Study On The Effect Of Polarization Mode On The Ferroelectricity Of PVDF Polymer

Ferroelectric materials have become one of the widely used materials along with the development of electronic science and technology.Among them,polymer ferroelectric materials process the potential to be used in wearable flexible,devices or other devices thanks to their unique mechanical properties.However,compared with inorganic ferroelectric materials,the ferroelectric properties of polymer ferroelectric materials are not powerful enough.Therefore,it is urgent to improve the ferroelectric properties of this group of materials so that increase the value of application.Polarization is a way to make charged particles or dipoles which inside the ferroelectric materials.And most of the polarized ferroelectric materials will exhibit piezoelectric and pyroelectric properties.In this work,by adjusting the polarization mode,the mechanism of the electric field on the PVDF molecular chain under different conditions has been explored,and different polarization processes are designed to improve the piezoelectricity and pyroelectricity of the PVDF film.The main research contents are as follows:By changing the strength,duration of electric field and the ambient temperature of polarization,the mechanism of molecular chain conformation of PVDF changed by electric field has been explored.Under the action of an electric field,the orientation polarization of dipole on the PVDF molecular chain changes the molecular chain conformation.The electric field provides energy for the orientation polarization of the dipole,and the duration time depends on the dipole relaxation time and the establishment time of new equilibrium.In addition,the thermal motion of molecules will increase with the increase of the polarization temperature,which is not conducive to turning polarization.The experimental results show that when the polarization time exceeds 20 minutes and the electric field strength is greater than 60 MV/m at room temperature,PVDF functional films with piezoelectric constant and pyroelectric coefficient of-6.9 pC/N and 7.69?C/m~2K could be obtained,respectively.Since the direct-current(DC)electric field will cause the twisting and folding of the PVDF molecular chain,in order to avoid this phenomenon and improve the ability of the electric field to change the conformation,a slow-acting electric field,alternating current(AC)electric field,was used to polarize the PVDF film.The influence of the waveform and frequency of AC electric field on the stretched PVDF film was explored.On this foundation,a new polarization mode“stepwise polarization”was proposed,which combines the advantages of AC and DC electric field.Through this method,a PVDF stretched film with more excellent ferroelectric properties was obtained.The experimental results showed that the AC electric field could improve the consistency of the dipoles on the PVDF molecular chain,and will not cause the chain defects.After the stepwise polarization treatment,the piezoelectric coefficient of the PVDF stretched film reached-9.8 pC/N,and the pyroelectric coefficient reached 19?C/m~2K near 53?.The increase was 48%and 43%respectively.In order to improve the ability of the electric field to change the conformation of the PVDF molecular chain,changing the main chain structure of the PVDF molecular chain is a great candidate.Taking advantage of the fact that the breakdown strength of PVDF increases with the decrease of temperature,a large electric field could be used to induce the dehydrogenation reaction at low temperature.Due to the defluorination reaction of PVDF,the removed hydrogen ions and fluoride ions will combine with each other under high temperature conditions.In the meantime,there are randomly unsubstituted dangling bonds on the PVDF molecular chain,which could form isolated C=C double bonds to reduce the rotation barrier of the single bond adjacent to the double bond.In this way,it's possible to improve the ability of the electric field to change the chain conformation.The test results showed that the dehydrogenation and defluorination reaction occurred on PVDF molecular chains under the action of large field strength or high temperature.The piezoelectric coefficient of PVDF stretched film can reach-13.5 pC/N through the new“stepwise low temperature polarization”method.Compared with the sample obtained by the stepwise AC polarization method which proposed previously,it has increased by 38%.