Polymers

A polymer is a long chain molecule made up of repeating units (monomers). Monomers link together in polymerisation reactions. The monomers must be the same unit, thought if two are used, it can be called a copolymer.

Polymers can be naturally occurring - wool, hair, cotton and rubber are all examples, or they can be synthetic, these are what we know as plastics.

Polymers are produced via addition and condensation polymerisation reactions. Different functional groups can also be added to change the properties of the product.

Thermo-softening plastics:
These are plastics that can be repeatedly heated, melted, reshaped and hardened by cooling without major changes.

Thermosetting plastics:
Do not melt when heated, instead, they will char and burn. These plastics must be molded or shaped during manufacture.

Addition Polymerisation:

The formation of different polymers using functional groups

This process uses ethene molecules at high pressure, to change the gas to a liquid. It is heated, under pressure with a catalyst, and the molecules join together to form a long chain, called polyethene (or polyethyne).
For polymerisation to occur, the monomer must have a double bond between two carbon atoms, which breaks so that monomers can bond together.
Functional groups can be used to replace certain hydrogen atoms, to produce different polymers with different properties.

Changing the Properties of Linear Polymers:
Polymers can be made to be: hard or soft, flexible or rigid, transparent or coloured, brittle or stretchy.

• extent of branching
• arrangements of side branches
• cross-linking of polymer chains
• degree of crystallinity
• length of polymer chains
• addition of plasticisers
• additives

Branching:

Short branches allow chains to stack closely, with strong dispersion forces, forming a high density plastic. If these branches are long and numerous, then the chains are further apart, and the plasic is softer and more flexible.

Side Branches:

Linear (unbranched) polymers can have certain hydrogen atoms replaced with a propene group. When all these groups are arranged on one side of the chain, the polymer is isotactic. Due to greater dispersion forces, these chains will pack tightly. However, when these chains occur at random intervals, on both sides of the chain, then the polymer is atactic. These chains cannot pack tightly, and so a low density, soft substance is formed. 

Cross-linking:

The chains that make up thermoplastics are held together by weak dispersion forces, however, when small amounts of cross linking are introduced, an elastomer is formed. When even more cross linking is present, a thermosetting plastic is made. 

Degree of Crystallinity:

When polymer chains are stacked in nice neat rows and organised lines, they are said to be crystalline. In other areas, with little to no organisation, the polymer is amorphorus. The more crystalline a structure is, the harder it is with higher tensile strength is has, and more opacity. Amorphorus polymers are more easily deformed and often transparent. 

Length of Polymer Chains:

As chain length increases, the strength, boiling point and melting point all increase. Long chains tend to get tangles and are less likely to slide over themselves. 

Addition of Plasticisers:

Plasticisers are small molecules added to plymers during production that cause the chains to more further apart. This means the final product is softer, and more flexible.