Study On Free Volume Regulation And Dielectric Characteristics Of Polar Polymers And Their Composite Films

Polymer dielectric film for electrical energy storage is the crucial component of polymer dielectric capacitor,which directly determines the energy storage density of dielectric capacitor,and the energy storage performance of dielectric materials is closely related to its dielectric properties（dielectric constant and breakdown strength,etc.）.The previous researches show that the free volume of the polymer significantly affects its dielectric properties,which has important research significance for regulating dielectric properties.In order to better design and prepare polymer dielectric films with superior dielectric properties and energy storage performance,it is necessary to further clarify the correlation between dielectric properties and free volume of polymers.In this study,based on the free volume and dielectric properties of polymers,different methods are utilized to regulate the free volume of different types of polymers,and the variations of free volume in different types of polymer systems are quantitatively analyzed by positron annihilation lifetime spectroscopy（PALS）.The effect of various regulation methods on the free volume and the origination of the free volume change were examined in different types of polymers（including high-elastic and highly polar polymer PVDF,glassy highly polar polymer PSF and glassy moderate-polar polymer PMMA）,and the relationships between the free volume and dielectric properties in different polymer systems were established,expecting to provide some theoretical guidance for the structural design and performance optimization of polymer dielectric materials.Additionally,the energy storage performances of different kinds of polymers were studied,and the polymer dielectric films with significantly improved energy storage performance were obtained.The main research contents and conclusions are as follows:（1）The results reveal that the size of free volume holes in PVDF films increases and then decreases with increasing irradiation dose due to electron irradiation-induced processes such as dehydrofluorination,chain scission,and crosslinking.The smaller the size of free volume holes,the greater the breakdown strength of the PVDF film,leading to a higher discharge energy density.When the irradiation dose was 390 k Gy,the size of free volume holes of the PVDF film was the smallest,and the breakdown strength reached the maximum value of 488MV/m,which was more than 60%higher than that of the unirradiated PVDF film.Due to the suppression of dielectric loss,the charge-discharge efficiency is maintained at a high level.Due to the high breakdown strength and high charge-discharge efficiency,the discharge energy density of PVDF film irradiated with 390 k Gy reaches 7.91 J/cm³,which is more than 160%higher than that of the unirradiated PVDF film.（2）In the high-elastic strongly polar PVDF composite film system in which the dielectric constants of polymer matrix and inorganic fillers are close,the variation of free volume is closely related to the destruction of hydrogen bonds inside PVDF.The effect of the change of free volume on dielectric properties is related to the frequency of the electric field.At low frequencies,the size increment of free volume holes will increase the orientation space of intrinsic polar groups,enhancing the orientation polarization and increasing the dielectric constant,which is the positive effect of free volume on dielectric properties.At high frequencies,the orientation polarization is significantly suppressed,and free volume holes with increased size become defects inside the material,which will reduce the density of polar groups and decline the dielectric constant,which is the negative effect of free volume on dielectric properties.Due to the dual effect of free volume on dielectric properties,the frequency dependence of the dielectric constant of PVDF composite membranes is tightly related to the size of free volume holes.（3）The PMMA/5HfO₂ composite film heat-treated at 150°C concurrently exhibits high discharge energy density of 10.45 J/cm³ and high charge-discharge efficiency of 88.45%at the electric field strength of 800 MV/m.More importantly,in the glassy moderate-polar polymer PMMA,the free volume effect of dielectric properties depends on the size of free volume holes or the glass transition.When the radius of free volume holes is less than 3.1（?）（in the glassy state）,the increase of the radius of free volume holes contributes to the increase of the dielectric constant.When the radius of free volume holes is larger than 3.2（?）（in the high-elastic state）,the increase of the volume hole radius decreases the dielectric constant.（4）In the glassy strongly polar polymer PSF,the improvement of dielectric properties brought by the ultralow addition of inorganic fillers originates from the size increase of free volume holes,which provides more sufficient orientation space for intrinsic polar groups,ultimately leading to a significant improvement of the dielectric constant.PALS is utilized to quantitatively analyze the free volume characteristics of the polymer system with ultralow loading of inorganic filler,and the free volume effect of dielectric properties of glassy strongly polar polymer system with ultralow loading of inorganic filler was experimentally clarified.（5）The free volume effects on dielectric properties of different types of polar polymers are distinct and related to various factors such as the type of polymer and frequency.