PVDF Thin Films Research Articles

In situ investigation of formation kinetics of microporous structure in PVDF thin films prepared via thermally‐induced phase separation (TIPS): Effects of film thickness and polymer concentration

AbstractIn this research, series of PVDF microporous films with various polymer concentrations and thickness were prepared through thermally‐induced phase separation in air condition with diphenyl carbonate as the diluent. Effects of polymer concentration and film thickness on the formation kinetics of microporous structure of PVDF thin films were in situ investigated via TII technique. The instantaneous heat conduction inside the films during film formation was also evaluated. Two types of parameter models were adopted in order to disclose the time and location dependence of TIPS film solidification kinetics, respectively. Hierarchical microstructures, crystallization behavior and thermal properties of the TIPS films were also characterized by SEM, DSC, and TGA, respectively. The current study could supply an insight into the fabrication of PVDF microporous films with target microstructure and desirable performance.

Dielectric and ferroelectric properties of PVDF thin film for biomechanical energy harvesting

Abstract The energy crisis has been global havoc due to increasing demand in energy sectors which further have a serious impact on declining fossil fuels. There is a quest to develop green alternative sources of energy such as biomass, geothermal, photovoltaics. The piezoelectric nanogenerator has also been a fascinating technology to harvest small level mechanical energy to an electrical output. The vibration energy from walking, rail/air vehicles, tires, sound waves are converted efficiently to usable energy. This report shows the fabrication of PVDF thin film using the solvent casting route. The various properties like structural, morphology, dielectric, piezoelectric, and ferroelectric are analyzed. Further, a piezoelectric nanogenerator was fabricated and results of poled and unpoled films have been investigated. The device output was utilized to light up a LED and charge a commercial capacitor.

Simulation and Optimization of stacked PVDF Membrane for Piezoelectric Application

In this paper, a comparative analysis of multilayer stacked PVDF thin film fabricated using two different techniques namely electrospinning and spin coating has been done. The simulation is carried out in COMSOL Multiphysics version 5.0 and the results are analyzed. PVDF is used as the fundamental material in the composition of the structure. The main properties that contribute to the comparative analysis are as follows: displacement, stress, acceleration, and electric potential. for a given material, all the properties are evaluated for a range of frequency. The variance of the properties for the same material under different fabrication techniques is evaluated and analyzed. The study provides comprehensive coverage of spin coating and electrospinning techniques and aims at understanding which technique can be adopted to enhance the efficiency and obtain the desired results.

High beta-phase processing of polyvinylidenefluoride for pyroelectric applications

It is well known that poly(vinylidene fluoride) (PVDF) crystallizes in semi polymorphic crystalline forms of α, β, γ,-phases. The piezo- as well as pyroelectric properties of PVDF is strongly dependent on its β-crystalline phase. These features make it suitable for sensors and battery applications. Thin films of PVDF are widely used in recent thermal camera sensors. In this study, optimum β-crystalline phase of PVDF thin film can be produced easily via a single-step process by crystallization from gel states in the presence of corona fields. FTIR- spectroscopy investigations indicate high volume fraction of β-phases for such films. The measured pyroelectric coefficient of 33 ± 2 C/m2K exhibits a comparable result with those reported from conventionally obtained β- phase PVDF foils which requires complex treatments.

Study of etching of patterned poly(vinylidene fluoride) film using dimethyl sulfoxide solvents

This paper focuses on the optimization of the etching rate of nano-patterns of Poly (vinylidene fluoride) (PVDF) thin films on glass substrates using Dimethyl Sulfoxide (DMSO). Soft imprint lithography is employed to transfer the nano-patterns onto the PVDF thin films systematically under optimized temperature and pressure. Atomic Force Microscopy (AFM) has been used to analyze the etching rate of patterned films.

