Dielectric Elastomers as Electromechanical Transducers (eBook)

INTRODUCTION: HISTORY OF DIELECTRIC ELASTOMER ACTUATORS

Dielectric elastomers, the name most commonly given to a new class of electroactive polymer actuators discovered in the early 1990s, are an exciting new actuator technology. The field is growing rapidly, whether measured by number of research papers, performance of the technology, or diversity of potential applications. This first book devoted exclusively to dielectric elastomers therefore seems timely, and the introduction is a good place to step back and take a big picture look at where the field has been and where it is going.

Dielectric elastomers are a technology that could have been discovered decades earlier. The notion of opposite charges attracting and like charges repelling has been known since the earliest days of electricity (indeed, the effect on dielectric materials is known as Maxwell stress referring to the work by James Maxwell on the foundations of the electromagnetic theory). The materials needed to show actuation, such as polymer films and carbon black, have been available since at least the 1940s if not earlier. In fact, Bar-Cohen and Breazeal [1] note that experiments done with electric charges on natural rubber by W. Roentgen date back to 1880 while M. Sacerdote measured the strain response of dielectrics to applied electric fields in 1899.

But just because a technology could be discovered does not mean that it will be discovered. As with other technologies, several seemingly disparate threads had to come together before dielectric elastomers could be realized. Certainly, long-term interest in piezoelectric and electrostrictive materials helped to set the stage for dielectric elastomer development. An important step in this regard was the excellent work by Scheinbeim, Zhang, Zhenyi and others in investigating electrostrictive polymers such as semi-crystalline polyurethanes [2]. As with piezoelectric and electrostrictive ceramic work before it, investigations into electrostrictive polymers treated Maxwell stress as a secondary factor that needed to be subtracted out from the more important electrostrictive measurements rather than a potentially powerful actuation mode worth pursuing in itself. Nonetheless, early electrostrictive polymer work was important in the evolution of dielectric elastomers in that it showed the potential of electroactive polymers beyond piezoelectric polymer materials such as polyvinylidene fluoride (PVDF), and it illustrated some of the important issues in electrode compliance that arise in high strain actuator materials.

Separate from the electrostrictive polymer work, several other developments came together in the early 1990s to lead to dielectric elastomer discoveries. One such development was the heavy interest in robotics in the late 1980s, with seminal work by Kornbluh [3], Ian Hunter, and others arguing for investigations into new actuation technologies for artificial muscles for robots. This work was important because it showed that, from a mechanical engineering perspective, there was no man-made analogue to natural muscle in performance. It was a very different perspective, and led to very different lines of research, than one might have focused on higher frequency applications such as sound generation. The work by Kornbluh, Higuchi [4] and others was also significant in that it suggested that electrostatic forces on micro scales might be used for such artificial muscles. Indeed, it is fair to say that dielectric elastomers might never have been discovered as a technology without the observation by many microelectromechanical systems (MEMS) researchers that very small gaps, and very thin films, can have much higher electric breakdown strength than corresponding macro structures.

These separate research threads came together in fundamental work by Pelrine and others at SRI International in the early 1990s [5, 6] showing that actuation using polymers with Maxwell’s stress alone was an exciting option for actuation. In retrospect, the idea that something as simple as an elastomer sandwiched between electrodes could give good actuation response was audacious given what was known at the time. But in quick succession it was shown that virtually all insulating elastomer films exhibit a visible response using graphite electrodes, with various silicone elastomers leading the pack.

From the beginning, dielectric elastomer research has been an international affair, and indeed the early work at SRI, a US organization, was sponsored by the Micromachine Center, an organization funded by the Japanese government (MITI/NEDO). In Europe, researchers such as Sommer-Larsen and De Rossi were already investigating new artificial muscle approaches in the early 1990s [7] and were quick to recognize the potential of dielectric elastomers. This growing interest was facilitated by the growth in the larger electroactive polymer community including developments in both electrostrictive and ionic polymers.

One other technical development is worth mentioning both because it was important in itself and because of what it says about the field. Throughout much of the 1990s, the peak dielectric elastomer performance was roughly constant and exemplified by silicones, with peak strains of 10–30%. To be sure, important discoveries in materials, actuator design, and basic theory were made throughout the 1990s, but performance improvements seemed to have levelled off. That situation changed in 1999 with the discovery of acrylic as a dielectric elastomer. Acrylic easily broke the 100% strain mark, and brought higher visibility to the field. Even a lay person could immediately recognize that there was something remarkable in seeing a material double in size at the flick of a switch. The ability to be able to easily purchase large quantities of high quality, high performing film also greatly enhanced research. This has been particularly true in applications and device development where the necessity to fabricate custom films might be an obstacle to many research groups.

Acrylic was discovered by Pelrine testing the adhesive on the back of a discarded child-proof lock. In one sense the discovery was serendipitous, but in a deeper sense dielectric elastomers have such a simple structure, and so many elastomers are commercially available, that trial-and-error tests of many different commercial films and chemistries has been an effective strategy. As the field matures, one can expect a greater reliance on materials synthesized and optimized specifically as dielectric elastomers rather than for other commercial purposes. Nonetheless, historically many of the most important breakthroughs in materials have come from exploratory testing of new materials long after the physics were ‘understood’. Ceramic superconductors and neodymium–iron magnets come to mind as two historically recent examples. Given the rich chemistries involved, it seems highly likely that dielectric elastomers will be identified with performance far superior to that of current acrylics designed as pressure sensitive adhesives, or silicones designed as sealants. But whether the next-generation materials will be designed and synthesized as super dielectric elastomers by intent, or discovered as part of a directed search of existing materials, is uncertain.

What is more certain is that rapid progress is being made in the technology on multiple fronts, much of it described in this book. Basic understanding of materials, failure mechanisms, and environmental aspects is growing. This fundamental understanding is taking place against a backdrop of intense application and device design interest. While the initial discoveries and research were motivated by the desire to develop improved muscle-like actuators, dielectric elastomers have also been shown to have great potential for applications as generators and sensors. Dielectric elastomers can potentially replace existing actuator, generator, and sensor technologies, or they may enable entirely new applications that are impractical using existing technologies (variable surfaces might be an example here). Dielectric elastomers are potentially competitive in such a broad range of applications that it is not surprising that commercial participation in dielectric elastomer research and development (R&D) has grown from nearly zero in the early 1990s to a major component today.

These trends will continue and will likely accelerate if dielectric elastomers can make the transition from research topic to practical use. Many challenges remain such as in lifetime, environmental tolerance, and manufacturing. Yet promising results on the practical side of the technology are being gradually realized, and one can be optimistic that the very flexibility and range of the technology in chemistries, device design, and potential applications will eventually bring it into common use. In the meantime, as a research community there is exciting work to be done, and beyond the academic interest this work may eventually have an important impact on society at large. With that in mind, we hope this book can further the field, and provide a reference for, and insights into, dielectric elastomers in years to come.

This book overviews both the technology itself and a representative selection of developing applications. This book is divided into five sections, roughly laid out starting from fundamentals and ending at the application end of the R&D spectrum. The first section focuses on fundamentals of dielectric elastomers. This section describes how dielectric elastomers work and how they fit into the larger picture of electroactive polymers. The...