Piezoelectric formation mechanisms and phase transformation of poly(vinylidene fluoride)/graphite nanosheets nanocomposites

The β-phase of poly(vinylidene fluoride) (PVDF) is of great technical importance because of its high dielectric constant and piezoelectric effect. In this work, we exfoliated and dispersed natural graphite to prepare 2D-graphite nanosheets (GNS) and prepared PVDF/GNS nanocomposites with different GNS volumes from 1 to 7 ml as film via solution casting method. The concentration of the supernatant was about 0.3 mg/mL. The effect of GNS on the β-phase formation and mechanisms of piezoelectric formation in the PVDF/GNS nanocomposites were investigated. The results showed that with the varying amounts of GNS, the crystalline structure, the morphology, and the dielectric and piezoelectric properties of PVDF/GNS nanocomposites changed. GNS acts as an effective nucleation agent with the orientation almost parallel to the surface of the film meanwhile increasing the amount of beta phase in the PVDF matrix with increasing amount of GNS, also the relative dielectric constant and dielectric loss of the nanocomposites increased with increasing amount of GNS. The value d₃₃ also increased with increasing amount of GNS, and reached a maximum (6.7 pC/N) with 6 ml GNS. The mechanism of piezoelectric formation was proposed based on experiment results of PVDF/GNS nanocomposites.     

Nanostructure and morphology of poly(vinylidene fluoride)/polymethyl (methacrylate)/clay nanocomposites: correlation to micromechanical properties

Nanocomposites based on poly(vinylidene fluoride) (PVDF)/poly(methyl methacrylate) (PMMA) with untreated clay were prepared in one step by reactive melt extrusion. Chemical reactions took place between the polymer matrices, the inorganic clay particles, and three reactive agents, leading to the PVDF/PMMA/clay nanocomposites. The microstructure characterizations were carried out by differential scanning calorimetry and wide-angle X-ray scattering (WAXS). The mechanical behavior was investigated by tensile experiments, impact tests, and microhardness measurements. The morphological characterization was carried out by optical and atomic force microscopy (AFM). The decrease of the melting and crystallization temperatures of the PVDF with the increasing PMMA content is attributed to the interactions between the oxygen of the PMMA carbonyl group and the PVDF’s hydrogen atom. WAXS analysis shows that there is neither an intercalation step nor total exfoliation in any composition. As the PMMA content increases, WAXS diagrams show either the PVDF α-crystallographic form, both, α- and β-forms, or only the β-form. For PMMA contents higher than 40 wt%, the materials became amorphous. The microhardness of the samples decrease for a PMMA content up to 20 wt%. The study by optical microscopy and AFM illustrates the significant effect in the presence of clay on the film’s surface morphology.     

Crystallization behavior of poly (vinylidene fluoride)/multi-walled carbon nanotubes nanocomposites

The non-isothermal and isothermal crystallization of poly (vinylidene fluoride) (PVDF)/multiple-walled carbon nanotubes (MWNTs) composites containing pristine (MWNT1) and carboxyl group (–COOH) functionalized MWNT (MWNT2) were investigated. The effects of MWNT on the crystallization behavior of PVDF were dependent on the dispersion state of MWNT. Pristine MWNT could increase the nucleation due to better dispersion, and thus, PVDF/MWNT1 composites exhibited higher crystallization peak temperatures (T _cps) and crystallinities (X _cs) compared with PVDF/MWNT2 composites. Meanwhile, the formation of MWNT network confined the growth of crystals. For the isothermal crystallization, MWNT acted as nucleating agents, and the crystallization rate constant k was increased with the addition of MWNT. Besides, the half crystallization time, t _0.5, was remarkably shortened with the increase of MWNT content, especially for the pristine MWNT.     

