Properties of Polymers

The following variables can be controlled when producing a polymer.

• The monomer polymerized or the monomers copolymerized.

• The reagent used to initiate the polymerization reaction.

• The identity and amount of the reagent used to crosslink the polymer chains.

• The temperature and pressure at which the polymerization occurs.

• The solvent in which the monomer is polymerized.

• The way the polymer is collected, which can produce either a more or less random alignment of the polymer chains or a fabric in which the chains are aligned in one direction.

Changing one or more of these parameters can affect the linearity of the polymer, its average molecular weight, the tacticity of side chains on the polymer backbone, and the density of the product.

It is also possible to change the properties of a polymer by adding either stabilizers or plasticizers. Stabilizers are used to increase the ability of a plastic to resist oxidation, to make it less sensitive to either heat or light, or as flame retardants. Plasticizers increase the flexibility of a plastic by acting as a lubricant, decreasing the friction between molecules as one polymer chain moves past another. They also increase the amount of empty space the so-called free volume within the polymer by opening up space between the polymer chains to increase the ease with which the chain ends, the side chains, and the main chain can move.

The result of all of these manipulations can be a polymer as strong as Kevlar, which is used to make bullet-proof vests, or a material as easy to rip as a piece of paper. It can be as hard as a bowling ball or as soft as a piece of tissue paper. It can be as brittle as the disposable polystyrene glasses used at parties or as elastic as a Styrofoam coffee cup.

The following list describes some of the important properties of a polymer

Heat capacity/ Heat conductivity The extent to which the plastic or polymer acts as an effective insulator against the flow of heat. (The polystyrene in disposable plastic glasses isn't a very good insulator. However, blowing air through styrene while it is being polymerized gives the Styrofoam used for disposable coffee cups, which is a much better insulator.)

Thermal expansion The extent to which the polymer expands or contracts when heated or cooled. (Silicone is often used to seal glass windows to their frames because it has a very low coefficient of thermal expansion.) Thermal expansion is also concerned with the question of whether the polymer expands or contracts by the same amount in all directions. (Polymers are usually anisotropic. They contain strong covalent bonds along the polymer chain and much weaker dispersive forces between the polymer chains. As a result, polymers can expand by differing amounts in different directions.)

Crystallinity The extent to which the polymer chains are arranged in a regular structure instead of a random fashion. (Some polymers, such as Silly Putty and Play Dough, are too amorphous and lack the rigidity needed to make a useful product. Polymers that are too crystalline often are also too brittle.)

Permeability The tendency of a polymer to pass extraneous materials. (Polyethylene is used to wrap foods because it is 4000 times less permeable to oxygen then polystyrene.)

Elastic modulus The force it takes to stretch the plastic in one direction.

Tensile strength The strength of the plastic. (The force that must be applied in one direction to stretch the plastic until it breaks.)

Resilience The ability of the plastic to resist abrasion and wear.

Refractive index The extent to which the plastic affects light as it passes through the polymer. (Does it pass light the way PMMA does, or does it absorb light like PVC?)

Resistance to electric current Is the material an insulator, like most polymers, or does it conduct an electric current? (There is a growing interest in conducting polymers, which can be charged and discharged, and photoconducting polymers that can pick up an electric charge when exposed to light.)