Crystallinity cotton fiber

Cellulose pulp Cotton fiber Crystalline cellulose Hydroxycellulose Powdered cellulose... 

Synonyms a-Amylose a-Cellulose Cellulose crystalline Cellulose powder Cellulose, powdered Cellulose pulp Cotton fiber Crystalline cellulose Hydroxycellu-lose Powdered cellulose Pyrocellulose Wood pulp, bleached Definition Natural polysaccharide derived from plant fibers Properties Colorless to wh, solid, odorless si, sol. in sodium hydroxide sol n. insol. 

An important chemical finishing process for cotton fabrics is that of mercerization, which improves strength, luster, and dye receptivity. Mercerization iavolves brief exposure of the fabric under tension to concentrated (20—25 wt %) NaOH solution (14). In this treatment, the cotton fibers become more circular ia cross-section and smoother ia surface appearance, which iacreases their luster. At the molecular level, mercerization causes a decrease ia the degree of crystallinity and a transformation of the cellulose crystal form. These fine stmctural changes iacrease the moisture and dye absorption properties of the fiber. Biopolishing is a relatively new treatment of cotton fabrics, involving ceUulase enzymes, to produce special surface effects (15). 

Nelson and Conrad29 have recently confirmed the viscosity behavior observed by Davidson26 and Nickerson and Habrle27 and have drawn a similar conclusion, namely, that after the rapid destruction of about 2 % of the intercrystalline network, hydrolysis occurs mainly on lateral crystallite surfaces. They also show that the apparent degree of crystallinity is reduced by fine grinding of cotton fibers. 

A few examples may serve to illustrate the implications of relative crystallinity and especially of intercrystalline reactivity. Two independent investigations27 29 have shown that cotton fibers are reduced to a... 

Raman spectroscopic measurement was also used to explain the improved thermal stability of nontreated (NoM-C) and silylated cotton fibers (Sil-C).20 In the region 3200-3500 cm-1, peaks become more intense and narrower, demonstrating an apparent increase in the -OH group concentration in the ordered phase and an increase in the crystallinity. This phenomenon was explained by the fact that the silylation reaction cannot take place in the crystalline phase. The increase in crystallinity is the result of easier segmental motion, which is facilitated by the reduction of secondary chemical bonds in the amorphous phase. These structural changes explain the higher thermal stability, since the OH groups in the amorphous phase are more sensitive to thermal dehydration. 

Fig. 9-1. Effect of crystallinity and hydrogen bonding on the acetylation of cotton fibers (Demint and Hoffpauir, 1957). (a) Original fibers, (b) Crystallinity has been destroyed by ethylene amine treatment. Subsequent drying has resulted in the formation of hydrogen bonds, (c) Crystallinity has been destroyed as above but because drying has been omitted no hydrogen bonds have been formed.

Some workers refer to cotton lint (the normal fibers) as cellulose to distinguish it from seed cotton (fiber still on the seed) or the entire plant. Herein, the word cellulose has only the strict chemical meaning linear p-(1 4)-D-glucan. In the cell wall, cellulose occurs in small, crystalline microfibrils that are arranged in multilayer structures (see Figure 5.1). An especially important layer is the primary wall (see Figure 5.2) although it is a small fraction of the mature, fully developed fiber. 

Detailed structures of many crystalline materials can be determined by diffraction methods. However, because of the complex hierarchy of the cotton fiber and its very small crystallites, diffraction experiments on cotton fibers cannot provide fine details of molecular structure. Instead, the best data on cellulose structure comes from other sources. One of the major points of interest is the finding that cellulose has many different crystalline forms, or polymorphs, depending on the sources and subsequent treatments. Historically, there are four polymorphs or allomorphs, I to IV, and subclasses have been identified for all but cellulose II. 

FIGURE 5.17 Cellulose diffraction patterns. Top left synchrotron radiation x-ray diffraction pattern for cotton fiber bundle. The fiber was vertical and the white circle and line correspond to a shadow from the main beam catcher and its support. (Credit to Zakhia Ford.) Top right electron diffraction pattern of fragments of cotton secondary wall. The much shorter arcs in the top right figure are due to the good alignment and small number of crystallites in the electron beam. (Credit to Richard J. Schmidt.) Bottom a synthesized powder pattern for cellulose, based on the unit cell dimensions and crystalline coordinates of Nishiyama et al. [209]. (Credit to Zakhia Ford.) Also shown are the hkl values for the Miller indices. The 2-theta values are for molybdenum radiation instead of the more commonly used copper radiation. 

The response of the cotton fiber to heat is a function of temperature, time of heating, moisture content of the fiber and the relative humidity of the ambient atmosphere, presence or absence of oxygen in the ambient atmosphere, and presence or absence of any finish or other material that may catalyze or retard the degradative processes. Crystalline state and DP of the cotton cellulose also affect the course of thermal degradation, as does the physical condition of the fibers and method of heating (radiant heating, convection, or heated surface). Time, temperature, and content of additive catalytic materials are the major factors that affect the rate of degradation or pyrolysis. 

IRG2959 (Scheme 12.1) was used as aprobe of molecular motion in cotton fibers. TR ESR spectra of IRG2959 consist of spectra of RPs in a liquid-like and in crystalline environments. Simulation of spectra led to the conclusion that radicals participate in 3D and in 2D motion. Observation of a contribution of SCRP suggested that approximately 50% of radicals are trapped in cages of cotton fibers during the time of the experiment (0.5 0.s). ... 

Measurements of optical birefringence and density were obtained in an attempt to determine if total polymer orientation and the amount of fiber crystallinity would correlate with aging behavior. These values are presented in Table III. Because the secondary wall of cotton comprises... 

Crystallinity indexes calculated according to the method described by Segal et al. (32) showed that the old cotton has a crystallinity of about 38 . Aqueous treatments increased the crystallinity of the historic cotton sample to about 45 . However, the crystallinity of contemporary cotton, which is about 70 , was not reached (30). This increase suggests that water acts as an internal plasticizer and allows a segmental reorientation which leads to an increase in crystallinity. Water-induced crystallization of amorphous cellulose fibers has been reported (17). Kalyanaraman (33) investigated orientation factors of cotton fibers from historic samples and found that the orientation values of the museums samples are smaller than the values of present-day cottons. He opined that cotton may have lost its orientation over time. In view of this... 

Much of the chemical behavior of cellulose fiber can be attributed to cellulose structure. Since cellulose is a highly crystalline polymer, it can absorb mechanical energy efficiently for mechanical stress reaction ( 5, 19). The mechanically activated thermal energy, in addition to rupture of main chains, may alter morphology or microstructure of cotton cellulose. Accordingly, the crystallinity and accessibility of cotton fiber may be influenced. 

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