Cellulose fibrous, crystallinity

Cellulose is a linear, nonbranched polysaccharide that can contain up to 10,000 P-d-(1-4) linked glucosyl units (Figure 2.8a) [56]. Cellulose has a flat ribbonlike conformation. Cellulose molecules can associate to form fibrous crystalline bundles that are highly insoluble and impermeable to water. 

The best-known renewable primary sources for natural fibers are wood and cotton. Both of these natural materials consist of basic fibrous units known as fibrils, the diameter of which is probably equivalent to that of the cellulose crystallites. Crystalline and amorphous regions alternate along the length of the fibrils. Several elementary fibrils may also be united in fibrillary structural units [84]. 

The entire spectrum of inorganic fibers can be divided into two classes, based on differences in the crystallinity of the solids (Ray, 1978). Synthetic fibers have been known as man-made mineral fibers (MMMF) and manmade vitreous fibers (MMVF). But fibrous materials can be approached or divided in other ways. For example, in the Concise Encyclopedia of Chemical Technology (1985) the entry for chemical fibers includes both manmade and natural polymers, with the discussion centering on carbon-based compounds such as acetates, acrylics, and cellulose. Fibers of other inorganic compounds were not mentioned in the encyclopedia under this entry, but silica glass fibers were described under the heading Optical Fibers. ... 

When crystalline cellulose I is treated with aqueous alkali solutions of sufficient strength, a process known as mercerization takes place. As a result of it, cellulose I is converted to cellulose II, the most stable or the four crystalline cellulose polymorphs. The conversion proceeds in the solid state, without apparent destruction or change in the fibrous morphology of the cellulose. As our diffraction analysis indicates, however, it is accompanied by a reversal of the chain packing polarity—from the parallel-chain cellulose I to the... 

PROP Fine white fibrous particles from treatment of bleached cellulose from wood or cotton. Insol in water and most org solvs. SYNS ABICEL n P-AMYLOSE ARBOCEL ARBOCEL BC 200 ARBOCELL B 600/30 AVICEL AVICEL 101 AVICEL 102 AVICEL PH 101 AVICEL PH 105 CELLEX MX a-CELLULOSE CELLULOSE 248 CELLULOSE (ACGIH.OSHA) CELLULOSE CRYSTALLINE CELUFI CEPO CEPO CFM CEPO S 20 CEPO S 40 CHROMEDIA CC 31 ... 

Cellulose is a fibrous, tough, water-insoluble, and crystalline substance. As a result of these characteristics, it is often converted to its derivatives in order to make it more useful. The most commonly used derivatives of cellulose are carbox-ymethylcellulose, methylcellulose, hydroxyethylceUulose, hydroxypropylcellulose, cellulose acetate, and cellulose xanthate (12). Among these derivatives, cellulose acetate and cellulose xanthate are cellulose esters, which are now widely used in the manufacturing of fibers, films, and in injection molding thermoplastics. 

Properties of Cellulose.—In its physical character cellulose possesses different properties such as fibrous, cellular or woody as has just been discussed. It is non-crystalline and probably colloidal. 

The principal commercial sources of chemical cellulose are purified cotton linters of about 99 per cent a-cellulose content and purified wood pulp of about 96 per cent cellulose content. Cellulose occurs in these materials as a fairly highly crystalline, high-molecular-weight polymer. It is in a fibrous form, which is insoluble in common reagents. Cellulose wUl not react to any significant d ee with acetic acid and will react with acetic anhydride without a catalyst only at very high temperatures, at which the cellulose is degraded. 

In the present study, the role of cellulose physical structure in alkaline reactions was investigated by comparing the alkaline degradation of highly crystalline (cellulose I) fibrous hydrocellulose with that of amorphous (noncrystalline) hydrocellulose. The amorphous substrate was taken as a cellulose model the reactivity of which would most closely approximate that of alkali-soluble cellulose. The availablity of such an approximation to the inherent reactivity of cellulose allowed evaluation of the effects of the more highly ordered structure of the fibrous hydrocellulose. 

Experimental Approach. The experimental study was a comparison of the alkaline degradations of fibrous and amorphous hydrocelluloses in oxygen-free 1.0 NaOH, at 60 and 80 C. The fibrous hydrocellulose was predominantly crystalline (cellulose I) and therefore served as a substrate which would undergo alkaline reactions with significant physical structure effects. In contrast, the amorphous hydrocellulose was noncrystalline (9,10). Thus, it was a substrate which would experience substantially less structural constraint during its alkaline reactions. 

In both the 60 and 80 C reactions, the fibrous hydrocellulose exhibited higher kpg values than the amorphous hydrocellulose (Table V). This appears to be due to the involvement of more molecules in crystalline domains of the fibrous substrate. The greater inhibition of chemical stopping by cellulose I than cellulose II domains may also have contributed to this effect by allowing more molecules in the fibrous hydrocellulose to peel to a point where the reducing endgroup would be inaccessible. 

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