Ultrathin ceramic nanowires for high interface interaction and energy density in PVDF nanocomposites

Dielectric nanocomposites with ceramic fillers have a crucial role in energy storage applications. Therefore, poly(vinylidene fluoride) (PVDF) based nanocomposites filled with 20 nm diameter, surface hydroxylated BaTiO₃ nanowires (BTnws) were produced by solution-casting method in this work, in which BTnws were synthesized via solvothermal method. The dielectric constant of BTnws/PVDF nanocomposites was 24 when the content of fillers was 10vol% at 100 Hz and the breakdown strength could increase up to 417 kV/mm before decreasing. The nanocomposites showed enhanced energy density performance and the maximum energy density could reach to 8.1J/cm³ at 320 kV/mm with 10vol% BTnws, nearly tripled that of pure PVDF at 300 kV/mm. Finite element and molecular dynamic simulation results revealed that thin BTnws could create dielectric homogeneity in the nanocomposites and have strong interface interaction with PVDF molecular. The ultrathin BTnws provided the possibility that single PVDF molecular could wrap on its surface, but this molecular wrapping pattern would not occur when the diameter of BTnws was large. Besides, the wrapping pattern could be reinforced by interactions between surface hydroxyl groups of BTnws and F atoms of PVDF molecular. Such contributions could induce good interface compatibility and lead to the improvement of energy density.     

Improving dielectric properties of poly(vinylidene fluoride) composites: effects of surface functionalization of exfoliated graphene

To investigate the effects of surface functionalization of exfoliated graphene (EG) on the crystalline form of β-phase and dielectric properties of poly(vinylidene fluoride) (PVDF), we prepared PVDF-based composites reinforced by different functionalized EG. The X-ray photoelectron spectroscopy results indicated that a wide variety of chemical functional groups such as C–OH, C–O–C, C=O, COOH and C–F could be introduced on the surface of modified EG. As confirmed by results of Fourier transform infrared spectrum and X-ray diffraction, the β-phase PVDF can be produced in the composites with the incorporation of functionalized EG. In the frequency ranging from 10² to 10⁷ Hz, the dielectric permittivity of PVDF composites shows an obvious increase owing to a variation of the carbonyl group (C=O) content. Among all the composites, the EG grafted with polymethyl methacrylate/PVDF composite has the highest dielectric permittivity and dielectric loss.     

Potential of Polyvinylidene Fluoride/Carbon Nanotube Composite in Energy,Electronics, and Membrane Technology: An Overview

Polyvinylidene fluoride (PVDF) is a semicrystalline thermoplastic and electroactive polymer with piezoelectric and pyroelectric properties, thermal stability, elasticity, and chemical resistance. PVDF exits in five different phases (α, β, δ, γ, and ε-phase). Unique properties of this polymer enhances its use in chemical, biomedical, and electronic industries such as supercapacitors, transducers, actuators, and batteries. Carbon nanotube (CNT) is used as reinforcement to exploit full potential of PVDF in energy, electronics, and membrane technology. The nanofiller affects morphology, piezoelectric, pyroelectric, electrical, dielectric, thermal, and mechanical properties of PVDF-based nanocomposite. CNT content and chemical modification influence properties as well as application of PVDF.     

A comparative assessment of poly(vinylidene fluoride)/conducting polymer electrospun nanofiber membranes for biomedical applications

Piezoelectric polymers, especially poly(vinylidene fluoride) (PVDF) are increasingly receiving interest as smart biomaterials for tissue engineering, energy harvesting, microfluidic, actuator, and biosensor applications. Despite possessing the greatest piezoelectric coefficients among all piezoelectric polymers, it is often desirable to increase the electrical outputs from PVDF for several of these applications. Blending with intrinsically conducting polymers (CP) in the form of nanofiber membranes is one of the facile methods to achieve the same. However, these polymers and their composites have so far been primarily investigated only for their physical property enhancements and in applications like energy storage while their biomedical applications and comparative assessment of their biocompatibility properties have not been yet explored. In this report, electrospinning of PVDF blends with polypyrrole (PPy), polyaniline (PANI), and a modified PANI with l -glutamic acid (PANI-LGA/P-LGA) is performed to obtain different electrically active material membranes. The PVDF:CP composite nanofibers are compared with respect to their nanostructures, β-phase content, and electrical conductivity. Further, biocompatibility of all the membranes was compared. It was found that incorporation of PPy, PANI, and P-LGA increased the electrical conductivity of PVDF while the β-phase content was also substantially enhanced. The highest biocompatibility with a pre-osteoblast cell line (MC3T3) was exhibited in the order p-LGA/PVDF > PANI/PVDF > PPy/PVDF, all being significantly higher than PVDF (p < .001). Although P-LGA/PVDF showed higher electrical conductivity, biocompatibility with MC3T3, it was found to be highly cytotoxic to a HeLa (cancer) cell line. It is concluded that such structure property relations would help in selection of materials for specific biomaterial applications.     