Dielectric Properties of γ‐Iirradiated, Stretched, and Poled PVDF Thin Films

AbstractThe effects of γ‐irradiation on the dielectric properties of stretched and poled polyvinylidene fluoride thin films synthesized by solvent cast method are reported. The films are subjected to γ‐irradiation with different doses (25, 50, and 75 kGy). XRD pattern is obtained to identify the presence of α/β phases. Dielectric constant and loss values of the stretched and poled films have been measured, before and after irradiation, and their nature of variation has been studied. β phase is retained even after the irradiation thereby confirming that the radiation damage is not significant. The dielectric loss is minimum in the frequency range 102 to104 Hz, giving the preferable frequency range of operation for devices

Immobilized ionic liquid induced electroactive β-phase in poly(vinylidene fluoride) thin films

Ferroelectric poly(vinylidene fluoride) (PVDF) has been proposed in the past decades as an electroactive material for various applications in microelectronic devices due to its ferro/piezo/pyroelectric properties. However, fabrication of smooth PVDF thin films with a high content of β-phase and controlled chain orientation is a long-standing problem. Here we report on the control over polymorphs of PVDF thin films by exploring the interfacial ion-dipolar interactions. The ion-dipolar interactions are provided by CF2 bonds of PVDF and imidazolium type ionic liquid immobilized onto substrates via biomimetic catecholic attachment chemistry. By simply melt crystallization of spin-coated PVDF thin films at elevated temperatures, the ferroelectric polar β-phase with a lying-down chain orientation is formed. The thin films exhibit excellent ferro/piezoelectric properties when applying electric field due to the immobilization of ionic liquids. The results may boost the applications of PVDF in microelectronic devices.

Third order nonlinear optical properties of β enhanced PVDF based nanocomposite thin films

PVDF based nanocomposite thin films have received great interest in energy harvesting, ferroelectric, pyroelectric and dielectric applications. In this novel study, we have exploited the electroactive β-phase (polar) formation in polyvinylidine fluoride–halloysite nanotube (PVDF–HNT) nanocomposite thin films fabricated by the spin coating technique for nonlinear optical applications. It was demonstrated that HNTs of different volume percentage loadings in the PVDF matrix were able to effectively nucleate PVDF in β (TTTT-all trans) conformation using X-ray diffraction and Infrared spectroscopy techniques. Closed aperture Z-scan measurements were performed for all the thin film samples with a CW laser as an excitation source at a wavelength of 632.8 nm. We observed a sign change in the nonlinear refractive index for PVDF. Nonlinear refractive index has a negative sign for pristine PVDF and a positive sign for HNT incorporated PVDF thin films. This anomalous behavior of change in the nonlinear refraction of PVDF is explained in our present work.

3D printing of electroactive PVDF thin films with high β-phase content

Bi-axially oriented electroactive PVDF thin films were prepared by a 3D printing process during which multiple parameters play a critical role in enhancing β-phase and crystallinity of the thin films. The PVDF molecular chains were rotated and stretched to form orderly arrangement by the pulling of in situ strong auxiliary electric potential (voltage) and the mechanical pulling force, and the aligned PVDF molecular chains improved the crystallinity of the thin films. Doping very small amount of multi-walled carbon nanotubes (MWCNTs) or graphene (GR) as a nucleating agent significantly increased the content of the β-phase in the films. Adding GR behaved is better than the MWCNTs in improving the β-phase due to the lamellar structure of graphene can generate a large number of micro electric fields stimulating the transformation of molecular chain from trans-gauche-trans-gauche(TG+TG−) to all-trans (TTTT). The content of β-phase in the PVDF/GR(0.03 wt%) composite thin film researched at 61.52%. Under the special environment of 3D printing, the printed PVDF thin films became very dense with high β-phase after depositing layer-by-layer. It exhibited good piezoelectric properties without post-poling treatment and mechanical stretching.

