The chemical structure of a polymer

A polymer is a molecule composed of a succession of identical (or similar) polymer units  

The chain thus has a skeleton, side-groups, and two end-groups. Therefore, the formula of a linear polymer can be represented as follows  

The simplest species is polyethylene, whose skeleton consists of a series of carbon atoms each of which is linked together. Thus, its formula is 

The polymer in solution that is the most currently used for configurational studies is polystyrene, which has the formula 

Sometimes other atoms like nitrogen or oxygen can periodically be found in the skeleton (polypeptides and polyoxyethylene). It may even happen that the skeleton does not contain any carbon atoms (polydimethylsiloxane) the carbon is then replaced by silicon. 

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The chemical structure of a polymer chain determines its statistical properties, such as its average dimensions in space and its flexibility. These parameters, in turn, affect various properties of a network consisting of these chains. A detailed understanding of the single chain is therefore important in this regard. 

The chemical structure of a polymer can be analysed by many of the techniques used to characterise molecular species (see Chapter 3). Multinuclear NMR, IR and UV-visible spectroscopy, for example, are widely used key characterisation tools. Most polymers will dissolve in at least some readily available solvents (although the rate of dissolution may be slow due to chain entanglement effects). In cases where polymers are insoluble, solid-state NMR techniques can be used to provide excellent structural characterisation. Due to structural imperfections, unknown end groups and incomplete combustion problems as a result of ceramic formation (Section 8.2.5), elemental analysis data obtained by... 

Understanding of the mechanism of radiation degradation of polymer molecules is essential for development of improved and new industrial processes, for radiation-induced modification of polymer properties, and for selection of polymers for use in radiation environments. This means that the detailed chemical reactions resulting from absorption of radiation must be known. This fundamental understanding must enable us to relate the chemical structure of a polymer to changes in its chemical, physical and material properties. Such structure-property relationships require a great deal of research work, but they are the key to further advancement on a scientific basis. 

In his phenomenological study [1], Briick ordered a number of polymers in a row describing their affinity to become positively or negatively charged when rubbed with each other. Briick s row and other comparable rows first demonstrated the fundamental relationship between the chemical structure of a polymer and its tribo-electrical charging properties. In a former work [2] we followed that idea and showed that the tribo-electrical charging of two polymer species charged in a fluidized bed clearly depends on the affinity of the polymer surface to take up electron... 

The chemical structure of a polymer usually is represented by that of the repeat unit enclosed by brackets. Thus the hypothetical homopolymern s/v-vA—A—A—A—A—A—A—Anx-s/N is represented by-[A]-n where n is the number of repeat, units linked together to form the macromolecule. Table given below shows the chemical structures of some common homopolymers together with the monomers from which they are derived and some comments upon their properties and uses. It should be evident that slight differences in chemical structure can lead to very significant differences in properties. 

The naming of polymers or envisaging the chemical structure of a polymer from its name is often an area of difficulty. At least in part this is because most polymers have more than one correct name, the situation being further complicated by the variety of trade-names which also are used to describe certain polymers. The approach adopted here is to use names which most clearly and simply indicate the chemical structures of the polymers under discussion. 

The chemical structure of a polymer determines whether it will be crystalline or amorphous in the solid state. Both tacticity (i.e., syndio-tactic or isotactic) and geometric isomerism (i.e., trans configuration) favor crystallinity. In general, tactic polymers with their more stereoregular chain structure are more likely to be crystalline than their atactic counterparts. For example, isotactic polypropylene is crystalline, whereas commercial-grade atactic polypropylene is amorphous. Also, cis-pol3nsoprene is amorphous, whereas the more easily packed rans-poly-isoprene is crystalline. In addition to symmetrical chain structures that allow close packing of polymer molecules into crystalline lamellae, specific interactions between chains that favor molecular orientation, favor crystallinity. For example, crystallinity in nylon is enhanced because of... 

The chemical structure of a polymer determines its flammability, or to be more specific, the chemical structure of the polymer degradation product dictates the... 

The chemical structure of a polymer profoundly affects its glass transition temperature. Some of the most significant factors are the presence of bulky groups, polar groups, and the strength of intermolecular forces. 