增容剂对PVDF/PUR-T复合材料的性能影响

采用流延成膜法制备了硅烷偶联剂KH–560和端氨基液体丁腈橡胶(ATBN)协同改性的聚偏氟乙烯(PVDF)/聚醚型热塑性聚氨酯弹性体(PUR–T)复合材料,初步探讨了单一增容剂(KH–560或ATBN)含量、复配增容剂含量与比例及成膜温度等条件对PVDF/PUR–T复合材料力学性能的影响,深入研究了复合材料的结晶性能、热稳定性能和亲/疏水性能。结果表明,与单一增容剂相比,复配增容剂的引入明显提高了复合材料的力学性能,并且当复配增容剂添加量为10%,复配比KH–560/ATBN=3/1,成膜温度为50℃时,其与PVDF/PUR–T基体间的界面粘结性明显得到改善,改性后复合材料断裂伸长率达到273%。PUR–T与复配增容剂的加入抑制了PVDF的结晶,复合材料的结晶度降低,但仍具有良好的热稳定性能,且疏水性得到提高。     

Suppression of AC conductivity by crystalline transformation in poly(vinylidene fluoride)/carbon nanofiber composites

Poly(vinylidene fluoride) (PVDF) is an important ferroelectric semi-crystalline polymer with multiple-phase behavior. In this study, remarkable effects of the various crystalline structures of PVDF nanocomposites on alternating current (AC) conductivity were discovered using carbon nanofibers (CNF). It was found that the transformation from α-phase to β-phase in PVDF, induced by the addition of CNFs, had a surprisingly suppressive effect on the AC conductivity of the nanocomposites. These unexpected results indicate that the decline in conductivity occurs after re-crystallization treatment (annealing) of the nanocomposites, and the reduction levels increase with increasing amounts of CNFs. Interestingly, the AC conductivity of annealed 5 wt% CNF/PVDF composites becomes even lower than that of re-crystallized nanocomposites with 3 wt% CNFs. These findings are believed to be very significant for fabrication and long-term service of PVDF composites in industry, which often involves exposure to repeated thermal cycling.     

Crystallization behavior of nano-composite based on poly(vinylidene fluoride) and organically modified layered titanate

To understand the effect of the nano-filler particles on the crystallization kinetics and crystalline structure of poly(vinylidene fluoride) (PVDF) upon nano-composite formation, we have prepared PVDF/organically modified layered titanate nano-composite via melt intercalation technique. The layer titanate (HTO) is a new nano-filler having highly surface charge density compared with conventional layered silicates. The detailed crystallization behavior and its kinetics including the conformational changes of the PVDF chain segment during crystallization of neat PVDF and HTO-based nano-composite (PVDF/HTO) have been investigated by using differential scanning calorimetric, wide-angle X-ray diffraction, light scattering, and infrared spectroscopic analyses. The neat PVDF predominantly formed α-phase in the crystallization temperature range of 110-150 °C. On the other hand, PVDF/HTO exhibited mainly α-phase crystal coexisting with γ- and β-phases at low T_c range (110-135 °C). A major γ-phase crystal coexists with β- and α-phases appeared at high T_c (=140-150 °C), owing to the dispersed layer titanate particles as a nucleating agent. The overall crystallization rate and crystalline structure of pure PVDF were strongly influenced in the presence of layered titanate particles.     

