Crystalline cellulose forms

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). 

Many cellulose derivatives form Hquid crystalline phases, both in solution (lyotropic mesophases) and in the melt (thermotropic mesophases). The first report (96) showed that aqueous solutions of 30% hydroxypropylceUulose [9004-64-2] (HPC) form lyotropic mesophases that display iridescent colors characteristic of the chiral nematic (cholesteric) state. The field has grown rapidly and has been reviewed from different perspectives (97—101). 

The pattern of variation of the multiplets differ among the samples. The relative intensities are not constant and they are not in the ratios of small numbers as would be expected if they arose from different points within a single unit cell. The spectral intensities are also not consistent within a single unit cell. The spectral intensities are also not consistent with the possibility of three independent crystal forms. According to Atalla therefore a model based on two independent crystalline forms seems most possible. In Fig. 5, the spectrum of pure cellulose II is given. 

The proposal of multiple crystalline forms in native celluloses implies that all native celluloses are compositions of two distinct forms, which has been earlier indicated for Acetobacter and Vallonia celluloses, 8). From the resolution of the NMR spectra an estimate of about 60-70 % of the la form in Acetobacter cellulose and of 60-70 % of the lb form in cotton was obtained. A further detailed analysis of conformational features in celluloses seemed to need X-ray diffractometric and Raman spectroscopic confirmation 19-56). 

The second choice is a simpler solution. According to Sarko and Muggli,66 all 39 observed reflections in the Valonia X-ray pattern are indexable by a two-chain triclinic unit cell with a = 9.41, b =8.15 and c = 10.34 A, a = 90°, 3 = 57.5°, and y = 96.2°. Ramie cellulose, on the other hand, is completely consistent with the two-chain monoclinic unit cell. Also, there are significant differences between their high-resolution solid-state l3C NMR spectra, indicating that Valonia and ramie celluloses, the two most crystalline forms, reflect two distinct families of biosynthesis. On this basis, the Valonia triclinic and the ramie monoclinic forms are classified69 as Ia and Ip, respectively. It has been shown from a systematic analysis of the NMR spectra by these authors, and from electron-dif-... 

The only pectic enzyme thus far obtained in crystalline form is the endo-D-galacturonanase from Acrocylindrium.209 Crystallization of the enzyme from a solution of ammonium sulfate was preceded by chromatography on calcium phosphate, Duolite CS 101, and DEAE-cellulose, and by starch-gel electrophoresis. 

Numerous disperse dyes are marketed in a metastable crystalline form that gives significantly higher uptake than the corresponding more stable modification. The molar free enthalpy difference can be used as a criterion of the relative thermodynamic stabilities of two different modifications [53]. Certain dyes can be isolated in several different morphological forms. For example, an azopyrazole yellow disperse dye (3.52) was prepared in five different crystal forms and applied to cellulose acetate fibres. Each form exhibited a different saturation limit, the less stable modifications giving the higher values [54]. 

This particular trimethylglucose is unique in that it was separated in crystalline form from the hydrolyzates of the methyl ethers of several naturally-occurring glucose polymers almost two decades before it was synthesized from glucose. These natural sources, which still furnish the most convenient routes for the preparation of 2,3,6-trimethyl-D-glucose, include maltose,124-128 cellobiose,127,128 lactose,122-181 starch,71,182 glycogen,188,184 cellulose,185-187 and lichenin. 188,189 The literature pub-... 

In the present work, we extend the method to compensate for the hydrogen bonds present in carbohydrates. The hydroxylated character of carbohydrate polymers influences between-chain interactions through networks of hydrogen bonds that occur during crystallization. Frequently, several possible attractive interactions exist that lead to different packing arrangements, and several allomorphic crystalline forms have been observed for polysaccharides such as cellulose, chitin, mannan and amylose. The situation is even more complex when water or other guest molecules are present in the crystalline domains. Another complication is that polysaccharide polymorphism includes different helix shapes as well. 

Bheda et al. ( ) showed that cellulose triacetate forms a mesophase in dichloroacetic acid. Navard and Haudin (18) examined the thermal behavior of liquid crystalline solutions of CTA in TFA. Navard et al. (23) studied the isotropic to anisotropic transitions of solutions of cellulose triacetate in TFA using differential scanning calorimetry. Navard and Haudin (S2) studied the mesophases of cellulose and cellulose triacetate calorimetrically. Navard et al. (83) report similar studies. Meeten and Navard (97) showed the twist of the cholesteric helicoidal structure of CTA and secondary cellulose in TFA is left-handed. 

