Thermosetting polymers do not melt when they are heated.

The idea

Thermosetting polymers have polymer chains that are joined to one another by strong covalent cross-links, forming a rigid three-dimensional network. When the material is heated, the chains cannot slide past each other because the cross-links hold them firmly in place. The thermal energy makes the atoms vibrate more, but the covalent bonds in the network are far too strong to be overcome by the temperatures normally used, so the polymer does not soften or flow. Instead it may char or decompose at very high temperatures. This rigid, infusible behaviour makes thermosetting polymers such as those used in electrical plugs, pan handles and some adhesives suitable where heat resistance and strength are needed.

What earns the marks

• Thermosetting polymers contain polymer chains joined together by strong covalent cross-links.
• The cross-links produce a rigid three-dimensional network rather than separate, free-moving chains.
• Heating makes the atoms vibrate more, but the strong covalent bonds of the network are not overcome at normal temperatures.
• Because the chains cannot slide over one another, the polymer does not soften or flow when heated.
• At very high temperatures the material may char or decompose rather than melt.

Common mistakes and how to put them right

• Saying thermosetting polymers have no intermolecular forces.

  Put it right: They do, but the covalent cross-links between chains dominate the behaviour.

• Claiming the cross-links break on heating so the polymer melts.

  Put it right: The covalent bonds are too strong to be overcome at normal temperatures.

• Confusing the two polymer types and stating that thermosetting polymers can be remelted and reshaped.

  Put it right: They cannot, because of their cross-linked network.

Exam tips

• Contrast the two polymer types directly: thermosoftening has weak forces between separate chains, whereas thermosetting has strong covalent cross-links between chains.
• Use the phrase 'three-dimensional network' to describe the cross-linked structure of a thermosetting polymer.
• Explain the consequence, not just the structure: because chains cannot slide, the polymer does not melt, which makes it useful for heat-resistant items.