Classification of polymers

Polymers can be classified in many ways, such as by source, method of synthesis, structural shape, thermal processing behavior, and end use of polymers. Some of these classifications have already been considered in earlier sections. Thus, polymers have been classified as natural and synthetic according to source, as condensation and addition (or step and chain) according to the method of synthesis or polymerization mechanism, and as linear, branched, and network according to the structural shape of polymer molecules. According to the thermal processing behavior, polymers are classified as thermoplastics and thermosets, while according to the end use it is convenient to classify polymers as plastics, fibers, and elastomers (Rudin, 1982). 

Polymers can be classified in many ways according to various criteria such as  

Linear, branched, cross-linked, ladder, star-shaped, combshaped, dendritic, and the like (Fig. 14.2) 

Amorphous or crystalline based on absence or presence of long-range ordered pattern among polymer chains 

Homopolymer or copolymer with different t3rpes of copolymer 

Fibers, plastics, or elastomers. Polymers (s3mthetic or natural) can be divided into various families fibers, elastomers, plastics, adhesives, and each family itself has subgroups 

The manner in which polymers are formed is also a distinguishing feature. Two common methods are described. 

A polymer is a giant molecule built up of relatively small, chemically bonded, repeating units. The molecular weight of such molecules may run from very low values into the millions, and an ordinary polymer generally 

The size of a polymer molecule is expressed in terms of the average number of repeat units in the molecule and is called the degree of polymerization (DP). From the known DP and the known molecular weight of the monomer (repeat unit), the average molecular weight of a polymer is easily computed  

The constitution of a polymer is generally described in terms of the structural units. When only one type of monomer unit is present in a polymer, it is called a homopolymer a polymer having two or more structural units is referred to as a copolymer. In linear polymers, the monomer units are joined together in a straight, open-chain fashion, whereas cross-linked polymers have a three-dimensional network. 

An ordered sequence of two or more monomers (a sequential polymer) such as a co(polyethylenemaleic anhydride)  

A random sequence of the monomers in which the distribution of each monomer is random, e.g., 

There are a number of methods of classifying polymers. One is to adopt the approach of using their response to thermal treatment and to divide them into thermoplastics and thermosets. Thermoplastics are polymers which melt when heated and resolidify when cooled, while thermosets are those which do not melt when heated but, at sufficiently high temperatures, decompose irreversibly. This system has the benefit that there is a useful chemical distinction between the two groups. Thermoplastics comprise essentially linear or lightly branched polymer molecules, while thermosets are substantially crosslinked materials, consisting of an extensive three-dimensional network of covalent chemical bonding. 

Another classification system, first suggested by Carothers in 1929, is based on the nature of the chemical reactions employed in the polymerisation. Here the two major groups are the condensation and the addition polymers. Condensation polymers are those prepared from monomers where reaction is accompanied by the loss of a small molecule, usually of water, for example polyesters which are formed by the condensation shown in Reaction 1.1. 

By contrast, addition polymers are those formed by the addition reaction of an unsaturated monomer, such as takes place in the polymerisation of vinyl chloride (Reaction 1.2). 

This system was slightly modified by R J. Flory, who placed the emphasis on the mechanisms of the polymerisation reactions. He reclassified polymerisations as step reactions or chain reactions corresponding approximately to condensation or addition in Carother s scheme, but not completely. A notable exception occurs with the synthesis of polyurethanes, which are formed by reaction of isocyanates with hydroxy compounds and follow step kinetics, but without the elimination of a small molecule from the respective units (Reaction 1.3). 

Polymer chains may be linear, or possibly with additional chemical groups forming branches along the primary chain. If a polymer is made up of the same repeating units of monomers, it is called a homopolymer otherwise, if it is made up of different types of monomers arranged in some sequence, then it is called a copolymer. 

Both thermoplastics and thermosets are used in injection molding. The main difference in processing is the mold temperature. For thermoplastics, the mold walls are colder than the melt, while for thermosets the mold walls are hotter than the material in the cavity. 

This book will only consider thermoplastics. Most of the mathematical simulation methods described later are, however, applicable to thermosets as well. The reader can see Sidi and Kamal (1982) for some discussions on injection molding of thermosets. 

Based on their chain conformation and morphology, thermoplastic polymers can be classified as 

Amorphous polymers comprise randomly configured molecular chains. Upon cooling, they change from rubber-like materials to glassy materials. The temperature at which the transition occurs is called the glass transition temperature (T ), At this temperature, there is a change in the slope of the specific volume/tem-perature curve. 

Thermoplastics have moderate crystalliuity. They cau uudergo large elougatiou, hut this elougatiou is uot as reversihle as it is for elastomers. Examples of thermoplastics are polyethyleue aud polypropyleue. 

