The origin of piezoelectricity in poly(vinylidene fluoride)

Electrostriction constant (K31) and Poisson's ratio of a film (μ31, μ21) and that of a crystal (μ′) were measured for uniaxially drawn and rolled poly(vinylidene fluoride) films. The highly stretched films showed the large values of MH (∼0.8). μc(∼ 4.0) and K31(∼ 30). These values depend strongly on the structures of films such as crystal structure and molecular orientation. The induced piezoelectric stress constant e 31 by poling was interpreted fairly well by the theory proposed by Wada and Hayakawa in terms of three factors; μ31, μ′c and K31. Contribution of each factor to the piezoelectricity was evaluated. Among them, the Poisson's ratio of a film has the largest effect which amounts to 50% of the total piezoelectric activity.