β-PVDF Films Research Articles

High [formula omitted]-phase PVDF films formed by uniaxial compression

Polyvinylidene fluoride (PVDF) is a semicrystalline polymer with excellent ferroelectric properties. The β crystal form has the best piezoelectric properties among its five crystal forms. Conventionally, PVDF films with a high β-phase are produced by uniaxial stretching of α-phase PVDF films. Uniaxial compression of α-PVDF samples of thickness 3.2 mm was done to obtain β-PVDF films of thickness about 100μm, with biaxial orientation. The effects of temperature and compression ratio on the compressed PVDF films were studied using Raman spectroscopy, FTIR spectroscopy and cross-polarized optical microscopy. Uniaxial compression of the α-phase PVDF sample resulted in transformation to the β-phase, accompanied by distortion of the originally present Maltese cross structures in the optical micrographs. The compressed samples exhibited β-phase fraction anisotropy along the radial and thickness directions of the sample at lower compression ratios. PVDF samples with about 80% β-phase were obtained for compression ratios more than 12 at temperatures of 60 and 90 °C.

Enhanced neuronal differentiation by dynamic piezoelectric stimulation.

Electroactive smart materials play an important role for tissue regenerative applications. Poly(vinylidene fluoride) (PVDF) is a specific subtype of piezoelectric electroactive material that generates electrical potential upon mechanical stimulation. This work focuses on the application of piezoelectric PVDF films for neural differentiation. Human neural precursor cells (hNPCs) are cultured on piezoelectric poled and non-poled β-PVDF films with or without a pre-coating step of poly-d-lysine and laminin (PDL/L). Subsequently, hNPCs differentiation into the neuronal lineage is assessed (MAP2+ and DCX+ ) under static or dynamic (piezoelectric stimulation) culture conditions. The results demonstrate that poled and coated β-PVDF films induce neuronal differentiation under static culture conditions which is further enhanced with mechanical stimulation. In silico calculations of the electrostatic potential of different domains of laminin, highlight the high polarity of those domains, which shows a clear preference to interact with the varying surface electric field of the piezoelectric material under mechanical stimulation. These interactions might explain the higher neuronal differentiation induced by poled β-PVDF films pre-coated with PDL/L under dynamic conditions. Our results suggest that electromechanical stimuli, such as the ones induced by piezoelectric β-PVDF films, are suitable to promote neuronal differentiation and hold great promise for the development of neuroregenerative therapies.

Force-responsive antibiofouling strategy based on the ultrasound-controlled piezoelectric effect

Designing antibiofouling coating is one of the most popular strategies for preventing the settlement of marine organisms on ship surface. Although the most classic biocidal coatings are effective, they can cause serious environmental and health problems. In order to improve the antifouling performance meanwhile not compromise the environmental problem, we report a cost-effective, durable and environment-friendly antibiofouling strategy based on the piezoelectric effect in this study. This strategy relies on the piezoelectric response of electroactive β-phase poly(vinylidene fluoride) (β-PVDF) film under ultrasonic stimulation and the release of charges, which subsequently induce reactive oxygen species (ROS) generation to attack microorganisms on the material surface. Furthermore, mechanism investigation confirms that in addition to the ROS effect, the surface potential also exerts an important impact on the antimicrobial behavior of electroactive β-PVDF. As a result, regulating antimicrobial behavior through piezoelectric response can not only kill the bacteria adhering to the material surface, but also enhance the bactericidal activity against planktonic bacteria and the survival rate is only 1.6% (vs control-Ti). An environmental strategy for preventing the settlement of marine organisms is provided through piezoelectric material-regulated antimicrobial behavior under wave force and this finding will offer fresh insights into the design of antibiofouling coatings.