Towards β-phase formation probability in spin coated PVDF thin films

This work aimed towards the study on variations in the percentage of β-phase in Poly vinylidene fluoride (PVDF) thin films deposited by spin coating technique. PVDF is a semi-crystalline polymer which exhibits α, β, γ and δ polymorphs. Among these polymorphs, generally it crystallizes in a non-polar α-phase, which is of little importance as far as its applications are concerned. However, the β-phase, which exhibits spontaneous polarity created tremendous interest and showed a path towards the devices based on its pyro- and piezoelectric properties. Fourier Transform Infrared (FTIR) spectroscopy and XRD techniques were used to study the percentage of formation of β-phase in spin coated PVDF thin films at different processing conditions viz. spin rotation speed (rpm), solution concentration and annealing temperature. We identified the β-phase percentage in PVDF thin films, which are (i) Deposited with different rotation speeds ranging from 1000 to 9000 rpm, (ii) Annealed at different temperatures viz.; room temperature to 100C, and (iii) Deposited at various solution concentrations. It is identified that percentage of formation of β-phase is high in the films deposited with 15(w/v)% solution concentration which is annealed at 100C. The films deposited at higher rpm is showing low enhancement in the β-phase with annealing temperature.     

Morphology, polymorphism behavior and molecular orientation of electrospun poly(vinylidene fluoride) fibers

The morphology, polymorphism behavior and molecular orientation of electrospun poly(vinylidene fluoride) (PVDF) fibers have been investigated. We found that electrospinning of PVDF from its N,N-dimethylformamide/acetone solutions led to the formation of β-phase. In contrast, only α- and γ-phase was detected in the spin-coated samples from the same solutions. In the aligned electrospun PVDF fibers obtained using a rotating disk collector, the β-phase crystallites had a preferred orientation along the fiber axis. The degree of orientation did not, however, vary significantly with the speed of the rotation disk collector, and the β-phase was also not significantly enhanced with the increase in the rotation speed or the decrease in the size of spinnerets. These facts indicated that the orientation was likely to be caused by Columbic force rather than the mechanical and shear forces exerted by the rotating disk collector and spinnerets. The Columbic force may induce local conformational change to straighter TTTT conformation, and hence promote the β-phase. The addition of 3 wt.% of tetrabutylammonium chloride (TBAC) into the polymer solutions effectively improved the morphology of the electrospun fibers, and led to almost pure β-phase in the fibers. With spin coating, PVDF-TBAC did not, however, show any strong β-phase diffraction peak. The synergistic β-enhancement effect of TBAC and electrospinning is possibly due to the fact that while TBAC could induce more trans conformers, electrospinning promotes parallel packing, and hence inter-chain registration.     

The effect of fibre concentration on the α to β-phase transformation, degree of crystallinity and electrical properties of vapour grown carbon nanofibre/poly(vinylidene fluoride) composites

Vapour growth carbon nanofibres/poly(vinilidene fluoride) - VGCNF/PVDF - composites prepared by solution casting were studied. The spherulitic crystallisation morphology of the pure polymer is maintained for the composites. Mechanical stretching of the composite films induces the α to β-phase transformation within the polymer matrix. This phase transition is accompanied by the destruction of the spherulitic microstructure in favour of a microfibrillar one. The incorporation of the VGCNF in the PVDF matrix increases the degree of crystallinity of the polymer composites for concentrations lower than ∼1%, remaining stable for higher VGCNF concentrations. With respect to the electrical properties, the stretching associated to the phase transformation induces a change in the conduction mechanism: the α-phase composite demonstrates a percolative behaviour on the measured conductivity whereas the β-phase demonstrates typical ionic conduction behaviour. Dielectric measurements in conjunction with the the two exponent percolation phenomenological equation demonstrates that in the β-phase an effective reduction in the ratio VGCNF length/domain length could induce the observed percolation behaviour.     

