Highly stretchable/soft silicone elastomers with tailored network structures

There is growing interest in developing stretchable/soft silicone elastomers for applications in advanced devices, including soft robotics, stretchable electronics, medical devices, and microfluidics. High stretchability permits various distortion scenarios and exceptional deformations, enabling wider applications of the devices. Besides, preparing silicone elastomers with a combined softness and elasticity resembling that of human soft tissue has been of great interest for use in soft robotics. Although various strategies have been developed for stretchable and soft silicone elastomers, none of them are versatile enough to enable the preparation of silicone elastomers that are both highly stretchable and very soft. This work aims to develop routes for molecular design of highly stretchable/soft silicone elastomers and to apply the resulting elastomers in one of the most promising fields, i. e. dielectric elastomer actuators (DEAs).

Firstly, elastomers, cured by forming concatenated rings, were substantiated through a series of experiments. In contrast to conventional silicone elastomers which are crosslinked, the novel silicone elastomers are prepared from heterobifunctional PDMS macromonomers without the use of crosslinkers. Size exclusion chromatography of extracts confirmed the formation of non-concatenated monocyclic PDMS. Swelling experiments confirmed mechanically stable  networks and high swelling ratios. Linear viscoelasticity measurements also demonstrated a behavior different from both PDMS melts and conventional crosslinked networks. The observed properties were explained by a formation of a network of concatenated rings which give rise to a continuous structure with a large degree of flexibility.

Secondly, a facile curing route was developed to tailor the stretchability and the softness of elastomersby creating highly entangled elastomers and bottle-brush elastomers. The curing chemistry was based onrealizing that silicone elastomers form when telechelic/multiple Si-H functional PDMS and platinumcatalyst are heated to 100℃ in air. This observation was explained through crosslinking reactions of the Si-H functional groups with otherwise inert constituents of the PDMS chain in the presence of oxygen (SiHcrosslinking) as elucidated by subsequent mechanistic studies. Combining the hydrosilylation and SiH crosslinking reactions in a one-pot reaction allowed formation of highly entangled elastomers and bottlebrush elastomers from commercial precursor polymers. The highly entangled elastomers showed high stretchability with maximum strains of 2800%, and the bottle-brush elastomers exhibited extremely lowsoftness with shear moduli of 1.2-7.4 kPa. The reported curing chemistry was used to prepare a range of silicone elastomers with carefully tailored mechanical properties.

To demonstrate the applications of the prepared materials, a highly stretchable silicone elastomer was applied in DEAs, aiming to decrease operation voltages by using thin prestretched films. The fabricated DEAs could be actuated to a 30% lateral strain at 4.3 kV for a 122 μm thick prestretched film, and to a 2.5% lateral strain at only 250 V for a 6.9 μm thick prestretched film. Lifetime and response speed tests further show that the DEAs are promising for applications where fast response speed is not strictly required.