The structure of polymers

The individual chain molecules within a polymer are usually organic compounds. These chain molecules consist of numerous identical units, called monomers. Typically, the number of monomers in a molecular chain is of the order of 10 to 10 , resulting in an overall molecular length of up to a few micrometres. The average number of monomers in the chain molecules of a polymer is called the degree of polymerisation. 

All molecules that can link in such a chain reaction can be used to synthesise polymers. Therefore, there exists a wide spectrum of polymers with strongly varying chemical and physical properties. A selection of technically important polymers will be presented in the next section. 

In between the molecular chains, there are no strong chemical bonds. Depending on the molecular structure, the strongly temperature dependent dipole, hydrogen, or van der Waals bonds are formed. 

Polymers form by two different types of polymerisation reactions, addition polymerisation and condensation polymerisation. These reactions are explained in 

The mechanical properties of polymers are mainly determined by the mobility of the chain molecules and will be discussed in detail in chapter 8. The mobility depends on the chemical structure of the polymer. A polymer with a carbon chain with single bonds, for instance, is flexible at each of the carbon atoms because a single bond between two carbon atoms can rotate freely. Double bonds, on the other hand, are rigid. The mobility is also affected by the presence of side groups. In this section, we will exemplify the structure of some polymers. 

The less simple polymers (like the epoxies, the polyesters and the formaldehyde-based resins) are networks each chain is cross-linked in many places to other chains, so that, if stretched out, the array would look like a piece of Belgian lace, somehow woven in three dimensions. These are the thermosets if heated, the structure softens but it does not melt the cross-links prevent viscous flow. Thermosets are usually a bit stiffer than amorphous thermoplastics because of the cross-links, but they cannot easily be crystallised or oriented, so there is less scope for changing their properties by processing. 

In this chapter we review, briefly, the essential features of polymer structures. They are more complicated than those of metal crystals, and there is no formal framework (like that of crystallography) in which to describe them exactly. But a looser, less precise description is possible, and is of enormous value in understanding the properties that polymers exhibit. 

Ethylene, C2H4, is a molecule. We can represent it as shown in Fig. 22.1(a), where the square box is a carbon atom, and the small circles are hydrogen. Polymerisation breaks 

Thermoplastics are the largest class of engineering polymer. They have linear molecules they are not cross-linked, and for that reason they soften when heated, allowing them to be formed (ways of doing this are described in Chapter 24). Monomers which form linear chains have two active bonds (they are bifunctional). A molecule with only one active bond can act as a chain terminator, but it cannot form a link in a chain. Monomers with three or more active sites (polyfunctional monomers) form networks they are the basis of thermosetting polymers, or resins. 

The simplest linear-chain polymer is polyethylene (Fig. 22.3a). By replacing one H atom of the monomer by a side-group or radical R (sausages on Fig. 22.3b, c, d) we obtain the vinyl group of polymers R = Cl gives polyvinyl chloride R = CIT3 gives 

---

The above discussion points out the difficulty associated with using the linear dimensions of a molecule as a measure of its size It is not the molecule alone that determines its dimensions, but also the shape in which it exists. Linear arrangements of the sort described above exist in polymer crystals, at least for some distance, although not over the full length of the chain. We shall take up the structure of polymer crystals in Chap. 4. In the solution and bulk states, many polymers exist in the coiled form we have also described. Still other structures are important, notably the helix, which we shall discuss in Sec. 1.11. The overall shape assumed by a polymer molecule is greatly affected... 

Whenever a phase is characterized by at least one linear dimension which is small, the properties of the surface begin to make significant contributions to the observed behavior. We shall examine the structure of polymer crystals in more detail in Sec. 4.7, but for now the following summary of generalizations about these crystals will be helpful ... 

MEARES, R, Polymers Structure and Bulk Properties, Van Nostrand, London (1965) MILLER, M. L., The Structure of Polymers, Reinhold, New York (1966)... 

So far the structure of polymers has been described with reference to the material with the simplest molecular structure, i.e. polyethylene. The general principles described also apply to other polymers and the structures of several of the more common polymers are given below. 

The glasslike sculpture is made of a polymer, which allows it to stand up to the outdoor weather. The repeating design, though random, recognizes the randomness and repetitiveness of the structure of polymers. 

Horkay, F Hecht, AM Zrinyi, M Geissler, E, Effect of Cross-Links on the Structure of Polymer Gels, Polymer Gels and Networks 4, 451, 1996. 

For SCVCP in general, DB strongly depends on the comonomer ratio (y=[monomer]o/[inimer]o) [73,74]. In the ideal case,when all rate constants are equal, for y>>l, the final value of DB decreases with y as DB=2/(y+l) which is four times higher than the value expected from dilution of inimer molecules by monomers. For low values of (yreactivity ratios, the structure of polymer obtained can change from macroinimers when the monomer M is much more reactive than the vinyl groups of inimer or polymer molecules to hyperstars in the opposite limiting case. 

The structure of polymers 8 to 10 contains the same two repeating units in different proportions. During thermolysis of 4 and 5 to polymers 8 and 9, respectively,... 