Achieving β-phase poly(vinylidene fluoride) from melt cooling: Effect of surface functionalized carbon nanotubes

Poly(vinylidene fluoride) (PVDF) based nanocomposites with different surface-functionalized multi-walled carbon nanotubes (MWCNTs) were prepared by melt mixing in a small scale compounder. With the incorporation of commercial functionalized MWCNTs, the β-phase in PVDF can be directly achieved from melt cooling, as verified by results of Fourier transform infrared spectrum and X-ray diffraction. Interestingly, nanocomposites with amino group functionalized MWCNTs showed the highest percentage of β-phase (17.4%) formation in PVDF, followed by those with hydroxyl groups (11.6%) and unmodified MWCNTs (9.4%). However, the nanocomposites containing MWCNTs with carboxyl groups which were thought to be able to well interact with the dipoles on PVDF chains have the lowest amount of β-phase, i.e. 4.7%. The analysis on the mechanism of the influence of surface functionalization of MWCNTs on the formation of β-phase in PVDF shows that the combined effects of the dispersion of MWCNTs and the nanotube–polymer interactions account for the formation of the β-phase in PVDF.     

Effect of mechanical stretching on electrical conductivity and positive temperature coefficient characteristics of poly(vinylidene fluoride)/carbon nanofiber composites prepared by non-solvent precipitation

Poly(vinylidene fluoride) (PVDF)/carbon nanofiber (CNF) composites with filler content ranging from 0.047 to 4.7 vol.% were prepared with non-solvent precipitation followed by melt compression. The morphology and electrical conductivity of the composites before and after mechanical stretching were examined. The results showed that CNFs were dispersed homogeneously in the PVDF matrix and a low electrical percolation threshold of 0.90 vol.% CNFs was obtained. Mechanical stretching led to a sharp decrease in the electrical conductivity of a composite containing 0.94 vol.% CNF. This was caused by the destruction of a conducting network structure when the fillers aligned along the stretching direction. This did not happen when the filler content was increased to 1.88 vol.%. The percolating composites displayed a positive temperature coefficient (PTC) effect with the effect being larger in stretched composites. This can be attributed to the presence of PVDF β-phase in stretched composites as revealed by X-ray diffraction and Fourier transform infrared spectroscopy.     

Tuning of shear thickening behavior and elastic strength of polyvinylidene fluoride via doping of ZnO-graphene

ZnO rice like nonarchitects are grafted on the graphene carbon core via a rapid microwave synthesis route. The prepared grafted systems are characterized via XRD, SEM, RAMAN, and XPS to examined the structural and morphological parameters. Zinc oxide grafted graphene sheets (ZnO-G) are further doped in β-phase of polyvinylidene fluoride (PVDF) to prepare the polymer nanocomposites (PNCs) via mixed solvent approach (THF/DMF). β-phase confirmation of PVDF PNCs is done by FTIR studies. It is observed that ZnO-G filler enhances the β-phase content in the PNCs. Non-doped PVDF and PNCs are further studied for rheological behavior under the shear rate of 1–100 s⁻¹. Doping of ZnO-G dopant to the PVDF matrix changes its discontinuous shear thickening (DST) behavior to continues shear thickening behavior (CST). Hydrocluster formation and their interaction with the dopant could be the reason for this striking DST to CST behavioral change. Strain amplitude sweep (10⁻³% -10%) oscillatory test reveals that the PNCs shows extended linear viscoelastic region with high elastic modulus and lower viscous modulus. Effective shear thickening behavior and strong elastic strength of these PNCs present their candidature for various fields including mechanical and soft body armor applications.     

Exceptional dielectric properties of chlorine-doped graphene oxide/poly (vinylidene fluoride) nanocomposites

Herein, we report a facile method to significantly enhance the dielectric performance of reduced graphene oxide-based polymer composites. Addition of thionyl chloride into graphene oxide (GO) dispersion induces synergistic modifications of the structure, chemistry, charge carrier density and electrical conductivity of GO, as well as the interfacial interaction and phase of the surrounding matrix in the poly (vinylidene fluoride) (PVDF) composite. The composites reinforced with a very low reduced chlorinated GO (Cl-rGO) content of 0.2 vol% deliver an exceptional dielectric constant of 364 with a moderate dielectric loss of 0.077 at 1 kHz. These values are well contrasted with the corresponding properties of the neat PVDF polymer with a constant of 28 and a loss of 0.0029. Synergistic effects arising from chlorination are identified, including the much enhanced electrical conductivity of Cl-GO sheets by more than 3 orders of magnitude through introducing charge-transfer complexes, the improved interfacial interactions between the fillers and the PVDF matrix through hydrogen bonds, and the transformation of PVDF to β-phase with an inherently high dielectric constant due to dipolar interaction. The comparison with the literature data confirms superior dielectric performance of the present Cl-rGO/PVDF composites.     

