Crystalline Polyvinylidene Fluoride Research Articles

Electrostatically Active Polymer Hybrid Aerogels for Airborne Nanoparticle Filtration

The role of electrostatic force on separation of airborne nanoparticles is evaluated in this work by considering a hybrid monolithic aerogel of syndiotactic polystyrene (sPS) and polyvinylidene fluoride (PVDF). The sPS part accounts for open pore structures in the monolith, while the PVDF chains contribute spontaneous polarity for particle capture by the electrostatic force. The hybrid aerogels are fabricated by thermoreversible gelation of sPS from a solution with PVDF in tetrahydrofuran followed by supercritical drying of the gel. sPS is present as the δ-form clathrate crystalline phase and PVDF as α- and γ-form crystalline phases in the hybrid. The presence of PVDF induces significant static charges on the surfaces of hybrid aerogels. The filtration efficiency is determined by passing airborne NaCl nanoparticles with diameter in the range 25-150 nm through the filter media. The experimental data reveal that air permeability of the hybrid system (∼10-10 m2) is close to that of sPS monoliths. The hybrid materials show filtration efficiency ≥99.999% in comparison to 98.889% observed for a sPS monolith with the same solid content.

Defects in crystalline PVDF: a density functional theory-density functional tight binding study.

We present a comparative density functional theory (DFT) and density functional tight binding (DFTB) study of structures, energetics, vibrational properties as well as electronic structures of the four crystalline phases of polyvinylidene fluoride (PVDF) with different types of defects. For pure phases, the relative energies of PVDF strands (i.e. absent van der Walls bonding) agree well between DFT (using a GGA or a hybrid functional) and DFTB. For crystals, DFTB needs to be calibrated due to deficiencies in the treatment of vdW interactions. Defect formation energies were computed in large-scale DFTB simulations. For single chain vacancies, they are 0.41, 0.59, 0.08 and 0.40 eV per monomer removed in α, β, γ, and δ PVDF, respectively. The energy required to form double vacancies is 0.38, 0.52, 0.33 and 0.39 eV per monomer removed, respectively, i.e. the effect is nearly additive except in the γ phase. Interstitial defects were found to be unstable and convert into vacancies. The relatively high defect formation energies (vs. kT at room temperature) imply that phase purity is feasible in PVDF. Vibrational contributions affect the relative phase energies by up to 0.1 eV but do not significantly affect the relative phase stability.

SPEEK/PVDF binary membrane as an alternative proton-exchange membrane in vanadium redox flow battery application

Sulfonated polyether ether ketone (SPEEK)/polyvinylidene fluoride (PVDF) blends are employed to prepare the ion-exchange membranes for vanadium redox flow battery (VRB) application. The addition of the highly crystalline and hydrophobic PVDF effectively confines the swelling behavior of SPEEK. The vanadium ion permeability of SPEEK/PVDF membranes is one order of magnitude lower than that of Nafion117 membrane and SPEEK membrane. In VRB single-cell test, the SPEEK/PVDF binary membranes exhibit higher energy efficiency (EE) than recast SPEEK and Nafion117 at 30 mA cm−2. The blend membrane with 30 wt% of PVDF (S0.7P0.3) shows EE of 83.2% at 30 mA cm−2, which is superior to that of the Nafion117 and recast SPEEK membranes. In the self-discharge test, S0.7P0.3 membrane also possesses twice longer duration in open circuit decay test. These results indicate that the addition of PVDF is a simple and efficient way to improve the ion selectivity of SPEEK. With all the good properties and low cost, the SPEEK/PVDF membranes are expected to have excellent commercial prospects as ion-exchange membranes for VRB system.

