Cellulose aggregation

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. 

Microcrystalline form, Avicel. Prepn and manuf of crystallite cellulosic aggregates Battista, fntf. Eng. Chem. 42, 502 (1950) Battista. Smith, U-S. pats. 2,978,446 and 3.141,-875 (1961 to Am. Viscose and 1964 to FMC). Non-fibrous powder. Particle shape rigid rods. Refractive index 1-55. Bulk density 18-19 lb/cubic foot. Practically insol. but... 

FIGURE 10.21 Model of cellulose aggregate in a cellulose solution. (From Schulz, L., Seger, B., and Burchard, W., Macromol. Chem. Phys., 201, 2008, 2001. Reprinted with permission of Wiley-VCH Verlag GmbH Co. KGaA and Professor W. Burchard.)... 

Thirty to one hundred strands of cellulose aggregate into so-called elemental fibrils (cross section approximately 2 mn) via H-bridges, which in turn aggregate into microfibrils (cross section approximately 10 30 mn) and these into macrofibrils (cross section approximately 500 nm). The orientation of the cellulose fibrils in the wood cell wall differs. The strongest cell wall layer, for example, shows a constitution parallel to the axis which accounts for the high tensile strength of wood. ... 

Fibrillar fines obtained from cellulosic fibres are known for their unique structure, material characteristics, and potential applications (Hubbe et al. 2008). An amorphous lignin and hemicellulose matrix separates the elementary nanofibrils in natural vegetable fibres. Based on raw material sources, pretreatment and subsequent defibrillation procedures will produce a broad spectrum of fibril structures as well as nomenclatures used to describe them. Thus, we find various terms adopted in the field, such as nanoscale-fibrillated cellulose, cellulosic fibrillar fines, cellulose aggregate fibrils, and microfibrillar cellulose. 

Figure 7 Schematic model of self-assembling process of synthetic cellulose on the surface of enzyme associations diffusion of monomers to active sites of the enzyme aggregate (a), which synthesizes cellulose molecules (b) and self-assembles them In situ Into dendritic cellulose aggregates with Dm = 2.1 and Ds = 2.3 in the reaction medium around the enzyme aggregates (c) The cellulose aggregates eventually growing into the dome (d). The cellulose aggregate surrounding the enzyme association has enough free space for diffusion of monomers from the reaction medium into the active sites and for diffusion of terminated polymers from the active sites Into the reaction medium as shown in part (e) and discussed in Section 2.13.3.2. From Tanaka, H. Koizumi, S. Hashimoto, T. et al. Macromolecules 2007, 40, 6304-6315. ...

At the late stage of the polymerization (t>60min), the exponent a decreases with time from 4 to 3.7, and hence increases from 2 to 2.3, as shown in Figure 8(h). This means that the surface of the cellulose aggregate becomes increasingly rough with time. This observation was supported by a Monte Carlo simulation based on the DLA model.3 ... 

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 ... 

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). 

---