Synergistic Effect of Hydrogen Bonds and Diffusion on the β-Crystallization of Poly(vinylidene fluoride) On Poly(methyl methacrylate) Interface

The PVDF thin film was prepared on PMMA sublayer. Compared to the PVDF on Si wafer, the PMMA layer shows great influence on the crystallization of PVDF under quenching thermal treatment. When the PMMA layer is thinner than 90 nm, mixture of α- and β-crystals form. Pure β-crystals are obtained when the PMMA layer is around 90 nm. With further thickening of the PMMA layer, pure β-crystals always form but with a decreased crystallinity. The nanodomains and needlelike morphology of β-crystals is observed. Moreover the formation mechanism of β-phase on PMMA interface is clarified. The hydrogen bonds at PVDF and PMMA interface is formed adopting the perpendicular orientation, which serve as the key role to induce β-PVDF. The amount of β-PVDF is not only determined by the interaction of PMMA and PVDF at interface, but also the diffusion extent of PMMA into the PVDF layer.

Preparation of β-Phase Poly(vinylidene fluoride) Films on Aluminum Substrate with the Addition of Hydrated Metal Salts.

Poly(vinylidene fluoride) (PVDF) is one of the most studied polymers that exhibits piezoelectric and ferroelectric properties. PVDF crystallizes into four different forms, namely α, β, γ, and δ phases. Generally, the nonpolar α phase, as a replacement for the polar β phase with the most superior ferroelectric and piezoelectric properties, is formed from the precursor melt and solution. Here, we report a method for the preparation of β-phase-dominant PVDF thin films by doping PVDF solution with a metal hydrated salt, such as Co(NO₃)₂·6H₂O, without any stretching procedure. The crystal structure of the film was analyzed using X-ray diffraction -2 scanning and Fourier transform infrared spectroscopy. The morphology of the film was observed using a field emission-scanning electron microscope. To determine the frequency dependence of dielectric properties, an LCR meter was used in the range of 50 Hz to 1 MHz. The crystalline phase and morphology of the electrostatic spray-deposited PVDF films depended on the chemical additive. Our results show that the addition of a metal hydrated salt can induce an in situ poling effect, consequently facilitating the preferred dipole orientation in the electrostatically sprayed PVDF films.

A Flexible Thin Film Single-Point Force Sensor from PVDF Film

On the basis of piezoelectric effect of PVDF film, the flexible thin film single-point force sensor with total dimension 75mmx20mmx1mm is devised to dynamically measure single-point force in biomedical or mechanical field. New flat interlayer structure with cavum is firstly introduced to enlarge the deformation of PVDF thin film, and a conductive supporting rack is firstly utilized to construct a detecting cavum and to realize the connection between PVDF film and down conductive electrode. Such structure is flat as the traditional sandwich structure; but its cavum may make sensor have a sensitive reaction and larger measurement range. Its measurement range is from 0 to 6217N and it works between -40°C and 80°C. The total thickness 1mm is so thin that the given sensor can be easily placed into aperture. Results of experiment and analysis illustrate that the developed sensor is available and that it may be used to dynamically sense a single-point force. The flat interlayer structure given is thin and flexible and flat and implantable. Such sensor makes that it becomes possible to dynamically measure single-point force in mechanical field or biomedical field.

Resistive switching and nanoscale chemical mapping of phase separation in PVDF/PMMA/F8T2 ternary thin films

In recent years organic ferroelectric/semiconducting blend film has attracted much attention due to its integration of both resistive switching and electrical rectifying properties. In all reported blend structures, P(VDF-TrFE), the copolymer of vinylidene fluoride and trifluoroethylene, is used as the ferroelectric phase, which is blended with organic semiconductors to form phase-separated structure. As another representative ferroelectric polymer, poly(vinylidene fluoride) (PVDF) has its own advantages of even larger polarization and low cost. However, PVDF has much complicated crystalline phases in which its non-polar α phase is the most thermally stable. Effective phase transformation from the α to the ferroelectric phases is still an open question especially for PVDF thin films. Till now seldom work has been reported on PVDF-based ferroelectric/semiconducting resistive films. Here we reported the fabrication and structural and electrical characterizations of PVDF-based ferroelectric/semiconducting ternary resistive films. Poly(9,9-dioctylfluorene-alt-bithiophene) (F8T2) was used as semiconducting phase which was phase separated with PVDF phase in PVDF/F8T2 blends. Poly(methyl metacrylate) (PMMA) was introduced into PVDF to promote its ferroelectric γ and β phases. This PVDF/PMMA/F8T2 ternary film showed good resistive switching performance with ON/OFF ratio up to 103, which was comparable to and even larger than those values obtained from blends with P(VDF-TrFE) as the ferroelectric phase. However, the introduction of PMMA also complicateed the basic understanding on phase separation and thus resistive switching mechanism in this ternary film. To clarify this, nanoscale chemical mapping was conducted to distinguish the local distributions of all three phases, the results of which showed obvious nanoscale phase separation in this ternary blend film.

