Long-chained molecules

Coarse-grained models have a longstanding history in polymer science. Long-chain molecules share many common mesoscopic characteristics which are independent of the atomistic stmcture of the chemical repeat units [4, 5 and 6]. The self-similar stmcture [7, 8, 9 and 10] on large length scales is only characterized by a single length scale, the chain extension R. 

SAMs are ordered molecular assembHes formed by the adsorption (qv) of an active surfactant on a soHd surface (Fig. 6). This simple process makes SAMs inherently manufacturable and thus technologically attractive for building supedattices and for surface engineering. The order in these two-dimensional systems is produced by a spontaneous chemical synthesis at the interface, as the system approaches equiHbrium. Although the area is not limited to long-chain molecules (112), SAMs of functionalized long-chain hydrocarbons are most frequently used as building blocks of supermolecular stmctures. 

There are two principal forces that govern the abdity of a polymer to crystallise the interchain attractive forces, which are a function of the chain stmcture, and the countervailing kinetic energy of the chain segments, which is a function of the temperature. The fact that polymers consist of long-chain molecules also iatroduces a third parameter, ie, the imposition of a mechanical force, eg, stretching, which can also enhance interchain orientation and favor crystallisation. 

Fibrous proteins can serve as structural materials for the same reason that other polymers do they are long-chain molecules. By cross-linking, interleaving and intertwining the proper combination of individual long-chain molecules, bulk properties are obtained that can serve many different functions. Fibrous proteins are usually divided in three different groups dependent on the secondary structure of the individual molecules coiled-coil a helices present in keratin and myosin, the triple helix in collagen, and P sheets in amyloid fibers and silks. 

One other variant in double-bond polymerisations may be mentioned here. Many conjugated dienes may be polymerised in such a way as to generate long chain molecules with residual double bonds in the chain. Well-known examples of such dienes are buta-1,3-diene and isoprene, which yield 1,4-polybutadiene and 1,4-polyisoprene respectively (Figure 2.4). Natural rubber has a formula corresponding to the 1,4-polyisoprene. 

In all of the examples given so far in this chapter the product of polymerisation has been a long chain molecule, a linear polymer. With such materials it should be possible for the molecules to slide past each other under shear forces above a certain temperature such that the molecules have enough energy to overcome the intermolecular attractions. In other words above a certain temperature the material is capable of flow, i.e. it is essentially plastic, whereas below this temperature it is to all intents and purposes a solid. Such materials are referred to as thermoplastics and today these may be considered to be the most important class of plastics material commercially available. 

It was pointed out in Chapter 3 that conventional vulcanised rubbers were composed of highly flexible long chain molecules with light cross-linking... 

Just how long-chain molecules can in fact be incorporated in regular crystal lattices, when the molecules are bound to extend through many unit cells, took a long time to explain. Finally, in 1957, three experimental teams found the answer this episode is presented in Chapter 8. 

The early years, when the nature of polymers was in vigorous dispute and the reality of long-chain molecules finally came to be accepted, are treated in Chapter 2, Section 2.1.3. For the convenience of the reader 1 set out the sequence of early events here in summary form. 

Generation of chemical linkages between long-chain molecules can be compared to two straight chains joined together by links. The rigidity of the material increases with the number of links. The function of a monomer is to provide these links. 

A polymer such as polyethylene is a long-chain molecule with repetitions of the same monomer. Due to topological constraints, the crystallization process of polymer chains is expected to be different from that of simple molecules as discussed so far [160]. 

A hypothetical chemical structure in the interfacial area of the PMPPlC-treated composite [72] is shown in Fig. 10. The long-chain molecules present in PMPPIC interact with polyethylene leading to van der Waals type of interaction. 

Bond breaking can occur at any position along the hydrocarbon chain. Because the aromatization reactions mentioned earlier produce hydrogen and are favored at high temperatures, some hydrocracking occurs also under these conditions. However, hydrocracking long-chain molecules can produce Ce, C7, and Cg hydrocarbons that are suitable for hydrode-cyclization to aromatics. 

These are particularly applicable to burners firing the heavier grades of oil which contain long-chain molecules called asphaltines. The superheating of the water in the emulsified fuel droplet enhances atomization. The effect is to provide secondary atomization to the droplet as the steam is formed. 

Fig. 18.6 Joining up of (a) long-chain molecules or (b) branched molecules to produce (c) a...

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. 

Soaps act as cleansers because the two ends of a soap molecule are so different. The carboxylate end of the long-chain molecule is ionic and therefore hydrophilic (Section 2.13), or attracted to w ater. The long hydrocarbon portion of the molecule, however, is nonpolar and hydrophobic, avoiding water and therefore more soluble in oils. The net effect of these tw O opposing tendencies is that soaps are attracted to both oils and water and are therefore useful as cleansers. 

Addition polymers are formed by the reaction of the monomeric units without the elimination of atoms. The monomer is usually an unsaturated organic compound such as ethylene, H2C=CHs, which in the presence of a suitable catalyst will undergo an addition reaction to form a long chain molecule such as polyethylene. A general equation for the first stage of such a process is... 

The hindering potentials must be of importance in the question of the flexibility of long-chain molecules such as synthetic polymers and naturally occurring macromolecules such as proteins and nucleic acids. Again, direct information on barriers in these molecules will be difficult to obtain but inferences can presumably be drawn from experiments on simpler analogues. 

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