Bottlebrush Dielectric Elastomers with Improved Properties for Actuation

Dielectric elastomer actuators (DEA) are elastic capacitors composed of a pair of compliant electrodes and a soft dielectric elastomer film sandwiched in between. This kind of stretchable capacitors can be actuated when charged, can generate electricity from mechanical deformations, and can sense a mechanical stress due to the change in capacitance. However, the driving voltage of DEA and the low dielectric permittivity of common elastomers, is hampering the exploitation of its full potential. To reduce the driving volt-age while keeping the same performance, some possible solutions appear to be: (i) reducing the film thickness, (ii) enhancing the relative permittivity, and (iii) lowering the elastic modulus. The film thickness can be reduced by processing approaches; the permittivity can be improved via chemical modification by polar groups. The elastic modulus can be lowered by reducing the network density, however, soft materials face electromechanical instability (EMI). To overcome the EMI, the use of a crosslinked bottlebrush network proved to be a promising solution. Therefore, this project aims to synthesize different bottlebrush polymers using various efficient synthetic methods, to crosslink the synthesized bottlebrush polymers into thin films with suitable and affective reactions, and to produce a library of crosslinked bottlebrush polymer networks. Next, to increase the dielectric permittivity, chemical modification methods of the materials by polar groups will be explored. Two strategies can be followed: a) post-polymerization modification of bottlebrush polymers with polar groups and b) use of macromonomers functionalized with polar groups. Additionally, after having achieved the synthesis of a collection of materials with different types and concentrations of dipoles via robust synthetic routs, the mechanical, dielectric, and electromechanical properties of the materials formed will be investigated, which may allow us to study the structure-property relationship. After gaining an in-depth understanding of the influence different structures have on key parameters, such as permittivity, strain at break, maximum extension ratio etc., and optimization of the material towards the most suitable applications will be achieved.Thus, at the beginning of the project, two synthetic paths were explored. The first is an in situ polymerization/crosslinking into thin films which should be achieved either by a free radical polymerization and crosslinking of acrylates macromonomers or by a ring-opening polymerization of macromonomers with epoxy functional groups in presence of a multi-functional epoxy crosslinker. The second approach was based on the synthesis of bottle-brush polymers either by atom transfer radical polymerization (ATRP) or by ring-opening metathesis polymerization (ROMP) followed by subsequent crosslinking of the bottle-brush polymers into thin films. As of now, the most promising approach to crosslinked bottlebrush polymers is starting from macromonomers suitable for ROMP which after polymerization can be easily crosslinked into thin films via the efficient and air insensitive thiol-ene reaction. Preliminary mechanical tests showed that soft elastomer can be achieved, but the strain at break still has to be improved.