In addition to providing information about the chemical structure of a polymer, vibrational spectroscopy can also give very useful information about the physical structure, because any two regions of the polymer that differ in the way the repeat units are arranged may exhibit detectable differences in their spectra. Furthermore, measurements of the strength of IR absorption or of Raman scattering can give quantitative information about the composition of any mixture. 

A determination of how Gat depends on the chemical structure of a polymer requires a definition of an entanglement. Various ideas have been proposed a fixed number of binary contacts per entanglement (Brochard and De Gennes, 1977) or per entanglement volume (Colby et al., 1992) a fixed number of strands per entanglement volume (Ronca, 1983 Lin, 1987 KavassaUs and... 

The hydrodynamic properties of macromolecules reported above have been calculated in the e order. The results contain the macroscopic scale length L which, in particular, depends on the chemical structure of a polymer. Of course, L-free expressions possess a universal meaning regardless of this structure. Such expressions can be obtained by combining the characteristic quantities (77], / (or D), (J ), and A2. 

The chemical structure of a polymer can also cause a contraction of the polymer coil compared to the unperturbed dimensions at theta-conditions. In this case the exponent a of the [ ]]-M-relationship shows values of a<0.5. A contraction of the coil occurs if the attractive intramolecular interactions between the polymer segments become larger than the interactions with the solvent molecules. In extreme cases, the solvent is forced out of the polymer coil and the chain segments start to form compact aggregates. The density of the polymer coil is then independent of the molar mass and the intrinsic viscosity is constant. In this case the exponent a is zero. An example is shown in Fig. 6.12 for compact glycogen in aqueous solution. 

Equivalent methods require information about the chemical structure of a polymer. End-group determinations allow the calculation of relative molar masses if the constitution of end groups is known (39). The sensitivity of end-group determinations depends on the experimental methods. C NMR spectrometry can yield information on polymers of molecular weight up to —8000. Titrations are applicable if the end group is either an acid or a base. They yield molecular weights up to —40,000. 

It is very difficult, if not impossible, to extrapolate the appearance of ecotoxic degradation metabolites or residues exclusively from the chemical structure of a polymer. Nevertheless, some basic guidelines concerning the presence (or absence) of heteroatoms and aromatic compounds in the polymer chain can be followed. The use of combined tests for biodegradability and ecotoxicity is strongly recommended. 

The advantages of the vibrational methods are that they are nondestructive, fast, and easy to use, and that remote measurement can be achieved through use of optic-fiber technology. Vibrational spectroscopic techniques provide methods of determining the chemical structure of a polymer and have the advantage that the methods are applicable to all polymers regardless of the phase or state of order in the system. The complete analysis of any type or shape of a polymer sample, from raw material via intermediate to final product, is possible on an as it is basis in the majority of cases. 

If the chemical structure of a polymer is altered during a viscoelastic experiment—in particular, if a cross-linked network is subjected to a reaction which increases or decreases the number of network strands while it is being investigated in the rubbery zone of viscoelastic behavior—the apparent mechanical properties will be profoundly influenced. For example, scission of the network strands will cause stress relaxation at constant strain" (Fig. 14-12) or creep under constant stress. Formally, if a single first-order chemical reaction is responsible, the relaxation may be described by a single relaxation time which is the reciprocal of the chemical rate constant, instead of the broad spectra which are characteristic of the usual mechanical processes. 

The mechanism of degradation depends mainly on the chemical structure of a polymer. The heat stability of different C-C links in the main chain decreases according to the following sequence ... 

The dynamic behaviour of polymer molecules is the bridging element in understanding the relationship between the chemical structure of a polymer and its physical properties. Molecular movement usually involves some change in the conformation of parts of the polymer chain. Since many readers may not have had experience of conformational analysis, it is now appropriate to consider the various forms of motion which are possible for polymers by starting with their small molecule analogues. 

The chemical structure of a polymer is one of the factors that determine its reactivity, see Table 1.9. That is why polymers with saturated unbranched hydrocarbon chain structure are the most oxidation resistant, reacting extremely slowly with oxygen under normal conditions and in the absence of light. Even in the course of several years of exposure or service, these products do not change their properties. Although the bond energy of C-C-bonds is clearly weaker than that of C-H bonds, C-H bonds are attacked almost exclusively radically. Here, neither degree of poly-... 

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