Molecular modeling of the piezoelectric effect in the ferroelectric polymer poly(vinylidene fluoride) (PVDF)

Abstract

In this work, computational molecular modeling and exploration was applied to study the nature of the negative piezoelectric effect in the ferroelectric polymer polyvinylidene fluoride (PVDF), and the results confirmed by actual nanoscale measurements. First principle calculations were employed, using various quantum-chemical methods (QM), including semi-empirical (PM3) and various density functional theory (DFT) approaches, and in addition combined with molecular mechanics (MM) methods in complex joint approaches (QM/MM). Both PVDF molecular chains and a unit cell of crystalline β-phase PVDF were modeled. This computational molecular exploration clearly shows that the nature of the so-called negative piezo-electric effect in the ferroelectric PVDF polymer has a self-consistent quantum nature, and is related to the redistribution of the electron molecular orbitals (wave functions), leading to the shifting of atomic nuclei and reorganization of all total charges to the new, energetically optimal positions, under an applied electrical field. Molecular modeling and first principles calculations show that the piezoelectric coefficient d 33 has a negative sign, and its average values lies in the range of d 33 ~ -16.6 to -19.2 pC/N (or pm/V) (for dielectric permittivity ε = 5) and in the range of d 33 ~ -33.5 to -38.5 pC/N (or pm/V) (for ε = 10), corresponding to known data, and allowing us to explain the reasons for the negative sign of the piezo-response. We found that when a field is applied perpendicular to the PVDF chain length, as polarization increases the chain also stretches, increasing its length and reducing its height. For computed value of ε ~ 5 we obtained a value of d31 ~ +15.5 pC/N with a positive sign. This computational study is corroborated by measured nanoscale data obtained by atomic force and piezo-response force microscopy (AFM/PFM). This study could be useful as a basis for further insights into other organic and molecular ferroelectrics.

Анализ Вычислительных И Экспериментальных Исследований Переключения Поляризации В ПВДФ И П (Вдф-ТрФЭ) Сегнетоэлектрических Пленках На Наноуровне

“…These studies continue the series of our previous investigations of ferroelectric and nanoscale properties of thin LB PVDF copolymer films [8,[16][17][18][19][20][21]. The analysis of computational and experimental data and new computed evaluations obtained allow us to estimate the sizes of possible transition region approximately to be of the order of 10 nm.…”

“…Computational studies of the polarization switching mechanism in such polymer films were performed on an idealized model, consisting of one, two and four polymer chains with limited structural units by means of the first-principles quantum theory [15][16][17][18][19][20][21].…”

“…Now these LB polymer films are widely explored experimentally, using various techniques, including nanoscale characterization by atomic force microscopy (AFM) and piezoresponse force microscopy (PFM) method [4][5][6][7][8][9][10][11], and theoretically, using various computational methods [12][13][14][15][16][17][18][19][20][21]. These novel ferroelectric nano-materials are very promising for nanobiotechnology, nanobiomedicine, bio-and microelectronics, and as a prospective component of various nanocomposites and "intellectual" templates with "smart surface" properties.…”

Shear Piezoelectricity in Poly(vinylidenefluoride‐co‐trifluoroethylene): Full Piezotensor Coefficients by Molecular Modeling, Biaxial Transverse Response, and Use in Suspended Energy‐Harvesting Nanostructures

“…Most studies assume only uniaxial models for polymers and organic-inorganic nanocomposites, [ 12,25 ] which may result in a limiting description of the response because of the assignment of a reduced number of piezoelectric parameters, as pertaining to systems much more symmetric than the actual ones. In this work, we demonstrate the biaxial shear activity in P(VDF-TrFE), which show the presence of two net components of the electronic polarization ( P x , P y according to the geometry displayed in Figure 1 b) in the plane perpendicular to the chains of macromolecules.…”

Local piezoresponse and polarization switching in nucleobase thymine microcrystals

“…For these calculations we used HyperChem 8.0 tools as described previously in a number of papers. 27,[34][35][36]59 Using this approach, we developed a simple molecular model for thymine molecular unit based on the reported unit cell of the crystal (consisting of four thymine molecules lying in one plane, Fig. 11).…”