Small, individual molecules are known as mers, or monomers (literally, “single mers”). When conditions are right, these small molecules can chemically “link” together to form long, chainlike structures. The macromolecules (giant molecules) that result may incorporate thousands of the original monomers. These long chain macromolecules are therefore known as polymers (“many mers”). The linking process itself is called polymerization. An example of this process is shown in Figure 12. Methane monomers can combine to form ethane, and eventually, polyethylene. Rubber and plastics are polymer-based materials.

Changes in physical properties as a result of polymerization are largely a factor of molecular weight. When molecules (each with their own total weight) join together to form a polymer, the sum of the molecular weights has a huge impact on the polymer’s physical properties. As a general rule, an increase in chain length (and thus molecular weight) also means an increase in strength and viscosity (resistance to flow).

Long polymeric chains are held in place by intermolecular forces (known as van der Waals forces) and by chain entanglement (as in a bowl of spaghetti). The intermolecular forces are heat-sensitive, so that as a polymer is heated, the molecular motion increases and the attractive forces between the molecules decrease. The polymer chains can then slide past one another.

Oil seal polymers are composed of branched, non-symmetrical molecules that cannot fit closely to one another. Because of this increased distance between the molecules, the van der Waals forces will be at their weakest, resulting in a random mass of twisted and entwined polymer chains. These polymers are said to be amorphous (see Figure 13).

Because their intermolecular forces are not very strong, amorphous polymers can be thought of as very viscous (thick) liquids that appear to be solids. All rubbers or elastomers are amorphous at room temperature.

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