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FL&O_section_5[1] - Chapter 5 Polymers Polymers...

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Chapter 5. Polymers. Polymers constitute another important class of materials. It is immediately obvious that they are different from metals and ceramics. They are generally lightweight, which reflects their composition: they are organic materials, consisting mostly of carbon and hydrogen atoms. Polymers are usually transparent or translucid (or they have strong colors because dyes have been incorporated in them) and excellent electric insulators. Their valence electrons reside in filled bands. Mechanically, polymers are much softer, weaker and less rigid than either metals or ceramics; they melt at relatively low temperatures. Clearly, a much weaker chemical bond is involved: polymers are held together by secondary bonds . Some primary bonds are involved in a class of polymers called thermosets which we will discuss later. Let us first examine the molecular structure of polymers and then analyze the chemical bonds responsible for their solidity. 5.1. Definition of a Polymer. The simplest polymer is polyethylene, which we will use to introduce the main characteristics of polymers. Look at the name: poly-ethylene means that its structural unit is composed of many ethylene molecules. Polyethylene is synthesized by a chemical reaction involving a catalyst, which is a peroxide radical, and ethylene gas. The peroxide attacks the double bond of ethylene and transforms the latter into a radical (a molecule with an unsatisfied, chemically active, bond). H H H H HO- + C = C HO – C – C - H H H H The new molecule attacks other ethylene molecules in a similar way H H H H H H H H HO – C – C - + C = C HO – C – C – C – C - H H H H H H H H The process continues until a long molecule is formed, consisting of many thousands of ethylene molecules; this is polyethylene which can be written as H H - [C-C] N - H H H H The repeat unit -C – C- is called a mer. H H
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The number N of mers is the degree of polymerization, this can be very large, often reaching N = 10,000, resulting in molecules as larger than one micrometer. These are called macromolecules . The molecular weight of the polymer is the sum of all atomic weights, in other words it is N times the molecular weight of the mer. The molecular weight of ethylene is M = 2x12 + 4x1 = 28. With N = 10,000, the molecular weight of the polyethylene chain is 280,000. In the processing of polymers, it is not possible to obtain all chains of the same length. Thus, in practice, the molecular weight given for a polymer is an average value . 5.2. Polymers and secondary bonds; thermoplastics. Now let us look at the bonding of polyethylene. The bonds between the carbon atoms in the chain (red in fig. 5.1) and between the carbon and hydrogen are covalent and based on the sp 3 hybrid valence electrons of carbon. This results in the shape of the molecule shown in figure 5.1. These valence electrons completely fill the valence band; therefore polymers are insulators and are often transparent.
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