A rubber band snaps back not because of some spring inside it but because of entropy: stretching straightens out tangled chains into a more ordered state, and letting go lets them coil into the configuration with far more possible arrangements. The more vulcanization bridges cross-link the chains, the stiffer the rubber, the more exactly it springs back to its old shape, and the less it can flow at all. Temperature is what separates the two great families of plastic: thermoplastics soften and can be remoulded again and again, while a thermoset already carries a covalent network, so heating it only burns it, never melts it.
A rubber band snaps back not because of some spring inside it but because of entropy: stretching straightens out tangled chains into a more ordered state, and letting go lets them coil into the configuration with far more possible arrangements. The more vulcanization bridges cross-link the chains, the stiffer the rubber, the more exactly it springs back to its old shape, and the less it can flow at all. Temperature is what separates the two great families of plastic: thermoplastics soften and can be remoulded again and again, while a thermoset already carries a covalent network, so heating it only burns it, never melts it.
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