Soft elastomer without cross-linking shows excellent elasticity dominated by knotted rings and entanglements

Conventional elastomer elasticity origins from the permanent elastomer networks formed by cross-links. Recently, an extremely soft silicone elastomer without cross-links has been shown to exhibit excellent strain recovery from extreme elongation (>5000%), namely 82% after 10 cycles [1]. This observation challenges conventional elasticity of cross-linked elastomers because a network with virtually no covalent crosslinks should not be able to recover strain to such extent. In this work, the microscopic structure of this elastomer was identified by use of nuclear magnetic resonance and size-exclusion chromatography. It was found that the elastic properties were dominated by the knotted rings and entanglements. Knotted rings are permanently trapped and form the network and can therefore be regarded as pseudo cross-linking. The knotted rings and entanglements in the elastomer can be controlled well by changing the preparation conditions.