Fabrication and Excellent Properties of Polyvinylidene fluoride/Graphene Composite Films as Thermal Interface Materials

The growing concern about thermal conductivityand electromagnetic shielding inelectronic equipment has promoted the development of interfacial film materials.In this work,polyvinylidene fluoride(PVDF)/graphene composite films with different graphene contents were fabricated by high-energy ball milling,cold isostatic pressing,scraping and coating,successively.High-energy ball milling is beneficial to the dispersion of graphene powder,while cold isostatic pressing can greatly enhance thermal conductivity and mechanical strength by reducing the voids in the film and increasing the contact area of graphene sheets.The thermal conductivity,tensile strength and electromagnetic shielding properties of the films were carefully investigated and compared.It was demonstrated that the thermal conductivity increased from 0.19 Wm-1.K-1 for pure PVDF to 103.9 W-m-1.K-1 for the composite film with PVDF:graphene=1:3.Meanwhile the electromagnetic shielding efficiency can reach 36.55 dB.The prepared PVDF/graphene composite films exhibit outstanding overall performance and have the potential for practical applications.

Wearable Triboelectric Nanogenerators Based on Printed Polyvinylidene Fluoride Films Incorporated with Cobalt-Based Metal-Organic Framework for Self-Powered Mobile Electronics

In this study,wearable triboelectric nanogenerators comprising bar-printed polyvinylidene fluoride(PVDF)films incorporated with cobalt-based metal-organic framework(Co-MOF)were developed.The enhanced output performance of the TENGs was attributed to the phase transition of PVDF from a-crystals toβ-crystals,as facilitated by the incorporation of the MOF.The synthesis conditions,including metal ion,concentration,and particle size of the MOF,were optimized to increase open-circuit voltage(VOC)and open-circuit current(I_(SC))of PVDF-based TENGs.In addition to high operational stability,mechanical robustness,and long-term reliability,the developed TENG consisting of PVDF incorporated with Co-MOF(Co-MOF@PVDF)achieved a VOC of 194 V and an I_(SC)of 18.8μA.Furthermore,the feasibility of self-powered mobile electronics was demonstrated by integrating the developed wearable TENG with rectifier and control units to power a global positioning system(GPS)device.The local position of the user in real-time through GPS was displayed on a mobile interface,powered by the battery charged through friction-induced electricity generation.

Dye-sensitized Solar Cells with Higher J_(sc) by Using Polyvinylidene Fluoride Membrane Counter Electrodes

A flexible counter electrode(CE) for dye-sensitized solar cells(DSCs) has been fabricated using a micro-porous polyvinylidene fluoride membrane as support media and sputtered Pt as the catalytic material.Non-conventional structure DSCs have been developed by the fabricated CEs. The Pt metal was sputtered onto one surface of the membrane as the catalytic material. DSCs were assembled by attaching the Ti O2 electrode to the membrane surface without Pt coating. The membrane was with cylindrical pore geometry. It served not only as a substrate for the CE but also as a spacer for the DSC. The fabricated DSC with the flexible membrane CE showed higher photocurrent density than the conventional sandwich devices based on chemically deposited Pt/FTO glass, achieving a photovoltaic conversion efficiency of 4.43%. The results provides useful information in investigation and development of stable, low-cost, simple-design, flexible and lightweight DSCs.

Effect of Packaging Material on Piezoelectric Response of Sensor with Electrospinning Polyvinylidene Fluoride (PVDF) Nanofiber Web

The polyvinylidene fluoride(PVDF)nanofiber web by electrospinning technology has the characteristics of fast response,high sensitivity,wide range of pressure,etc.,and provides new sensitive materials for the sensor testing the dynamic pressure such as foot pressure during walking.Because of the nanofiber mesh structure,it must be packaged to collect piezoelectric charge and bear strong mechanical behavior before industrial practice.The PVDF nanofiber web is usually packaged by incorporating a pair of flexible electrode as well as the lead of signal output.This present work will introduce the detailed packaging process and technology of PVDF nanofiber web,and three different types of packaging electrode materials(adhesive copper foil tape,indium tin oxide(ITO)thin plate,and adhesive conductive cloth)in previously published literatures are compared by the piezoelectric response of their sensor prototypes to a periodic mechanical activation.The results showed that the surface property of packaging material had a significant effect on the piezoelectric response of sensor by PVDF nanofiber web.For PVDF nanofiber web sensor,therefore,it needed a deep investigation on the specific packaging technology in terms of different working conditions.

