Slide-ring elastomers consist of mobile cross-links that can rearrange themselves within the network in contrast to conventional elastomers with fixed junctures. This unique feature affects the macroscopic mechanical properties of the sliding elastomers by imparting a distinct sliding elasticity that is caused by the distribution entropy of the sliding crosslinks. Slide-ring silicone elastomers exhibit two distinct time dependent elastic responses that can be credited to the conformational entropy of the polysiloxane chains and the distribution entropy of the threaded rings. In this work, the transition between rubber elasticity of the silicone chains and the sliding elasticity of the rings has been observed through linear viscoelastic studies. The extensional properties of the elastomers further corroborated the presence of two distinct time dependent viscoelastic profiles. This novel network structure presents the potential to design more intricate dielectric elastomer transducers with two distinctive modes of behavior determined by the operational speed of the system.
Enabling desirable actuations at low voltages for silicone based dielectric elastomer actuators (DEAs) is challenging. Reducing the thickness and increasing the softness of the silicone film are key approaches for this purpose. In this work, a super-stretchable silicone elastomer was characterized and used as DEA. The prepared elastomer can be stretched uniaxially to 2400% strain, allowing a significant thickness reduction through pre-stretches. Besides, it shows a moderate average elastic modulus of 0.32 MPa even at strains of 1000%-1500%. These properties favor its application in DEAs. Actuation results show that the elastomer was not only actuated to a high strain but also actuated at attractive voltages. Specifically, a 1 mm-thick elastomer with a pre-strain of 200%×200% was actuated 45% in area at 4 kV, and a 0.25 mmthick elastomer film with pre-strain of 600%×600% showed a 3% actuation strain at only 120 V. Considering its easy fabrication and excellent actuation performance at low voltages, the elastomer is promising in the application in DEAs.
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.
Slide-ring elastomers have mobile cross-links that can slide on their axial polymers in a manner similar to a pulley on a zip line. This supramolecular network structure imparts unique mechanical properties to the elastomers, such as high deformability and low hysteresis upon cyclic loading, that are often favorable for dielectric elastomer actuators (DEAs). The utilization of this type of dynamic network for actuation has been limited by the low compatibility of slide-ring materials and common elastomer platforms used in DEAs. Here, a synthetic pathway is proposed to allow for incorporation of slide-ring cross-linkers into silicone networks.
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