Towards comprehensive understanding of piezoelectricity and its relaxation in VDF-based ferroelectric polymers
Document Type
Article
Publication Date
9-1-2023
Abstract
Piezoelectric resonance spectroscopy was employed to elucidate the microscopic mechanisms of piezoelectricity and its relaxation for the uniaxially-drawn and poled films of polyvinylidene fluoride and the vinylidene fluo-ride/ trifluoroethylene copolymer with a composition of 75/25 mol. The former was semicrystalline whereas the latter consisted of extended-chain crystals. Accurate measurements of dielectric frequency spectra in the mHz-toGHz range revealed piezoelectric resonance superimposed on broad dielectric relaxation. Analyses of the resonance spectra of the length, width and thickness vibrations allowed for evaluation of all elastic and piezoelectric tensor components. The single-crystalline copolymer substantiated the crystalline relaxation associated with intramolecular rotational fluctuations of trans segments and longitudinal chain softening. The most informative were the piezoelectric e(31), e(32), and e(33) constants expressing the charge responses induced by chain elongation, interchain separations parallel and perpendicular to thickness, respectively. It was found that e(31) showed relaxation from small negative to large positive, and e(33) was much larger than e(32) both being negative and non-relaxational. These results were discussed in reference to those of Form I crystals whose polar axis was p/6 tilted due to (110) twin. We identified three microscopic mechanisms (i) elongation of relaxing chain reduces the effective monomer dipole moment mu due to reduction of rotational fluctuations, (e(31) > 0), (ii) the increase in interchain distance reduces mu due to decrease in positive local field, (e(32), e(33) < 0), (iii) the strain-induced reduction of dipole density (dimensional effect) is effective in experimental e(33) but is absent in e(32) and e(31). The piezoelectric tensor components determined in this study were consistent with the electrostrictive coupling measured from the strain-induced change in dielectric permittivity and the strain proportional to the square of polarization.
Keywords
Ferroelectric polymer, Poly(vinylidene fluoride), VDF/TrFE copolymer, Piezoelecric resonance, Dielectric relaxation
Divisions
PHYSICS
Publication Title
Polymer
Volume
283
Publisher
Elsevier
Publisher Location
125 London Wall, London, ENGLAND