Structural levels, cellulose

FIGURE 1.9 The different structural levels of a typical cellulose fiber the fiber wall consists of closely packed cellulose microfibrils oriented mainly in the direction of the fiber. 

The electron micrographs of the enzyme-treated sprucewood holo-cellulose revealed the loci of the removed substances when compared with untreated samples. The relative intensity of degradation at the ultra-structural level corresponded to the results obtained by chemical analysis of the dissolved carbohydrates. 

Due to the 3 hydroxyl groups available for oxidation within one anhydroglucose unit and due to the polymeric character of the cellulose a great variety of structural modifications and combinations is possible. As with other types of chemical changes at the cellulose molecule also in this case the oxidation can affect different structural levels differently. Depending on the oxidative stress imposed on the cellulose, the individual hydroxyls within the AGU and within the polymer chain are involved to varying extent and may respond to further treatment and reactions in a specific way. Despite their low concentration in the imol/g range, oxidative functionalities are one of the prime factors to determine macroscopic properties and chemical behavior of cellulosic materials (Fig. 1). 

For convenience, multicomponent polymeric materials based on cellu-losics may be grouped into three classes, namely (a) combinations of wood with plastics (WPC), (b) mechanical mixtures in the form of fibers, such as cotton/polyester staple-mixed fibers and cellulosic fiber-filled polymer sheets, and (c) incorporations of cellulosics at a hyperfine structural level. The latter can be further ramified, for instance as follows ... 

The structure of cellulose or nitrocellulose may be discussed on three distinct levels (14, 18). 

The use of electron microscopy in the study of celluloses, particularly in their native state, has resulted in important advances beginning with investigations that were undertaken at the time of the introduction of the earliest electron microscopes. The early work has been ably reviewed by a number of authors. Of particular note among these is the coverage of the subject in the treatise by Preston. Here, we will focus on studies that have been important to advancing the understanding of the structure of cellulose at the submicroscopic level. 

Native cellulose is a linear condensation homopolymer with a complex stmcmre. Three structural levels can be considered [4, 7] ... 

On the molecular level, cellulose is a linear polymer of P-(l 4)-D-gluco-pyranose units in " Ci conformation. The fully equatorial conformation of 3-linked glucopyranose residues stabilizes the chair structure, minimizing its flexibility. 

High performance composite materials can be obtained with a good level of dispersion, mainly when the hierarchical structure of cellulose and use of a water soluble polymer to form the matrix are considered. For most materials applications, the main biopolymers of interest are cellixlose and starch. The ease of adhesion that occurs in cellulose has contributed to its use in paper and other fiber-based composite materials. 

The molecular mechanism of the fatigue process was investigated for a series of spinning polymers, such as polyamide 6 and polycarbonates [875-860], poly(ethylene terephthalate) and polypropylene [861], polyacrylonitrile and viscose-type cellulose [830, 862-864]. The majority of published papers are focused on the aspects concerning the other structural levels, i.e. supramolecular and morphological one. 

The microfibrillar structure of cellulose has been established beyond doubt through the application of electron microscopy [64-65] and great variations in dimensions, depending on origin, have been reported [47-48, 66], The application of transmission electron microscopy [67-72] has established with certainty that the microfibril is the basic crystalline element of native cellulose [1, 67, 68-70, 73], Different levels of structural organisation of cellulose are now well characterized. 

In Chapter 6, an attempt was made to identify suspect physiological cross-links, tentatively assigned from HPLC dafa, in addition to those already established in literature. Two novel cross-links, denoted chromatographic fractions IV and V-2, were purified from bovine root dentin, and the structure of V-2 was elucidated. During the analysis of human denfin as described in Chapter 5, V-2 appeared below detection level. A peak was observed for IV. Part of the material with the same retention as IV was not retained on cellulose in butanol/acetic acid/water = 4 1 1 (vol.). Co-elution of a non-cross-linking amino acid with IV could therefore not be excluded, and the results were omitted. 

The deck may be nailable, eg, wood or light weight concrete, or not, eg, steel or structural concrete. The felts or mats may be oiganic (cellulose), or fiber glass. The roof slope ranges from dead level (0—2.1 cm/m), to flat (2.1—12.5 cm/m), to steep (12.5—25 cm/m). 

An important chemical finishing process for cotton fabrics is that of mercerization, which improves strength, luster, and dye receptivity. Mercerization involves brief exposure of the fabric under tension to concentrated (20—25 wt %) NaOH solution (14). In this treatment, the cotton fibers become more circular in cross-section and smoother in surface appearance, which increases their luster. At the molecular level, mercerization causes a decrease in the degree of crystallinity and a transformation of the cellulose crystal form. These fine structural changes increase the moisture and dye absorption properties of the fiber. Biopolishing is a relatively new treatment of cotton fabrics, involving cellulase enzymes, to produce special surface effects (15). 

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