A novel elastic electret material is likely to generate important active components for soft electronics, energy harvesters, stretchable sensors, and transducers.
Electrets are materials that have quasipermanent charges on their surfaces and thus a permanent electric field. The most common ways to generate polymer electrets are by poling a polymer in a strong electric field above its glass transition temperature (Tg) and subsequently cooling while the voltage is still on, or by corona discharge. Unfortunately, most of the electret materials reported today lack stretchability, which limits their application whenever dimensional changes of the active material are desired. Dr. Opris and colleagues overcomes this limitation by introducing a new class of electrets, which are elastic under relevant conditions.
The new electrets are composites of polar nanoparticles (NPs) blended into a chemically cross-linked elastomer matrix, exemplified here for an easily available silicone. The polar NPs used in this composite consist of a high Tg polymer that contains moieties with large permanent dipole moments. They are prepared by convenient nanoprecipitation. The composites are processed into the desired form (typically a thin film) and are then cross-linked. To obtain the final material the initially randomly oriented polar moieties in the NPs in the now cross-linked film are poled by a high electric field while the film is heated above the Tg of the NPs. Under these conditions, the polar moieties orient to the electric field and the achieved orientation is subsequently frozen by cooling the material below Tg of the NPs.
By using this approach, an all organic elastic electret is prepared: the piezoelectric coefficient of which amounts to d33 of 26.7 pCN-1, which is comparable to the gold standard – poly(vinylidene fluoride). The importance hereof is also seen in abundantly available high Tg polar polymers and elastic matrices, which can be combined in the described way. This will, in future, allow for the creation of a large variety of elastic electrets with tailor-made properties.