Cellulosic fibers, crystallinity

Mercerized cellulose fibers have improved luster and do not shrink further. One of the main reasons for mercerizing textiles is to improve their receptivity to dyes. This improvement may result more from the dismption of the crystalline regions rather than the partial conversion to a new crystal stmcture. A good example of the fundamental importance of the particular crystal form is the difference in rate of digestion by bacteria. Bacteria from cattle mmen rapidly digest Cellulose I but degrade Cellulose II very slowly (69). Thus aHomorphic form can be an important factor in biochemical reactions of cellulose as well as in some conventional chemical reactions. 

The primary driving forces behind investigation of new solvents include environmental concerns and the abiUty to form Hquid crystals in the new solvent systems. By analogy with Kevlar, a synthetic aromatic polyamide fiber, spinning from a Hquid crystalline solution should yield cellulose fibers with improved strength, as has been demonstrated in laboratory experiments. 

Cellulosic fibers contain a partly crystalline cellulose phase as the main constituent. The grafting reactions can usually occur only with the amorphous or disordered cellulose. Only highly swollen cellulose with an expanded lattice may react throughout the ordered regions. 

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. ... 

The elastomers exhibited rubber-like behavior. From an examination of electron photomicrographs of cross sections of the elastomers, the fibrillar structure of the cellulose fibers apparently formed a network, and poly (ethyl acrylate) was distributed uniformly among the fibrils. The rigid crystalline regions of the cellulose fibers apparently stabilized the amorphous, grafted poly (ethyl acrylate) to determine the mechanical properties of the elastomers (43, 44). For example, typical elastic recovery properties for these elastomers are shown in Table X. 

Major obstacles in the hydrolysis of cellulose are the interference of lignin (which cements cellulosic fibers together) and the highly ordered crystalline structure of cellulose. These obstacles necessitate a costly pretreatment step in which elementary cellulosic fibrils are exposed and separated. 

Finally, before carrying out the calculation, it is necessary to sketch the boundary between the crystalline peaks and the amorphous background. This line can be calculated if an amorphous sample has been used as a reference, such as for PET and cellulose fibers. If no amorphous standards are available, the background is drawn manually, following a line parallel to the theoretical curve (jt,5) (total scattering power summing up coherent and incoherent scattering). 

Corrections of the apparent crystallinity values of fibers materials have been carried out by taking into account a disorder parameter k, following Ruland s method. Peculiar care was taken about samples preparation (cutting and pelleting of fibers), data collection and reduction, which will be briefly described. Crystallinity and disorder parameter measurements have been performed on main textile fibers (polyester, polyamide, aramid, polypropylene, cellulosic fibers) and the results will be discussed comparatively, with those got by more conventional x-ray crystallinity determinations. The complementarities of these different approaches will be illustrated with several examples. For instance,... 

Molecular orientations of grafted celluloses can be changed to a small extent. Interactions of monomer solutions with cellulosic fibers to decrease crystallinities and to change lattice type of the products from those of unmodified fibers have been reported. X-ray diffraction methods are used to determine the changes (24). 

Cellulose is a partly crystalline and highly hydrogen-bonded substrate. Consequently it is much less accessible to grafting than starch (2). The present paper describes grafting experiments with the Mn -initiator applied to never-dried pulp fibers from wood and to fibers from cellulose derivatives of low degrees of substitution, i.e. cellulosic substrates known to be more accessible to chemical reactions than other cellulose fibers after drying. 

The degree of crystallinity of the fibers and the structure of the approximately 80% crystallinity, kraft with 60%, and regenerated cellulose fiber with around 50% show differing degrees of accessibility. A cotton-based paper does have longer life, under adverse conditions, than one made from rayon. However, given acid conditions, all the cellulose fibers finally do degrade and become brittle. 

Machell and Richards and Colbran and Davidson. . . assumed that degradation ceased not only when the reducing terminal unit of cellulose was converted into a metasaccharinic acid residue but also when the reactant end of the molecule reached inaccessible (crystalline) regions of the cellulose fiber... . The available data indicate that the less ordered portions of cellulose are the more reactive. Thus the rate of oxidation of... 

Flax. Flax is also a cellulosic fiber but has a greater degree of crystallinity than cotton. The morphology of flax is quite different from that of cotton. Flax fibers have a long cylindrical shape with a hollow core. The fibers range in length from V2 to 2V2 inches, with a diameter of 12 to 16 microns. Flax staple is comprised of bundles of individual fibers. Historians believe that flax was among the first fibers to be used as... 

Crystallinity long has been recognized as one of the characteristics of native cellulosic fibers. Indeed, native cellulose was observed to diffract x-rays, in the manner characteristic of three-dimensionally ordered molecular systems, before the hypothesis of polymeric structure had been proposed by Staudinger (1,2). 

Paper derives its principal properties from the cellulose fiber of which it is made. In comparison with many other fibers, cellulose fibers are not highly pliable or fatigue resistant. They are partly crystalline and partly amorphous and both portions are rather rigid when dry. The amorphous regions are relatively open, however, and they absorb moisture of humidity, which acts to flexibilize the fiber and the paper it forms. 

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