Study On The Electrostrictive And Piezoelectric Mechanisms And Structure-Properties Relationship In Poly(Vinylidene Fluoride)-Based Polymers

Compared with the counterpart in ceramics,ferroelectric polymers are featured by light weight and easy processing,which are attractive for potential applications such as actuation,mechanical sensors,and energy harvesting in new wearable and foldable electronic devices.There are two fundamental electromechanical coupling effects in ferroelectric polymers:electrostrictive effect and piezoelectric effect.Although a significant amount of studies have been conducted on the electrostrictive and piezoelectric effects of ferroelectric polymers,its electrostrictive and piezoelectric mechanisms are still controversial due to the complex semicrystalline structures of ferroelectric polymers.In order to continue to enhance the electrostrictive and piezoelectric properties of ferroelectric polymers,it is necessary to have a fundamental understanding of its electrostrictive and piezoelectric mechanisms.In this thesis,direct and inverse piezoelectric coefficient measurements were designed,poly（vinylidene fluoride）（PVDF）-based ferroelectric polymers are the research object,and the electrostrictive and piezoelectric properties of different samples were studied.Synchrotron small-angle X-ray scattering（SAXS）and wide-angle X-ray diffraction（WAXD）and other characterizations were performed to analyze the microstructure of different samples to reveal the electrostrictive and piezoelectric mechanisms of PVDF-based ferroelectric polymers.The results in this thesis not only extend our understanding of electrostrictive effect and piezoelectric effect in ferroelectric polymers,but also provide guidance for further enhancing the electrostrictive and piezoelectric properties of ferroelectric polymers in the future.The detailed research content and conclusions are as follows:The theoretical models of electrostrictive effect and piezoelectric effect were proposed.Two origins of electrostrictive effect in ferroelectric polymers were revealed.The first is called mechano-electrostriction,which is resulted from the poling field-induced conformational transformations.The second is called electrostatic repulsion-electrostriction,which is resulted from electrostatic repulsion among highly polarized/aligned ferroelectric nanodomains in the transverse direction under a high electric field.Poly（vinylidene fluoride-co-trifluoroethylene-co-chlorotrifluoroethylene）[P（VDF-Tr FE-CTFE）]random terpolymers films with different thermal annealing histories were prepared.First,the semicrystalline structure of the P（VDF-Tr FE-CTFE）terpolymer films was studied by synchrotron SAXS and WAXD analyses.A three-phase model was employed,namely,crystals,oriented and isotropic amorphous fractions（OAF and IAF）.Many conformational defects caused by the bulky CTFE units exist in the crystalline lamella to form taut-tie molecules（TTM）in the crystalline lamella,dividing it into many nanosized crystals（～1.3 nm thick）.It is this unique crystalline structure with nanocrystals and mobile TTM/OAF that enabled the relaxor ferroelectric behavior for the P（VDF-Tr FE）-based terpolymers.Through electrostriction measurements and nonlinear dielectric analysis,an inverse correlation was observed between the ferroelectric nonlinearity and the electrostrictive coefficient under a high poling electric field（>100 MV/m）.By increasing the temperature,the electric field-induced ferroelectric domain size and ferroelectric nonlinearity are reduced,the electrostatic repulsion among electric field-induced ferroelectric nanodomains is significantly enhanced,resulting in greatly increasing of the electrostriction coefficient Q₃₁.A series of poly（vinylidene fluoride-co-trifluoroethylene）（P（VDF-Tr FE））random copolymers with VDF content from 50 to 65 mol.%were prepared,and the ferroelectric,electrostrictive,inverse piezoelectric and dielectric properties of different samples at room temperature have been studied in detail.It was found that the inverse piezoelectric properties of samples with different composition at room temperature were greatly different.For P（VDF-Tr FE）52/48QSAP sample,an extremely high inverse piezoelectric coefficient d₃₁,57.6±2.4pm/V,was observed,which is the highest inverse piezoelectric coefficient d₃₁ observed in polymers so far.But with the increasing of VDF content,the inverse piezoelectric coefficient d₃₁ of P（VDF-Tr FE）55/45QSAP and P（VDF-Tr FE）65/35QSAP samples dropped sharply to30.1±1.7 pm/V and 9±3.3 pm/V,respectively.Through synchrotron SAXS,WAXD and other characterizations,it was found that the secondary crystals(SC_OAF,T_m≈40°C)in OAF is the reason for the ultrahigh inverse piezoelectric properties of P（VDF-Tr FE）sample with low VDF content.This electric field-induced SC_OAF has a strong mobility at room temperature,which gives the sample a high dielectric constant,and significantly improves the electrostatic interaction between the oriented amorphous fraction and the ferroelectric domains in the crystals.This leads to a significant enhancement of the remanent polarization-biased electrostriction（i.e.inverse piezoelectricity）.It was found that at room temperature,the P（VDF-Tr FE）52/48QSAPSP sample exhibited much lower piezoelectric properties than the P（VDF-Tr FE）52/48QSAP sample.Further BDS research indicated that compared with the P（VDF-Tr FE）52/48QSAP sample,the dipole mobility in SC_OAF of the P（VDF-Tr FE）52/48QSAPSP sample significantly decreased.It was the lower dipole mobility that decreased the piezoelectric performance for the P（VDF-Tr FE）52/48QSAPSP sample.However,upon heating toward the melting temperature（～58°C）of the SC_OAF of the P（VDF-Tr FE）52/48QSAPSP sample,the dipole mobility was regained,and high piezoelectric performance was achieved at 50°C:the inverse piezoelectric coefficient d₃₁=62.5±7.2 pm/V.For the P（VDF-Tr FE）55/45QSAP sample,as a result of the increase of SC_OAFcontent and the improvement of dipole mobility,the inverse piezoelectric coefficient d₃₁reached as high as 77±5 pm/V at 55°C.The in-situ stretching SAXS and WAXD experiments of P（VDF-Tr FE）52/48QSAP and P（VDF-Tr FE）65/35QSAP samples were conducted and the results indicated that there are reversible changes of the long period,the content of crystals,and the content of OAF for P（VDF-Tr FE）52/48QSAP sample during stretching.This is mainly due to the reversible stretching-induced crystallization in OAF of P（VDF-Tr FE）52/48QSAP sample.And this reversible stretching-induced crystallization is exactly the direct piezoelectric mechanism in the P（VDF-Tr FE）copolymer:the stretching-induced crystals caused the change of the macroscopic dipole moment of the sample.Because this stretching-induced crystallization is reversible（it melts and disappears immediately after stretching）,so the change in the macroscopic dipole moment of the sample is also reversible.Full atomistic molecular dynamics simulations of the direct and inverse piezoelectricity were carried out,and the results showed that the inverse piezoelectric coefficient d₃₁ increased with the increasing the electric field,while the direct piezoelectric coefficient d₃₁ remained mostly unchanged with the increase of strain.On the other hand,the experimental results showed that for P（VDF-Tr FE）55/45QSAP and P（VDF-Tr FE）65/35QSAP samples,the inverse piezoelectric coefficient d₃₁ is almost the same as the direct piezoelectric coefficient d₃₁.But for the P（VDF-Tr FE）52/48QSAP sample,the inverse piezoelectric coefficient d₃₁ is greater than the direct piezoelectric coefficient d₃₁.The simulation and experimental results consistently showed that when the electrostatic interaction between the crystalline ferroelectric domains and the oriented amorphous fraction is significantly enhanced(especially,when relaxor SC_OAF exists),the inverse piezoelectric coefficient d₃₁ of the ferroelectric polymers is not equal to the direct piezoelectric coefficient d₃₁.