Christiane Löwe

Frequency dependent dielectric and mechanical behavior of elastomers for actuator applications

The low frequency mechanical and dielectric behavior of three different elastomers has been investigated by dynamic mechanical analysis and dielectric spectroscopy, with the aim of accounting for the frequency dependence of the characteristics of the corresponding dielectric elastomer actuators. Satisfactory agreement was obtained between the dynamic response of the actuators and a simple model based on the experimental data for the elastomers, assuming that the relatively large prestrains employed in the actuators to have little influence on the frequency dependence of their effective moduli. It was thus demonstrated that the frequency dependence of the actuator strain is dominated by that of the mechanical response of the elastomer, and that the frequency dependence of the dielectric properties has a relatively minor influence on the actuator performance.

High k Dielectric Elastomeric Materials for Low Voltage Applications

In principle EAP technology could potentially replace common motion-generating mechanisms in positioning, valve control, pump and sensor applications, where designers are seeking quieter, power efficient devices to replace conventional electrical motors and drive trains. Their use as artificial muscles is of special interest due to their similar properties in terms of stress and strain, energy and power densities or efficiency. A broad application of dielectric elastomer actuators (DEA) is limited by the high voltage necessary to drive such devices. The development of novel elastomers offering better intrinsic electromechanical properties is one way to solve the problem. We prepared composites from cross-linked silicone elastomers or thermoplastic elastomers (TPE) by blending them with organic fillers exhibiting a high dielectric constant. Well characterized monomeric phthalocyanines and modified doped polyaniline (PANI) were used as filler materials. In addition, blends of TPE and an inorganic filler material PZT were characterized as well. We studied the influence of the filler materials onto the mechanical and electromechanical properties of the resulting mixtures. A hundredfold increase of the dielectric constant was already observed for blends of an olefin based thermoplastic elastomer and PANI.

Phthalocyanine and encapsulated polyaniline nanoparticles as fillers for dielectric elastomers

The dielectric constant (ε) of a polymer can significantly be increased by blending it with conducting fillers. Given our interest in developing highly efficient and long-lasting actuators for muscle replacement, we set out to explore all key issues which could help to reduce the required voltage and at the same time ensure long term stability. The presentation describes experiments which prove that the water content in carboxylic acid-decorated phthalocyanines (Pcs), commonly falsely referred to oligo-Pcs, is a critical factor determining the absolute value of ε. Several publications on ε values of these oligo-Pcs led to contradicting conclusions because the effect of water was not sufficiently considered. The water content is relevant because o-Pcs are often used as fillers to increase ε of polymer matrices. This presentation also describes an experimental evaluation on whether or not as-prepared polyaniline (PANI) and poly(divinyl benzene)- encapsulated (PDVB) PANI can be reasonably used as high ε fillers in matrix materials. For this purpose several blends with polystyrene-polybutadiene block copolymer gels (PS-b-PB) and polydimethyl siloxane (PDMS) were prepared and their dielectric properties investigated. The former part of this presentation has in part already been published (D. M. Opris et al. Chem. Mater. 20(21), 6889-6896, 2008), the latter is completely new.

Dielectric elastomer materials for actuators and energy harvesting

The success of dielectric elastomer materials in actuator technology as well as in energy harvesting is much influenced by the material parameters, e.g. breakdown field, dielectric constant, and elastic modulus which have a direct impact on the driving voltage. By increasing the dielectric constant of a material the activation voltage can be decreased, however this increase is very often associated with a decrease in the breakdown field. In this proceeding, dielectric elastomer materials based on polydimethylsiloxanes with increased strain at break and high breakdown fields are presented.

Elastomer actuators: systematic improvement in properties by use of composite materials

Dielectric elastomer actuators (DEAs) have attracted increasing attention over the last few years owing to their outstanding properties, e.g. their large actuation strains, high energy density, and pliability, which have opened up a wide spectrum of potential applications in fields ranging from microengineering to medical prosthetics. There is consequently a huge demand for new elastomer materials with improved properties to enhance the performance of DEAs and to overcome the limitations associated with currently available materials, such as the need for high activation voltages and the poor long-term stability. The electrostatic pressure that activates dielectric elastomers can be increased by higher permittivity of the elastomer and thus may lead to lower activation voltages. This has led us to consider composite elastomeric dielectrics based on thermoplastic elastomers or PDMS, and conductive polyaniline or ceramic (soft doped PZT) powder fillers. The potential of such materials and strategies to counter the adverse effects of increased conductivity and elastic modulus are discussed.