Characteristic analysis of biaxially stretched PVDF thin films

ABSTRACTBy use of biaxial stretching process of poly(vinylidene fluoride) fabrication to gain high piezoelectricity is the focus in this article. The influence of different stretch ratio and temperature are investigated and compared to uniaxial stretching process. The Fourier transform infrared spectroscopy and X‐ray diffraction are used to observe the β‐phase fraction and the degree of crystallinity. The piezoelectric coefficients (d33, d31) and the sensitivity are also measured. All the above characteristic examinations show the same consequences that equi‐biaxial stretching process employed with same stretch ratio in both axial directions of poly(vinylidene fluoride) fabrication is the focus in this article. And heating temperature would have adequate piezoelectricity approximate to uniaxial stretching. In observation of surface morphology by using scanning electron microscopy, uniformity in biaxial direction is gained. However, biaxial stretching with higher stretch ratio of R = 4 × 4 would produce porosity and crack. Via atomic force microscopy, biaxial stretching with higher stretch ratio attains better surface chain orientation and smoothness. To sum up, the biaxial stretching approach has the advantage of low stretch ratio requirement to preserve good fabrication quality. Set up with stretch ratio in‐between R = 3 × 3 and R = 4 × 4 at 80 °C is suggested to use in biaxial stretching. © 2018 Wiley Periodicals, Inc. J. Appl. Polym. Sci. 2018, 135, 46677.

Focused electron-beam-induced deposition for fabrication of highly durable and sensitive metallic AFM-IR probes

We report on the fabrication of metallic, ultra-sharp atomic force microscope tips for localized nanoscale infrared (IR) spectrum measurements by using focused electron-beam-induced deposition of platinum or tungsten. The tip length can be controlled by changing the duration time of the electron beam. Probes of 12.0 ± 5.0 nm radius-of-curvature can be routinely produced with high repeatability and near-100% yield. The near-field-enhancement appears stronger at the extremity of the metallic tip, compared with commercial pristine silicon-nitride probe tip. Finally, the performance of the modified metallic tips is demonstrated by imaging PVDF and PMMA thin films, which shows that spatial resolution is greatly enhanced. In addition, the signal intensity of the localized nanoscale IR spectrum is increased offering greater sensitivity for chemical IR imaging.

Knife edges technique for focused beam profiling of the photothermal imaging system

In this paper, we demonstrate knife edge techniques for measuring the focused beam profile of the photothermal imaging system. This quick and simple method of profiling the focused beam from Helium-Neon (He-Ne) laser is presented and detected by the PVDF thin film as a sensor. The study of focused beam profile is crucial so as to provide information of the focused beam such as beam resolution, depth of focus and the level of astigmatism of the beam. In knife edge technique, the blade moves across the beam in x and y-direction at varying axial position.

