Reliability of Dielectric Elastomers

Dielectric elastomers are soft, voltage-responsive polymers. They deform when an electric field is applied and this deformation allows the dielectric elastomers to convert electrical energy into mechanical energy, or mechanical into electrical energy, which in other words mark dielectric elastomers as electromechanical transducers. A dielectric elastomer transducer that converts electrical into mechanical energy is called a dielectric elastomer actuator and it can be used in many soft devices.

The soft robotics/technology fields are growing fast, so there is a need to develop new soft and flexible, human-like materials to further these technologies. Dielectric elastomers are one of the most promising candidates for soft technology applications. Dielectric elastomer actuators first were used in industrial applications as a replacement for some parts of rigid machinery. Due to their easy preparation, good mechanical properties and convenience, soon they were applied in many new fields. For example, dielectric elastomer actuators can be used in daily necessity devices, consumer electronics, musical instruments and as biomedical, environmental and welfare devices. Even though dielectric elastomers are
widely used, scientist still do not completely understand all the fundamentals of the material. Therefore, dielectric elastomers need to be further investigated from a fundamental point of view to be fully exploited in before mentioned devices as well as novel devices. In this thesis, the reliability of dielectric elastomers is studied. First, mechanical and electrical properties are investigated. Then, electromechanical instability is studied. Later, the effect of electrodes and stability-instability of a developed device is investigated. Finally, application of unfavorable phenomenon, occurring in dielectric elastomer actuators, is discussed. Soft and stretchable device properties depends on dielectric elastomer materials
mechanical and electrical properties. Commercial elastomers are commonly used as materials for dielectric elastomer actuators of familiarity, but often commercial silicone elastomers do not meet all necessary requirements for these applications. Therefore, five of the most commonly used commercial silicone elastomers are blended with each other in various proportions, to optimise their mechanical and electrical properties. Then, dielectric elastomer electrical and mechanical properties are investigated. Young’s modulus, ultimate stress and strain, dielectric permittivity, breakdown strength, viscosity, leakage current, and optical transmittance, of these blends are investigated, mapped and compared to identify the best compositions for fabricating soft and stretchable devices.

Dielectric elastomer actuators are usually limited by electromechanical instabilities and dielectric breakdown strength. Therefore, in this thesis, electromechanical instability of dielectric elastomers is studied. The breakdown process is very complex and cannot be studied only by using static breakdown test method. Therefore, static breakdown tests are coupled with a high-speed camera. Tests are carried out with differently constructed elastomers and electrodes. Experimental insights into the electromechanical processes are discussed and a theoretical explanation for the observed actuation mechanism is presented.

Electrode properties are equally important for efficient work of stretchable electronic device. In this thesis, stability-instability of electrodes is studied in stretchable, transparent, electromechanical devices that are made of hydrogel electrodes and silicone elastomer. Lifetime of the device is tested by applying cyclic voltage and recording the numbers of cycles when the elastomer suffers electric breakdown. Several experimental observations associated with electric field concentration along the edges of the hydrogels are reported and discussed. Even though, field concentration is regarded as an issue, the application of the field concentration in electroluminescent devices is studied in the thesis. It is presented, that amplification effect of field concentration in hydrogel-elastomer electroluminescent devices is used to reduce the working voltage. Two types of electroluminescent hydrogel-elastomer devices are prepared and lifetimes of these devices are tested.