In all cases, plastic materials consist of a mass of very large molecules. Although some polymeric materials have a structure which is heterogeneous and complex, most plastics consist of long chains of repeating small molecular units, or monomers, covalently bonded together. In the bulk material, polymer chains are mechanically intertwined and may also be chemically bonded together or crosslinked at various points along their lengths to form a network structure. These materials are of high molecular weight and hence are referred to as high polymers or macromolecules.
Polymer chains may be linear or branched depending on the nature of the monomer and the polymerization reaction. In homo-polymers, the monomer units are all of one chemical type. Hydrocarbon polymers such as polyethylene are based on carbon and hydrogen alone. Other carbon chain polymers may have atoms of other elements such as chlorine incorporated into the polymer structure. The replacement of carbon atoms in the backbone itself by atoms of other elements produces het-erochain polymers. In co-polymers, the monomer units are of two or more types. There are three principal types of arrangement of the units in co-polymers, alternating, ran-
dom and block. Polymer terminology is illustrated in Figure 4.1.
Poly(vinyl acetate) (PVAC), one of the simplest polymers, consists of up to 20 000 repeating units of vinyl acetate joined together. This can be expressed by writing the chemical formula as [CH2CHOCOCH3]n where n specifies the number of monomer units in the chain and is called the degree of polymerization (DP). Synthetic polymers are formed by two major classes of chemical reactions, addition reactions and condensation reactions (Hall, 1981). Because of the nature of the reactions by which polymers are formed, not all molecules of a given compound are identical in size. Even under controlled conditions of manufacture there will be a range of molecular weights and the DP quoted for any product will be an average. The plastic may also contain some unre-acted monomer and small quantities of other impurities. All these factors are important because the mechanical, chemical and ageing properties depend to a considerable extent on them (Brydson, 1991).
The simplest macromolecules are composed of fundamental units linked end to end in chains with relatively few branches or side chains. The links or primary (covalent) bonds along each chain are strong compared to the secondary forces (e.g. Van der Waals forces and hydrogen bonding) holding adjoining chains together and it is therefore possible to separate the molecules from each other. Such materials are usually both soluble and fusible, that is they are thermoplastic, becoming soft and fluid when heated and returning to the solid state on cooling. If secondary forces are sufficiently strong, a linear polymer may be insoluble or restricted in solubility and it may be infusible or fusible only at temperatures at which the polymer begins to decompose. If the composition is such that the chain molecules are flexible, but the inter-chain forces are very weak, random coiling occurs and the material shows long range elasticity. Such materials are known as elastomers (e.g. rubber). If more than two primary linkages can be formed by each fundamental unit, the structure of the polymer becomes one of chains with cross chains and may approach a fully crosslinked network which is effectively a single molecule. Polymers of this type are much less tractable than linear polymers being both insoluble and infusible. The name thermoset was originally given to them because they were only formed under the influence of heat as the result of chemical reaction. They are usually used as resin pre-polymers, the crosslinking being deferred until it can be completed in situ.
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