Graphene sheets segregated by barium titanate for polyvinylidene fluoride composites with high dielectric constant and ultralow loss tangent

In this paper, we report a unique method to develop polyvinylidene fluoride (PVDF) composites with high dielectric constant and low loss tangent by loading relatively low content of graphene-encapsulated barium titanate (BT) hybrid fillers. BT particles encapsulated with graphene oxide (BT-GO) were prepared via electrostatic self-assembly and subsequent chemical reduction resulted in BT-RGO particles. SEM morphology revealed that RGO sheets were segregated by BT particles. The hybrid fillers have two advantages for tuning dielectric properties: loading extremely low content of RGO can be exactly controlled and individual RGO sheets segregated by BT particles would prevent leakage current. As a result, PVDF composites filled with BT-RGO displayed improved dielectric properties before percolative behavior occurred. Composites filled with 30 vol% BT-RGO have a dielectric constant and loss tangent (tan δ) value of 67.5 and 0.060 (1 kHz), respectively. By contrast, dielectric constant and tan δ of composites filled with 30 vol% BT-GO and BT were 57.7 and 38.3, 0.076 and 0.042 (1 kHz), respectively. The improvement of dielectric constant is attributable to the formation of microcapacitors by highly conductive RGO sheets segregated by BT particles. Meanwhile, the distance between adjacent RGO sheets is large enough to prevent leakage current from tunneling conductance, by which tan δ is remarkably constrained. The composites could achieve excellent dielectric properties by loading relatively low amount of ceramic fillers, which indicates that this method can be used as guideline for reduce the usage amount of ceramic fillers.     

聚偏氟乙烯及其共聚物基压电复合材料的研究进展

聚合物压电材料聚偏氟乙烯(PVDF)及其共聚物P(VDF-TrFE)、P(VDF-HFP)是一类典型的具有压电性的有机高分子材料,它们由于具有良好的力学性能、耐腐蚀性、生物相容性以及易于加工等特点而受到研究人员的广泛关注.但相对于传统无机类压电陶瓷材料来说,聚合物压电材料的压电常数仍相对较低,因此提升这类聚合物压电材料...     

Effects of particle size of BaTiO3 powder on the dielectric properties of BaTiO3/polyvinylidene fluoride composites

The effects of particle size of fine BaTiO₃powder on dielectric properties of BaTiO₃/polyvinylidene fluoride (PVDF) composites were investigated. When the frequency of the applied field was less than 100 kHz, the dielectric constant and loss for BaTiO₃/PVDF composites decreased with increasing BaTiO₃particle size. When the frequency was greater than 100 kHz, the opposite results were obtained. The resistivity increased with increasing BaTiO₃particle size, whereas the open areas of hysteresis loops decreased gradually. The degree of poling efficiency for BaTiO₃/PVDF composites increased with increasing BaTiO₃particle size. The BaTiO₃particle size dependence of the dielectric properties of BaTiO₃/PVDF composites is explained by space charge effects at the interface between BaTiO₃and PVDF, and domain configurations (single or multi-domain) of the BaTiO₃powders.     

Phase transformation and thermomechanical characteristics of stretched polyvinylidene fluoride

Phase transformation and thermomechanical characteristics of the polyvinylidene fluoride (PVDF) are investigated using a microtesting machine and a thermomechanical analyzer. The phase transformation from the - to β-phase of the PVDF resulted from increased PVDF chain length caused by the stretching procedure. The results show that the phase transformation changed due to increased stretching ratios with smaller changes for stretching ratios over 4. Young's modulus along the stretching direction was higher than that along the transverse of stretching. The thermomechanical curve variation can be divided into two parts: one part is a gradual slope before about 50 °C, and the other is similar to exponential growth. Thermomechanical change, thermal elongation, and expansion greatly influence the stretching ratios of 5 in the stretching direction. The higher stretching ratio easily causes the shrink influence of the material due to the higher cohesion of the amorphous regions. Therefore, the observed results can provide useful information to optimize for piezoelectric or pyroelectric PVDF applications.     

XRD studies on barium titanate (BaTiO3)/polyvinylidene fluoride (PVDF) composites

XRD studies were carried out on barium titanate (BaTiO₃)/polyvinylidene fluoride (PVDF) composites in the ranges 98° to 103° and 10° to 50°. The structures of PVDF/BaTiO₃ composites were affected significantly. In the composites, elimination and suppression of some of the peaks of PVDF and shifting of some of the peaks of BaTiO₃ was observed. The observed changes are attributed to the internal stresses.     

