveaux procédés de production (polymère, filature-extrusion, traitement), les nouvelles fibres de polymeres naturels (lyocel, chitine, chitosan) et des polymeres synthétiques (polyméthylenetérephtalate, poly-hydroxy-acides, polyamida 4.6, mélamine-formaldéhyde, PBO, polymeres superabsorbants).Peer Reviewe

Unusual conductance of polyyne-based molecular wires

We report a full self-consistent ab initio calculation of the current-voltage curve and the conductance of thiolate capped polyynes in contact with gold electrodes. We find the conductance of polyynes an order of magnitude larger compared with other conjugated oligomers. The reason lies in the position of the Fermi level deep in the HOMO related resonance. With the conductance weakly dependent on the applied bias and almost independent of the length of the molecular chain, polyynes appear as nearly perfect molecular wires.Comment: 4 pages, 5 figures, 3 table

Acción de los polímeros de carboxilato en el lavado sin fosfatos de tejido de algodón aprestado.

A partir de un tejido de algodón sin apresto y aprestado con la resina DMDHEU, se evaluó la eficiencia detersiva a 25°HF obtenida con la inclusión de los copolímeros de ácido acrílico/ácido maléico (sal de sodio) y zeolita como coadyuvante en la formulación detergente. Los indicados polímeros fueron estudiados variando el peso molecular y su proporción porcentual en peso en la cadena. Los tejidos fueron previamente ensuciados con impurezas del tipo ácido oleico/negro de humo. En las formulaciones detergentes, además de los polímeros mencionados, se utilizaron el DBSS, alcohol graso etoxilado con 6 mOE y zeolita A. La eficiencia detergente se obtuvo mediante una planificación experimental de acuerdo a un diseño factorial central compuesto bajo variación de dos factores: la concentración de polímero y zeolita.Untreated and DMDHEU resin-treated cotton fabric were used to evaluate detergency at 25° obtained by the inclusion of acrylic acid/maleic acid (sodium salt) co-polymers and zeolyte as a coagulant aid in the detergent formulation. These polymers were studied by varying the molecular weigth and its proportion in weight in the chain. The tissues were previously stained with oleic acid/carbon black type impurities. In the detergent formulations, in addition to the above-mentioned polymers, DBSS, fatty alcohol ethoxylated with 6mOE, and zeolyte were used. Detergency was obtained by means of experimental planning in accordance with a central factorial design composed under the variation of two factors: polymer and zeolyte concentration.À partir d'un tissu en coton sans apprêt et d'un tissu apprêté avec de la resine DMDHEU, nous avons évalué l'efficacité détersive a 25°HF obtenue en ajoutant des copolymeres d'acide acylique/acide maléique (sel de sodium) et de la zéolithe comme co-adjuvant dans la formulation détergente. Ces polymeres ont été étudiés en modifiant le poids moléculaire et leur proportion en pourcentage par rapport au poids de la chaîne. Les tissus ont été préalablement salis avec des impuretés du type acide oléique/noir de fumée. Dans les formulations détergentes, en plus des polymeres cités, nous avons utilisé le DBSS, un alcool gras éthoxylé avec 6 mOE et de la zéolithe A. Nous avons obtenu l'efficacité détergente a l'aide d'une planification expérimentale suivant une conception factorielle centrale composée, compte tenu de la variation de deux facteurs: la concentration en polymère et en zéolithe.Peer Reviewe

Los polímeros en formulaciones detergentes.

En este trabajo, atendiendo al desarrollo actual en la fabricación de polímeros con el objeto de ser utilizados en las formulaciones detergentes se presentan varias de sus estructuras químicas. Se indican algunas de las acciones de estos compuestos en el lavado, tales como: a) Prevención de la precipitación de sales inorgánicas. b) Coadyuvantes de detergencia. c) Prevención de la redeposición de la suciedad e d) Inhibición del sangrado de tejidos teñidos. Se indica, asimismo, la acción de estos compuestos frente al medio ambiente.Given today's progress in the production of polymers used in detergent formulations, this paper presents some of their chemical structures. Some of the actions of these compounds on washing are mentioned: such as a) Prevention of precipitation of inorganic salts b) Detergency aids; c) Prevention of soil redeposition and d) Inhibition of bleeding of dyed fabrics. The effect of these compounds versus environment is also indicated.Vu le développement actuel de la fabrication de polymères destinés aux formulations de détergents, ce travail présente plusieurs de leurs structures chimiques et décrit certaines actions de ces composés sur le lavage, telles que: a) prévention de la précipitation des sels inorganiques, b) co-adjuvants de détergence, c) prévention de la redéposition de saletés et d) inhibition du dégorgement des tissus teints, ainsi que l'effet de ces composés sur l'environnement.Peer Reviewe

Radiation effects on polymeric materials

It is important to study changes in properties of polymeres after irradiation with charged particles, with ultraviolet radiation, and with combinations of both. An apparatus for this purpose has been built at the NASA Langley Research Center. It consists of a chamber 9 inches in diameter and 9 inches high with a port for an electron gun, another port for a mass spectrometer, and a quartz window through which an ultraviolet lamp can be focused. The chamber, including the electron gun and the mass spectrometer, can be evacuated to a pressure of 10 to the 8th power torr. A sample placed in the chamber can be irradiated with electrons and ultraviolet radiation separately, sequentially, or simultaneously, while volatile products can be monitored during all irradiations with the mass spectrometer. The apparatus described above has been used to study three different polymer films: lexan; a polycarbonate; P1700, a polysulfone; and mylar, a polyethylene terephthalate. All three polymers had been studied extensively with both electrons and ultraviolet radiation separately, but not simultaneously. Also, volatile products had not been monitored during irradiation for the materials. A high electron dose rate of 530 Mrads/hr was used so that a sufficient concentration of volatile products would be formed to yield a reasonable mass spectrum