Crystallinity of cotton

Wakelin, J. H., Virgin, H. S., and Crystal, E. (1959). Development and comparison of two X-ray methods for determining the crystallinity of cotton cellulose. Journal of Applied Physics. 30,1654-1662. 

Table II shows effects of the disorder parameter on the calculated crystallinity of cotton, nylon 66 (PA 66) and PET fiber samples. When k = 0, no correction for distortion is made during calculation of crystallinity. Accordingly, values of X become smaller as the integration intervals increase. On the nomogram of K values, it is possible to determine the disorder parameter value that maintains as approximately constant this crystallinity when using the different intervals the deviation from the constancy is used, in the computing program to determine the best value of k and to estimate the errors of these analyses. The disorder parameter k is higher in cotton and PA 66 than in all PET fibers. Besides, one can see that apparent crystallinity values (disorder parameter not considered) are lower than the true ones.

Glycol cleavage. The initial periodate oxidation of cellulose, like other chemical reactions, was largely limited to the readily accessible component and has also been used to indicate the accessibility of cellulose substrates [151] (Table 1). Rowland and Cousins [232], based on the influence of periodate oxidation in the crystallinity of cotton, observed about 40% of the component being noncrystalline. Since the m-diol unit is generally more reactive than the /ran.9-diol, the cleavage of the mannose residues would proceed faster than that of the glucose or xylose residues. 

Crystallinity of cotton cellulose was measured using a density method by means of a density gradient column (Techne Inc., Model DC-2). Xylene and carbon tetrachloride were used to make up the solution. Based on the density data, crystallinity of cellulose can be calculated from the following equation (13) ... 

Figure 4. Change in crystallinity of cotton cellulose during cutting and milling. Key V, cut fiber , milled fiber O, milled powder.

It should be noted that average values are given in Table I. E tecially in the case of x-ray determinations, estimates of crystallinity will depend strongly on the calculation methods which have been reviewed fay Tripp ( ). Thus using Karwicker s data, the standard error and coefficient of variation (%) for crystallinity of cotton by x-rey determinations are 2.5 and 15, respectively, and by acid hydrolysis are only 1.1 and 5.9, reflectively. The various chemical reaction and sorption techniques will now be discussed in more detail. 

Table 10.4 Crystallinity of cotton fabrics after different treatments...

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). 

Grafting of the unacetylated samples gives higher values in the case of wood pulp than in cotton linters. This is explained by the crystallinity of both pulps, which is 80% and 56.8% for cotton linters and wood pulp, respectively. 

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. 

Polymers with rigid, cyclic structures in the polymer chain, as in cellulose and poly(ethy-leneterephthalate), are difficult to crystallize. Moderate crystallization does occur in these cases, as a result of the polar polymer chains. Additional crystallization can be induced by mechanical stretching. Cellulose is interesting in that native cellulose in the form of cotton is much more crystalline than cellulose that is obtained by precipitation of cellulose from... 

Place 10.3 g (0.05 mol) of a-acetamidocinnamic acid (Expt 8.21) and 200 ml of 1 m hydrochloric acid in a 500-ml round-bottomed flask and boil the mixture steadily under reflux for 3 hours. Remove a small quantity of green oil by rapidly filtering the hot reaction mixture through a small plug of cotton wool loosely inserted into the stem of a preheated glass filter funnel, cool the filtrate to room temperature and leave it at 0 °C for 48 hours. Collect the crystalline product by filtration, wash it with a small quantity of ice-cold water and dry it in a vacuum desiccator over anhydrous calcium chloride and potassium hydroxide pellets. The yield of phenylpyruvic acid, which is sufficiently pure for most purposes, is 4.4 g, m.p. 157 °C (decomp.). A further 1.7 g of product of comparable purity (total yield 74%) separates from the aqueous acidic filtrate when this is set aside at 0 °C for about one week. 

An important parameter influencing the mode of action of cellulases is the accessibility of the cellulose to the enzymes. The molecular weights of cellulases range between 30 and 80 kDa. A comparison of the size of cellulase (3-8 nm) and the pore size of cotton swollen in water (1-7 nm) shows very clearly that cellulases can penetrate the cellulose to a limited extent only. In addition, the enzyme reaction takes place preferentially on amorphous cellulose because the more compact, crystalline cellulose structures do not offer any space for such macromolecules. Thus - provided of enzyme and process parameters have been selected correctly -cellulases act mainly on the textile surface. In this way interesting effects on cellu-losic fibers can be achieved. 

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.

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. 

---