Aggregation cellulosics

Natural fibers consist of aggregated cellulose chains arranged in a hierarchical structure. These elementary fibrils are composed of cellulose chains called cellulose macro fibrils [16]. Figure 6.3 shows a fransmission elecfron microscopy (TEM) image of cellulose microfibrils, or MFC [17],... 

Cellulose consists of several thousand o-glucose units linked by l- 4-/3-glyco-side bonds like those in cellobiose. Different cellulose molecules then interact to form a large aggregate structure held together by hydrogen bonds. 

Another important aspects of solubilization are the physical state of the dissolved polymer as well as the thermo-chemistry and kinetics of the dissolution reaction. It is known that a clear cellulose solution is a necessary, but not sufficient condition for the success of derivatization. The reason is that the polymer may be present as an aggregate, as will be discussed below. Additionally, dissolution of activated cellulose requires less time at low temperature, e.g., 2 h at 40 °C, and more than 8 h at 70 °C [106]. These aspects will be commented on below. 

As indicated earlier, clear cellulose solutions are not necessarily molec-ularly dispersed they may contain aggregates of still ordered cellulose molecules [107]. The structure of these aggregates has been described in terms of a fringed micellar structure. Figure 2a shows a schematic possi-... 

In summary, although clear, light-colored cellulose solutions are required to start the synthesis, there is no guarantee, a priori, that the targeted DS will be obtained. The reasons are that the state of aggregation of cellulose is dependent on the structural characteristics of the starting material, is sensitive to the pre-treatment employed, and the impurities present. This may result in non-reproducible aggregation states, and may lead to oscillation in cellulose reactivity. Typically, effects of these oscillations may not be readily apparent, because ... 

Recently, use of LiCl/DMAc and LiCl/l,3-dimethyl-2-imidazolidinone as solvent systems for acetylation of cellulose by acetic anhydride/pyridine has been compared. A DS of 1.4 was obtained the substituent distribution in the products synthesized in both solvents was found to be the same, with reactivity order Ce > C2 > C3. Therefore, the latter solvent system does not appear to be better than the much less expensive LiCl/DMAc, at least for this reaction. It appears, however, to be especially efficient for etherification reactions [178]. It is possible, however, that the effect of cellulose aggregation is more important for its reaction with the (less reactive) halides than with acid anhydrides this being the reason for the better performance of the latter solvent system in ether formation, since it is more efficient in cellulose dissolution. 

Chitosan samples with degrees of deacetylation of 65,73,85, and 92% were almost completely adsorbed onto the surfaces of cellulosic fibers, especially onto the surfaces of fines in a variety of cellulosic systems used in industrial operations. Adsorption increased as the degree of deacetylation of chitosan increased. The aggregation of the fine cellulosic particles was maximum at a dosage of about 10 mg/kg. The interactions between chitosan and the cellulosic substrates were dominated by a bridging mechanism at about pH 7 [32]. 

Mark (1895-1992) and Meyer (1893-1952) suggest that materials like caoutchouk or cellulose are made of small molecules aggregated to micells. 

In the transmission electron microscopy (TEM) images, the starch nanoplatelets (SNPs) are believed to aggregate as a result of hydrogen bond interactions due to the surface hydroxyl groups [13] (Fig. lA). Blocking these interactions by relatively large molecular weight molecules obviously improves the individualization of the nanoparticles. The acetylated starch and cellulose nanoparticles (SAcNPs and CelAcNPs) appeared more individualized and monodispersed than their unmodified counterparts with a size of about 50 nm (Fig. IB C). 

Anthraquinone leuco dyes are widely known as vat dyes.10 Vat dyes possess extensively conjugated aromatic systems containing two or more carbonyl groups, e.g., anthraquinone, indigoid chromophores. The colored form of vat dyes are insoluble in water. The dyes are applied by a process whereby the dye is converted to the reduced form (leuco dye) which is soluble in water and can penetrate into a cellulosic fiber. On exposure to the atmosphere the leuco form is oxidized to the original quinoid form which then precipitates as an aggregate. Vat dyes generally have excellent chemical and photochemical stability. 

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