The creep behaviour of poly(vinylidene fluoride)/“bud-branched” nanotubes nanocomposites

The creep behaviour of poly(vinylidene fluoride) (PVDF)/multiwall carbon nanotubes nanocomposites has been studied at different stress levels and temperatures. To fine-tune the ability to transfer stress from matrix to carbon nanotubes, bud-branched nanotubes, were fabricated. The PVDF showed improved creep resistance with the addition of carbon nanotubes. However, bud-branched nanotubes showed a modified stress–temperature-dependent creep resistance compared with carbon nanotubes. At low stress levels and low temperatures, bud-branched nanotubes showed better improvement of the creep resistance than that of virgin carbon nanotubes, while at high stress levels and high temperatures, the virgin carbon nanotubes presented better creep resistance than that of bud-branched nanotubes. DSC, WAXD, and FTIR were employed to characterise the crystalline structures and dynamic mechanical properties were characterised by DMA testing. The Burgers’ model and the Findley power law were employed to model the creep behaviour, and both were found well describe the creep behaviour of PVDF and its nanocomposites. The relationship between the structures and properties was analysed based on the parameters of the modelling. The improved creep resistance for PVDF by the addition of nanotubes would benefit its application in thermoset composite welding technology.     

Dielectric and magnetic properties of ferrite/poly(vinylidene fluoride) nanocomposites

Particulate composite films of poly(vinylidene fluoride) and CoFe₂O₄ and NiFe₂O₄ were prepared by solvent casting and melt processing. The well-dispersed ferrite nanoparticles nucleate the piezoelectric β-phase of the polymer, but the different ferrites nucleate the whole polymer crystalline phase at different filler concentrations. The macroscopic magnetic and dielectric response of the composites demonstrates a strong dependence on the volume fraction of ferrite nanoparticles, with both magnetization and dielectric constant increasing for increasing filler content. The β-relaxation in the composite samples is similar to the one observed for β-PVDF obtained by stretching. A superparamagnetic behavior was observed for NiFe₂O₄/PVDF composites, whereas CoFe₂O₄/PVDF samples developed a hysteresis cycle with coercivity of 0.3 T.     

Large dielectric constant and high thermal conductivity in poly(vinylidene fluoride)/barium titanate/silicon carbide three-phase nanocomposites

Dielectric polymer composites with high dielectric constants and high thermal conductivity have many potential applications in modern electronic and electrical industry. In this study, three-phase composites comprising poly(vinylidene fluoride) (PVDF), barium titanate (BT) nanoparticles, and β-silicon carbide (β-SiC) whiskers were prepared. The superiority of this method is that, when compared with the two-phase PVDF/BT composites, three-phase composites not only show significantly increased dielectric constants but also have higher thermal conductivity. Our results show that the addition of 17.5 vol % β-SiC whiskers increases the dielectric constants of PVDF/BT nanocomposites from 39 to 325 at 1000 Hz, while the addition of 20.0 vol % β-SiC whiskers increases the thermal conductivity of PVDF/BT nanocomposites from 1.05 to 1.68 W m(-1) K(-1) at 25 °C. PVDF/β-SiC composites were also prepared for comparative research. It was found that PVDF/BT/β-SiC composites show much higher dielectric constants in comparison with the PVDF/β-SiC composites within 17.5 vol % β-SiC. The PVDF/β-SiC composites show dielectric constants comparable to those of the three-phase composites only when the β-SiC volume fraction is 20.0%, whereas the dielectric loss of the PVDF/β-SiC composites was much higher than that of the three-phase composites. The frequency dependence of the dielectric property for the composites was investigated by using broad-band (10(-2)-10(6) Hz) dielectric spectroscopy.     

Interface characterization and thermal degradation of ferrite/poly(vinylidene fluoride) multiferroic nanocomposites

Flexible multiferroic 0–3 composite films, with CoFe₂O₄, Ni_0.5Zn_0.5Fe₂O₄ or NiFe₂O₄ ferrite nanoparticles as filler and polyvinylidene fluoride (PVDF) as the polymer matrix, have been prepared by solvent casting and melt crystallization. The inclusion of ferrite nanoparticles in the polymer allows to obtain magnetoelectric nanocomposites through the nucleation of the piezoelectric β-phase of the polymer by the ferrite fillers. Since the interface between PVDF and the nanoparticles has an important role in the nucleation of the polymer phase, thermogravimetric analysis was used in order to identify and quantify the interface region and to correlate it with the β-phase content. It is found that an intimate relation exists between the size of the interface region and the piezoelectric β-phase formation that depends on the content and type of ferrite nanoparticles. The interface value and the β-phase content increase with increasing ferrite loading and they are higher for CoFe₂O₄ and Ni_0.5Zn_0.5Fe₂O₄ ferrite nanoparticles. The composites shows lower thermal stability than the pure polymer due to the existence of mass loss processes at lower temperature than the main degradation of the polymer. The main degradation of the polymer matrix, nevertheless, shows increased degradation temperature with increasing ferrite content.     