Development of PVDF membranes for membrane distillation via vapour induced crystallisation

This study explores vapour induced crystallisation technique for developing polyvinylidene fluoride (PVDF) membranes for membrane distillation (MD). It works based on the observation that a sufficiently long exposure of PVDF film under humid air allows the occurrence of crystallisation of semi-crystalline PVDF. Three PVDF polymer grades with different molecular weights (MWs) were used to investigate crystallisation behaviour during vapour induced phase separation (VIPS). Later, the performances of four membranes, prepared at exposure times of 30, 60, 90 and 120min and using PVDF-370kDa, were evaluated in a lab-scale direct contact MD (DCMD) set-up. Vapour induced crystallisation was proven to be an effective way to adjust PVDF crystallinity, being higher at longer exposure time. There exists a transition time in humidity exposure (between 60 and 120min) at which a shift of membrane morphology occurs from highly amorphous, flat and smooth to rough and grainy morphology. This finding is persistent for the three PVDF MWs. This shift in morphology has clear consequences on the membrane properties and its MD performance. The increase in degree of crystallinity led to higher contact angle, larger pore size and reduced membrane thickness. The combination of these properties led to an overall improvement in membrane performance manifested in an increase in its flux and salt rejection.

Influence of pyrazole on the photovoltaic performance of dye-sensitized solar cell with polyvinylidene fluoride polymer electrolytes

We investigate the influence of the pyrazole content on the polyvinylidene fluoride (PVDF)/KI/I2 electrolytes for dye-sensitized solar cells (DSSCs). The solid polymer electrolyte films consisting of different weight percentage ratios (0 20, 30, 40, and 50 %) of pyrazole doped with PVDF/KI/I2 have been prepared by solution casting technique using N,N-dimethyl formamide (DMF) as a solvent. The as-prepared polymer electrolyte films were characterized by various techniques such as Fourier transform infrared spectroscopy (FT-IR spectroscopy), differential scanning calorimetry (DSC), X-ray diffractometer (XRD), alternate current (AC)-impedance analysis, and scanning electron microscopy (SEM). The 40 wt% pyrazole-PVDF/KI/I2 electrolyte exhibited the highest ionic conductivity value of 9.52 × 10−5 Scm−1 at room temperature. This may be due to the lower crystallinity of PVDF and higher ionic mobility of iodide ions in the electrolyte. The DSSC fabricated using this highest ion conducting electrolyte showed an enhanced power conversion efficiency of 3.30 % under an illumination of 60 mW/cm2 than that of pure PVDF/KI/I2 electrolyte (1.42 %).

Swelling of semi-crystalline PVDF by a PMMA-based nanostructured diblock copolymer: Morphology and mechanical properties

The influence of a self-organizing diblock copolymer on the morphology, crystallinity and mechanical properties of a high molecular weight polyvinylidene fluoride (PVDF) is investigated. A spherically organized nanostructured diblock copolymer containing 25 wt% of soft acrylate block and 75 wt% of rigid polymethylmethacrylate (PMMA) is incorporated in PVDF in proportions varying between 12.5 and 70 wt% copolymer. DSC and DMA experiments prove the miscibility of the glassy PMMA block with the amorphous fraction of PVDF, whereas the soft block is fully segregated, preserving the nanostructuration of the diblock copolymer in the blend. Transmission electron microscopy and SAXS experiments show that the copolymer is confined into the inter-lamellar gallery of PVDF, giving rise to a swelling of the crystalline/amorphous lamellar morphology. The long period Lp varies from 120 Å in pure PVDF to 200 Å in the presence of 25 wt% copolymer. WAXS experiments and ATR spectroscopy show that the polar β polymorph is favored in the blends and that the PVDF crystallinity is preserved. Tensile tests demonstrate a large improvement of the elongation at break, without significant loss of strength for samples containing lower amounts of copolymer.

High-performance flexible lead-free nanocomposite piezoelectric nanogenerator for biomechanical energy harvesting and storage

Environmentally friendly and flexible piezoelectric nanogenerators (PENGs) have attracted considerable interest for use in autonomous micro-/nano-systems, wearable electronics, and implantable biomedical devices. Herein, a flexible, lead-free, solution-processable and efficient PENG is demonstrated, based on a highly piezoelectric nanocomposite thin film of barium titanate nanoparticles (BT NPs) embedded into a highly crystalline polyvinylidene fluoride–trifluoroethylene (P(VDF–TrFE)) polymer for charge storage. The nanocomposite PENG (nc-PENG) with high loading of BT NPs up to 40wt% in the nanocomposite can produce output voltage as high as 9.8V and output power density of 13.5µW/cm2 under cyclic bending, comparable to the output of conventional inorganic polycrystalline lead zirconate titanate PENGs. The high performance of these nc-PENGs is primarily attributed to the very high effective piezoelectricity of the highly crystalline P(VDF-TrFE), strengthened by the BT NPs. The nc-PENG was also demonstrated to harvest biomechanical energy from simple body movements and store the generated electricity during mechanical and biomechanical motions in a rechargeable microbattery.

