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
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 thermosoftening polymer. Its tangled polymer chains can uncoil and slide past each other, making it a flexible material.
Thermosetting polymers
Thermosetting polymers have different properties to thermosoftening polymers. Once moulded, they do not soften when heated and they cannot be reshaped. Vulcanised rubber is a thermoset used to make tyres. Its polymer chains are joined together by cross-links, so they cannot slide past each other easily.
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