Nylon pyroelectricity

The bending piezoelectricity in drawn and polarized polymer films was studied in detail by Kawai (1) (1970). Kitayama and Nakayama (1971) reported a very high piezoelectricity in composite films of polymer (PVDF, nylon 11, PVC) and powdered ceramics (barium titanate, PZT) after poling. In the case of PVDF and nylon, the piezoelectric constant increase by a factor of 102 when the ceramics make up 50% of the volume. The pyroelectricity and optical nonlinearity of polarized PVDF films have been studied by Bergmann, McFee, and Crane (1971). 

Ferroelectricity has also been found in certain copolymer compositions of VF2 with trifluoroethylene, F3E, [6-11] and tetrafluoroethylene, F4E, [12-15] and in nylon 11 [16]. Specifically, copolymers of vinylidene fluoride and trifluoroethylene (VF2/F3E) are materials of great interest because of their outstanding ferroelectricity [9,17-18], together with a parallel strong piezo- [7] and pyroelectricity [19]. These copolymers exhibit, in addition, an important aspect of ferroelectricity that so far has not been demonstrated in PVF2 the existence of a Curie temperature at which the crystals undergo reversibly a ferroelectric to a paraelectric phase transition in a wide range of compositions [9, 17-18],... 

Piezoelectricity links the fields of electricity and acoustics. Piezoelectric materials are key components in acoustic transducers such as microphones, loudspeakers, transmitters, burglar alarms and submarine detectors. The Curie brothers [7] in 1880 first observed the phenomenon in quartz crystals. Langevin [8] in 1916 first reported the application of piezoelectrics to acoustics. He used piezoelectric quartz crystals in an ultrasonic sending and detection system - a forerunner to present day sonar systems. Subsequently, other materials with piezoelectric properties were discovered. These included the crystal Rochelle salt [9], the ceramics lead barium titanate/zirconate (pzt) and barium titanate [10] and the polymer poly(vinylidene fluoride) [11]. Other polymers such as nylon 11 [12], poly(vinyl chloride) [13] and poly (vinyl fluoride) [14] exhibit piezoelectric behavior, but to a much smaller extent. Strain constants characterize the piezoelectric response. These relate a vector quantity, the electrical field, to a tensor quantity, the mechanical stress (or strain). In this convention, the film orientation direction is denoted by 1, the width by 2 and the thickness by 3. Thus, the piezoelectric strain constant dl3 refers to a polymer film held in the orientation direction with the electrical field applied parallel to the thickness or 3 direction. The requirements for observing piezoelectricity in materials are a non-symmetric unit cell and a net dipole movement in the structure. There are 32-point groups, but only 30 of these have non-symmetric unit cells and are therefore capable of exhibiting piezoelectricity. Further, only 10 out of these twenty point groups exhibit both piezoelectricity and pyroelectricity. The piezoelectric strain constant, d, is related to the piezoelectric stress coefficient, g, by... 

In crystal phase Nylon 11 molecules are packed in sheets with hydrogen bonds between oxygen atoms and NH groups of neighboring chains, and dipole moments of Nylon 11 chains are aligned. The piezoelectric and pyroelectric coefficients of Nylon 11 are smaller than those for PVF2 (see Table 48.4 for comparison). 

Kawai s (7) pioneering work almost thirty years ago in the area of piezoelectric polymers has led to the development of strong piezoelectric activity in polyvinylidene fluoride (PVDF) and its copolymers with trifluoroethylene and tetrafluoroethylene. These semicrystalline fluoropolymers represent the state of the art in piezoelectric polymers. Research on the morphology (2-5), piezoelectric and pyroelectric properties (6-70), and applications of polyvinylidene fluoride 11-14) are widespread in the literature. More recently Scheinbeim et al. have demonstrated piezoelectric activity in a series of semicrystalline, odd numbered nylons (75-77). When examined relative to their glass transition tenq>erature, these nylons exhibit good piezoelectric properties (dai = 17 pCTN for Nylon 7) but have not been used commercially primarily due to the serious problem of moisture uptake. In order to render them piezoelectric, semicrystalline polymers must have a noncentrosynunetric crystalline phase. In the case of PVDF and nylon, these polar crystals cannot be grown from the melt. The polymer must be mechanically oriented to induce noncentrosynunetric crystals which are subsequently polarized by an electric field. In such systems the amorphous phase supports the crystalline orientation and polarization is stable up to the Curie temperature. 

Nylon-5,7 has only y form, which can be produced by thermal or mechanical treatments. Nyk)n-S,7 has a net dipole moment, and both poled os well os unpoled samples exhibit pyroelectricity. Litt and Lin [26] reported a pyroelectric constant of 100 pC/em K. The effea of water on piezoelectricity of odd nylons has been studied by Kim [27]. The piezoelectric constant increases with increasing water content, probably due to the crystalline change. Lin et al. [10] reported a pyroelectric coefficient of loi-100 Ocm K for nylon-5,7. 

M. H. Litt and J. C lin. Dielectric and pyroelectric properties of nylon 5.7 as a fuictioo of molecular otientatioo. Ferroeleetria 5 171 (1984X... 

V. Oelftmdbeim and D. Katz. Pyroelectric response and ctysial strocture of nylon-11, Fer-roeleorics 13 111 (1981). 

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