Fibrous cellulosic structure

Transient Conduction in Fibrous Materials and Paper. In this section a study is presented which focusses on transient ionic currents in paper and fibrous materials. The phenomenon that we observed in the case of paper samples closely resembles the transient phenomena observed and reported for dissociating species in dielectric fluids 18). For the case of paper, the conductive medium is the water network associated with the fibrous cellulose structure. The ionic species can be transported through this structure by applying an electric field. 

Over time, a number of possibilities have been explored to produce cellulose-based nanometric fillers. In particular, microfibrillated cellulose (MFC) (Section 6.2.1) is formed by fibrous cellulose structures with the length of several tens of microns and generally a few tens of nanometers thick, as it consists of aggregates of microfibrils, natmally occurring as an effect of the hierarchical structure of cellulose in plants [3]. In other words, MFC is formed by long, flexible and entangled cellulose nanofibers, where both amorphous and crystalline phases are present [4]. 

As reported previously, the morphology of fibrous cellulose-polyvinyl copolymers, determined by electron microscopy, depends on the method of free-radical initiation of the copolymerization reaction, the experimental conditions during the reaction, and the type of vinyl monomer used. Variations in the shape of the fibrous copolymer cross section, in layering effects in the copolymer structure, and in location and distribution of the polyvinyl polymer within the fibrous structure were shown (1,2,7,29,52). 

An Interpretative review of the reactions initiated by macrocellulosic free radicals with vinyl monomers to yield block and graft copolymers of fibrous cellulose was made. Macrocellulosic radicals are usually formed by interactions with radiation or chemical redox systems. Important factors in these heterogeneous reactions are lifetimes and accessibilities of the radicals and interactions of solutions of monomer with fibrous cellulose. Changes in organochemical, macromolecular, and morphological structures in cellulosic fibers through formation of copolymers are made. 

Graft copolymerization reactions of fibrous cellulose with vinyl monomers were initiated at free radical sites formed on the cellulose molecule by interaction with radiation, by reaction with Ce4+ ions in acidic solution, or by H abstraction by OH radicals formed by reaction of Fe2+ ions with H202 in aqueous solution. The effects of experimental conditions on the location of these sites on the cellulose molecule and on the reactions were studied by ESR spectroscopy. The molecular weights of the grafted copolymers and the distribution of the polymers within the fibrous cellu-losic structure were determined. Some of the properties of the copolymers are discussed. 

In summary, the results which are presented in this section suggest that the charge transport of ions within paper and paperlike structures is essentially the same as that of the transport properties associated with aqueous electrolyte systems. Furthermore, the transient current behaviour which has been observed in these fibrous cellulosic systems show characteristics similar to the ionic transient current conduction exhibited in both dielectric fluids and aqueous ionic systems. 

Colloidal, aqueous dispersions of microcrystalline cellulose can be prepared by boiling cotton linters for 15 minutes with 2.5 N hydrochloric acid, followed by subjecting the resulting (level-off D.P.) cellulose to vigorous mechanical agitation. The microcrystals ( Avicel ), which are thereby freed from their fibrous packed structure, have been put to a variety of uses in the production of food gels and structural materials. ... 

The collagenous tissues of animals are in some respects similar to the cellulosic structures of plants. As a major fibrous constituent of skins,... 

The mechanism of action of glucanases is strongly influenced by the shape and structure of the active site. exo-Enzymes normally have a pocket or a tunnel-shaped active site, which forces the enzyme to attack a chain end of the polysaccharide substrate. " The active site of endo-enzymes has the shape of a cleft. e polymeric backbone (provided that the active site can accommodate the sequence of tire polysaccharide). Furthermore, some glucanases also contain a binding domain, which enables reversible sorption onto (and, thus, hydrolysis of) nondissolved polysaccharides, such as granular starch and fibrous cellulose. 

Organic fillers are of natural origin and abundantly available at low cost. Most of them are fibrous in structure and are composed largely of cellulose, with lesser amounts of lignin and other components. Sources of organic fillers are plentiful and renewable natural (cellulose) and wood flour, flock, nutshell flours, starch, corncobs, and rice hulls. They improve physical, chemical, and electrical properties of compounds. Organic fillers are also used with other types of additives to reduce the cost of the molding compound and to improve processability. 

The morphological or supermolecular structure of fibrous cellulose is easily altered by formation of graft copolymers. The distribution of copolymer within the fibrous cellulose primarily depends on... 

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. 

Both inorganic and organic sorbent materials are suitable for apphcation in partition PLC. The relevant material in this connection is cellulose. Celluloses are natural products with the universal chemical formula (CgHioOj),. These native celluloses have a fibrous structure and they need to be groimd and purified before use in PLC. Besides native cellulose, microcrystalline cellulose can also be used in partition PLC. In this case the cellulose has been recrystaUized and is rod-shaped. The specific surface area of celluloses is in the range of about 2 m /g. 

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