Thermoplastics, such as polystyrene and polyvinyl chloride (PVC), can be repeatedly soften when heated. This is because the chain molecules in them are moveable relative to each other. The mobility increases with increasing temperature. In heat-resistant polymers (bakelite and epoxy resin) cross-links between the chains inhibit this process, so they when heated, not softened.
Elastomers - rubber and similar polymers, natural and synthetic rubber. To obtain the necessary number of links between the molecular chains of these materials are subjected to special treatment. As a result, they gain flexibility, i.e. the ability to be stretched, and after removal of the load to take its original shape.
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