Molecular weight reduction cellulose

From among the natural carbohydrate polymers, we mention here cellulose, chitin and its deacetylated form chitosan, hyaluronic acid (hyaluronan), and heparin. Apart from cellulose, the monomer-unit sequences are not strictly regular, but the structures given below are representative. Chitosan, hyaluronic acid, and heparin are water-soluble because they carry electrically-charged functions. Since cellulose and chitin are insoluble in water, most of their radiation chemistry has been done in the solid state, as discussed below. Yields of molecular-weight reduction have usually been determined by viscosimetry and, more recently, by the laser light-scattering technique. 

The first synthetic fiber for tires was rayon. Cellulose is initially treated with sodium hydroxide to form an alkali cellulose. It is then shredded and allowed to age in air, where it is oxidized and undergoes molecular weight reduction to enable subsequent spinning operations. Treatment with carbon disulfide produces cellulose xanthate, which is then dissolved in sodium hydroxide to form viscose. The material undergoes further hydrolysis and is then fed into spinnerets to produce the fiber. This fiber is passed through a bath of sulfuric acid and sodium sulfate, where the viscose fibers are further coagulated. 

Membrane Sep r tion. The separation of components ofhquid milk products can be accompHshed with semipermeable membranes by either ultrafiltration (qv) or hyperfiltration, also called reverse osmosis (qv) (30). With ultrafiltration (UF) the membrane selectively prevents the passage of large molecules such as protein. In reverse osmosis (RO) different small, low molecular weight molecules are separated. Both procedures require that pressure be maintained and that the energy needed is a cost item. The materials from which the membranes are made are similar for both processes and include cellulose acetate, poly(vinyl chloride), poly(vinyHdene diduoride), nylon, and polyamide (see AFembrane technology). Membranes are commonly used for the concentration of whey and milk for cheesemaking (31). For example, membranes with 100 and 200 p.m are used to obtain a 4 1 reduction of skimmed milk. 

At this point it seems of interest to include a graph obtained on a quite different polymer, viz. cellulose tricarbanilate. Results from a series of ten sharp fractions of this polymer will be discussed in Chapter 5 in connection with the limits of validity of the present theory. In Fig. 3.5 a double logarithmic plot of FR vs. is given for a molecular weight of 720000. This figure refers to a 0.1 wt. per cent solution in benzophenone. It appears that the temperature reduction is perfect. Moreover, the JeR-value for fiN smaller than one is very close to the JeR value obtained from Figure 3.1 for anionic polystyrenes in bromo-benzene. As in the case of Fig. 3.1, pN is calculated from zero shear viscosity. The correspondence of Figs. 3.1 and 3.5 shows that also the molecules of cellulose tricarbanilate behave like flexible linear chain molecules. For more details on this subject reference is made to Chapter 5. 

The peeling of xylans is basically similar to that of cellulose (Fig. 2). Extended alkali treatment of a rye flour arabinoxylan at room temperature resulted in only a 29% reduction in molecular weight [265]. As anticipated, the polysaccharide, after reduction with sodium borohydride, was completely stable to alkali. 

When oxycelluloses are placed in alkaline solution, large reductions in molecular weight take place because of the scission of glycosidic bonds. As a consequence, the oxidation of cellulose does not, in general, lead directly to rupture of the molecules, but renders those linkages near the point of attack extremely susceptible to alkaline cleavage. < Several mechanisms have been proposed for explaining these effects. 

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