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A5.3 Elastomers

These are flexible polymers, which can be stretched to many times their original dimensions and which return (more or less) to their original size and shape afterwards.
Rubber is the obvious example of an elastomer.
These have some cross-linking, so they are thermosets but they have twisted or curled-up chains, so once the stretching force is removed, the intermolecular forces return the molecules to their original positions.