POLYMERS

The term polymer comes from "poly," meaning many, and "mer," meaning units. Hence, polymers are composed of many units—in this case, structural units called monomers. A monomer is any unit that can be converted into a polymer. Similarly, a dimer is a combination of two monomers; a trimer is a combination of three monomers, and so on.

Organic chemistry

Alkanes ( paraffins) are composed of all C–C, saturated bonds and have the general formula C_nH_2n+2. The alkenes have one C=C double bond and alkynes have a carbon–carbon triple bond such as Methane (CH₄) and Ethane (C₂H₆). The alkenes have one C=C double bond, and alkynes have a carbon–carbon triple bond. Many chemical reactions involving organic molecules, particularly polymerization reactions, involve functional groups. A functional group is an atom or groups of atoms that show a relative constancy of properties when attached to different carbon chains. For example alcohols when a hydroxyl group, –OH, attaches to hydrocarbons as R-OH

Polymerization

Polymerization is the process of combining many small molecules known as monomers into a covalently bonded chain or network. During the polymerization process, some chemical groups may be lost from each monomer. This is the case, for example, in the polymerization of PET polyester. The monomers are terephthalic acid (HOOC-C₆H₄-COOH) and ethylene glycol (HO-CH₂-CH₂-OH) but the repeating unit is -OC-C₆H₄-COO-CH₂-CH₂-O-, which corresponds to the combination of the two monomers with the loss of two water molecules. The distinct piece of each monomer that is incorporated into the polymer is known as a repeat unit or monomer residue.An interesting and important structural characteristic of many polymers is that they are easily transformed from the amorphous to the crystalline state. Unlike inorganic glasses, which generally need a great deal of energy to devitrify (crystallize), or metals that require nonequilibrium conditions to form amorphous structures, many polymers have amorphous to crystalline transition temperatures that are near room temperature.

Types of Polymers

There are many types of materials which incorporate long chain structures with many repeated units. One useful way of categorising polymers for the requirements of electronic assembly is by functional behaviour. In the strictest sense these categories are not fixed, or even particularly precise, some materials can fit into more than one category:

• Elastomers are flexible or 'rubbery' materials which can readily be deformed, and return rapidly to almost their original shape and size once released from stress, thus making them able to form reliable seals. Natural and synthetic rubbers are common examples of elastomers
• Plastics are materials which can be shaped or moulded under appropriate conditions of temperature and pressure, and then hold their shape. In contrast to elastomers, plastics have a greater stiffness and lack reversible elasticity
• Some plastics, such as nylon and cellulose acetate, can be formed into fibres.