Cellulose high-crystalline

Each kind of CNs has a characteristic size, aspect ratio, morphology, crystallinity, crystal structure, and properties. Figure 13.7a shows the microstructure of the T-CNs prepared by acid hydrolyzation. T-CNs have a high-aspect-ratio [20-30 nm wide, 0.5-4 pm in length], are about 100% cellulose, highly crystalline [85-100%] and containing a high fraction of crystal structure [ 90%] [19, 28-30]. After sonicatlon, the T-CNs became much... 

The solubility of chitin is remarkably poorer than that of cellulose, because of the high crystallinity of chitin, supported by hydrogen bonds mainly through the acetamido group. Dimethylacetamide containing 5-9% liCl (DMAc/IiCl), and N-methyl-2-pyrrohdinone/LiCl are systems where chitin can be dissolved up to 5%. The main chain of chitin is rigid at room temperature, so that mesomorphic properties may be expected at a sufficiently high concentration [67,68]. 

It is more difficult to control the enzymatic processing of wool. Hence there is a greater danger of fibre damage compared with cellulosic fibres. Since cellulose is a highly crystalline... 

Physical Form. Natural cellulose is a highly crystalline, white solid with a molecular weight varying from 300,000 to greater than 1,000,000. 

Submicrofibril and triple-stranded left-hand helical microfibrils are found in tobacco primary cell wall and bacterial A. xylinum cellulose. We suspect from our results and the literature survey outlined in reference (1) that the triple stranded structures are prominent in the primary plant cell wall. The highly crystalline cellulose of plant and algae secondary cell wall appears by X-ray fiber diffraction (18,19) and TEM lattice imaging (20-23) to be largely crystalline arrays of planar straight chains of (l-4)-/3-D-glucan chains. 

Because cellulose consists of regions of high and relatively low crystallinity, processes to disrupt cellulose operate at two levels intercrystalline and intracrystalline. The conditions required to swell intercrystalline regions are relatively mild, whereas drastic processes are required to affect the high-crystalline regions. These two areas overlap strongly but need to be distinguished. 

The standard procedure by Saeman et al. (I) involves manual stirring of the polysaccharide with 72% H2S04, standing at 30°C, and secondary hydrolysis at 100° or 120°C in a steam autoclave. While certain resistant polysaccharides are still incompletely depolymerized, decomposition of the more sensitive monosaccharides formed cannot be avoided. An alternative method by using trifluoroacetic acid was applied successfully for plant cell wall polysaccharides by Albersheim et al. (2) and for dissolving pulps and hemicelluloses by Fengel et al. (3). Highly crystalline cellulose was not well dissolved and not completely hydrolyzed by CFsCOOH. 

Most experiments were performed with cotton or cotton linters as highly crystalline celluloses. Table I shows conditions leading to complete dissolution. A minimum amount of an acid which forms a cellulose ester (sulfuric or trifluoromethylsulfuric acid) (Entries 5 7-14) is necessary for the reaction. The dissolution is accelerated by a temperature increase (Entries 10-12 13, 14) and leads to water-soluble cellulose acetate hydrogensulfate. Whereas this primary hydrolysis can be achieved within 1-5 min, the deesterification and complete hydrolysis of the soluble cellulose derivative proved to be much more difficult. This is in contrast to the generally accepted view that the main resistance to the hydrolysis of cellulose lies in the crystalline nature or low accessibility determining the heterogeneous first step of the reaction. 

From these experiments we conclude that for pure and highly crystalline celluloses, a fast decomposition to glucose with minimum by-products is possible. Carbohydrates, containing other sugar units than glucose, are hydrolyzed accompanied by destruction of monosaccharides. The conditions for complete breakdown of cellulose are too severe for other polysaccharides. The problem of achieving optimum hydrolysis conditions is, of course, not a new one and must be solved for each polysaccharide or polysaccharide-containing material. The problem was not pursued further. 

Highly crystalline 0-chitin is obtained from pogonophore tubes and the spines of certain diatoms, and is analogous in both crystallinity and morphology to the cellulose obtained from Valonia cell walls. Intensity data (10) were obtained for 61 observed non-meridional reflections for a specimen of dispersed (sonicated) crystallites of pogonophore tube (Oligobrachia... 

Early on, before the existence of macromolecules had been recognized, the presence of highly crystalline structures had been suspected. Such structures were discovered when undercooling or when stretching cellulose and natural rubber. Later, it was found that a crystalline order also existed in synthetic macromolecular materials such as polyamides, polyethylenes, and polyvinyls. Because of the polymolecularity of macromolecular materials, a 100% degree of crystallization cannot be achieved. Hence, these polymers are referred to as semi-crystalline. It is common to assume that the semi-crystalline structures are formed by small regions of alignment or crystallites connected by random or amorphous polymer molecules. 

Because of the extraordinary supramolecular structure and exceptional product characteristics as high-molecular and high-crystalline cellulosics with a water content up to 99%, nanocelluloses require increasing attention. This review assembles the current knowledge in research, development, and application in the field of nanocelluloses through examples. The topics combine selected results on nanocelluloses from bacteria and wood as well as their use as technical membranes and composites with the first longtime study of cellulosics in the animal body for the development of medical devices such as artificial blood vessels, and the application of bacterial nanocellulose as animal wound dressings and cosmetic tissues. 

Regenerated cellulose Stable in most organic solvents (typically prepared from cellulose acetate as precursor) High crystalline content 4-9... 

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