Improving essential properties of dielectrics for electro-electrets, piezo-electrets and ferroelectric polymer electrets via physico-chemical routes

Abstract

The performance of electro-electrets (dielectric elastomers) for actuator and sensor applications relies on high relative permittivity and low elastic modulus. Recent advances in the chemical modification of silicone elastomers allow for the stable attachment of molecular dipoles to the elastomer network, which leads to an overall improvement by a factor of 6 through an increase of the dielectric permittivity and a decrease of the elastic modulus. Similar and alternate routes may also be possible for polyurethane and other dielectric elastomers. Space-charge electret films and ferroelectret systems depend on the thermal and long-term stability of trapped charges on the surface or in the bulk of the respective polymer materials. Recently, chemical surface treatments of electret polymers have been developed and have resulted in much higher charge stabilities even on standard polymers such as polyethylene, but also on fluoropolymers that already exhibited rather good charge stabilities. The treatment relies on gas or fluid exposure and is suitable not only for polymer films, but also for piezoelectret systems with open channels. Ferroelectric polymers from the polyvinylidene-fluoride (PVDF) family show useful piezo- and pyroelectric properties if they can be prepared in the relevant all-trans conformation that leads to parallel packing of the molecular dipoles in the crystalline β phase. Recent successful experiments with the addition of ionic liquids to a PVDF solution yielded β-phase crystallites without further processing and thus allowed for the preparation of pyro-and piezoelectric polymer films from solution even by means of coating onto substrates. Similar routes may also be available for VDF copolymers and for inducing favorable orientations of polar polymer units inside ferroelectric polymer films. The above-mentioned and other related advances in the research on dielectrics for electret applications share the common feature that they are only possible through a combination of physics and chemistry at the molecular level or the nano-scale. If the new approaches are sustainable and successful also in the relevant industry, they may lead to significant improvements of electret materials and possibly of other dielectrics as well.