Structure of Plastics

Plastics are composed of polymer molecules and various additives, polymers are long-chain molecules (also called giant molecules or macromolecules), which are formed by polymerization; that is, linking and cross-linking of different monomers.

A monomer is the basic building block of polymer. The word mer (from the Greek Meros, meaning part) indicates the smallest repetitive unit, similar to the term unit cell in crystal structures. Thus, polymer means many mers or units. Monomers are organic materials. They include carbon atoms joined in covalent bonds (electron sharing) with other atoms such as hydrogen, oxygen, nitrogen, fluorine, chlorine, silicon, and sulfur.

A typical monomer is the ethylene molecule as these molecules can be made to attach themselves to other ethylene molecules by a chemical reaction involving heat, pressure, and a catalyst. In this reaction, the double bonds between the carbon atoms open and he molecules arrange them in a long line.

The polymer is called a linear polymer because of its linear structure. The number of molecules in this chain (length of chain) is known as the molecular weight. A linear molecule does not mean that it is straight. These molecules (chain) are of different lengths and their arrangement is amorphous (without any long-range order). This arrangement is often described as a bowl of spaghetti, or worms in a bucket, all intertwined with each other. Because of the differences in the length of the chains, the molecular weight of a polymer is determined on a statistical basis.

Although within each long-chain molecule there is covalent bonding (primary bonds), no such intermolecular bonding exists between different chains. The bonds between different chains (intramolecular), and between the overlapping portions of the same chain, are known as secondary bonds. Secondary bonds are van der waals bonds, hydrogen bonds, and ionic bonds, Secondary bonds are all much weaker than the covalent bonds within the chain.

The difference in strength between the two bonds is on the order of one to two orders of magnitude. This difference is important, because it is the weaker secondary bonds that determine the overall strength of the polymer. (Thus, for instance, an object is only as strong as its weakest part.)

If the repeating units in a chain are all of the same type, the macromolecule is known as a homopolymer. However, in order to obtain certain special properties, two or three different types of monomers can be combined in a polymer. These are known as copolymers and terpolymers, respectively.

Branching and linking
The properties of a copolymer depend not only on the two monomers, but also on their arrangement in the molecular structure. Branching, for instance, interferees with the relative movement of the molecules and affects the resistance to deformation. Another kind of chain is cross-linking, which is spatial (three-dimensional) network structure with adjacent chains together. Cross-linking has great influence on the properties of the polymer, such as in the vulcanization of rubber.

Thermoplastics, such as acrylic, nylons, polyethylene, and polyvinyl chloride, are linear-chain molecules. Thermosetting plastics, such as epoxies, phenolics, and silicones, have cross-linked chains. An example of cross-linking is the vulcanization of rubber for automobile tires, each tire being one giant molecule.

Plastics mode of linear chain structures are called thermoplastics, and those made of cross-linked chain are called thermosetting plastics, or simply, thermosets.