Morphological, viscoelastic and thermal properties of poly(vinylidene Fluoride)/POSS nanocomposites

Nanocomposites of poly(vinylidene fluoride) and polyhedral oligomeric silsesquioxane were prepared through melt blending. Morphology, viscoelastic and thermal properties were investigated. Up to 1 wt.% the processing conditions were efficient to prevent formation of large POSS agglomerates. In the nanocomposites with higher POSS contents these conditions could not avoid it, because of the strong interaction among POSS molecules. The presence of two different crystalline phases in nanocomposite was evidenced by X-ray diffraction and Fourier Transformed Infra-Red Spectroscopy. The nanocomposite with 5 wt.% content had the highest values for degree of cristallinity. The polyhedral oligomeric silsesquioxane molecules are acting as lubricant in the system, once lower values for storage modulus as well as for viscosity were observed.     

Preparation and characterization of poly (styrene-acrylonitrile) (SAN)/clay nanocomposites by melt intercalation

Poly (styrene-acrylonitrile) (SAN)/clay nanocomposites have been prepared by melt intercalation method from pristine montmorillonite (MMT), using hexadecyl trimethyl ammonium bromide (C16) and hexadecyl triphenyl phosphonium bromide (P16) as the reactive compatibilizers between polymer and clay. The influence of the reactive compatibilizers proportion relative to the clay on the structure and properties of the SAN/clay nanocomposites is investigated by X-ray diffraction (XRD) and transmission electron microscopy (TEM), high-resolution electron microscopy (HREM), thermogravimetric analysis (TGA) and dynamic mechanical analysis (DMA). The effects of the two different clays (MMT and organic modified MMT) on the nanocomposites formation, morphology and property are also studied. The results indicate that the SAN cannot intercalate into the interlayers of the MMT and results in microcomposites. In the presence of the reactive compatibilizers, the dispersion of clay in SAN is rather facile and the SAN/clay nanocomposites reveal an intermediate morphology, an intercalated structure with some exfoliation and the presence of small tactoids. The appropriate proportion with 3 wt% reactive compatibilizers to 5 wt% MMT induces well-dispersed morphology and properties in the SAN matrix. The TGA analyses show that the thermal stability properties of the SAN/clay nanocomposites have been improved compared with those of the pristine SAN. The DMA results show that the storage modulus and glass transition temperature (Tg) of the SAN/clay nanocomposites have remarkably enhancements compared with the pristine SAN. At last the intercalation mechanism of the technology is discussed.     

聚偏氟乙烯/有机累托土纳米复合超滤膜的制备及应用

利用溶液插层法制备了聚偏氟乙烯/有机累托土纳米复合超滤膜,研究了纳米掺杂含量对纳米片层分散状态的影响,测定了膜的各项性能,并将膜用于3种水样的处理。结果表明:纳米粘土的加入对复合膜的性能有很大影响;少量粘土的加入,能同时提高膜水通量和截留性能,在纯水通量性能测试时,其阻力增大系数由4.79下降到2.16;在对3种水样进行处理时,粘土的加入不仅能大大地提高膜在实际水处理中的抗污染能力,显著提高膜的反冲洗通量恢复率,还能获得比初始膜更好的出水水质。     

Improving the mechanical properties of thermoplastic starch/poly(vinyl alcohol)/clay nanocomposites

Thermoplastic starch/poly(vinyl alcohol) (PVOH)/clay nanocomposites, exhibiting the intercalated and exfoliated structures, were prepared via melt extrusion method. The effects of clay cation, water, PVOH and clay contents on clay intercalation and mechanical properties of nanocomposites were investigated. The experiments were carried out according to the Taguchi experimental design method. Montmorillonite (MMT) with three types of cation or modifier (Na⁺, alkyl ammonium ion, and citric acid) was examined. The prepared nanocomposites with modified montmorillonite indicated a mechanical improvement in the properties in comparison with pristine MMT. It was also observed that increases in tensile strength and modulus would be attained for nanocomposite samples with 10%, 5% and 4% (by weight) of water, PVOH and clay loading, respectively. The clay intercalation was examined by X-ray diffraction (XRD) patterns. The chemical structure and morphology of the optimum sample was also probed by FTIR spectroscopy and transmission electron microscopy (TEM).     