Solution blow spinning of piezoelectric nanofiber mat for detecting mechanical and acoustic signals

Solution blow spinning (SBS) technique can produce nanofibers (NFs) mat in large-scale production. In this work, the SBS was used to fabricate piezoelectric polyvinylidene fluoride (PVDF) NF membranes that can be utilized for energy harvesting applications. The effect of operating air pressure from (2–5 bar) on the surface morphology of the NFs has been studied. The structural analysis for crystalline polymorph β-phase for PVDF powder, casted film, electrospinning and SBS NFs has also been presented with the aid of Fourier-transform infrared spectroscopy and X-ray diffraction (XRD). Piezoelectric characteristics of PVDF NFs mats were tested by applying impact impulse with different weights from different heights between 1 and 10 cm. The sensitivity of the voltage response increased from 1.71 mV/g to 8.98 mV/g, respectively. Besides, the SBS generated PVDF mat is found to be sensitive to pressure forces in a range of few Newtons with the generated voltage according to detected sensitivity of 80 mV/N based on the analysis of the impact of a few Hertz mechanical vibrations. In addition, the produced SBS NFs were applied as an acoustic signal detector within different acoustic frequencies. The results suggest that the β-phase PVDF nanofibrous membrane produced via the SBS technique has a great potential to be used as a piezoelectric sensor.     

Enhanced electroactive phase,toughness and dielectric properties of poly(vinylidene fluoride) with addition of MMA-BA-IL copolymer

Methyl methacrylate (MMA), butyl acrylate (BA) and 1-butyl-3-vinylimdazolium tetrafluoroborate ([BVIM][BF₄]) copolymer (MMA-BA-IL) was prepared and used to enhance the electroactive phase content, toughness and dielectric properties of poly(vinylidene fluoride) (PVDF). Fourier transform infrared spectroscopy (FTIR), X-ray diffraction (XRD) and differential scanning calorimetry (DSC) tests indicated that crystal transformation of PVDF from α-phase to β/γ-phase occurred due to ion-dipole interaction between PVDF and [BVIM][BF₄]. Scanning electron microscope (SEM) results showed MMA-BA-IL copolymer dispersed in the PVDF uniformly and the partial replacement of MMA components by [BVIM][BF₄] decreased the miscibility between PVDF and MMA-BA copolymer. MMA-BA-IL copolymer improved the tensile ductility and impact toughness of PVDF. When the content of MMA-BA-IL was beyond 10 wt%, the elongation at break was higher than 400% and the impact strength was higher than 600 J/m. Deformation mechanism researches proved that shear yielding of the PVDF matrix and debonding/cavitation of the MMA-BA-IL copolymer particles were the major toughening mechanisms. The addition of MMA-BA-IL copolymer enhanced the dielectric properties of PVDF significantly. When the MMA-BA-IL content was 15 wt%, the dielectric constant of the PVDF/MMA-BA-IL blend increased to 54.3 at the 100 Hz frequency, which improved by 246% relative to that of the pure PVDF.     

Dielectric properties of poly(vinylidene fluoride) composites based on Bucky gels of carbon nanotubes with ionic liquids

“Bucky gels” of carbon nanotubes were successfully prepared by grinding multi‐walled carbon nanotubes (MWNTs) and ionic liquids (ILs) for several hours. A series of poly(vinylidene fluoride) (PVDF) composites with Bucky gels was obtained through simple melt compounding. The Raman spectrum showed significant interaction among the ILs, MWNTs, and PVDF. The dielectric behavior of the PVDF composites based on unmodified and IL‐modified MWNTs was studied from 40 Hz to 30 MHz. The addition of ILs significantly enhanced the dielectric property of the PVDF/IL/MWNT ternary composites, which was much higher than that of the sum of PVDF/IL and PVDF/MWNT binary composites. The SEM results revealed that both MWNTs and ILs uniformly dispersed throughout the PVDF/IL/MWNT composites because of the strong interaction between them. The DSC and XRD results showed that the addition of ILs in the composites changed the crystallinity and crystal form of the PVDF. POLYM. COMPOS., 36:94–101, 2015. © 2014 Society of Plastics Engineers     

