A-level Physics (Advancing Physics)/Polymers

A simple polymer consists of a long chain of monomers (components of molecules) joined by covalent bonds. A polymer usually consists of many of these bonds, tangled up. This is known as a bulk polymer.

Types

A bulk polymer may contain two types of regions. In crystalline regions, the chains run parallel to each other, whereas in amorphous regions, they do not. Intermolecular bonds are stronger in crystalline regions. A polycrystalline polymer consists of multiple regions, in which the chains point in a different direction in each region.

Polycrystalline glass

Amorphous rubber

Properties

Transparency

Polymers which are crystalline are usually opaque or translucent. As a polymer becomes less polycrystalline, it becomes more transparent, whilst an amorphous polymer is usually transparent.

Elasticity

In some polymers, such as polyethene, the chains are folded up. When they are stretched, the chains unravel, stretching without breaking. When the stress ceases, they will return to their original shape. If, however, the bonds between the molecules are broken, the material reaches its elastic limit and will not return to its original shape.

Stiffness

Polymer chains may be linked together, causing the polymer to become stiffer. An example is rubber, which, when heated with sulfur, undergoes a process known as vulcanization. The chains in the rubber become joined by sulfur atoms, making the rubber suitable for use in car tyres. A stiffer polymer, however, will usually be more brittle.

Plasticity

When a polymer is stretched, the chains become parallel, and amorphous areas may become crystalline. This causes an apparent change in colour, and a process known as 'necking'. This is when the chains recede out of an area of the substance, making it thinner, with fatter areas on either side.

Conductivity

Polymers consist of covalent bonds, so the electrons are not free to move according to potential difference. This means that polymers are poor conductors.

Boiling Point

Polymers do not have boiling points. This is because, before they reach a theoretical boiling point, polymers decompose. Some polymers do not have melting points for the same reason.

Questions

1. Different crystalline structures have different refractive indexes. Why does this mean that a polycrystalline polymer is translucent?

2. What sort of polymer is a pane of perspex?

3. What sort of polymer does the pane of perspex become when shattered (but still in one piece)?

4. What sort of polymer is a rubber on the end of a pencil?

5. What happens to the translucency of an amorphous polymer when it is put under stress?

Worked Solutions