Thin poly(ionic liquid) and poly(vinylidene fluoride) blend films with ferro‐ and piezo‐electric polar γ‐crystals

ABSTRACTFerro‐ and piezo‐electric poly(vinylidene fluoride) (PVDF) thin film is reported to be obtained by using a poly(ionic liquid) (PIL) [poly(2‐(dimethylamino)ethyl methacrylate) methyl chloride quaternary salt] through solution route. The short range interactions between localized cationic ions of PIL and polar >CF2 of PVDF are responsible for modified polar γ‐PVDF (T3GT3Ḡ) formation. Modification in chain conformation of PVDF is confirmed by FTIR, XRD, and DSC studies suggesting the miscible PVDF–PIL (PPIL) blend. Up to 40 wt % loading of PIL in PVDF matrix enhances relative intensity of γ‐phase up to 50% in the entire crystalline phase. The P‐E hysteresis loop of PVDF‐PIL blends at 25 wt % PIL loading (PPIL‐25) thin film at sweep voltage of ±50 V shows excellent ferroelectric property with nearly saturated high remnant polarization ∼6.0 µC cm−2 owing to large proportion of γ‐PVDF. However, non‐polar pure PVDF thin film shows unsaturated hysteresis loop with 1.4 µC cm−2 remnant polarization. The operation voltage decreases effectively because of the polar γ‐phase formation in PPIL blended film. High‐sensitivity piezo‐response force microscopy shows electromechanical switching property at low voltages in PPIL‐25 thin films through local switching measurements, making them potentially suitable as ferroelectric tunnel barriers. © 2018 Wiley Periodicals, Inc. J. Polym. Sci., Part B: Polym. Phys. 2018, 56, 795–802

Electroactive PVDF thin films fabricated via cooperative stretching process

ABSTRACTFerroelectric polymers have obtained much attention in low cost and flexible electronics. As one representative ferroelectric polymer, poly(vinylidene fluoride) (PVDF) has been comprehensively studied. Due to its complicated phase composition, fabrication of electroactive phases has been an open question especially for PVDF thin films deposited on solid substrates. Here cooperative stretching process is introduced for the fabrication of electroactive PVDF thin films. PVDF thin films are coated on stretchable poly(vinyl alcohol) substrates and mechanically stretched under optimized stretching parameters. Structural, spectral, and electrical measurements indicate that cooperative stretching process can effectively convert the nonpolar α phase to the electroactive β and γ phases companied by an enhancement of film crystallinity. Stretched PVDF films present a reverse piezoelectric coefficient of −37 pm/V, comparable with the results from films fabricated by the other techniques. © 2018 Wiley Periodicals, Inc. J. Appl. Polym. Sci. 2018, 135, 46324.

Influence of various Activated Carbon based Electrode Materials in the Performance of Super Capacitor

Various activated carbon based electrode materials with different surface areas was prepared on stainless steel based refillable super capacitor model using spin coating. Bio Synthesized Activated Carbon (BSAC), Activated Carbon (AC) and Graphite powder are chosen as electrode materials in this paper. Electrode materials prepared using binder solution which is 6% by wt. polyvinylidene difluoride, 94% by wt. dimethyl fluoride. 3M concentrated KOH solution is used as aqueous electrolyte with PVDF thin film as separator. It is tested for electrochemical characterizations and material characterizations. It is observed that the Specific capacitance of Graphite, Biosynthesized active carbon and Commercially available activated carbon are 16.1F g-1, 53.4F g-1 and 107.6F g-1 respectively at 5mV s-1 scan rate.

Effects of Gamma Irradiation on Polyvinylidene Fluoride Thin Films

Polyvinylidene fluoride thin films were synthesized by Sol-Gel method with spin rate of 3000 rpm for 30 sec on ITO glass substrates and were annealed at 170 C. The films were irradiated by Gamma radiation with different doses (10, 30, 40 and 50 kGy). XRD and FTIR spectra have been obtained to identify the presence of α / β phases. Mean crystallite size was calculated by Scherer’s equation. Different vibrational bands were identified and percentage of β phase was determined by FTIR analysis. Optical properties like band gap, refractive index, optical activation energy have been determined. Surface morphology and compositions of pristine and gamma irradiated PVDF thin films were confirmed respectively, by SEM and Energy dispersive X-ray analysis. The comparison of the structural and optical optical properties of pristine PVDF polymer film has been made with those of the Gamma irradiated films.