Polyvinylidene fluoride-modified BaTiO3 composites with high dielectric constant and temperature stability

Polyvinylidene fluoride (PVDF)-modified X7R-type BaTiO₃ (BTO) composites were prepared by hot pressing, and the dielectric properties were investigated. The dielectric constant of the PVDF–BTO composites at 1 kHz increased significantly with increasing the volume fraction of BTO up to 0.5, and good temperature stability of dielectric constant was obtained for the composites, which benefited from the temperature-stable dielectric constant of the modified BaTiO₃. Two significant dielectric relaxations were observed for the PVDF–BTO composites, and they fit the Vogel–Fulcher and Arrhenius fittings, respectively.     

Enhanced electroactive β phase in three phase PVDF/CaCO3/nanoclay composites: Effect of micro‐CaCO3 and uniaxial stretching

In this study, electroactive polar phase transformation and crystallinity of poly(vinylidene fluoride) (PVDF)‐based composites, such as PVDF/CaCO₃/nanoclay, is explored as a function of micro‐CaCO₃ fraction and draw ratio (R) of uniaxial stretching. Composites including PVDF/clay, PVDF/CaCO₃ and most importantly PVDF/CaCO₃/clay with varying fraction of micro‐CaCO₃ were extruded into homogenous and flexible cast films. Characterization via Fourier transform infrared spectroscopy, X‐ray diffraction, and differential scanning calorimetry (DSC) confirmed the presence of β phase in all the composites incorporated with micro‐CaCO₃ and nanoclay either individually (i.e., PVDF/CaCO₃ and PVDF/clay films, respectively) or together (i.e., PVDF/CaCO₃/nanoclay composites). Interestingly, a gradual but significant improvement in this electroactive phase (β phase) was obtained with successive increment in CaCO₃ content into a fixed composition of PVDF and nanoclay (PVDF/CaCO₃/clay composites). Further increment in β phase content was obtained via uniaxial stretching to different draw ratios and at a temperature of 90 °C, where for PVDF/CaCO₃/clay (especially, 100–35‐3 and 100–40‐3) samples almost no α phase was observed irrespective of R. Conversely, the crystallinity of melt extruded samples decreased gradually all the way with CaCO₃ concentration in PVDF/CaCO₃/clay composites compared to the neat PVDF while increased gradually with increasing draw ratio. © 2017 Wiley Periodicals, Inc. J. Appl. Polym. Sci. 2017 , 134, 44940.     

Binary and ternary nanocomposites based on PVDF,PMMA, and PVDF/PMMA blends: polymorphism,thermal, and rheological properties

Binary and ternary nanocomposites based on poly(vinylidene fluoride) (PVDF), poly(methyl methacrylate) (PMMA), and PVDF/PMMA blends were successfully prepared through a melt-mixing process, using a commercial organoclay (15A) as the nanofiller. The 15A was more finely dispersed (intercalated/partially exfoliated) within the PVDF matrix compared with the PMMA matrix. A higher PVDF content in the ternary composite essentially led to a superior degree of 15A dispersion. The 15A addition induced the polar β-form PVDF crystals, whereas the presence of PMMA in ternary composites reduced the efficiency in promoting β-form formation by 15A. Adding 15A also enhanced the nucleation of PVDF, but the enhancement was inferior while PMMA was present. Conversely, the crystal growth of PVDF was retarded with the existence of 15A, and the PVDF/15A binary composite exhibited the greatest retardation. The equilibrium melting temperature (T_m°) of PVDF in the neat state and in the blends increased after 15A addition. The PVDF/15A binary composite possessed an evidently higher β-form T_m° than the α-form T_m° of neat PVDF (~20.1 °C rise). Similar effects on the individual components, 15A declined the thermal stability of PVDF but increased that of PMMA in the ternary composites. Rheological property measurements revealed that the ternary composites performed in-between that of individual PVDF/15A and PMMA/15A binary composites. A percolation of 15A (pseudo)network structure was developed in the composites, and a more elastic behavior was observed with increasing PVDF content in the composites.