Effect of rolling temperature on the microstructure and electric properties of β-polyvinylidene fluoride films

Due to its outstanding dielectric, ferroelectric and piezoelectric properties among polymers, β-poly(vinylidene fluoride) (PVDF) has been widely used in flexible sensors, transducers and energy harvesters. These applications trigged tremendous interest infabrication of β-PVDF films with high electric performance over decades. Herein, the traditional solution casting method combined with a novel low-temperature double rolling approach was developed to fabricate β-PVDF films with superior dielectric, ferroelectric and piezoelectric properties. A systematic investigation was performed to study the effect of rolling temperature on the microstructure and electric properties of original β-PVDF obtained from solution casting. The results showed that rolling at temperatures between 25 and 60 °C could slightly influence the crystal forms of the β-PVDF films, but induce a well-oriented β-phase structure. Moreover, rolling at temperatures below 60 °C can significantly reduce the crystallite size of PVDF. With declined crystallite size, improved chain orientation and morphological continuity along the rolling direction, the double rolled β-PVDF exhibited notably enhanced dielectric and ferroelectric properties. Furthermore, due to the higher breakdown field and lower coercive electric field in comparison with the original β-PVDF films, the rolled β-PVDF films can be efficiently poled to obtain excellent piezoelectric properties. A maximum piezoelectric coefficient d33 of 29pC/N was obtained in the rolled β-PVDF films with a rolling temperature of around 50 °C.

In vivo demonstration of the suitability of piezoelectric stimuli for bone reparation

The potential of piezoelectric biomaterials for bone tissue engineering is demonstrated. This work proves that the use of piezoelectric poly(vinylidene fluoride) (PVDF), able to provide electrical stimuli upon mechanical solicitation to the growing cells, enhances bone regeneration in vivo. Poled and non-poled β-PVDF films and randomly oriented electrospun fiber mats have been used to test their osteogenic properties in Wistar rats by analyzing new bone formation in vivo. After 4weeks, defect implanted with poled β-PVDF films demonstrated significantly more defect closure and bone remodeling, showing the large potential of piezoelectric biomaterials for bone repair.

Fabrication of microstructure array directly on β-phase poly(vinylidene fluoride) thin film by O2 reactive ion etching

The ability to pattern a thin film of poly(vinylidene fluoride) (PVDF), a piezoelectric, pyroelectric and ferroelectric polymer, has potential applications in the fields of microelectromechanical systems (MEMS), nonlinear optics and nonvolatile ferroelectric random access memory technology. Low pressure O2 reactive ion etching (RIE) was employed to realize fine pitch microstructures on a β-phase PVDF (β-PVDF) film for the first time; a line and space (70/130 μm) microstructure array with a height of over 30 μm was fabricated. Different to the traditional method of PDMS molding, the proposed technology did not result in significant loss of piezoelectricity. Furthermore, unlike the x-ray photo-etching method, there was no etching saturation limit with the proposed method. Here, we introduce the fabrication process technology in detail and report on the etching properties of the β-PVDF film under different process conditions. The effect of process variables, such as supplied gas flow, applied RF power and etch time, on the PVDF etching characteristics were investigated in detail. The RF power and etch time showed a more predominant influence on PVDF etching progress than the gas flow. The etched depth was linearly increased with the etch time and the amount of RF power. Etching rates over 10 μm h−1 were achieved and increased linearly with the applied RF power. By means of a responding photomask design and control of process conditions, much finer and higher microstructure arrays are also possible.

Influence of nanoparticle size and concentration on the electroactive phase content of PVDF in PVDF-CoFe2O4-based hybrid films

The synthesis and characterization of a new magnetoelectric polymer-oxide hybrid flexible class of materials were reported. It consists of ferrimagnetic and magnetostrictive cobalt ferrite (CoFe2O4) nanoparticles (NPs) dispersed into a ferroelectric and piezoelectric polyvinylidene fluoride beta-phase (β-PVDF) film. 6-nm and 9-nm sized CoFe2O4 NPs were produced by forced hydrolysis in polyol. β-PVDF films were obtained by drying a solution of PVDF/DMF at low temperature and by varying the amount and/or size of NPs inside them. The resulting composites were characterized by X-ray diffraction, transmission electron microscopy, infrared spectroscopy (attenuated total reflectance), differential scanning calorimetry and near-field microscopy (AFM). We clearly demonstrate that the presence of NPs leads to a decrease of the whole PVDF crystallinity within the hybrids but also that small variations of the size and amount of nanoparticles influenced the crystallization of the electroactive polar phase of PVDF. Higher percentages of polar PVDF phase were found in hybrid films processed at 80 °C and containing 9-nm CoFe2O4 NPs. Self-standing films of neat beta-phase PVDF (left) and PVDF–CoFe2O4 (15 wt.%) hybrid (right).

