Bridges between cellulose

Fig 1. Electron micrograph of a platinum/carbon replica prepared by the fast-freeze, deep-etch, rotary-shadow replica technique printed in reverse contrast. Cell walls of onion parenchyma have an elaborate structure with many thin fibres bridging between thicker cellulosic microfibrils. Scale bar represents 200nm. 

Such enzymatic catalyzed polycondensations have allowed the synthesis of a number of natural polysaccharides, but has also allowed the production of nonnatural polysaccharides such as cellulose-xylan hybrids and functionalized hyaluronan, chondroitin sulfate, and chondroitin. Such work illustrates the ever-narrowing bridge between natural and synthetic polymers and polymer syntheses. 

FIGURE 1.17 Detail of the cellulose fiber displayed in Fig. 1.16, which clearly shows the establishment of a micrometric gas bridge between the two gas pockets trapped inside the fiber s lumen (bar = 10 pm photograph by Gerard Liger-Belair). 

Fig. 5.4 Formation of hydrogen bridges (...) between butyl amine and cellulose [6,11]...

It has been proposed that in alkali cellulose, alkali (NaOH) and water bridge the cellulose molecules (23). However, it does not seem likely that all direct intermolecular hydrogen bonds in alkali cellulose are disrupted, inasmuch as the fibrous morphology is retained. Additionally, there have not been any reports of any precipitates of con lexes between oligomers and alkali in aqueous solutions. 

Moreover, it can be seen that an anhydride of ccllobiose which contains an oxygen bridge between the carbon atom 1 of the reduced ring and the carbon atom 4 of the other ring, cannot be constructed without strain. This is important, because such a formula can occasionally be assigned to certain degradation products of cellulose... 

The purpose of this chapter is to deal with the surface modification of cellulose fibres in order to provide them with specific functionalities, so that they can play determining roles in such applications as reinforcing elements for composite materials, self-contained composite structures, anti-pollution aids, hybrid materials, superhydrophobic surfaces and conductive and magnetic materials. Other types of surface modifications, such as those associated with dying or the manufacture of chromosorb, enzymatic and ion-exchange supports, fall outside the scope of this review. Within the structure of this book, this chapter constitutes in many ways a bridge between the chemistry associated with bulk modification treated in Chapter 16 and the processing and properties of composite materials in Chapter 19, with the addition of more specific aspects. 

Urea-formaldehyde resins and similar aminoplast precondensates form the greatest proportion of all the resins used as additives. Mono-methylated and dimethylated ureas are used, as are the analogous condensation products of formaldehyde with melamine. The monomeric compounds penetrate into the intermicellar space in the cellulose in aqueous solution, and there harden with heat to form insoluble resins (cf. also Section 28.2). Since the formation of mono- and dimethylated urea is reversible, CH2O occurs in equilibrium. Formaldehyde can form methylene cross-link bridges between the individual chains. In addition, longer cross-linking... 

Electrophoresis, a process of separating compounds on the basis of their electric charges, is used to separate and identify mixtures of amino acids and proteins. Electrophoretic separations can be carried out with paper, starch, agar, certain plastics, and cellulose acetate used as solid supports. In paper electrophoresis, a paper strip saturated with an aqueous buffer of predetermined pH serves as a bridge between two electrode vessels (Figure 18.4). Next, a sample of amino acids is applied as a colorless spot on the paper strip. (The amino acid... 

In starch, the orientations of the bonds bridging between adjacent glucose rings are such as to make a zigzag structure, like that in cellulose, impossible. Unbranched starch (amylose) mole-... 

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