Effect Of Uniaxial Stretching On Crystallization And Properties Of PVDF-based Dielectrics

Flexible polyvinylidene fluoride(PVDF)(PVDF)-based dielectric polymers have attracted considerable attentions due to their unique piezoelectric,thermoelectric and ferroelectric properties.Among four crystal phases α,β,γ and δ of PVDF,β phase has a high polarity and a good piezoelectric property after polarization.In this paper,we prepare PVDF,P(VDF-CTFE)and P(VDF-TrFE)films by solution casting method.The mechanical properties of PVDF and P(VDF-CTFE)are studied by uniaxial stretching.In addition,the dependences of dielectric,energy storage and piezoelectric properties of these films on their crystal structiures before and after stretching are investigated by related testing incuding FTIR,XRD,DSC,ferroelectric and dielectric testing devices.By such,some statements are proposed and listed as follows.1)The PVDF films with different molecular weights are treated by low temperature quenching process.the crystalline regions of the PVDF films were found to be in the form of a mixed crystalline phase.After stretching,PVDF changed from a phase to β phase or γ phase,and finally induced into β phase.The mass fraction of β phase increases slightly with increasing molecular weight,which leads to a lower Tm than unstretched sample.In P(VDF-CTFE),the ferroelectric properties of the samples increased with the increase of stretching rate and the content of CTFE monomer.Comparetively,the ferroelectric properties of P(VDF-TrFE)decreased with the increases of TrFE content,and the Tm and Tc decrease accordingly.2)The hysteresis loop test of stretched and un-stretching show that the un-stretching PVDF and P(VDF-CTFE)films have double hysteresis loops,which show the characteristics of an antiferroelectrics.After stretching,the hysteresis loop of P(VDF-CTFE)show that the sample has a phase transition from linear dielectric to relaxor ferroelectric.And P(VDF-TrFE)50/50mol%film is relaxor ferroelectrics,P(VDF-TrFE)55/45,65/35,70/30,75/25 and 80/20mol%films show the characters of ferroelectrics.The maximum polarization of 13.56 μC/m2 and remnant polarization of 9.059 μC/m2 of P(VDF-TrFE)75/25mol%are obtained.After stretching,εr of PVDF and P(VDF-CTFE)is higher than that of unstretched samples.Furthermore,εr of P(VDF-CTFE)increases with stretching rate.In the low frequency range,εr decreases slowly with the increase of frequency.However,it decreases rapidly in high frequency region(>100 kHz).3)Despite of stretching processing,unipolar D-E loop and energy storage density show that the Dm of PVDF and P(VDF-CTFE)increases slightly with the molecular weight.And the stretched PVDF and P(VDF-CTFE)possess the higher energy storage density Than unstretched ones.Moreover,P(VDF-TrFE)has the largest energy loss rate in the coercive field,and loss decreases with electric field.4)The piezoelectric value(d33)of stretched PVDF film decreases continously with increases of temperature.However,under a wide temperaturte range from 25 to 110℃,P(VDF-TrFE)75/25mol%piezoelectric film maintains a high and stable d33 of about-20pC/N.When the temperature increases to 115 ℃,its d33 reduces from-21.26 pC/N to-4.55 pC/N eventually.