Functionalized Nanoporous Track-Etched b-PVDF Membrane Electrodes for Heavy Metal Determination by Square-Wave Anodic Stripping Voltammetry

Track-etched functionalized nanoporous β-PVDF membrane electrodes, or functionalized membrane electrodes (FMEs), are electrodes made from track-etched, poly(acrylic acid) (PAA) functionalized nanoporous β-poly(vinylidene fluoride) (β-PVDF) membranes with thin porous Au films sputtered on each side as electrodes. To form the β-PVDF nanoporous membranes, β-PVDF films are irradiated by swift heavy ions. After irradiation, radical tracks are stable in the membranes. Chemical etching removes some of the radical tracks revealing nanopores. Radicals, remaining in the pores, initiate radio grafting of PAA from the pore walls of the nanoporous β-PVDF. PAA is a cation exchange polymer that adsorbs metal ions, such as Pb2+, from aqueous solutions thus concentrating the ions into the membrane. After a calibrated time the FME is transferred to an electrochemical cell for square-wave anodic stripping voltammetry analysis.

Effect of poling state and morphology of piezoelectric poly(vinylidene fluoride) membranes for skeletal muscle tissue engineering

This work reports on the influence of polarization and morphology of electroactive poly(vinylidene fluoride), PVDF, on the adhesion and morphology of myoblast cells. Non-poled, “poled+” and “poled−” β-PVDF films were prepared by solvent casting followed by corona poling. Furthermore, random and aligned electrospun β-PVDF fiber mats were also prepared. It is demonstrated that negatively charged surfaces improve cell adhesion and proliferation and that the directional growth of the myoblast cells can be achieved by culturing the cell on aligned fibers. Therefore, the potential application of electroactive materials for muscle regeneration is demonstrated.

Fibronectin adsorption and cell response on electroactive poly(vinylidene fluoride) films

Due to the large potential of electroactive materials in novel tissue engineering strategies, the aim of this work is to determine if the crystalline phase and/or the surface electrical charge of electroactive poly(vinylidene fluoride), PVDF, have influence on the biological response in monolayer cell culture. Non-polar α-PVDF and electroactive β-PVDF were prepared. The β-PVDF films were poled by corona discharge to show negative or positive electrical surface charge density. It has been concluded that hydrophilicity of the PVDF substrates depends significantly on crystalline phase and polarity. Furthermore, by means of atomic force microscopy and an enzyme-linked immunosorbent assay test, it has been shown that positive or negative poling strongly influences the behavior of β-PVDF supports with respect to fibronectin (FN) adsorption, varying the exhibition of adhesion ligands of adsorbed FN. Culture of MC3T3-E1 pre-osteoeblasts proved that cell proliferation depends on surface polarity as well. These results open the viability of cell culture stimulation by mechanical deformation of a piezoelectric substrate that results in varying electrical charge densities on the substrate surface.

Enhanced proliferation of pre-osteoblastic cells by dynamic piezoelectric stimulation

This work reports on the influence of the polarization of electroactive poly(vinylidene fluoride), PVDF, on the biological response of cells cultivated under static and dynamic conditions. Non-poled and “poled +” β-PVDF with and without a titanium layer were thus prepared. A thin titanium layer was deposited on PVDF films in order to obtain a more homogeneous surface charge. The MC3T3-E1 osteoblast cell culture exhibited different responses in the presence of PVDF films. The positively charged β-PVDF films promote higher osteoblast adhesion and proliferation, which is higher under dynamic conditions on poled samples, showing that the surface charge under mechanical stimulation improves the osteoblast growth. Therefore, electroactive membranes and scaffolds can provide the necessary electrical stimuli for the growth and proliferation of specific cells.

Functionalized nanoporous track-etched β-PVDF membrane electrodes for lead(ii) determination by square wave anodic stripping voltammetry

Track-etched functionalized nanoporous β-PVDF membrane electrodes, or functionalized membrane electrodes (FMEs), are electrodes made from track-etched, poly(acrylic acid) (PAA) functionalized nanoporous β-poly(vinylidene fluoride) (β-PVDF) membranes with thin porous Au films sputtered on each side as electrodes. In order to form the β-PVDF nanoporous membranes, β-PVDF films are irradiated by swift heavy ions. After irradiation, radical tracks are left in the membranes. Etching removes some of the radical tracks revealing nanopores. The remaining radicals initiate radio grafting of PAA from the pore walls of the nanoporous β-PVDF. PAA is a cation exchange polymer that adsorbs metal ions, such as Pb2+, from aqueous solutions thus concentrating the ions into the membrane. After a calibrated time, the FME is transferred to an electrochemical cell for analysis. A negative potential is applied to the Au film of the FME for a set time to reduce the adsorbed ions onto the Au film working electrode. Square-wave anodic stripping voltammetry was performed on the FME to determine the Pb2+ ion concentration in the original solution based on calibration. The zero current intercept of the calibration for Pb2+ is 0.13 ppb (μg L−1) and three times the sample blank deviation (3S/N) is 0.050 ppb.

