Polymers properties- Higher tier
Strong covalent bonds join atoms to each other in individual polymer molecules. Weak intermolecular forces attract polymer molecules towards each other. The properties of solid materials like polymers depend on:
  * how their molecules are arranged
  * The strength of the forces between these molecules.
A polymer will melt when the intermolecular forces are overcome. The stronger the forces, the more energy are needed to break them, and the higher the material’s melting point.
Many polymers, such as polyethene, contain long molecules that lie side by side. These can uncoil and slide past each other, making the material flexible. Long polymer chains have stronger forces of attraction than shorter ones: they make stronger materials.
Cross-linking is where the polymer chains are chemically joined together in places, by covalent bonds. The polymer molecules cannot slide over each other so easily. This makes materials tougher and less flexible, and they cannot be easily stretched. Cross-linking also gives materials high melting points.
Vulcanised rubber has cross-links. Its polymer molecules are cross-linked by sulfur atoms. It is tough but flexible, and used for making tyres.

Polymers have properties which depend on the chemicals they are made from, and the conditions in which they are made. For example, poly(ethene) can be low-density or high-density depending upon the catalyst and reaction condition used to make it. The table summarises some differences in their properties:

LDPE low-density poly(ethene) HDPE high-density poly(ethene)
Branches on polymer molecules Many Few
Relative strength Weak Strong
Maximum useable temperature 85°C 120°C
Thermosoftening polymers
two seperate polymer chains
Polymer with no cross-links
Thermosoftening polymers soften when heated and can be shaped when hot. The shape will harden when it is cooled, but can be reshaped when heated up again. Poly(ethene) is a...