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Material hierarchy: Polymer
Polymers are large molecules built from monomers. A polymer is a compound consisting of long-chain molecules, each molecule made up of repeating units connected together. There may be thousands, even millions of units in a single polymer molecule. The word is derived from the Greek words poly, meaning many, and meros (reduced to mer), meaning part. Most polymers are based on carbon and are therefore considered organic chemicals.
With the exception of natural rubber, nearly all of the polymeric materials used in engineering are synthetic. The materials themselves are made by chemical processing, and the products are made by solidification processes.
Polymers can be separated into plastics and rubbers.
A polymer molecule consists of many repeating mers to form very large molecules held together by covalent bonding. Elements in polymers are usually carbon plus one or more other elements such as hydrogen, nitrogen, oxygen, and chlorine. Secondary bonding (van der Waals) holds the molecules together within the aggregate material (intermolecular bonding). Polymers have either a glassy structure or mixture of glassy and crystalline. There are differences among the three polymer types. In thermoplastic polymers, the molecules consist of long chains of mers in a linear structure. These materials can be heated and cooled without substantially altering their linear structure. In thermosetting polymers, the molecules transform into a rigid, three-dimensional structure on cooling from a heated plastic condition. If thermosetting polymers are reheated, they degrade chemically rather than soften. Elastomers have large molecules with coiled structures. The uncoiling and recoiling of the molecules when subjected to stress cycles motivate the aggregate material to exhibit its characteristic elastic behavior.
The molecular structure and bonding of polymers provide them with the following typical properties: low density, high electrical resistivity (some polymers are used as insulating materials), and low thermal conductivity. Strength and stiffness of polymers vary widely. Some are strong and rigid (although not matching the strength and stiffness of metals or ceramics), while others exhibit highly elastic behavior.
Polymers have moduli that are low, roughly 50 times less than those of metals, but they can be strong—nearly as strong as metals. A consequence of this is that elastic deflections can be large. They creep, even at room temperature, meaning that a polymer component under load may, with time, acquire a permanent set. And their properties depend on temperature so that a polymer that is tough and flexible at 20°C may be brittle at the 4°C of a household refrigerator, yet creep rapidly at the 100°C of boiling water. Few have useful strength above 200°C. If these aspects are allowed for in the design, the advantages of polymers can be exploited. And there are many. When combinations of properties, such as strength-per-unit-weight, are important, polymers are as good as metals. They are easy to shape: complicated parts performing several functions can be molded from a polymer in a single operation. The large elastic deflections allow the design of polymer components that snap together, making assembly fast and cheap. And by accurately sizing the mold and pre-coloring the polymer, no finishing operations are needed. Polymers are corrosion resistant and have low coefficients of friction. Good design exploits these properties.
A polymer, pronounced PAHL ih muhr, is a large molecule formed by the chemical linking of many smaller molecules into a long chain. The small molecular building units are called monomers. Monomers are joined into chains by a process of repeated linking known as polymerization. A polymer may consist of thousands of monomers. Some polymers occur naturally. Others are synthetic.
Many common and useful substances are polymers. For example, starch and wool are naturally occurring polymers. Starch is formed by plants from a simple sugar called glucose, and wool is a variety of protein. Nylon and polyethylene, a tough plastic material, are examples of synthetic polymers. Rubber, another polymer, occurs naturally and is also made synthetically.
A chain molecule has a definite length, but, like a piece of string, it can assume a variety of shapes. This combination of molecular length and flexibility gives polymers many useful and unique properties. For example, rubber and many other polymers can be stretched to several times their normal length without breaking. The chains simply straighten into more extended shapes. Because of the large size of the molecules, polymers do not dissolve easily. They also have high viscosity (resistance to flowing).