Ferroelectric polymers
Polyvinylidene fluoride (PVDF) and its copolymers with trifluoroethylene (TrFE), tetrafluoroethylene (TFE) and hexafluoropropylene (HFP) as well as the so-called odd-numbered poly\-amides %(which have an odd number of carbon atoms between two amide groups in the polymer chain) are semi crystalline polymers which show pronounced ferroelectric-hysteresis behaviour and related electromechanical and thermo electrical response, in particular the piezo- and pyroelectric effect. Since the discovery of piezoelectricity on PVDF in 1969 this class of polymers has been the subject of numerous structural, dynamic-mechanical, thermal and electrical investigations in order to explain how the preparation-induced phase structure determines mechanical and electrical behaviour and how this phase structure can be changed by external loads and fields. Many attempts have been successfully undertaken in order to use these properties commercially in sensor and actuator applications (piezoelectric transducers, motion detectors, ultrasound detectors) and in organic electronics (OFETs). An important advantage of ferroelectric polymers in comparison with anorganic ferroelectrics concerns their processability as flexible free-standing film of only few micrometer thickness or as thin submicrometer film on a substrate.
As in anorganic ferroelectrics ferroelectricity of these polymers is a property of a polar crystalline phase, but the structure and elastic properties of the amorphous phase embedding the polar crystallites strongly determine the macroscopic ferroelectric behaviour. There exist several polar and non-polar crystalline phases depending on processing conditions and prehistory of the film. Moreover, intermediate phases between amorphous and crystalline phases must be considered in order to explain dynamic-mechanical and electromechanical properties. Recent investigations on ferroelectric polymers in our group focus on microscopic processes which induce the formation of a ferroelectric phase through the interaction of molecular dipoles with ionic additives and the orientation of molecular dipoles under the influence of electric fields in a soft state (solution, liquid). These investigations involve the development of special preparation techniques and the characterisation of the induced structure and polarisation by nonlinear current-voltage measurements (ferroelectric hysteresis), dielectric relaxation spectroscopy (DRS), dynamic-mechanical analysis (DMA) and differential scanning calorimetry (DSC).