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  • 1
    ISSN: 1432-0886
    Source: Springer Online Journal Archives 1860-2000
    Topics: Biology , Medicine
    Description / Table of Contents: Résumé Le Collembole Bilobella grassei est largement répandu en Europe méridionale. L'étude d'une population de Toulouse (France) montre un caryotype formé de 2 n = 12 chromosomes, le mâle étant hétérozygote de type XY. Les cellules d'un grand nombre de tissus sont polyploïdes. Seules les glandes salivaires contiennent des chromosomes polytènes de deux types: courts et mal structurés dans la région antérieure, longs et bien structurés en bandes dans les lobes postérieurs réduits à 5 à 7 cellules géantes. La description de ces chromosomes montre une grande variabilité dans une même population, variabilité due à l'âge des individus, à la fréquence des accidents chromosomiques, au fonctionnement du chromosome, peut-être aussi à l'amplification génétique et à l'autonomie de replication au niveau de certaines bandes. La structuration en bandes apparait comme un épiphénomène qui peut s'estomper localement sur les chromosomes longs (écharpes hétérochromatiques) ou presque totalement sur les chromosomes courts des lobes antérieurs.
    Notes: Abstract This study is restricted to a population of B. grassei from Toulouse (France) because of considerable differences between populations. The chromosome number is 2 n = 12 (♀ = 10 + XX, ♂ = 10 + XY). Although, as seen from cellular and nuclear sizes, numerous tissues are polyploid, only in salivary glands polytene chromosome are formed. They are of two types: In the anterior part of the glands the chromosomes are short and of irregular structure with poorly developed banding pattern; in the posterior parts of the glands they are long and distinctly banded. A detailed description of these latter chromosomes is given. Homologues are mainly unpaired with only short zones of occasional contact. The chromosomes show a considerable variability in the expression of the bands, heterochromatic regions, and Balbiani rings even within the population. These variations stem from structural rearragements and functional states of cells and individuals. The underlying factors are discussed.
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  • 2
    ISSN: 0016-6480
    Source: Elsevier Journal Backfiles on ScienceDirect 1907 - 2002
    Topics: Biology , Medicine
    Type of Medium: Electronic Resource
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  • 3
    ISSN: 0016-6480
    Source: Elsevier Journal Backfiles on ScienceDirect 1907 - 2002
    Topics: Biology , Medicine
    Type of Medium: Electronic Resource
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  • 4
    ISSN: 1432-1939
    Source: Springer Online Journal Archives 1860-2000
    Topics: Biology
    Description / Table of Contents: Résumé A l'aide d'une méthode statistique (Analyse factorielle des Correspondances de Benzécri) les auteurs étudient la répartition des microarthropodes dans des mousses recouvrant un rocher en sous-bois, et dans les milieux édaphiques avoisinants. L'étude porte sur 26 espèces d'Insectes Collemboles et 45 espèces d'Acariens Oribates. Par la comparaison des populations d'une série de relevés, ils montrent que le peuplement se comporte comme un “continuum” allant des couches édaphiques proprement dites aux mousses de type aérien en passant par les mousses sur le sol. Il existe aussi un gradient de peuplement des mousses sèches bien drainées, de la face Sud du rocher aux milieux édaphiques profonds de la face Nord. D'autre part l'affinité relative des espèces pour les différents relevés indique de façon très nette leur spécialisation écologique et l'importance des facteurs hygro-thermiques agissant sur chacune d'elles. A une variation continue des biotopes correspond une variation continue des peuplements.
    Notes: Summary A statistical method (Benzécri's Factorial Analysis of Correspondances) is used to study the distribution of microarthropods in mosses growing on a forest rock and in surrounding soil. 26 species of Insects (Collembola) and 45 species of Acari (Oribata) were investigated. Comparison of data shows a graduate variation of the population from the soil itself to the aerial mosses, passing by soil mosses. A population gradient is shown from dry and highly drained mosses of south face of the rock to the deep edaphic biotops of the north face. Relative specific affinity for the various samples makes conspicuous their ecologic specialization and the importance of humidity and temperature as distribution factors. A continuous variation of biotops is characterized by a continuous variation of populations.
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  • 5
    ISSN: 0021-8995
    Keywords: Chemistry ; Polymer and Materials Science
    Source: Wiley InterScience Backfile Collection 1832-2000
    Topics: Chemistry and Pharmacology , Mechanical Engineering, Materials Science, Production Engineering, Mining and Metallurgy, Traffic Engineering, Precision Mechanics , Physics
    Notes: The transesterification reaction in the molten state of ester groups of ethylene vinyl acetate (EVA) copolymers and ethylene acrylic ester (EMA) copolymers has been used to crosslink the chains of this polymeric system. The relative EVA copolymers (or EMA copolymers) concentration dependence of the network formation by co-crosslinking of EVA/EMA miscible blends has been assessed. EVA/EMA networks were characterized by swelling experiments, rheological measurements, and determination of the extent of the reaction through a chromatographic technique. All results can be compared in a master curve. The influence of the polydispersity and the microstructure of EVA and EMA samples was put in evidence. On the other hand, a scaling law (v2∼Mc-3/5) was observed in agreement with predictions of the Flory-Rhener expression. © 1994 John Wiley & Sons, Inc.
    Additional Material: 4 Ill.