Preparation and Characterization of the Modified Polyvinylidene Fluoride (PVDF) Hollow Fibre Microfiltration Membrane

A novel thermally induced graft polymerization technique was used to modify a polyvinylidene fluoride （PVDF） hollow fibre microfiltration membrane. An artificial neural network （ANN） was applied to optimize the prepared condition of the membrane. The optimized dosing of acrylic acid （AA）, acrylamide （AM）, N, N＇- methylenebisacrylamide （NMBA） and potassium persulphate （KSP） designed by ANN was that AA was 40.63 ml/L; AM acted as 6.25 g/L; NMBA was 1.72 g/L and KSP was 1.5 g/L, respectively. The thermal stability of the PVDF modified hollow fibre membrane （PVDF-PAA） was investigated by thermogravimetric （TG） and differential scanning calorimetry （DSC） analysis. The polycrystallinity of the PVDF-PAA membrane was evaluated by X-ray diffraction （XRD） analysis. The complex formation of the modified membrane was ascertained by Fourier transform infrared spectroscopy （FTIR）. The morphology of the PVDF-PAA membrane was studied by environmental scanning electron microscopy （ESEM）. The surface compositions of the membrane were analyzed by X-ray photoelectron spectroscopy （XPS）. The adsorption capacity of Cu^2＋ ion on the PVDF-PAA hollow fibre membrane was also investigated.

Preparation and Properties of Electrospun Polyvinylidene Fluoride (PVDF)/Polyurethane (PU) Composite Nanofiber Membranes

Polyvinylidene fluoride(PVDF)/polyurethane(PU)composite nanofiber membranes were prepared by mixing PVDF and PU at different mass ratios.The microstructure and the crystal structure of the composite nanofiber membranes were analyzed by scanning electron microscopy(SEM),Fourier transform infrared(FTIR)spectroscopy and X-ray diffraction(XRD).The hydrophilicity,mechanical properties and piezoelectric properties were also tested.Results showed that when the mass ratio of PVDF to PU was 9∶1,the fiber membrane had the best microstructure,and the crystal form of PVDF changed fromαcrystal toβcrystal in the electrospinning.The addition of PU improved the hydrophilicity,mechanical properties,and piezoelectric signal of the fiber membrane.When the mass ratio of PVDF to PU was 9∶1,the tensile strength reached the peak value of(10.39±0.41)N,and the output voltage reached the maximum value of(1.98±0.12)V.

Effect of Polyvinylidene Fluoride Hollow Fiber Membranes on Mass Transfer of Samarium

The influence of swelling and stripping acidity on the mass transfer coefficient based on water phase and the inner diameters of membranes were studied with P507-HCl-Sm as working system in the two different kinds of hollow fiber membranes. Effects of extractant concentration, H+ concentration in aqueous phase and Sm3+ concentration on extraction rate were discussed and the corresponding reaction series were obtained. According to the investigations on the interfacial kinetics, the reaction kinetics equation and reaction rate constant were obtained.

PREPARATION AND CHARACTERIZATION OF ION EXCHANGE MEMBRANES BASED ON POLYVINYLIDENE FLUORIDE

A new ion exchange membrane based on polyvinylidene fluoride (PVDF) and sulfonated poly(styrene-divinylbenzene) was prepared by in-situ polymerization. The incorporation of sulfonic groups into the polyvinylidene fluoride composite membrane was confirmed by infrared spectroscopy (IR), ion exchange capacity (IEC) and energy dispersive X-ray analysis (EDAX). Area resistance, IEC and water uptake of the treated membrane were evaluated. When 20% of the crosslinked membrane was sulfonated at 80degreesC for 22 h, the PVDF ion exchange membrane can attain 0.8 Omega . cm(2) area resistance in NaCl aqueous solution at 25degreesC, IEC is as high as 2.43 millimoles per grain of the wet membrane. The hydrophilicity of PVDF membrane is also significantly improved after treatment. When 60% of crosslinked membrane was sulfonated at 80degreesC for 6 h, water uptake of the treated membrane can attain 64.7%.