Segregated poly(vinylidene fluoride)/MWCNTs composites for high-performance electromagnetic interference shielding

Conductive polymer composites (CPCs) that contain a segregated structure have attracted significant attentions because of their promising for fulfilling low filler contents with high electromagnetic interference (EMI) properties. In the present study, segregated poly(vinylidene fluoride) (PVDF)/multi-walled carbon nanotubes (MWCNTs) composites were successfully prepared by mechanical mixing and hot compaction. The PVDF/MWCNTs samples with 7 wt% filler content possess high electrical conductivities and high EMI shielding effectiveness (SE), reaching 0.06 S cm⁻¹ and 30.89 dB (in the X-band frequency region), much higher than lots of reported results for CNT-based composites. And the EMI SE greatly increased across the frequency range as the sample thickness was improved from 0.6 to 3.0 mm. The EMI shielding mechanisms were also investigated and the results demonstrated absorption dominating shielding mechanism in this segregated material. This effective preparation method is simple, low-cost, and environmentally-friendly and has potential industrial applications in the future.     

Effect of filler size and concentration on the structure and properties of poly(vinylidene fluoride)/BaTiO<Subscript>3</Subscript> nanocomposites

The effect of filler size and content in the thermal, mechanical, and electrical response of poly(vinylidene fluoride) (PVDF)/BaTiO₃ nanocomposites has been investigated. Dielectric constant increases significantly with increasing filler content and decreasing filler size. Space charge effects at the interface between BaTiO₃ and PVDF strongly influence the dielectric response. The electroactive β-phase of PVDF is nucleated by the presence of the ceramic filler, the effect being strongly dependent on filler size and independent on filler content. This filler/matrix interaction is also responsible for the variations observed in the activation energy of the thermal degradation of the polymer. Smaller particles lead to larger relative contact areas and are responsible for the main variations observed in the thermal, mechanical, and electrical properties of the composites.     

Energy harvesting performance of BaTiO3/poly(vinylidene fluoride–trifluoroethylene) spin coated nanocomposites

The energy harvesting efficiency of poly(vinylidene fluoride–trifluoroethylene) spin coated films and its nanocomposites with piezoelectric BaTiO₃ have been investigated as a function of ceramic filler size and content. It is found that the best energy harvesting performance of ∼0.28 μW is obtained for the nanocomposite samples with 20% filler content of 10 nm size particles and for 5% filler content for the 100 and 500 nm size fillers. For the larger filler average sizes, the power decreases for filler contents above 5% due to increase of the mechanical stiffness of the samples. Due to the similar dielectric characteristics of the samples, the performance is mainly governed by the mechanical response. The obtained power values, easy processing and the low cost and robustness of the polymer, allow the implementation of the material for micro and nanogenerator applications.     

Poly(vinylidene fluoride)/poly(methyl methacrylate)/TiO2 blown films: preparation and surface study

Blown films of poly(vinylidene fluoride) (PVDF) and poly(methyl methacrylate) (PMMA) blends and PVDF/PMMA/TiO₂ composites were prepared by melting-extrusion for the first time. The crystalline structure and surface morphology PVDF/PMMA (DFMA) blown films were investigated using differential scanning calorimeter (DSC), atomic force microscope (AFM), and X-ray diffractometry (XRD). PVDF/PMMA/TiO₂ blown films were further prepared and underwent surface treatment. The results show that PVDF/PMMA/TiO₂ blown films present good mechanical properties, and acrylic acid surface-grafted films exhibit good adhesion capability and long-lasting hydrophilicity, making them attractive as encapsulation materials.     