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  • 6
    ISSN: 0032-3888
    Keywords: Chemistry ; Chemical Engineering
    Source: Wiley InterScience Backfile Collection 1832-2000
    Topics: Chemistry and Pharmacology , Mechanical Engineering, Materials Science, Production Engineering, Mining and Metallurgy, Traffic Engineering, Precision Mechanics , Physics
    Notes: The transesterification reaction catalyzed by dibutyltin oxide has been used to crosslink miscible blends (at about 200°C) of ethylene vinyl acetate (EVA) and ethylene methyl acrylate (EMA) copolymers. Then, microwave heating at 2.45 GHz with the fundamental TE01 mode was used for activation of this crosslinking reaction. The microwave treatment was carried out in a continuous process through a resonant cavity on line with a twin screw extruder equipped with a strip die or a circular die, which imposed the shape of the samples in the waveguide. Therefore, a shape factor due to the geometry of the samples and their orientation with respect to the electromagnetic field was defined. Last, this technique was applied to the microwave crosslinking of an EVA/EMA blend dispersed in a polypropylene matrix and consequently offers a new route to control the morphology of the polymer blends.
    Additional Material: 6 Ill.
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  • 7
    Electronic Resource
    Electronic Resource
    New York, NY [u.a.] : Wiley-Blackwell
    Journal of Applied Polymer Science 60 (1996), S. 1765-1771 
    ISSN: 0021-8995
    Keywords: Chemistry ; Polymer and Materials Science
    Source: Wiley InterScience Backfile Collection 1832-2000
    Topics: Chemistry and Pharmacology , Mechanical Engineering, Materials Science, Production Engineering, Mining and Metallurgy, Traffic Engineering, Precision Mechanics , Physics
    Notes: A new concept of conversion distribution applied to the continuous chemical modification and polymerization of polymers in a twin screw extruder was discussed. The kinetics and the residence time distribution function (RTD) are two undissociable parameters in reactive extrusion because their combination reflects the degree of homogeneity of the modified polymer at the die die exit of the extruder. A function G defining the distribution of the modification rate allowed us to evaluate the distribution of the extent of the conversion around the mean conversion. This study has underlined the predominant role of the kinetics on the structural homogeneity of the modified product. This new concept was developed, within a framework of reactive processing, from previous works on chemical modification of polymers and polycondensation in corotating twin screw extruder. © 1996 John Wiley & Sons, Inc.
    Additional Material: 8 Ill.
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  • 8
    ISSN: 0032-3888
    Keywords: Chemistry ; Chemical Engineering
    Source: Wiley InterScience Backfile Collection 1832-2000
    Topics: Chemistry and Pharmacology , Mechanical Engineering, Materials Science, Production Engineering, Mining and Metallurgy, Traffic Engineering, Precision Mechanics , Physics
    Notes: The stereoselective substitution of chlorine atoms was used to graft anthracene and phenyl groups onto the poly(vinyl chloride) (PVC) backbone as ultraviolet (UV) tracers for the assessment of the residence time distribution (R.T.D) of PVC chains during the extrusion of rigid and plasticized PVC. The residence time distribution obtained using these-grafted tracers was compared with that obtained using a free UV tracer (anthracene). The polymeric tracer had a narrower R.T.D and a shorter mean residence time. This difference of behavior between a free and grafted tracers was attributed to the compatibility of these tracers with the PVC.
    Additional Material: 8 Ill.
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  • 9
    ISSN: 0032-3888
    Keywords: Chemistry ; Chemical Engineering
    Source: Wiley InterScience Backfile Collection 1832-2000
    Topics: Chemistry and Pharmacology , Mechanical Engineering, Materials Science, Production Engineering, Mining and Metallurgy, Traffic Engineering, Precision Mechanics , Physics
    Notes: Transesterification may be used to crosslink a copolymer such as ethylene vinyl acetate with poly(ethylene acrylate-co-propylene) in the presence of dibutyltin oxide as a catalyst. A rheological study of the mechanism of this exchange reaction has been made: The kinetics of the crosslinking reaction was determined by studying the time and temperature dependence of the dynamic storage modulus G′(t, T)ω. Kinetic curves obtained for different reactive blends (EVA, PP/EVA, and PPf/EVA) allowed for the evaluation of the activation energy of the reaction and, thus, specified the appropriate parameters (temperature and time) for carrying out this reaction in a molten state.
    Additional Material: 9 Ill.
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  • 10
    ISSN: 0887-6266
    Keywords: polyethylene ; poly(butylene terephthalate) ; blend ; rheology ; Palierne's model ; morphology ; differential scanning calorimetry ; crystallization ; Physics ; Polymer and Materials Science
    Source: Wiley InterScience Backfile Collection 1832-2000
    Topics: Chemistry and Pharmacology , Physics
    Notes: Crystallization at high supercooling of polybutylene terephthalate (PBT) droplets dispersed in a molten polyethylene (PE) matrix was investigated through rheological and DSC experiments. The Palierne's emulsion model was used as a theoretical framework for studying the viscoelastic behavior of the blends in different ranges of temperature: on the one hand, when the two polymers are molten (T 〉 225°C) and on the other hand, when PBT droplets are at high supercooling in the molten PE matrix (130°C 〈 T 〈 205°C). From rheological experimental evidences it was shown that molten and solidified droplets coexist at high supercooling. The Palierne's model was then successfully adapted to take into account the three phases (molten PE, molten PBT droplets, and solidified PBT droplets). The evolution of the behavior with the temperature is consistent with the growing amount of crystallized droplets. Moreover, a calculation taking into account the droplets size distribution and the number of nuclei is introduced to explain the crystallization behavior of three different blend ratios.© 1998 John Wiley & Sons, Inc. J. Polym. Sci. B Polym. Phys. 36: 2573-2585, 1998
    Additional Material: 9 Ill.
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