ZrO2/聚偏氟乙烯复合材料介电性能及松弛行为

通过熔融法制备了ZrO₂/聚偏氟乙烯（PVDF）复合材料,采用SEM、XRD和FTIR对其形貌和结构进行分析,结果表明,ZrO₂分布较为均匀,ZrO₂/PVDF复合材料主要含α相和少量γ相。采用宽频介电谱（BDS）测试,ZrO₂/PVDF复合材料介电常数ε'随ZrO₂含量的增加而增加,而介电损耗tanδ保持定值,表明ZrO₂的加入可以显著提高ZrO₂/PVDF复合材料的介电性能。经计算ZrO₂/PVDF复合材料介电模量M″和活化能,发现有玻璃化转变峰、缺陷峰和界面极化峰存在,而加入ZrO₂后,ZrO₂/PVDF复合材料活化能增加。      

Polydopamine coating layer on graphene for suppressing loss tangent and enhancing dielectric constant of poly(vinylidene fluoride)/graphene composites

Graphene with polydopamine (PDA) coating layer which displays promoted dispersibility in organic solvent was prepared through self-polymerization of dopamine onto graphene oxide (GO) and subsequent chemical reduction. The PDA coated reduced GO (RDGO) is homogeneously incorporated into poly(vinylidene fluoride) (PVDF) matrix, which exhibit a percolation threshold at 0.643 wt%. The dielectric constant of PVDF with 0.70 wt% RDGO increases to 176, about 17 times of neat PVDF. Importantly, the loss tangent is suppressed to 0.337 due to reduction of the concentration and mobility of ionizable carboxylic groups by PDA. The enhancement of dielectric constant probably rises from duplex interfacial polarization induced by graphene–semiconductor interface, and semiconductor–insulator interface. The composites displays advantages in excellent dielectric properties and good flexibility and processability guaranteed by low loading of RDGO, which is suitable for the development of dielectric materials for energy storage.     

聚偏氟乙烯膜的疏水改性

在空气除湿过程中,膜液体分离技术凭借其对空气无污染、效率高、能耗少等优点,已越来越得到人们的重视.但是在长期的使用过程中,经常发现液体会泄漏到气体中.为了解决这一问题,通过对聚偏氟乙烯(PVDF)膜进行改性,使其表面有一层薄薄的α,β二羟基聚二甲基硅氧(PDMS)致密层.采用扫描电镜、接触角测量表征改性前后的膜结构变化...     

Dielectric constant of rolled polyvinylidene fluoride

The contribution of dipole orientation to dielectric constant is markedly affected by rolling. The intensity of the peak is increased and the maximum shifted slightly to higher temperatures and frequencies. The shift is much larger with the peak. Its intensity, however, is reduced by rolling. Long before the maximum of the peak is reached, annealing effects irreversibly change the sample. With annealing, the values of the dielectric constant gradually approach those of the unrolled material. The observed effects can be explained by a substantial orientation of chains in the roll plane, which is perpendicular to the applied electric field. Such an orientation increases the average contribution of dipoles in crystal defects to polarisation as observed in the peak. Stronger interaction between partially aligned chains in the amorphous matrix hampers the rotation of chain segments in the electric field, and hence reduces the peak and shifts it to higher temperatures.     

聚偏氟乙烯超疏水分离膜的制备

采用溶液涂覆-浸没相分离法对聚偏氟乙烯膜(PVDF)进行表面复合改性,制备了超疏水分离膜。初步考察了涂覆液中PVDF固含量和涂覆条件(浸泡时间、预蒸发时间、凝固浴组成和凝固浴温度)对复合膜疏水性能的影响。实验结果表明,当涂覆液中PVDF含量为1.88%(质量分数)时,膜丝有最大接触角136°;复合膜的接触角随浸泡时间的延长呈现先增大后减小的趋势,当浸泡时间为40s时,接触角最大,达到133°;在较短时间内(0～5s),预蒸发时间对复合膜的接触角影响不大;复合膜的接触角随着凝固浴中DMAc含量的增加而逐渐减小,随着凝固浴温度的增大而增大,当凝固浴温度为65℃时,膜表面的接触角增至153°。     

聚偏氟乙烯微滤膜的表面改性

采用改良的热诱导聚合技术,在聚偏氟乙烯(PVDF)微滤分离膜表面接枝具有离子交换性能的羧酸基团.采用热重(TG)、差示扫描量热(DSC)、X衍射(XRD)、红外光谱(IR)、环境扫描电镜(ESEM)和X衍射光电子能谱(XPS)对改性PVDF微滤分离膜(PAA-PVDF分离膜)进行表征,研究了改性PVDF分离膜对Cu2 的去除效果.试验结果表明,热诱导聚合技术在PVDF分离膜表面成功接枝了羧酸基团,PAA-PVDF分离膜对Cu2 的去除效果优良.     

