Fibre structures

Cellulose fibres originate from the seed of the plant (cotton), its stem (bast fibres) or its leaves (sisal, alfa), having, as a consequence, different percentages of cellulose, lignin and hemicellulose. Basically, any plant may be used as a source of cellulose fibres, and it is a matter of historical development, availability and abundance that cotton, hemp and linen (flax) are today the most used cellulosic fibres. 

Cellulose, the base of all cellulosic fibres, is a complex composite material which structurally comprises three hierarchical levels  

On the molecular level, cellulose is a linear polymer of P-(l 4)-D-gluco-pyranose units in Ci conformation. The fully equatorial conformation of 3-linked glucopyranose residues stabilizes the chair structure, minimizing its flexibility. 

The spatial arrangement of elementary fibrils and larger aggregates determines the morphology and properties of a fibre and in maturing cotton the arrangement of these structural units differs, depending on the state of fibre development. 

The natural crystal is made up from meta-stable Cellulose I with all the cellulose strands parallel and no inter-sheet hydrogen bonding. The reason for this meta-stable crystalline form is that the conversion of glucose into cellulose takes place on the surface of an enzyme having about 30 active sites. As a consequence polymer molecules grow together in the same direction, producing a parallel faces crystal. 

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The nitration of cellulose is unusual in that uniform reaction takes place even though the fibrous structure is retained. This is explained by the fact that nitration is an equilibrium reaction unaffected by fibre structure, the extent of nitration being determined by the strength of the nitrating acid. 

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. 

Sayers, Z. Synchrotron X-Ray Scattering Studies of the Chromatin Fibre Structure, 145, 203-232 (1987). 

Formation of covalent bonds between dimethyl phosphite, glucose and amino groups in wool keratin, stabilising the loosened fibre structure. 

So the function of special optical fibres for sensing is to produce a sensitive response to changes in the fibre surroundings. Such requirements on optical hardware as durability to the analyte, transparency (i.e. minimum optical losses) in a wide spectral range and common availability should be pointed out. Related to the these requirements, the choice of the fibre material as well as of the fibre coating and fibre structure belong to fundamental tasks in the design of fibre-optic sensors. 

The simplest case of fibre structure is the step-index one, characterized by a constant circular refractive index profile in the core and polymer cladding of lower refractive index (Figure 3). The refractive indexes of the core and cladding depend on the materials used. The cores of these structures can be prepared from melts as well as from preforms without radial and azimutal variations of the refractive index. To obtain suitable mechanical... 

Figure 9. Sectorial S-fibre structure Figure 10. Spectral dependence of the attenuation of (microphoto), core diameter 30 pm. the s-fihre and PCS fibre in solution of methylene...

The kinetics of alkaline hydrolysis of a series of eleven vinylsulphone reactive dyes fixed on cellulose have been investigated at 50 °C and pH 11. Bimodal hydrolytic behaviour was observed under these conditions, the reaction rates being rapid at first but becoming slower as the concentration of fixed dye remaining gradually decreased. These results were attributed to differences in the degree of accessibility of the sites of reaction of the dyes within the fibre structure [87]. 

The formation of crosslinks in silk fibroin increases the tenacity and resistance to deformation of the fibres, as reflected in the initial modulus and the yield point. This protective effect conferred by fixation of the bifunctional dye Cl Reactive Red 194 was not shown by the monofunctional Orange 16, which is unable to form crosslinks. The loss in tenacity of undyed silk that is observed on treatment at 90 °C and pH 7 for 2 hours is attributable to lowering of the degree of polymerisation (DP) by hydrolysis of peptide bonds. The crosslinking action of bifunctional dyes tends to compensate for this loss in DP and provides an intermolecular network that helps to maintain the physical integrity of the fibre structure [124] ... 

Fig. 1. Schematic view of muscle fibre structure and fibre sub-units.

We were unable to explain the large difference in the tensile strengths of a single fibre and a wrapped fibre, nor the high tensile strength of the fibres treated with water glass. The latter was contrary to our expectations that the concentrated alkali in the water glass would affect the fibre structure. 