热致相分离制备聚偏氟乙烯膜

以聚偏氟乙烯(PVDF)为基材,选用3种稀释剂邻苯二甲酸二甲酯(DMP),间苯二甲酸二甲酯(DMIP),水杨酸甲酯(MS)为稀释剂,通过热致液-固相分离制备了微孔膜.结晶度随PVDF含量的增加先增加后降低,在40%(wt,下同)时结晶度最大.结晶温度随着PVDF-MS,PVDF-DMIP,PVDF-DMP的顺序降低.考察了不同降温条件对聚偏氟乙烯膜的结构的影响.在液-固相分离的前提下,通过不同的降温条件,得到球粒堆积的微观结构.     

Crosslinked aqueous dispersion of silylated poly (urethane–urea)/clay nanocomposites

Stable water-borne crosslinked silylated poly (urethane–urea) (CSPU)/clay nanocomposites, reinforced with various amounts of the organically modified clay, were prepared by a polyaddition reaction of toluene diisocyanate (TDI) or isophorone diisocyanate (IPDI), polytetramethylene glycol and dimethylol propionic acid. This was followed by end-capping the free NCO groups of the PU prepolymer with phenylamino propyl trimethoxysilane and self-crosslinking. The particle size, viscosity and storage stability of these nanocomposites were measured. The particle size and viscosity of the IPDI-based nanocomposites were higher than the TDI-based ones. Intercalation of the silicate layer in the CSPU matrix were conformed by X-ray diffraction pattern and transmission electron microscopy studies. The mechanical properties of the SPU/clay nanocomposites were tested by tensile, dynamic mechanical, and nano-indentation measuring techniques and the respective properties were found to be enhanced by the reinforcing effect of organophilic clay. Modulus and hardness increased with an increase in the clay content in the CSPU matrix. Thermal stability, water and xylene resistance of the nanocomposites increased, as compared to pure CSPU and these properties increased with an increase in clay content. The mechanical properties, water and xylene resistance of the TDI-based nanocomposites were higher compared to the IPDI-based nanocomposites. A marginal reduction in transparency was observed with the addition of clay. Storage stability results confirmed that the prepared nanocomposite dispersions were stable.     

Influence of zeolite structure and chemistry on the electrical response and crystallization phase of poly(vinylidene fluoride)

Zeolites with framework types LTL, LTA, FAU, and MFI were synthesized and used as fillers to prepare PVDF/zeolite composites. The obtained composites showed structural and electrical dependence on the pore system and chemical content of the inorganic host. The larger polymer-zeolite electrostatic interactions of the Y and A zeolites lead the polymer to crystallize in the electroactive γ-phase, which in the case of the L zeolite is prevented due to the reduced interaction area. The solvent and water encapsulation ability of the zeolite as well as improve of the dielectric response of the composite is directly related to the Si/Al ratio, leading zeolites with lower Si/Al ratios to larger dielectric responses and encapsulation efficiencies in the composites. These effects show also some dependency on the dimensionality of the pore system; the zeolite L-containing 1D channels showing superior dielectric performance than the 3D pore system of zeolite Y.     

Time-domain dielectric relaxation in polyamide 6, poly(vinyl chloride), ethylene-vinyl acetate copolymer and poly(vinylidene fluoride)

The time-domain dielectric responses of polyamide 6 (PA6), poly(vinyl chloride) (PVC), ethylene-vinyl acetate copolymer (EVA) and poly(vinylidene fluoride) (PVDF) to a voltage step were measured at different temperatures. From the variation of the sample capacitance,C, with time, the ratioF _d/C was determined, whereF _d = (dC/dlnt)_max is the maximum (inflexion) slope of the capacitance versus log(time) dipole response curve, and C is the difference between the initial and the extrapolated equilibrium capacitance values. A modified KohIrausch-WiIIiams-Watts (stretched exponential) function provided a good fit to the measuredC(t) data. For low temperatures, typically below – 20°C,F _d/C, is about 0.1, characteristic of highly cooperative relaxation, while at higher temperatures the ratio approaches 1/e, characteristic of nearly independent (Maxwellian) relaxation. This is in contrast to corresponding analyses of mechanical relaxation in solids for which the constant is almost always near 0.1 at room temperature.