Cellulose molecules, mechanical

Naturally occurring cellulose is extremely mechanically stable and is highly resistant to chemical and enzymatic hydrolysis. These properties are due to the conformation of the molecules and their supramolecular organization. The unbranched pi 4 linkage results in linear chains that are stabilized by hydrogen bonds within the chain and between neighboring chains (1). Already during biosynthesis, 50-100 cellulose molecules associate to form an elementary fibril with a diameter of 4 nm. About 20 such elementary fibrils then form a microfibril (2), which is readily visible with the electron microscope. 

Macromolecular properties of grafted cellulosic fibers usually measured are differential solubility in either polymeric or cellulosic solvents, mechanical or physical properties, and abrasion resistances. The molecular weights of the grafted or block polymers and of cellulose, both before and after formation of macrocellulosic radicals, have been determined. The number of grafted or block polymer molecules per cellulose molecule calculated has usually been much less than one. Grafted cellulosic fibers exhibit second order transition temperatures, dependent on the composition of the grafted polymer (3, 4). 

It is also known that chemical and physical abnormalities in the cellulose molecule can produce weak bonds (30,31) thus, structural abnormalities may set up strain at localized points. At such positions, the rate constants for hydrolysis may be increased by as much as 10,000 (31). In the presence of acid, these properties would be enhanced and would assist UV grafting. Such a mechanism may even be the predominant process by which acid accelerates UV grafting. 

Cellulose contains adsorbed water which is held in the glucopyranose structure by hydrogen bonding, hence the separation proceeds via a partition mechanism. Cellulose materials are used almost exclusively for separating hydrophilic substances, for instance, amino acids and sugars in contrast to silica gel and alumina which are used for the separation of lipophilic compounds. Similar eluants, as for the PC application, can be selected. The partial structure of the cellulose molecule is shown in Figure 3.4. 

Electron-microscope investigation of cultures of bacteria has shown that the cellulose elements are not a physical appendage of the cell, but occur free and are scattered within the medium.This discovery has raised questions as to the processes or steps involved in the formation of the microfibrils and the extent to which they are influenced by cell organelles or carried out by exogenous chemical interactions and mechanical forces. Ohad and coworkers considered that the steps involved may be resolved into (a) polymerization of the activated, monomeric precursor to form cellulose molecules of high molecular weight, (b) transport of the molecule from the site of synthesis to that of crystallization, (c) crystallization or fibril formation, and (d) orientation of fibrils during deposition. 

The above results have obvious implications for the biosynthesis of cellulose mlcrofIbrlls. The parallel chain structure of cellulose I rules out any kind of regularly folded chain structure, and reveals the mlcrofibrils to be extended chain polymer single crystals, which leads to optimum tensile properties. Work by Brown and co-workers (22) on the mechanism of biosynthesis points to synthesis of arrays of cellulose chains from banks of enzyme complexes on the cell wall. These complexes produce a bundle of chains with the same sense, which crystallize almost immediately afterwards to form cellulose I mlcroflbrlls there is no opportunity to rearrange to form a more stable anti-parallel cellulose II structure. Electron microscopy by Hleta et al. (23) confirms the parallel sense of cellulose chains within the individual mlcroflbrlls stains reactive at the reducing end of the cellulose molecule stain only one end of the mlcroflbrll. 

The mechanism of action of carboxymethylcellulose (CMC) will be discussed here as an example. Here carboxylic acid group — COOH has a surface active effect with respect to the ceramic particle surface by reacting to form hydrogen bonds with the surface groups of the ceramic particle. The free OH groups of the cellulose molecule act like those of polymeric alcohols with respect to the addition of water molecules. 

The formation of cellulose fibrils in plant cell walls is associated with distinct aggregates of spherical structures in the plasmalemma, originally postulated by Preston before they were found. It is now uncertain whether these contain an appreciable part of the required glycosyltransferase, or are simply a kind of organising or spinning mechanisms for bringing cellulose molecules together to form fibrils. 

Fig. 4 Structure and properties of nanocellulose, (a) Hierarchical assembly of cellulose molecules into cellulosic fibers. Adapted, with permission, from [131]. Copyright 2012 Elsevier, (b) Proposed mechanism of formation of CNF cross-linked with metal cations. Reproduced, with permission, from [132]. Copyright 2013 American Chtanical Society, (c) Effect of the type of metal cation on the frequency-dependent storage modulus of CNF hydrogels, probed by dynamic frequency sweeps (25 °C) at a strain rate of 0.5 %. Adapted, with permission, from [132]. Copyright 2013 American Chemical Society, (d) Polarization optical microscopy photograph of a biphasic 8.78 % (w/w) CNC suspension. Adapted, with permission, from [133]. Copyright 1996 American Chemical Society, (e) Polarization optical microscopy photograph of a CNC suspension. Scale bar. 200 pm. Reproduced, with permission, from [134]. Copyright 2000 Amaiean Chemical Society...

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