Synthetic rubber (elastomers) are high molecular weight polymers with long flexible chains and weak intermolecular forces. They have low crystallinity (highly amorphous) in the unstressed state, segmental mobility, and high reversible elasticity. Elastomers are usually cross-linked to impart strength. 

Fessenden, R., and Fessenden, J., Organic Chemistry, 4th Ed., Brooks/Cole Publishing Co., 1991, p. 926. 

Rodriguez, E, Principles of Polymer Systems, 3rd Ed., Hemisphere Publishing Corp., New York, 1989, p. 108. 

Stevens, M. P, Polymer Chemistry, Addison Wesley Publishing Co., London, 1975, p. 156. 

Given the versatility of polymers, they can be classified according to different criteria. In this section, we review some of these classifications. 

A second major classification of polymers was proposed by Flory [1] in 1953. This is based on the kinetic mechanism of the polymerization reaction. Hory classifies polymerizations into two categories  

1 Step-growth Polymerization The simplest scheme of this polymerization involves the reaction of a difunctional monomer AB, which contains both functional groups A and B in the molecule. For example, A can be an amine and B a carboxylic acid group. Another scheme involves the reaction between two difunctional monomers of the type AA and BB. In any case, each polymer linkage will have involved the reaction of the functional groups A and B coming from two molecules (monomers or chains). Some examples of polymers synthesized by this mechanism are polyurethane, polyamide, and polyester. 

The chain growth occurs by steps at each step, a reaction between the functional groups belonging to two monomers or chains occurs. If Mj denotes monomer, M2 dimer, M3 trimer, and so on, the mechanism can be schematically represented as follows  

The size of the chains increases gradually and relatively slowly. 

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In Sec. 1.4 we discussed the classification of polymers into the categories of addition or condensation. At that time we noted that these classifications could be based on the following ... 

Table 1. Classification of Polymers According to Properties and Processing...

Table Chemical Classification of Polymers Monomer structure Polymer...

Classification of polymers varies depending on the country and application. Each case must be examined in its context and the following data are only examples ... 

There are several ways of classification of polymers based on some special considerations. The following are some of the common classifications of polymers ... 

The classification of polymers according to polymerization mechanism, like that by structure and composition, is not without its ambiguities. Certain polymerizations show a linear increase of molecular weight with conversion (Fig. 1-lc) when the polymerization... 

Figure 1.55 Classification of polymers according to macroscopic stmcture (a) linear, (b) branched and (c) networked.

Classification of Polymers According to Their Response to Ionizing Radiation... 

The classification of polymers for oral drug delivery can be done by using various means. To make this discipline readily accessible to the novice reader, the hydro-phobic-hydrophilic nature of the polymer was chosen to group polymers since the mechanism of biomacromolecule release from most hydrophobic polymeric devices is similar the mechanism of release from most hydrophilic polymeric devices also have similar mechanisms. Hydrophobic polymers are described first, followed by hydrophilic polymers. 

Polymer identification starts with a series of preliminary tests. In contrast to low molecular weight organic compounds, which are frequently satisfactorily identified simply by their melting or boiling point, molecular weight and elementary composition, precise identification of polymers is difficult by the presence of copolymers, the statistical character of the composition, macromolecular properties and, by potential polymeric-analogous reactions. Exact classification of polymers is not usually possible from a few preliminary tests. Further physical data must be measured and specific reactions must be carried out in order to make a reliable classification. The efficiency of physical methods such as IR spectroscopy and NMR spectroscopy as well as pyrolysis gas chromatography makes them particularly important. 

Classification of polymers on the basis of their mechanical behaviour... 

On the basis of the general behaviour described in the preceding sections it is possible to develop a classification of polymers for practical use. This is given in Table 2.11. The nomenclature is to agreement with a proposal by Leuchs (1968). 

TABLE 2.11 Classification of polymers on the basis of mechanical behaviour (nomenclature according to Leuchs, 1968)... 

Yet another type of classification of polymers is based on the type of repeating unit. A homopolymer has one type of repeat unit. Copolymers are polymers that have more than one type of monomers or repeat units. If the monomers in a copolymer are distributed randomly along the chain, it is called a regular or random copolymer. If, on the other hand, a sequence of one type of monomer is followed by a sequence of another type of monomer, it is called a block copolymer. If the main chain is one type of monomer and the branch chains are of another monomer, it is called a graft copolymer. 

The most fundamental classification of polymers is whether they are naturally occurring or synthetic. Common natural polymers (often referred to as biopolymers) include macromolecules such as polysaccharides e.g., starches, sugars, cellulose, gums, etc.), proteins e.g., enzymes), fibers e.g., wool, silk, cotton), polyisoprenes e.g., natural rubber), and nucleic acids e.g., RNA, DNA). The synthesis of biodegradable polymers from natural biopolymer sources is an area of increasing interest, due to dwindling world petroleum supplies and disposal concerns. 

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