Biaxially oriented PVDF films

Certain polymers, such as polyvinylidene fluoride (PVDF) and polyvinyl fluoride (PVF), possess special properties in the film form, caileApiezoelectricity and pyroelectricity. Piezoelectricity is electric polarization of a film produced by mechanical strain in some crystals. The polarization is proportional to the amount of strain and changes sign with it. The reverse is true and an electrical polarization induces a mechanical strain in piezoelectric sensors. Pyroelectricity is electric polarization of a film induced by thermal absorption in some polymer crystals. The induced polarization is proportional to the level of thermal change. These properties can be used in the manufacture of transducers, microphones, loudspeakers, pressure gauges, pickup heads, hydrophones, motion sensors, and other devices from biaxially oriented PVDF films. Table 13.37 gives the properties of a piezoelectric film of polyvinylidene fluoride. 

In another example, unoriented PVDF films were stretched at 150°C in the longitudinal direction at a stretch ratio of 3.5 to 1. A sample of the longitudinally oriented film was stretched in the transverse direction at 60°C at a stretch ratio of 3.4 to 1, thus producing a biaxially oriented film. Dielectric constant and loss factor values were measured for these films (Table 6.10). 

Takasc et al. (63] have found that rctiumcni polarizatkms of 0.1 Cfm can be achieved in deulcratcd PVDF, and Bauer [64] reported the same for biaxially oriented films. Bauer and co-workers have developed a standard stress gauge using PVDF and in the process have demonstrated [6S] that they can achieve a remanent polarizalioo of 0.092 0.002 C/m using a patented poling process. 

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