Polyvinylidene Fluoride/Acrylonitrile Butadiene Rubber Blends Prepared Via Dynamic Vulcanization

Dynamically vulcanized blends based on polyvinylidene fluoride (PVDF)/acrylonitrile butadiene rubber (NBR) were prepared and characterized. The mixing torque and dynamic rheology analyses showed that the NBR phase increased the viscosity of the blends. Scanning electron microscopy (SEM) results showed that the NBR phase was in the form of spherical particles dispersed in the PVDF phase during dynamic vulcanization. Comparing PVDF-rich and NBR-rich blends, the size of the rubber particles in the NBR-rich blends were larger than those in PVDF-rich blends. Differential scanning calorimetry (DSC) results showed that the addition of the NBR phase reduced the PVDF crystallinity and Tm. Thermal gravimetric analysis (TGA) results showed that the dynamically vulcanized PVDF/NBR blends had a higher residual char mass than the neat PVDF and NBR. For PVDF-rich blends, the PVDF can be highly toughened by NBR; the Izod impact strength of the PVDF/NBR (70/30) blend was 77.5 kJ/m2, which was about six times higher than that of pure PVDF. For rubber-rich blends, the PVDF component was beneficial to the mechanical properties of the blends, which can be used as thermoplastic elastomers.

Application of a bio-derivative, rosin, as a binder additive for lithium titanium oxide electrodes in lithium-ion batteries

Rosin, which is an extract of pine trees, is used as an eco-friendly binder additive to a conventional solvent-based binder, polyvinylidene fluoride (PVdF). The composite binder system is used to evaluate the performance of the Li4Ti5O12 (LTO) electrodes using a range of physical and electrochemical characterization techniques. The addition of rosin decreases the crystallinity of PVdF, disperses the conducting agent and binder well, and ultimately leads to an increase in the diffusivity and cyclic capacity of lithium ions. When the amount of rosin additive to the PVdF binder is 40 wt%, the LTO electrode exhibits the highest cyclic capacity and diffusivity; of 164 mAh g−1 at the 110th cycle and 4.28 × 10−10 cm2 s−1, respectively.

Thermally stimulated discharge current (TSDC) characteristics in β-phase PVDF–BaTiO3 nanocomposites

β-phase polyvinylidene fluoride (PVDF)–BaTiO3 nanocomposite samples have been prepared by solution mixing method. XRD data represent that the crystallinity of PVDF decreases with increase in loading level of BaTiO3 nanoparticles. DSC curve represents that the melting point of PVDF is lightly affected by loading concentration of BaTiO3. The morphology and microstructure of PVDF and PVDF embedded by BaTiO3 nanofillers were investigated by using inverted contrast microscopy (ICM) and scanning electron microscopy (SEM). FTIR interferrometry is proven that PVDF and BaTiO3 are not chemically interacting; therefore, interaction of BaTiO3 is van der Waals type of interaction. The thermally stimulated discharge current (TSDC) of PVDF and PVDF–BaTiO3 nanocomposites sample was characterized by single peak. The observed TSDC peak is discussed on the basis of dipolar and interfacial polarization.

Beta-phase enhancement in polyvinylidene fluoride through filler addition: comparing cellulose with carbon nanotubes and clay

The beta-phase in polyvinylidene fluoride (PVDF) is responsible for its ferro-, piezo- and pyroelectric properties, key for memory, sensing and actuation applications. Filler addition as a route to enhance the smart beta-phase in PVDF is presented in this study. PVDF composites with varying concentrations of microcrystalline cellulose, carbon nanotubes and kaolinite clay were prepared by solution-sonication method and subsequently cast into thin standing composite films. Crystallinity of PVDF and its composites were found to be similar. Phase evolution studies showed that addition of microcrystalline cellulose yields a beta-phase fraction comparable to carbon nanotubes and significantly higher than that of clay. The mechanism of interaction between microcrystalline cellulose and PVDF is also proposed.

A nano-frost array technique to prepare nanoporous PVDF membranes.