Cotton thread treated with concentrated NaOH shrinks and has an increased luster. Tire resulting "Mercerized" cellulose has changed into other crystalline forms. The major one is cellulose II, in which the chains in the sheets are antiparallel.693 Cellulose II may also occur to some extent in nature. Many other modified celluloses, e.g., methylcellulose, in which some -OH groups have been converted to methyl ethers70 are important commercial products. 

The enzyme was purified from Candida utilis in 1965 by Rosen et al. (8Q). Dried yeast was allowed to autolyze in phosphate buffer at pH 7.5 for 48 hr, and the enzyme was isolated in crystalline form from these autolysates by a procedure which included heating to 55° at pH 5.0, fractionation with ammonium sulfate, and purification on phospho-cellulose columns from which the enzyme was specifically eluted with malonate buffer containing 2.0 mM FDP. Crystallization was carried out by addition of ammonium sulfate in the presence of mM magnesium chloride. The Candida enzyme was more active than the mammalian FDPases at room temperature and pH 9.5 the crystalline protein catalyzed the hydrolysis of 83 /nnoles of FDP per minute per milligram of protein. The enzyme was completely inactive with other phosphate esters, including sedoheptulose diphosphate, ribulose diphosphate, and fructose 1- or fructose 6-phosphates. Nor was the activity of the enzyme inhibited by any of these compounds. Optimum activity was observed at concentrations of FDP between 0.05 and 0.5 mM higher concentrations of FDP (5 mM) were inhibitory. 

All the cellulose regeneration processes, with or without chemical modification, cause cellulose molecules to organise in a different crystalline form, called cellulose II and sometimes cellulose IV (especially in MODAL-HWM fibres)... 

There are at least four crystalline forms of cellulose, based on different packing of the primary chain (Blackwell, 1982), and three forms of granular starch, based on the packing of double helices (Noel et al., 1993). The differences are largely in the unit-cell dimensions and the crystallization and precipitation temperatures. One form of starch, precipitated with alcohol, is in a symmetrical molecular arrangement and is readily dispersible in cold water (Kerr, 1950). Mannan and dextran yield different crystals at low and high temperatures, and there was not only a polymorphic difference, but a conformational difference in cellulose (Quenin and Chanzy, 1987). Curdlan appears to have three polymorphs—anhydrous, hydrated, and annealed. 

The cellulose polymer chains are considered to run in the direction of the fibrils and are largely in crystalline form. The molecules, however, periodically get out of the ordered relation. The fibril at this point becomes amorphous and of lower density. This region being more open absorbs moisture, which acts to plasticize the fiber. The amorphous region is also the point at which acid or enzyme can penetrate to cut the molecule. 

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. 

Some cotton cellulose is noncrystalline or amorphous in the sense of lacking definite crystalline form. One reason is that cotton cellulose has a broad molecular weight... 

The most important alternative crystalline form is cellulose II. This form can result from treatment of cellulose in concentrated alkali, such as 23% NaOH, followed by rinsing in water. This is also the main form that results from crystallization of dissolved cellulose, such as regeneration of rayon. Supercritical water can also effect the transformation [216]. The treatment of cotton in milder alkali, for industrial mercerization, amounts mainly to disruption and decrystallization rather than transformation to crystalline II. Cellulose II can occur as the native state when the normal biosynthesis and subsequent crystallization is disrupted [217-219]. 

Cellulose powders can be created by cutting fibers into small particles, perhaps with a Wiley mill (Arthur H. Thomas Company, Swedesboro, New Jersey). On a laboratory x-ray system, powder diffraction patterns take 30 min. The positions of the peaks indicate the polymorphic form (I-IV) the powder diffraction pattern is often used as a fingerprint for comparison with the known pattern for a given crystalline form [207]. The breadth of the peaks is related to the extent of crystallinity (Figure 5.17, bottom). Using the Scherrer formula [245,246] and assuming no other distortions, the crystallite size can be calculated. Values for cotton perpendicular to the molecular axis are around 40 A. That corresponds to a 6x6 array of... 

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