Design Requirements for Amorphous Piezoelectric Polymers

Abstract An overview of the piezoelectric activity in amorphous piezoelectricpolymers is presented. The criteria required to render a polymerpiezoelectric are discussed. Although piezoelectricity is a couplingbetween mechanical and electrical properties, most research hasconcentrated on the electrical properties of potentially piezoelectricpolymers. In this work, we present comparative mechanical data as afunction of temperature and offer a summary of polarization andelectromechanical properties for each of the polymers considered. Introduction Kawai's [1] pioneering work almost thirty years ago in the area of piezoelectric polymers has led to thedevelopment of strong piezoelectric activity in polyvinylidene fluoride (PVDF) and its copolymers withtrifluoroethylene and tetrafluoroethylene. These semicrystalline fluoropolymers represent the state of theart in piezoelectric polymers. Research on the morphology [2-5], piezoelectric and pyroelectric properties[6-10], and applications of polyvinylidene fluoride [11-14] are widespread in the literature. More recentlyScheinbeim et al. have demonstrated piezoelectric activity in a series of semicrystalline, odd numberednylons [15-17]. When examined relative to their glass transition temperature, these nylons exhibit goodpiezoelectric properties (d_l = 17 pC/N for Nylon 7) but have not been used commercially primarily due tothe serious problem of moisture uptake. In order to render them piezoelectric, semicrystalline polymersmust have a noncentrosymmetric crystalline phase. In the case of PVDF and nylon, these polar crystalscannot be grown from the melt. The polymer must be mechanically oriented to inducenoncentrosymmetric crystals which are subsequently polarized by an electric field. In such systems theamorphous phase supports the crystalline orientation and polarization is stable up to the Curietemperature.Nalwa et al. have also examined piezoelectricity in a series of polythioureas [18-19]. Though nothighly crystalline, these thiourea polymers have a very high degree of hydrogen bonding which stabilizesthe remanent polarization in such systems after poling.The literature on amorphous piezoelectric polymers is much more limited than that for semicrystallinesystems. This is in part due to the fact that no amorphous piezoelectric polymers have exhibitedresponses high enough to attract commercial interest. Much of the previous work resides in the area ofnitrile substituted polymers including polyacrylonitrile (PAN) [20-22], poly(vinylidenecyanidevinylacetate) (PVDCN/VAc) [23-26], polyphenylethernitrile (PPEN) [27-28], and poly(1-bicyclobutanecarbonitrile) [29]. The most promising of these materials are the vinylidene cyanidecopolymers which exhibit large dielectric relaxation strengths and strong piezoelectricity. The carbon-chlorine dipole in polyvinylchloride (PVC) has also been oriented to produce a low level ofpiezoelectricity [30,31]. Motivated by a need for high temperature piezoelectric sensor materials, NASAhas recently begun research in the development of amorphous piezoelectric polymers. In this paper anamorphous, aromatic piezoelectric polyimide developed at NASA [32] is presented along with otheramorphous and paracrystalline piezoelectric polymers shown in Table 1. The purpose of this overview isto explain the mechanism and key components required for developing piezoelectricity in amorphouspolymers and to present a summary of polarization and electromechanical properties of currently