Fibre heterogeneous

The methods available today may be considered under two headings, homogeneous acetylation, in which the acetylated cellulose dissolves into a solvent as it is formed, and the heterogeneous technique, in which the fibre structure is retained. 

Pulsed deuteron NMR is described, which has recently been developed to become a powerftd tool for studying molectdar order and dynamics in solid polymers. In drawn fibres the complete orientational distribution function for the polymer chains can be determined from the analysis of deuteron NMR line shapes. By analyzing the line shapes of 2H absorption spectra and spectra obtained via solid echo and spin alignment, respectively, both type and timescale of rotational motions can be determined over an extraordinary wide range of characteristic frequencies, approximately 10 MHz to 1 Hz. In addition, motional heterogeneities can be detected and the resulting distribution of correlation times can directly be determined. 

Heterogeneous liquid membrane electrodes. This type, which has become of considerable practical importance, consists of a liquid ion-exchange layer or a complex-forming layer within a hydrophobic porous membrane of plastic (PTFE, PVC, etc.), sintered glass or filtering textile (glass-fibre, etc.). The construction of such an electrode is depicted in Fig. 2.12. 

Asakura, T., and Yao, J. M. (2002). C-13 CP/MAS NMR study on structural heterogeneity in Bombyx mori silk fibre and their generation by stretching. Protein Sci. 11, 2706-2713. 

Asakura, T., Yao, J. M., Yamane, T., Umemura, K., and Ultrich, A. S. (2002b). Heterogeneous structure of silk fibres from Bombyx mod resolved by C-13 solid-state NMR spectroscopy./. Am. Chem. Soc. 124, 8794-8795. 

The first reported attempts of what was then called "absolute or total asymmetric synthesis" with chiral solid catalysts used nature (naturally ) both as a model and as a challenge. Hypotheses of the origin of chirality on earth and early ideas on the nature of enzymes strongly influenced this period [15]. Two directions were tried First, chiral solids such as quartz and natural fibres were used as supports for metallic catalysts and second, existing heterogeneous catalysts were modified by the addition of naturally occuring chiral molecules. Both approaches were successful and even if the optical yields were, with few exceptions, very low or not even determined quantitatively the basic feasibility of heterogeneous enantioselective catalysis was established. 

Experimental results. Some carbon fibre specimens reveal several orders of 001 particularly in electron diffraction patterns Figure 15 shows a plot of (3 against l2, equation (3), for an electron diffraction pattern from the skin region of a high-modulus material. L(oOl)> usually referred to as Lc, is 3.5 nm and a = 2%. A full description of electron-diffraction analysis in several similarly heterogeneous carbon fibres has been published (23). Figure 15 also includes a plot from the 001 electron diffraction profiles of a carbon whisker, an exceptionally perfect graphite material. This specimen, with an Lc of 10 nm, has zero distortion, and represents the only case where we have found no distortion in a fibrous specimen. 

Christl, I., Knicker, H., Koegel-Knabner, I., and Kretzschmar,R. (2000). Chemical heterogeneity of humic substances Characterization of size fractions obtained by hollow-fibre ultrafiltration. Euro J. Soil Sci. 51,617-625. 

O-nitration of cellulose with mixtures of acids is a widely applied type of esterification reaction in a heterogeneous system. One of the factors influencing the reaction rate here is the rate of diffusion of the acid into the fibre. 

The topography of cross-sectional surfaces produced by cutting and polishing usually reflects the differences in mechanical properties of its heterogeneities. In general, this is due to the fact that the rate of material removal of hard components like fibres and metals is smaller than that of soft polymeric components. Easily deformable components like rubber inclusions, however, tend to be more compressed rather than abraded and their surfaces are quite often above that of the surrounding material of the final cross-section. 

Exemplified by paper and board materials with a heterogeneous, open network of fibres with large air spaces or channels. Low molecular weight substances in particular can migrate rather rapidly with little hindrance offered. 

Thermal residual stresses are inherent to fibre reinforced composites due to the heterogeneity of the thermo-mechanical properties of their two constituents. Such stresses build up when composite structures are cooled down from the processing temperature to the test temperature. Residual stresses will be present on both a fibre-matrix scale (micro-scale), and on a ply-to-ply scale (macro-scale) in laminates built up from layers with different orientations. It is recognised that these stresses should be taken into account in any stress analysis. 

Low-volume (under 11) but very high density (1-2 x 10 mH) heterogeneous systems (e.g. hollow-fibre bioreactors). 

Heterogeneous group of products including thatch, bamboo, rattan, wrapping leaves and fibres... 

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