Piezoelectric polymers and copolymers and their applications

Discusses the two materials that are of most immediate interest to those interested in transducer applications which are polyvinylidene fluoride (PVDF) and the copolymer of vinylidene fluoride and trifluoroethylene (VDF-TrFE). The first of these, PVDF, has been known since 1969 and is a material with relatively high piezoelectric constants. The structure and understanding of the manner in which its piezoelectric behaviour is produced has been described by many authors. For example, Broadhurst and Davies (1984) describe how, in the crystal phase, particular molecular configurations are preferred as a result of the energetics involved and they provide a series of very informative diagrams showing these preferential arrangements. There are at least four crystalline structures which are stable at room temperature and three of these phases have net dipolar moments, the β-phase being dominant. The copolymer of VDF with TrFE is particularly interesting because the Curie temperatures or transitions between the ferroelectric and paraelectric states occur at temperatures below the melt temperature. This means that they can be measured readily, but more importantly perhaps, these copolymers exhibit piezoelectric effects of much greater magnitude than that of PVDF