Elastomers can also be formed as thermoplastic materials. In these the role of the cross-links is replaced by hard domains that form by the aggregation of 'hard' sections on the individual chains as shown opposite. For (a) styrene-butadiene-styrene (S-B-S) segments form the molecule with S being the rigid mer and B the soft flexible mer. In (b) styrene-isoprene-styrene (S-I-S) it is the isoprene segment that is flexible. When these copolymer molecules form a solid, the structure has the form shown in the lower picture. The hard styrene domains from several molecules aggregate and form tie-points for the individual molecules. At high enough temperatures the bonding in these tie points is not enough to prevent molecular sliding and melting of the elastomer.

From: Callister, "Materials Science and Engineering," Wiley (1997)