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381.
Because of their exceptional reactivity, fluorine and fluorinated gases are of primary importance for the modification of the surface properties of materials. This study is devoted to surface treatment of thin nitrile gloves, made of carboxylated nitrile butadiene rubber latex, using either direct fluorination (10% F2gas diluted in N2) or plasma-enhanced fluorination in radio-frequency cold plasmas using fluorinated gases (CF4, CHF3). Mechanisms of fluorination of these co-elastomers have been proposed on the basis of the assignment of the different components of the XPS spectra. Several mechanisms have been observed depending on the fluorination conditions. Although the modification of nitrile gloves is already effective for fluorination reactions at room temperature, an important activation is observed for experiments carried out at 90°C. When the treatments are carried out at room temperature, a gradual fluorination occurs: in the case of 10% diluted F2 gas, monofluorinated C—F groups are the species most found at the surface and perfluoro groups CF
n
are present in lower amount. An addition reaction takes place at the
CH=CH
double bonds of the polybutadiene entities, leading to
CHF=CHF
units. Whatever the fluorination method, thermal activation yields a more massive fluorination of the surface that finally leads to perfluorinated
CF2
groups and terminal —CF3 groups. 相似文献
382.
383.
Corminboeuf O Quaranta L Renaud P Liu M Jasperse CP Sibi MP 《Chemistry (Weinheim an der Bergstrasse, Germany)》2003,9(1):28-35
Chiral Lewis acid catalysis has emerged as one of the premiere method to control stereochemistry. Much effort has gone into the design of superior ligands with increasing steric extension to shield distant reactive sites. We report here an alternative and complementary approach based on a "chiral relay". This strategy focuses on the improved design of achiral templates which may relay and amplify the stereochemistry from ligands. The essence of this strategy is that the chiral Lewis acid would effectively convert an achiral template into a chiral non-racemic template. This approach combines the advantages of enantioselective catalysis (substoichiometric amount of the chiral inducer) with the ones of chiral auxiliary control (efficient and predictable stereocontrol). 相似文献
384.
385.
386.
The stereoselectivity of radical reactions using cyclic iodohydrins and 2-alkoxy iodides was investigated on a simple model system obtained from indene (see 1a ? d ). The low level of stereoselectivity inherent to this type of systems could neither be overcome by using large protective group on the O-atom of 1c nor by complexation with Lewis acids. However, starting from the free alcohol 1c , it was possible to obtain very high selectivities (trans/cis > 100:1) by forming an aluminium alkoxide derivative upon treatment with methylaluminium bis[2,6-di(tert-butyl)-4-methylphenoxide] (MAD) before running the radical reaction. Despite the high steric demand of these complexes, the reactions gave satisfactory yields even for the formation of C? C bonds. 相似文献
387.
Eight new acetylenic compounds have been isolated and identified from the culture medium of the fungus Eutypa lata. Structural elucidation and biological activity are discussed. 相似文献
388.
Nonmagnetic particles in a carrier ferrofluid acquire an effective dipolar moment when placed in an external magnetic field. This fact leads them to form chains that will roughen due to Brownian motion when the magnetic field is decreased. We study this process through experiments, theory and simulations, three methods that agree on the scaling behavior over 5 orders of magnitude. The rms width goes initially as t(1/2), then as t(1/4) before it saturates. We show how these results complement existing results on polymer chains, and how the chain dynamics may be described by a recent non-Markovian formulation of anomalous diffusion. 相似文献
389.
We show that a polymer tip, integrated by free-radical photopolymerization at the end of a telecommunication optical fiber, allows high-efficiency coupling between the fiber and an infrared laser diode. A coupling efficiency of 70% (1.5-dB loss) was achieved. We obtained this result by controlling the radius of curvature of the tip, the origin of which is discussed in terms of the photochemical influence of oxygen during tip formation. The experimental data were found to be in agreement with results of electromagnetic calculations based on the finite-element method. 相似文献
390.
Tusche C Meyerheim HL Jedrecy N Renaud G Ernst A Henk J Bruno P Kirschner J 《Physical review letters》2005,95(17):176101
We present x-ray diffraction experiments and multiple-scattering calculations on the structure and transport properties of a Fe/MgO/Fe(001) magnetic tunnel junction (MTJ). Coherent growth of the top Fe electrode on the MgO spacer is observed only for Fe deposition in ambient oxygen atmosphere leading to a coherent and symmetric MTJ structure characterized by FeO layers at both interfaces. This goes in parallel with calculations indicating large positive tunnel magnetoresistance (TMR) values in such symmetric junctions. The results have important implications for achieving giant TMR values. 相似文献