Morphology of Polyvinylidene Fluoride Based Gel Polymer Electrolytes

Two series of polyvinylidene fluoride (PVDF) based gel polymer electrolytes, with different LiClO 4 or propylene carbonate (PC) content, were prepared and analyzed by infrared spectrometer, differential scanning calorimetry, scanning electron microscope and complex impedance spectrometer. The results show that there are great interactions between PVDF, PC and lithium cations. Both LiClO 4 and PC content lead to evident change of the morphology of the gel polymer electrolytes. The content of LiClO 4 and PC also influences the ionic conductivity of the samples, and an ionic conductivity of above 10 -3 S·cm -1 can be reached at room temperature.

Soft Template-Induced Porous Polyvinylidene Fluoride Membrane for Vanadium Flow Batteries

Vanadium flow batteries(VFBs)are considered ideal for grid-sc ale,long-duration energy storage applications owing to their decoupled output power and storage capacity,high safety,efficiency,and long cycle life.However,the widespread adoption of VFB s is hindered by the use of expensive Nafion membranes.Herein,we report a soft template-induced method to develop a porous polyvinylidene fluoride(PVDF)membrane for VFB applications.By incorporating water-soluble and flexible polyethylene glycol(PEG 400)as a soft template,we induced the aggregation of hydrophilic sulfonated poly(ether ether ketone),resulting in phase separation from the hydrophobic PVDF polymer during membrane formation.This process led to the creation of a porous PVDF membrane with controllable morphologies determined by the polyethylene glycol content in the cast solution.The optimized porous PVDF membrane enabled a stable VFB performance for 200 cycles at a current density of 80 mA/cm^(2),and the VFB exhibited a Coulombic efficiency of 95.2%and a voltage efficiency of 87.8%.These findings provide valuable insights for the development of highly stable membranes for VFB applications.

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.

Superhydrophobic polyvinylidene fluoride/polyimide nanofiber composite aerogels for thermal insulation under extremely humid and hot environment

Excellent thermal insulating materials are highly demanded in various applications including buildings, aerospace and sport equipment. However, in practical applications,the performance of thermal insulating materials usually deteriorates under diverse temperature and humidity conditions.Therefore, it is highly essential to construct a bulk material that exhibits outstanding thermal insulation performance under extremely humid and hot environment. In this work, we have conceived a green and effective strategy to fabricate a superhydrophobic and compressible polyvinylidene fluoride/polyimide(PVDF/PI) nanofiber composite aerogel via electrospinning and freeze-drying technique. Interestingly, the PVDF nanofibers and PI nanofibers function as the hydrophobic fibrous framework and mechanical support skeleton,respectively, forming a robust three-dimensional framework with good mechanical flexibility. The PVDF/PI aerogel possesses outstanding superhydrophobic feature(water contact angle of 152°) and low thermal conductivity(31.0 m W m^(-1)K^(-1))at room temperature. Significantly, even at 100% relative humidity(80℃), the PVDF/PI aerogel still exhibits a low thermal conductivity of only 48.6 m W m^(-1)K^(-1), which outperforms the majority of commercial thermal insulating materials. Therefore, the novel PVDF/PI aerogel is promising as an excellent thermal insulating material for the applications in high-temperature and humid environment.

Efficacy of a novel endotoxin adsorber polyvinylidene fluoride fiber immobilized with L-serine ligand on septic pigs

A novel adsorber,polyvinylidene fluoride matrix immobilized with L-serine ligand (PVDF-Ser),was developed in the present study to evaluate its safety and therapeutic efficacy in septic pigs by extracorporeal hemoperfusion.Endotoxin adsorption efficiency (EAE) of the adsorber was firstly measured in vitro.The biocompatibility and hemodynamic changes during extracorporeal circulation were then evaluated.One half of 16 pigs receiving lipopolysaccharide (Escherichia coli O111:B4,5 μg/kg) intravenously in 1 h were consecutively treated by hemoperfusion with the new adsorber for 2 h.The changes of circulating endotoxin and certain cytokines and respiratory function were analyzed.The 72 h-survival rate was assessed eventually.EAE reached 46.3% (100 EU/ml in 80 ml calf serum) after 2 hcirculation.No deleterious effect was observed within the process.The plasma endotoxin,interleukin-6 (IL-6),and tumor necrosis factor-α (TNF-α) levels were decreased during the hemoperfusion.Arterial oxygenation was also improved during and after the process.Furthermore,the survival time was significantly extended (>72 h vs.47.5 h for median survival time).The novel product PVDF-Ser could adsorb endotoxin with high safety and efficacy.Early use of extracorporeal hemoperfusion with the new adsorber could reduce the levels of circulating endotoxin,IL-6,and TNF-α,besides improve respiratory function and consequent 72 h-survival rate of the septic pigs.Endotoxin removal strategy with blood purification using the new adsorber renders a potential promising future in sepsis therapy.