In order to control this disadvantageous effect, de-foamers and de-aeraters are added19. The defoamer s task is to control the foam formation due to the tensio-active substances, while the de-aerater removes gas bubbles, in solution and in the fibre structure. In this way, the contact surface between fabric and bleaching solution is kept maximal, which can only favour the rate of the process. By adding the de-aerater, lower quantities of tensio-active substances are needed, which helps to minimise foam formation. 

Dall Acqua et al.45 reported the development of conductive fibres made by cellulose-based fibres embedded with polypyrrole. Several efforts with cotton, viscose, cupro and lyonell have followed. The conductivity is directly related to the amount of polypyrrole, oxidant ratio and fibre structure with significant differences between viscose and lyonell. Polymerisation occurs uniformly inside the fibre bulk, by producing a coherent composite polypyrrole/cellulose. The mechanical and physical properties of cellulose fibres were not significantly modified as they are the best available45. 

Metallisation of fibres is not only a physical process determined by absorption capacity of the fibres for the metal and diffusion capacity of the metal in the fibre structure, but also depends on chemical parameters such as chemical structure of the fibres, presence of functional groups, reactivity of the fibre and the metal, oxidation state of the metal and the presence, necessity and reactivity of supporting chemicals (e.g. reducing agent). Therefore, it was necessary first to study metallisation at different types of fibres in order to investigate which structure is most useful for further research. In this respect, viscose, cotton, natural silk and polyacrylonitrile fibres were investigated because of their different structure and properties and their availability in the New Independent States of the former Soviet Union (Uzbekistan, Kazakhstan, Kyrgyzstan). 

During the production of cation-containing PAN fibres, it was found that absorption of Ni(II) in the fibre structure and adsorption at the surface of the fibre through formation of complexes with cyanide and carboxylic acid... 

Using fibres, the first effect also applies but, in addition, absorption of Ni(II) in the fibre structure (not only on the surface) occurs. It should also be noted that in the fibre, the functional groups are close to each other, which means that complex formation can occur much faster com-... 

Finally, a difference in reduction, absorption and adsorption rate of Ni(II) can be observed between freshly formed and thermolixated PAN fibre. Proof was not found for this effect, but probably the absorption capacity of the thermolixated fibre is reduced, resulting in a decrease in the absorption of Ni(II) in the PAN-fibre structure. 

Galvanisation of the metallised fibre will improve its properties as electrical conductor because of the formation of a continuous metallic coating at the surface of the fibre. In this respect, the seed layer formed during metallisation is crucial for a good adhesion between the metal layer and the PAN-fibre structure. 

Wirtz, S., P.R. Galle, and M.F. Neurath. 1999. Efficient gene delivery to the inflamed colon by local administration of recombinant adenoviruses with normal and modified fibre structure. Gut 44 800. 

Sumitomo Chemical Co. produces a fibre that is a mixture of alumina (85%) and silica (15%). The fibre structure consists of fine crystallites of spinel. Si02 serves to stabilize the spinel structure and prevents it from transforming to a-alumina [14], The flow diagram of this process is shown in Fig. 3.2. 

Industrial Applications of Natural Fibres Structure, Properties and Technical Applications... 

Transformation of this structure into a fibre structure by a mechanism called "micronecking"... 

Plastic deformation of the fibre structure Macroscopically, a sharp neck can generally be observed. 

Hearle, J. W. S., and Peters, R. H. (Eds.), Fibre Structure, The Textile Institute, Manchester, Butterworths, London, 1963. 

Scholie 4.6,15 Nous pouvons maintenant apporter les precisions suivantes a 4.5.5. Alors que dans les questions faisant intervenir exclusivement des limites projectives ( produits fibres, structures alg briques...), on peut, d aprks les resultats de l Exposk I et 4.5 5, identifier indiffkremment kune sous-... 

The reticular fibre structure of the sinusoids breaks apart, while liver cells attempt to remedy the parenchymatous defects from the area of the lobules. In chronic, alcohol-... 

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