Thermosoftening polymers melt when they are heated.

The idea

Thermosoftening polymers consist of long, flexible carbon-based chains held together only by weak intermolecular forces. When heated, the thermal energy supplied makes the chains vibrate more strongly until these weak forces between chains are overcome, so the chains slide over one another and the solid softens and eventually flows as a liquid. Because no covalent bonds between chains are broken, the change is physical and reversible: on cooling, the chains settle back into a rigid arrangement and the polymer hardens again. This is why thermosoftening polymers such as poly(ethene) can be repeatedly melted and remoulded, making them suitable for plastic bottles, bags and food containers.

What earns the marks

• Thermosoftening polymers are made of long polymer chains that are not joined to each other by covalent bonds.
• The forces between separate chains are weak intermolecular forces, not covalent bonds.
• Heating transfers energy that makes the chains vibrate; when the vibrations are strong enough the weak forces between chains are overcome.
• Once the intermolecular forces are overcome, the chains can slide over one another, so the material softens and can flow.
• The process is physical and reversible: cooling allows the weak forces to re-form and the polymer hardens again.

Common mistakes and how to put them right

• Thinking that heating breaks covalent bonds within the polymer chains.

  Put it right: In fact only the weak intermolecular forces between chains are overcome, so the chains themselves stay intact.

• Describing the change as a chemical reaction.

  Put it right: It is a physical change because the polymer can reharden on cooling and be remelted.

• Saying the polymer 'dissolves' when heated.

  Put it right: It softens and flows because chains slide past each other, not because a solvent is present.

Exam tips

• Link the melting behaviour directly to structure: weak forces between chains are overcome, not the strong covalent bonds along each chain.
• Use the word 'intermolecular' when describing the forces between polymer chains to show precise understanding.
• Give a named example such as poly(ethene) and state that it can be remelted and remoulded, which is why it is used for bottles and bags.