Using polyvinylidene fluoride to improve ignition and combustion of micron-sized boron powder by fluorination reaction

Boron has a promising application in the field of propellants due to its high calorific value.However,the difficulty of ignition and the poor combustion efficiency of boron(B)have severely limited its efficient application.In response to this issue,this paper proposes to improve the ignition and combustion performance of micron-sized boron by the Polyvinylidene Fluoride(PVDF)coating.The effect of PVDF content on the B combustion performance was systematically studied using a Thermogravimetry-Differential Scanning Calorimetry(TG-DSC),a Transmission Electron Microscope(TEM),an X-Ray Diffractometer(XRD),a laser Particle Size Analyzer(PSA),and a high-speed camera.The results show that PVDF can significantly reduce the initial oxidation temperature of B powder and increase its reaction heat.When the PVDF content is 23wt%,the reaction heat and the combustion intensity of B powder reach the maximum and are significantly higher than those of the uncoated B powder.Moreover,the fluorination reaction that occurs during the combustion process not only can effectively shorten the combustion time of B powder,but also has a positive effect on its flame intensity and propagation speed,and it significantly reduces B particle agglomeration,which improves the combustion efficiency significantly.This study lays the foundation for the application of PVDF modified B in B-based solid propellants.

EFFECT OF SOLUTION EXTRUSION RATE ON MORPHOLOGY AND PERFORMANCE OF POLYVINYLIDENE FLUORIDE HOLLOW FIBER MEMBRANES USING POLYVINYL PYRROLIDONE AS AN ADDITIVE

Polyvinylidene fluoride （PVDF） hollow fiber membranes prepared from spinning solutions with different polyvinyl pyrrolidone （PVP） contents （1% and 5%） at different extrusion rates were obtained by wet/dry phase process keeping all other spinning parameters constant. In spinning these PVDF hollow fibers, dimethylacetamide （DMAc） and PVP were used as a solvent and an additive, respectively. Water was used as the inner coagulant. Dimethylformamide （DMF） and water （30/70） were used as the external coagulant. The performances of membranes were characterized in terms of water flux, solute rejection for the wet membranes. The structure and morphology of PVDF hollow fiber were examined by BET adsorption, dry/wet weight method and scanning electron microscopy （SEM）. It is found that the increase in PVP content and extrusion rate of spinning solution can result in the increase of water flux and decrease of solute rejection. The improvements of interconnected porous structure and pore size are induced by shear-thinning behavior of spinning solution at high extrusion rates, which could result in the increase of water flux of hollow fiber membranes. The increase of extrusion rate also leads to the increase of membrane thickness due to the recovery effect of elastic property of polymer chains.

Anion exchange membranes with a semi-interpenetrating polymer network using 1,6-dibromohexane as bifunctional crosslinker

An anion exchange membrane(AEM)is generally expected to possess high ion exchange capacity(IEC),low water uptake(WU),and high mechanical strength when applied to electrodialysis desalination.Among different types of AEMs,semi-interpenetrating polymer networks(SIPNs)have been suggested for their structural superiorities,i.e.,the tunable local density of ion exchange groups for IEC and the restrained leaching of hygroscopic groups by insolubility for WU.Unfortunately,the conventional SIPN AEMs still struggle to balances IEC,WU,and mechanical strength simultaneously,due to the lack of the compact crosslinking region.In this work,we proposed a novel SIPN structure of polyvinylidene difluoride/polyvinylimidazole/1,6-dibromohexane(PVDF/PVIm/DBH).On the one hand,DBH with two cationic groups of imidazole groups are introduced to enhance the ion conductivity,which is different from the conventional monofunctional modifier with only one cationic group.On the other hand,DBH has the ability to bridge with PVIm,where the mechanical strength of the resulting AEM is increased by the increase of crosslinking degree.Results show that a low WU of 38.1%to 62.6%,high IEC of 2.12—2.22 mmol·g^(-1),and excellent tensile strength of 3.54—12.35 MPa for PVDF/PVIm/DBH membrane are achieved.This work opens a new avenue for achieving the high-quality AEMs.