Effect of drawing on the dielectric properties and polarization of pressed solution cast β-PVDF films

Poly(vinylidene fluoride), PVDF, samples containing exclusively the polar β phase were obtained by crystallization from N,N-dimethylformamide (DMF) solution at 60 °C and subsequent pressing. Some of these samples were uniaxially drawn at 120 °C at draw ratio of 4, resulting in oriented films. Oriented and unoriented samples were characterized as to relative fraction of β phase, degree of crystallinity and orientation by infrared spectroscopy (FTIR), differential scanning calorimetry (DSC), and wide-angle X-ray diffraction (WAXD). The dependence on frequency of the real (\( \varepsilon_{\text{r}}^{\prime} \)) and imaginary (\( \varepsilon_{\text{r}}^{\prime\prime} \)) components of the relative permittivity of the samples was determined between 104 and 1.7 × 107 Hz. The coercive field and stable and metastable remanent ferroelectric polarization were determined from the hysteresis loop obtained by the ramp voltage technique, described in detail in this work. The results allowed verification of the effect of drawing on structure and of the resulting structure on the dielectric properties, remanent polarization, and coercive field of β-PVDF. A possible effect of the crystal–amorphous interphase region on the metastable remanent polarization is suggested. The results obtained with the oriented and unoriented samples were compared with those obtained for films commercialized by Piezotech S.A.

Making porous membranes by chemical etching of heavy-ion tracks in β-PVDF films

Production of porous membranes using heavy ion bombardment and subsequent chemical etching of poly(vinylidene difluoride) (PVDF) films has been reported several years ago. However, porous membranes with pore diameter in the nanometer scale requires a better understanding of the chemical etching mechanism. In this work PVDF foils irradiated with Sn ions (2.85MeV per nucleon) were exposed to several etching conditions which involved permanganate oxidation in different alkaline environments. The solution of KOH 9molL−1 and saturated in KMnO4 was the best etching reactant for PVDF. Functional groups created in the alkaline and oxidative attack by permanganate were studied by FT-IR and UV–vis spectroscopy. The spectroscopic data reveals that the formation of pores occurs by a two-step mechanism: (i) double bonds as a result of dehydrofluorination induced by alkaline media and (ii) oxidation of these double bonds in permanganate solution. The etching temperature and time can be attuned to prepare track-etched membrane with a desired pore diameter in the range of few hundred nanometers. Temperatures ranged between 55°C and 65°C were optimal to produce cylindrical pores. Temperatures higher than 85°C induce conical-shaped track-etched pores while temperatures lower than 50°C slow down the chemical attack. The addition of a phase-transfer agent enhances the chemical attack and allows the decrease of the etching temperature and/or time.

Characterization of poled and non-poled β-PVDF films using thermal analysis techniques

β-poly(vinylidene fluoride)—β-PVDF—exhibits ferroelectric properties due to the special arrangement of the chain units in the crystalline phase. The ferroelectric phase can be optimised by poling the original stretched film, that tends to align the randomly organised crystallites against the applied field. In this work, polarised and non-polarised β-PVDF from the same batch are characterised by mechanical tests and a series of thermal analysis techniques, including DMA, TMA and DSC. The films exhibit mechanical anisotropy, and in the longitudinal direction the poled film presents larger mechanical properties, due to the higher structural organisation. Poled and non-poled show similar crystallinity levels but the melting temperature probed by DSC is higher for the non-poled film; for both films the melting peak exhibits a complex shape, indicating a heterogeneous crystalline organization. Two relaxation processes were found (β and αc) by dynamic mechanical analysis (DMA). The β process, attributed to segmental motions in the amorphous phase, appears at the same temperature in both directions, but its intensity was found to be higher for the films tested in the longitudinal direction. For a given direction, the poled films exhibit lower peaks, due to the more organised amorphous structure. At higher temperature, the αc-relaxation was related to the contraction of the films in the longitudinal direction during heating, recorded by thermal mechanical analysis (TMA). The onset of molecular mobility within the crystalline phase allows for cooperative diffusion processes in the amorphous phase, generating the randomisation of the initially oriented structure.