The reaction was carried out by adding a THF solution of 9-BBN [equimolar to the polypropylene oxide) (PPO) chain ends] drop by drop to PPO or PPO diallylether and stirring the resulting mixture for 5-7 hours (scheme 3). The structure of polymers obtained was confirmed by 1H- and nB-NMR spectra. From the differential scanning colorimetric (DSC) measurement, no peak due to the melting point was observed to show that the polymer was fully amorphous. 

L. Mandelkern, The structure of polymers crystallized in the bulk. In M. Dosiere (Ed.), Crystallization of Polymers, NATO ASI Series, Kluwer Academic Publishers, The Netherlands, 1995, p. 25. 

Future challenges for polymerization model catalysts are to study the structure of polymers below their melting point in what is called the nascent morphology. Such work can be undertaken on silica-supported chromium catalysts as discussed above, or on so-called single-site catalysts, such as metallocenes, applied on flat silica supports. 

However, before proceeding to the determination of Molecular weights of polymers we will take up an elementary discussion of the use of x-ray diffraction, spectroscopic techniques and electromicroscopic techniques, etc. in determining the structure of polymers. 

Over the years Herman Mark has been known as polymer science s advocate, early explorer, spokesman, representative, teacher, and senior citizen. Starting, as we have seen, as a young man when the concept of high molecular weight was not accepted, Dr. Mark and his many associates confirmed the structure of polymers and helped open whole new areas of scientific research. 

All of these chemical species have importance in the production of polymeric materials. There are several shorthand techniques for writing down the structures of polymers. The carbon-based polymer molecules using the stick representation are made up of atoms connected by covalent bonds (represented here by the straight lines between the carbon and the hydrogen and the carbon-to-carbon molecules), as shown in Fig. 2.6. To reiterate, carbon is always tetravalent, having four covalent bonds, and a schematic of the paired electrons for two of the incorporated carbon molecules can be seen in the bottom of Fig. 2.6. Thus each stick represents two electrons. For the two highlighted carbon atoms in the polyethylene molecule of Fig. 2.6, the electron representation is shown, where there are four covalent bonds associated with each carbon and each bond is made up of two shared electrons represented by the black dots. This polymer molecule is made up of only carbon and hydrogen with no double bonds, and it represents a linear form... 

A functional polymer according to Definition 3.6 of the present document is a polymer that exhibits specified chemical reactivity or has specified physical, biological, pharmacological, or other uses that depend on specific chemical groups. Thus, several terms concerned with properties or the structure of polymers are included in Section 3 whenever they are closely related to specific functions. Terms that are defined implicitly in the notes and related to the main terms are given in bold type. 

The refractive index is an important quantity for characterizing the structure of polymers. This is because it depends sensitively on the chemical composition, on the tacticity, and - for oligomeric samples - also on the molecular weight of a macromolecular substance. The refractive indices (determined using the sodium D line) of many polymers are collected in the literature. In order to characterize a molecule s constitution one requires knowledge of the mole refraction, Rg. For isotropic samples, it can be calculated in good approximation by the Lorentz-Lorenz equation ... 

The value of the modulus and the shape of the modulus curve allow deductions concerning not only the state of aggregation but also the structure of polymers. Thus, by means of torsion-oscillation measurements, one can determine the proportions of amorphous and crystalline regions, crosslinking and chemical non-uniformity, and can distinguish random copolymers from block copolymers. This procedure is also very suitable for the investigation of plasticized or filled polymers, as well as for the characterization of mixtures of different polymers (polymer blends). 

These are long chain molecules consisting of multiples of repeat units (monomers). These are linked by covalent bonds in a three-dimensional network which is characteristic of a polymer. The magnitude of the length of a polymeric molecule can extend up to several hundred nanometres. The dimensions of individual polymer molecules and their arrangement define the structure of polymers and their properties. Many catalytic processes are aimed at producing polymers as we describe in the following chapters. (Polymers can also be used as catalyst supports.)... 

Copolymerization is a facile method to diversify the structure of polymer materials. However, if the polymerizabiHties of comonomers are far from each other, copolymerization is essentially difficult, resulting in the formation of a mixture of the homopolymers and/or the copolymer with block sequences. This is the case for the anionic copolymerization of epoxide and episulfide, where the po-lymerizabihty of episulfide is much higher than that of epoxide, and the copolymer consisting mostly of -S-C-C-S- and -O-C-C-O- homo sequences is formed [87]. As mentioned in the previous sections, the zinc complex of /-methylpor-phyrin brings about polymerization of both epoxide and episulfide. 

Thus the effects of the rate of application of stress and the ambient temperature must be recognized when polymers are used as structural materials, and definite rates and temperatures must be specified for tests, such as those for tensile and flexural strengths cited in Chapter 3. A knowledge of the structure of polymers is essential for the understanding of these effects, which differ from the effects of stress and temperature on all other materials of construction. 

M. L. Miller, The Structure of Polymers, Wiley-lnterscience, New York (1964). 

---