A matter of attraction: Electric charges localised on dielectric polymers enable electromechanical transduction

Significant amounts of localised or trapped charges, as well as large electric dipoles from pairs of such charges, are usually detrimental for electrical insulation. The same charges can, however, lead to useful electro-mechanical and mechano-electrical effects (or inverse and direct piezoelectricity, respectively) if they are properly arranged in dielectric materials with extremely low conductivities. The resulting space-charge and dipole electrets are widely applied, e.g. in microphones, air filters, radiation dosimeters, ultrasonic transducers, etc. Furthermore, the coupling between localised charges and anisotropic mechanical properties may also give rise to pyroelectric and electrocaloric effects, as well as to soft and flexible or stretchable sensors and actuators with large figures of merit. Basically, the performance of all such transducer materials relies on the attraction (and repulsion) of electric charges and thus directly on the electromagnetic interaction, i.e. on one of the fundamental interactions that Nature provides. The beneficial transducer effects of localised excess charges of both polarities in dielectric polymer materials are being exploited in recent developments in the areas of anisotropic polymer ferroelectrets with quasi-ferroelectric behavior, of soft and basically incompressible electro-electrets (dielectric elastomers) with extremely large strains (used in sensors, energy harvesters and actuators), of molecular-dipole electrets with significant ferro-, pyro- and piezo-electricity, and of space-charge polymer electrets with locally stabilised charges.