Study On Preparing PVDF Film With Ultrahigh β-phase Content Achieved By Melt Stretching And Its Piezoelectric Properties

Compared with traditional ceramic piezoelectric materials,the piezoelectric polymer represented by poly（vinylidene fluoride）（PVDF）has the advantages of light weight,good piezoelectric properties and low production cost,which has a broad application prospect in modern high-tech fields.It has been shown that the piezoelectric function of PVDF mainly depends on the internal crystalline phase structure,that is,electroactiveβ-phase.Obtaining high content ofβ-phase is the premise and basis to realize the excellent piezoelectric properties of material.At present,β-phase PVDF films are mainly prepared by solid-state drawing,strong electric field poling and adding nano-fillers.But these methods are often low efficient,complex or sacrificing other properties of material.For this reason,we have proposed a new melt stretching strategy in this thesis.Through the precise control of melt crystallization by stretching flow,the high-quality PVDF film with ultra-highβ-phase content was prepared,and its piezoelectric properties and affecting factors were systematically studied.The main research work is as follows:（1）A melt stretching method for preparing PVDF film with ultra-highβ-phase content directly from melt was proposed.By applying high stretching ratio（160 ℃,40×）to the slightly cross-linked PVDF melt,the formation ofβ-phase with a nanofibrils morphology was induced.The nanofibrils with a diameter of about 21.6nm were arranged parallel to each other throughout the whole film space.It was shown that the relative content ofβ-phase was as high as 96%and the crystallinity was 56.4%,which were much higher than the general value reported in PVDF.In addition,due to the nano-scale oriented nanofibrils morphology,the film had high transparency of～90%,high mechanical strength and good flexibility.（2）Based on the PVDF film filled withβ-phase nanofibrils,the flexible piezoelectric device was fabricated by combining the electric field poling.The device could monitor the change of dynamic pressure（0-250 k Pa）and temperature（3.3m V/℃）with a broad response range and good sensitivity.When the piezoelectric device was made into a wearable flexible sensor,it could well capture the human motion signal such as finger bending and tapping,as well as weak physiological signal such as respiration,pulse and pronunciation.This confirmed that the piezoelectric film had excellent sensing performance with high sensitivity and good repeatability.（3）PVDF/BTO composite films were prepared by melt stretching（160 ℃,20×）after introducing the barium titanate（Ba Ti O₃,BTO,0-20%）nanoparticles into the slightly cross-linked PVDF network.The addition of fillers could affect the morphology ofβ-phase by influencing the micro-flow field distribution,resulting in an increase in the distance between lamellar crystal and a decrease in theβ-phase nanofibril diameter from 28.7nm to 19.7nm.Meanwhile,theβ-phase content did not change much,still as high as 90%.In addition,due to the high dielectric properties of BTO,the dielectric constant of composite film increased from 12.4（BTO,0%）to15.4（BTO,20%）.Thanks to the retention ofβ-phase nanofibrils morphology,the composite film still exhibits a high tensile strength of more than 90 MPa.（4）Based on the PVDF/BTO composite film obtained by melt stretching,the corresponding piezoelectric device was prepared by electric field poling.It was found that the addition of BTO not only retained the flexibility and working stability of PVDF film,but also greatly improved the piezoelectric properties.For the film with20%filler content,the sensitivity was 5.2 V/k Pa at low pressure（～0.5 k Pa）and 41.6m V/k Pa at high pressure（～10 k Pa）,significantly higher than that of pure PVDF.In addition,as a wearable flexible sensor,the film could also effectively monitor human motion and physiological signal.Because of its good piezoelectric performance,the composite film serving as a small piezoelectric generator was able to collect mechanical energy and convert it into electric energy（power density 5.8 m W/m²）,thus supplying power for microelectronic components like lighting LED lamps.The excellent piezoelectric properties of composite film are mainly due to the combined effects of highβ-phase content in PVDF matrix and BTO piezoelectric fillers.