Mechanism of polarization and piezoelectric behavior in crystallizable ferroelectric polymers from the standpoint of propagation of soliton waves

Published experimental data concerning the process of poling and the piezoelectric response in poly(vinylidene fluoride)-based ferroelectric polymers are discussed in terms of the soliton mechanism of propagation of excitation energy throughout the polymer. For the poling processes, the earlier proposed model of the twist conformational defect, which can travel with a subsonic velocity in a crystal, is considered. To have increased values of remanent polarization in uniaxially drawn ferroelectric polymers, a structure that satisfies certain conditions should be formed. The thickness of amorphous interphases between the faces of microfibrillar crystals must be as low as possible. It is desirable to have a microstructure of interlamellar regions with a high packing density due to an increased amount of all- trans tie chains. Such topology and microstructure of interlamellar phases ensures low scattering of dynamic conformational defects described by soliton waves. Experimental data show that the generation of a conformational defect in a crystal (emergence of a soliton wave) is limited by the mobility in the interfibrillar amorphous phase in tie chains close to crystals. Higher values of remanent polarization are observed in textured films, in which the aforementioned micro-Brownian dynamics is characterized by reduced values of activation parameters. Such conditions, for example, are created when isotropic films with certain initial morphology are textured by uniaxial cold drawing. Meeting the above requirements for the structure of an oriented film also creates conditions for field-induced postcrystallization, which also facilitates the formation of high remanent polarization. In the case of the lateral piezoelectric effect, the role of the dynamic behavior of vacancies that are formed in a polymer crystal by elastic loading of an oriented polymer film along the draw axis is considered. In such a model, the piezoelectric effect must be the strongest for structures that provide the lowest damping of soliton waves that describe the motion of vacancies. It was found that these favorable structures could also be produced by means of cold drawing used to form the above morphology.