Frost, the solid deposition of water vapor from humid air, forms on the surface of a solid substrate when its temperature drops below the freezing point of water. In this study, we demonstrate how this natural phenomenon can be applied to develop novel nanoporous materials. The solvent annealing of polyvinylidene fluoride (PVDF) infiltrated into nanopores induced template-directed dewetting thus preparing nanoembossing films. Then, water nanodroplets formed on the cold polymer nanopatterned surfaces following the embossing patterns, similar to dew formation on the ground. Subsequently, the nanodroplets were frozen and then removed by freeze-drying. This nano-frost array technique produced nanoporous PVDF membranes with an average thickness of 250 (± 48) nm. It was revealed that the nanopatterned surface formed by solvent annealing played an important role in achieving a nano-frost array with an adjustable size. Additionally, the freezing process led to significant changes of the PVDF crystallinity and polymorphism. Our results prove that the nano-frost array technique can be broadly used to design ordered nanoporous structures and provide new prospects in nanomaterial fields.

Mechanical, electrical, and dielectric properties of polyvinylidene fluoride/short carbon fiber composites with low‐electrical percolation threshold

AbstractIn this investigation, polyvinylidene fluoride (PVDF)/short carbon fiber (SCF) composites have been prepared by solution casting technique to enhance electrical and dielectric properties with very low‐electrical percolation threshold (0.5 phr SCF). The effect of SCF content on mechanical, thermal and morphological properties of the composites have also been investigated. The mechanical properties of the composites are found to reduce compared to neat PVDF due to poor polymer–filler interaction which can be concluded from FESEM micrographs showing poor bonding between PVDF and SCF. The PVDF/SCF composites exhibit either positive temperature coefficient effect of resistivity or negative temperature coefficient effect of resistivity depending on the loading of SCF in the polymer matrix. The change in conductivity during heating–cooling cycle for these composites shows electrical hysteresis along with electrical set. The melting point of the composites marginally increases with the increase in fiber loading in PVDF matrix as evidenced from DSC thermograms. X‐ray diffraction analysis reveals the crystallinity of PVDF decreases with the increase in SCF loading in matrix polymer. © 2013 Wiley Periodicals, Inc. J. Appl. Polym. Sci. 2014, 131, 39866.

Conductive Materials for Proton Exchange Membrane Fuel Cell Bipolar Plates Made from PVDF, PET and Co‐continuous PVDF/PET Filled with Carbon Additives

AbstractThe aim of this work was to develop and characterise electrically conductive materials for proton exchange membrane fuel cells and bipolar plates (BPPs). These BPPs were made from highly conductive blends of polyethylene terephthalate (PET) and polyvinylidene fluoride (PVDF), as matrix phase. The conductive materials were developed from carefully formulated blends composed of conductive carbon black (CB) powder and, in some cases, graphite synthetic flakes mixed with pure PET, PVDF or with PVDF/PET systems. They were first developed by twin‐screw extrusion process then compression‐molded to give BPP final shape. As the developed blends have to meet properties suitable for BPP applications, they were characterised for their rheological properties, electrical through‐plane resistivity (the inverse of conductivity), oxygen permeability, flexural and impact properties. Results showed that lower resistivity was obtained with PVDF/CB blends due to the higher interfacial energy between the PVDF matrix and CB and also the higher density and crystallinity of PVDF, compared to those of PET. It was also observed that the lowest resistivity values were obtained with mixing PVDF and PET at controlled compositions to ensure PVDF/PET co‐continuous morphology. Also, slow cooling rates helped to attain the lowest values of through‐plane resistivity for all studied blends. This behaviour was related to the higher crystallinity obtained with low cooling rates leading to smaller amorphous regions in which carbon particles are much more concentrated.