X-ray high pressure study of polyvinylidene fluoride

An X-ray high pressure study at room temperature of both phase I and phase II crystal structures of polyvinylidene fluoride has been carried out. At room temperature both phases are stable up to pressures greater than 14 kbar. The variation of lattice compressive strains with pressure could be fitted to the Tait equation with little scatter and the variation of the unit cell parameters with pressure computed. The bulk lattice compressibilities of both phase I and phase II was found to be considerably less than that of polyethylene with the lowest compressibility being found for the phase I structure. The linear lattice compressibilities are extremely anisotropic with the lowest compressibility being in the chain direction as expected. However, at the highest pressures, for the case of phase II it was observed that this anisotropy was greatly reduced. Applications of these data to the unique piezo-electric activity of PVF₂ are pointed out.     

聚偏氟乙烯膜的超疏水改性研究

为提高疏水膜的疏水性能,使其可在膜蒸馏、膜吸收等领域有更广泛的应用.采用溶液相转移法制备超疏水性聚偏氟乙烯(PVDF)分离膜,考察了铸膜液中PVDF和非溶剂(低分子二醇类化合物PG)的浓度对膜润湿性能的影响.结果表明,通过改变铸膜液中PVDF、PG的浓度,能使PVDF膜的表面静态接触角从75.1°提高到161.7°,滚动角仅为15.8°.还研究了PVDF复合膜的制备条件对膜润湿性能的影响,结果表明,在一定的非溶剂浓度范围,增加复合膜涂覆液中非溶剂PG的加入量,有利于得到较高的复合膜表面接触角,但膜丝在涂覆液中的浸泡时间也需要相应延长.当非溶剂PG的质量分数为39.1%、浸泡时间为50 s时,复合膜表面接触角达到了155°.     

聚偏氟乙烯分离膜的亲水改性

利用醇类与醛类化合物的缩醛化反应,生成枝状或网状非水溶性的亲水性大分子,以制备具有持久亲水性的聚偏氟乙烯中空纤维膜.根据溶液的浊度变化来确定交联程度.反应体系为戊二醛与聚乙烯醇(PVA)在溶剂二甲基乙酰胺中的缩醛化反应,反应温度为70℃,反应时间30 min,催化剂为盐酸(质量分数36.5％).改性后的聚偏氟乙烯(PVDF)分离膜的亲水性、水通量明显提高,蛋白吸附性降低,有较好的化学稳定性.随着PVA浓度的增加,膜材料的接触角相应变小,亲水性能增强,蛋白吸附性降低.     

Binary synergistic enhancement of dielectric and microwave absorption properties: A composite of arm symmetrical PbS dendrites and polyvinylidene fluoride

Arm symmetrical PbS dendrite (ASD-PbS) nanostructures can be prepared on a large scale by a solvothermal process. The ASD-PbSs exhibit a three-dimensional symmetrical structure, and each dendrite grows multiple branches on the main trunk. Such unique ASD-PbSs can be combined with polyvinylidene fluoride (PVDF) to prepare a composite material with enhanced dielectric and microwave-absorption properties. A detailed investigation of the dependence of the dielectric properties on the frequency and temperature shows that the ASD-PbS/PVDF composite has an ultrahigh dielectric constant and a low percolation threshold. The dielectric permittivity is as high as 1,548 when the concentration of the ASD-PbS filler reaches 13.79 vol.% at 102 Hz, which is 150 times larger than that of pure PVDF, while the composite is as flexible as pure PVDF. Furthermore, the maximum reflection loss can reach–36.69 dB at 16.16 GHz with a filler content of only 2 wt.%, which indicates excellent microwave absorption. The loss mechanism is also elucidated. The present work demonstrates that the addition of metal sulfide microcrystals to polymer matrix composites provides a useful method for improving the dielectric and microwave-absorption properties.