Electrical Response of β-PVDF in a Constant Uniaxial Strain Rate Deformation

The microstructure of β-PVDF has great influence on its piezo- and pyroelectric responses. The microstructure of β-PVDF drastically changes upon a mechanical deformation perpendicular to the preferred chain orientation, mainly above the yielding point. The voltage (open-circuit response) developed in β-PVDF films was monitored while the material is subjected to a constant strain rate program.

Dynamic mechanical analysis and creep behaviour of β-PVDF films

In this work, tensile dynamic mechanical analysis (DMA) was used to characterise the solid-state rheological properties of a commercial -polyvinylidene fluoride (PVDF) film in the two main directions (longitudinal and transversal to the stretch direction). The -relaxation, assigned to the segmental motions within the amorphous phase, is observed at ∼−25 ◦ C. Thec-relaxation is observed above room temperature. This relaxation is the main responsible for the anelastic properties of the material and to the short-term creep behaviour. Longer-term creep tests along the longitudinal direction were also performed and the Eyring model was applied, in order to characterize the flow-process within the polymeric structure. The results suggest that long-term creep is mostly controlled by the deformation of amorphous tie-chains that connect adjacent crystalline lamellae. © 2003 Published by Elsevier B.V.

Chain Reorientation in β-PVDF Films Upon Transverse Mechanical Deformation Studied by SEM and Dielectric Relaxation

Chain reorientation may be induced in polyvinylidene fluoride (PVDF) in its β-phase by applying a deformation perpendicular to the pre-oriented polymeric chains. This reorientation begins right after the yielding point and seems to be completed when the stress-strain curve stabilizes. We have studied the chain reorientation mechanism by scanning electron microscopy (SEM) and dielectric relaxation methods and we have analyzed its influence in the α- and β-relaxations of the polymer. SEM images confirm the existence of a reorientation upon transversal deformation. The decrease of crystallinity with increasing reorientation and the dependence of the dielectric permittivity upon deformation indicate that a model of stress-induced melting followed by an incomplete recrystallization properly describes the reorientation mechanism.

Mechanical Characterization and Influence of the High Temperature Shrinkage of β-PVDF Films on its Electromechanical Properties

Tensile dynamic mechanical analysis at 1 Hz was used to characterize the solid rheological properties of β-PVDF films. Both the elastic and loss moduli and the specific damping capacity were monitored against temperature, allowing the study of the effect of anisotropy upon the viscoelastic properties of the films. The temperature range covered the β- and α-relaxations. These results are compared to dielectric relaxation results in order to elucidate the electrical and mechanical contributions to the observed relaxations. Further, an important shrinking effect upon heating above 364 K has been observed, that influences the material properties. This geometrical effect has been monitored by thermal mechanical analysis. The thermal coefficients of linear expansion have been calculated, giving two different regimes for this parameter. The variations at a molecular level have been monitored by FTIR.

Patterned ferro-, pyro- and piezoelectricity in poly(vinylidene fluoride) by means of a laser-induced irreversible β→α phase transformation

Materials with patterned pyro- or piezoelectricity are useful for a range of applications such as sensor arrays with reduced cross talk between individual elements, piezoelectric gratings for direction-sensitive acoustic-wave detection/emission, or motion-sensitive pyroelectric sensors. Here, the successful patterning of pyro- and piezoelectric poly(vinylidene fluoride) (PVDF) films, by controlled scanning of a focused laser beam across their top electrodes, is reported. The resulting patterns are based on the differences between the ferroelectric polar β phase and the paraelectric nonpolar α phase of PVDF. Nonpolar α-phase areas are generated in a poled β-PVDF film through a local heating process that yields a β→α phase transition throughout the thickness of the film. An important feature of this process is the fact that the PVDF film remains intact and can still be used even in applications that require free-standing films. Infrared spectroscopy and pyroelectrical depth profiling are employed to prove the β→α phase transition and the accompanying pyroelectricity pattern, respectively.