Atmospheric plasma fluorination as a means to improve the mechanical properties of short-carbon fibre reinforced poly (vinylidene fluoride)

The impact of fluorination of carbon fibres on the properties of short fibre reinforced polyvinylidene fluoride (PVDF) composites was studied. As received and continuously atmospheric plasma fluorinated (APF) carbon fibres were cut to an average fibre length of 2 mm. Short fibre composites (SFC) containing 5, 10 and 15 wt.% carbon fibres were manufactured using a twin-screw mixer. Test specimens were produced by injection moulding. The mechanical properties of the SFC were studied using tensile and compression testing. As expected, the incorporation of short-carbon fibres into PVDF led to an increase in strength and stiffness. The tensile strength and Young’s modulus of the SFC containing APF-treated carbon fibres increased by up to 17% and 190%, respectively. Furthermore, the compressive strength and modulus of the SFC containing APF-treated carbon fibres also increased by 19% and 35%, respectively. APF of carbon fibres results only in a marginal increase in the bulk matrix crystallinity of PVDF as determined by DSC. Scanning electron micrographs of fracture surfaces from tensile tested specimens exhibited a typical brittle failure mode with low fibre loading fraction. Despite the presence of up to 5% of voids and visible resin rich regions at fracture surface, SFC containing APF-treated fibres suggest better bonding at the fibre/matrix interface which led to the much enhanced mechanical properties.

The Study on Poly (Vinylidene Fluoride)/Silica Hybrid Composites Made by Sol—gel Process

In this study, the PVDF (Poly (vinylidene fluoride))/silica hybrid composites are prepared by mixing PVDF and silica, which was synthesized by sol—gel process. We changed R (the molar ratio of water/TEOS) during the sol—gel process, and made a film. The crystallinity of PVDF in the hybrid is measured by DSC (Differential scanning calorimetry), and the morphologies are observed with SEM pictures. We found that the silica particles could be well dispersed in PVDF, and R-value influenced the particle size and the crystallinity of PVDF. The concentration of TEOS also influenced the particle size and shape of silica. Higher concentration of TEOS made the silica particles have round shape and also lowered the crystallinity of PVDF.

Haloform adsorption on crystalline copolymer films of vinylidene fluoride with trifluoroethylene

Reversible bromoform adsorption on crystalline polyvinylidene fluoride with 30% of trifluoroethylene, P(VDF–TrFE 70:30) was examined by photoemission and inverse photoemission spectroscopies. The adsorption of bromoform on this polymer surface is associative and reversible. Molecular bromoform adsorption appears to be an activated process at 120 K with enhanced adsorption following the initial adsorption of bromoform. Strong intermolecular interactions are also implicated in the presence of a weak shake off or screened photoemission final state, whose intensity scales with the unscreened photoemission final state.

On the possibility of synthesizing one-dimensional carbon during radiative carbonization of PVDF

The modification of x-ray photoelectron spectra (XPS) and C KVV spectra of a partially crystalline polyvinylidene fluoride (PVDF) film under the long action of soft x-rays and secondary electrons followed by argon ion bombardment of its surface is investigated. Deep radiative carbonization leads to the formation of carbynoid structures (chain carbon) on the PVDF surface. Hence, the carbon XPS of the carbonized sample differs from those obtained for graphite and PVDF. Ion bombardment shows the instability of the carbonized sample surface, giving rise to formation of sp 2 hybrid bonds of carbon atoms. The obtained results are indirect experimental evidence that, before ion bombardment, sp-type bonds are dominant on the carbonized PVDF surface.

Molecular Adsorption and Fragmentation of Bromoform on Polyvinylidene Fluoride with Trifluoroethylene

AbstractBromoform absorption on crystalline polyvinylidene fluoride with 30% of trifluoroethylene, P(VDF-TrFE 70:30) was investigated by photoemission and inverse photoemission and found to be associative and reversible. Molecular bromoform adsorption appears to be an activated process at 120 K with enhanced adsorption following the initial adsorption of bromoform. Strong intermolecular interactions are also implicated in the presence of a weak shake off or screened photoemission final state.

Crystalinity Properties of Carbon Nanotube-Polyvinylidene Fluoride Composites

AbstractSingle-wall and multi-wall carbon nanotube blends (0 to 0.5 vol% ) with polyvinylidene fluoride (PVDF) have been prepared using solution cast method and characterized. By acid treatment, it has been observed that nanotube has been well functionalized and uniformly dispersed into the polymer. X-ray diffraction analysis coupled with differential scanning calorimetry (DSC) has revealed that carbon nanotube alters the crystallinity of PVDF and thereby enhances the β-phase in PVDF. Experimental results have demonstrated that enhancement of β-phase is a function of carbon nanotube concentration.