Cellulose terms Links

In terms of tonnage the bulk of plastics produced are thermoplastics, a group which includes polyethylene, polyvinyl chloride (p.v.c.), the nylons, polycarbonates and cellulose acetate. There is however a second class of materials, the thermosetting plastics. They are supplied by the manufacturer either as long-chain molecules, similar to a typical thermoplastic molecule or as rather small branched molecules. They are shaped and then subjected to either heat or chemical reaction, or both, in such a way that the molecules link one with another to form a cross-linked network (Fig. 18.6). As the molecules are now interconnected they can no longer slide extensively one past the other and the material has set, cured or cross linked. Plastics materials behaving in this way are spoken of as thermosetting plastics, a term which is now used to include those materials which can in fact cross link with suitable catalysts at room temperature. 

The importance of proper RO membrane selection has already been discussed. A review of commercially available RO membranes revealed five different basic membranes that could provide organic recovery. Cellulose acetate and cellulose acetate blends, aromatic polyamide, polyamide thin-film composite, cross-linked polyimine thin-film composite (FT-30), and polybenzimidazole were available when this work was performed. Only the first four types were commercially available. All membranes were available with excellent salt rejection (>97 sodium chloride). Two types of membranes, cellulose acetate and FT-30, have shown short-term (<2-months intermittent use) resistance... 

The first synthetic plastics were the phenol-formaldehyde resins introduced by Baekeland in 1907 [1], Melamine and urea also react with formaldehyde to form intermediate methylol compounds which condense to cross-linked polymers much like phenol-formaldehyde resins. Paper, cotton fabric, wood flour or other forms of cellulose have long been used to reinforce these methylol-functional polymers. Methylol groups react with hydroxyl groups of cellulose to form stable ether linkages to bond filler to polymers. Cellulose is so compatible with these resins that no one thought of an interface between them, and the term reinforced composites was not even used to describe these reinforced systems. 

Several years ago we began to express the results of our thermal and photochemical degradation experiments in terms of Equation 4. Figure 1 shows some of the initial data obtained, relating to exposure of test sheets in the Fade-ometer. Just as Daruwalla and Narsian (16) observed, there seems to be a break in the curve. If the first stage indeed represents weak links, about 3 such bonds per 10,000 in the original cellulosic chain (0.03 ) are apparent, a figure similar to the fraction of labile acid-sensitive links determined by Sharpies (21) and labile photochemically sensitive links reported by Daruwalla (16) when radiation at 2540 A was used. 

From the evidence presented, the structural formula of native cellulose may be schematically represented as in Fig. 2, neglecting for the moment the concepts of cross linking, and the fact that by common usage of the term, a cellulosic fiber material containing no detectable, or many, end groups is still called cellulose. 

A term more general than polymer is that of macromolecule. Macromolecules are chemical compounds formed from at least one thousand atoms linked by covalent bonds. They are common as natural substances like cellulose, proteins, lignin, etc., and also as synthetic compounds including plastics, fibers, elastomers, coatings, and adhesives. Many synthetic and some natural macromolecules have repetitive structures and are known as polymers. For example, cellulose is made from p-D-glucose residues interconnected by p-glucoside (1->4) links, polystyrene is made from 1-phenylethylidene units, etc. 

In describing a polymer, it is often more important to know the number of monomer units linked together than the actual mean molecular weight M. The term degree of polymerization, often abbreviated to D.P., is used to express this factor. Thus in cellulose the molecular weight of the single... 

In contrast to the fibrous and well-oriented polysaccharides (such as cellulose and chitin), amylose in its natural state is found only in more-or-less spherical starch granules. Amylose is the minor constituent of most starches, and is a linear polymer of a-D-(l—>4)-linked D-glucose residues. The birefringence of starch grains was already well known at the turn of the century, and the cross of isocline pattern between crossed Nicols was interpreted in terms of radially arranged crystallites. Later,... 

Since the early part of the twentieth century, cellulose has been recognized as the linear / -1,4-linked homopolymer of anhydroglucose shown schematically below. More recently, it has been noted that it is more accurate to define it as the homopolymer of anhydrocellobiose, since such definition explicidy incorporates the identity of the linkage. Given this definition, it might be anticipated that the nature and the chemical behavior of cellulose could be understood in terms of the chemistry of its monomeric constituents together with... 

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