Improvement of hydrophilicity of porous PVDF membranes with LiCl additives

Porous polyvinylidene fluoride（PVDF）membranes blended with LiCl are prepared through the phase inversion method to obtain a good support layer for air dehumidification.The hydrophilicity of the resulting membrane is evaluated by water contact angle measurements and vapor adsorption tests.The moisture permeation performance of the membrane is measured by permeation tests in terms of total mass transfer coefficients and moisture permeability rates.It is found that water contact angles and water vapor adsorption capacities increase with the increasing LiCl content in the casting solution.As the LiCl content increases,the total mass transfer coefficient increases slightly at a low LiCl content（below 2.5%）and then improves greatly at a high LiCl content（above 2.5%）,whereas the moisture permeation rate increases.The results demonstrate that LiCl can remarkably improve the hydrophilicity of PVDF membranes,and then greatly enhance moisture permeation performance.

Construction of Renewable Superhydrophobic Surfaces via Thermally Induced Phase Separation and Mechanical Peeling

We report a simple preparation method of a renewable superhydrophobic surface by thermally induced phase separation （TIPS） and mechanical peeling. Porous polyvinylidene fluoride （PVDF） membranes with hierarchical structures were prepared by a TIPS process under different cooling conditions, which were confirmed by scanning electron microscopy and mercury intrusion porosimetry. After peeling off the top layer, rough structures with hundreds of nanometers to several microns were obtained. A digital microscopy determines that the surface roughness of peeled PVDF membranes is much higher than that of the original PVDF membrane, which is important to obtain the superhydrophobicity. Water contact angle and sliding angle measurements demonstrate that the peeled membrane surfaces display super- hydrophobicity with a high contact angle （152°） and a low sliding angle （7.2°）. Moreover, the superhydrophobicity can be easily recovered for many times by a simple mechanical peeling, identical to the original superhydrophobicity. This simple preparation method is low cost, and suitable for large-scale industrialization, which may offer more opportunities for practical applications.

聚偏氟乙烯改性氧化石墨烯智能柔性膜的湿电性能研究

石墨烯在湿气产电、水蒸发诱导产电、湿气响应驱动器等领域显示出强大的应用潜力,强化石墨烯基材料的湿电性能具有重要的现实意义。将一定量的聚偏氟乙烯(polyvinylidene fluoried,PVDF)引入到氧化石墨烯(graphene oxide,GO)薄膜中,在维生素C存在时,促使成膜过程中GO发生部分还原,最终形成一种均匀分散的PVDF复合膜。红外谱图显示,复合膜具有GO和PVDF的双重特征;扫描电子显微镜扫描结果表明,PVDF在GO内部呈现树叶状分布,提高了膜的稳定性;X射线衍射采集数据表明了GO存在部分还原现象;湿气响应实验表明,得到的复合材料的端电压和端电阻随湿度变化呈现规律性变化,具有很好的响应性能,为智能材料的研发奠定了基础。

Engineering a High-Selectivity PVDF Hollow-Fiber Membrane for Cesium Removal

In this study,a copper ferrocyanide/silica/polyvinylidene fluoride(CuFC/SiO2/PVDF)hollow-fiber composite membrane was successfully synthesized through a facile and effective crosslinking strategy.The PVDF hollow-fiber membrane with embedded SiO2 was used to fix the dispersion of CuFC nanoparticles for cesium(Cs)removal.The surface morphology and chemical composition of the composite membrane were analyzed using scanning electron microscopy and X-ray photoelectron spectroscopy(XPS).The composite membrane showed a high Cs rejection rate and membrane flux at the three layers of CuFC and 0.5%SiO2,and its Cs rejection rate was not affected by variation in the pH(pH=4–10).The modified membrane could be effectively regenerated many times using ammonium nitrate(NH4NO3).The Cs selectivity performance was verified by an efficient Cs rejection rate(76.25%and 88.67%in 8 h)in a solution of 100 lg L 1 of Cs with 1 mmol L 1 of competing cations(K+and Na+).The CuFC/SiO2/PVDF hollowfiber composite membrane showed a particularly superior removal performance(greater than 90%)in natural surface water and simulated water with a low Cs concentration.Therefore,the CuFC/SiO2/PVDF hollow-fiber composite membrane can be used directly in engineering applications for the remediation of radioactive Cs-contaminated water.