Soft silicone elastomers with no chemical cross-linking and unprecedented softness and stability

Abstract

For silicone elastomers used as actuators, softness is key for enabling actuation at low voltages. Recently, an extremely soft (Young’s modulus < 50 kPa) silicone elastomer without cross-links has been reported by Goff et al. Besides its extreme softness, the elastomer was reported to almost completely recover (82%) from a 10-cycle elongation of more than 5000%. This observation challenges conventional elasticity theory of cross-linked elastomers because a network without covalent crosslinks should not be able to strain-recover to such extent. In this work, the elastomer is hypothesized to be formed from concatenated rings through heterodifunctional uni-molecular ring closure. It is found that the elastic properties of this uncross-linked elastomer can be described by the dynamics of concatenated rings, which act as pseudo-crosslinks and pseudo-entanglements. Isolated rings and dangling rings function as external solvents and internal solvents respectively, thereby contributing to the unprecedented softness. The ability to precisely control the ratio between concatenated and dangling rings is expected to lead to even softer dielectric elastomers paving the way forward for ultra-soft robotics without significant mechanical losses.