Thick Polyvinylidene Fluoride Research Articles

Three-dimensional composite electrospun nanofibrous membrane by multi-jet electrospinning with sheath gas for high-efficiency antibiosis air filtration

Three-dimensional (3D) composite polyvinylidene fluoride (PVDF)/polyacrylonitrile (PAN) electrospun nanofibrous membranes combining both thick and thin nanofibers have been fabricated by the method of multi-jet electrospinning with sheath gas to realize high-efficiency air filtration under a low pressure drop. The thin PAN nanofibers form a dense membrane, with a strong capturing ability on the ultra-fine particles, while the thick PVDF nanofibers play a 3D supporting effect on the thin PAN nanofibers. In this case, the combination results in a fluffy membrane with higher porosity, which could achieve the airflow passing through the membrane without the air pressure drop. The effects of the composite manner of thick nanofibers and thin nanofibers are investigated, in order to optimize the air filtration performance of the 3D composite nanofibrous membrane. As a result, the maximum quality factor for air filtration could reach up to 0.398 Pa−1. The particle-fiber interaction model was used to simulate the air filtration process as well, and the simulation results were fairly consistent with the experimental results, providing a guidance method for the optimization of composite nanofibrous membrane for high-efficiency air filtration. More interestingly, a cationic poly[2-(N,N-dimethyl amino) ethyl methacrylate] (PDMAEMA) was added in the PVDF solution to obtain a composite air filtration membrane with excellent antibiosis performance, which achieved the highest inhibition rate of approximately 90%. In short, this work provides an effective way to promote antibiosis air filtration performance by using an electrospun nanofibrous membrane, and might also effectively accelerate the biological protection application of current air filtration membranes.

A Focused Optically Transparent PVDF Transducer for Photoacoustic Microscopy

This papers reports the development of a novel focused optically-transparent ultrasound transducer with a wide bandwidth for photoacoustic microscopy (PAM). It consists of a 9- $\mu \text{m}$ -thick polyvinylidene fluoride (PVDF) film coated with Indium-tin oxide (ITO) and metal electrodes, which is laminated onto a concave glass lens. A new fabrication process was developed to ensure the formation of continuous ITO coatings on the curved transducer surface without fracturing. Both optical and acoustic experiments were conducted to characterize the performance of the transducer. The testing results show that the transducer has an optical transmittance of 60% (@ 532 nm), an acoustic center frequency of 24 MHz, and an acoustic bandwidth of 26 MHz. With an acoustic numerical aperture (NA) of 0.23, it provides a 0.13-mm acoustic focal spot and a 1.6-mm focal depth. For demonstration, a PAM setup was built for conducting PA imaging on different targets. In contrast to existing PAM systems, the use of the optically-transparent focused transducer allows a simpler and more compact configuration and easier alignment of the optical excitation and acoustic focal points. Such features are especially useful for the development of miniaturized PAM systems for handheld, wearable and even endoscopic applications.

In Situ Quantification of the Swelling of Graphite Composite Electrodes by Scanning Electrochemical Microscopy

The physical swelling of uncharged graphite composite electrodes due to electrolyte-binder interactions is investigated by scanning electrochemical microscopy (SECM) using 2,5-di-tert-butyl-1,4,-dimethoxybenzene as a redox mediator. A series of approach curves at the same location is conducted in order to quantify in situ and locally the physical swelling. The film thickness change δ film amounted to 9.1 μm on average for a 80 μm thick uncharged graphite composite electrode in LP40 electrolyte between 1.1 and 5.9 h. Curves of δ film vs. t usually reach a saturation within 12 h. The swelling ratio χ varies from 0.3% to 17.6% for uncharged graphite composite electrodes from the same batch in the same electrolyte. In contrast, the 8 μm thick polyvinylidene fluoride (PVDF) model sample swelled by χ = 99%. Approach curves demonstrate that swelling of the PVDF is the main cause for the physical swelling of uncharged graphite composite electrodes. Both PVDF model sample and uncharged graphite composite electrodes show locally different swelling ratios by SECM imaging. Based on these results a swelling model is proposed, where the uncharged graphite composite electrode swells physically on average by at least χ = 11% and the local topography is changing during swelling.

Vibration Mode-Dependent Energy Harvesting Characteristics of Magnetoelectric Composite Cantilevers

In this letter, we simply fabricate magnetoelectric (ME) composite cantilevers through bonding 30 μm thick Metglas and 52 μm thick polyvinylidene fluoride with an epoxy, and report resonant and energy harvesting characteristics with applied DC and AC magnetic fields ranging from 0 to 3 kA/m. Generated ME voltages from a 40 mm long, 12.3 mm wide, and 112 μm thick cantilever are measured and analyzed as a function of the applied magnetic field. In our experiment, higher ME voltages are generated when both the DC and AC magnetic fields are present and the ME voltages around the second flexural bending mode are approximately five times higher than those around the first mode. The higher ME voltage with the second mode is also confirmed with finite element analysis.

Broadband dielectric relaxation spectra of PVDF copolymer films

The dielectric permittivity of polyvinylidenefluoride (PVDF) and its copolymer containing trifluoroethylene (TrFE) is investigated in the frequency domain with variation of the temperature. A pronounced relaxational behavior is observed in the films at high frequencies. In thick commercial PVDF films a Vogel-Tamman-Fulcher (VTF) dependence of the relaxation time versus the temperature is found yielding a thermally activated process. In thin Langmuir-Blodgett P(VDF-TrFE) films the relaxation times determined from the dielectric permittivity in the frequency range are not consistent with the relaxation times determined from the temperature range. The data extracted from the temperature domain also here show a VTF behavior. Nevertheless, the relaxation times calculated from the frequency domain cannot be fitted with the VTF formula. Thus, the time-temperature superposition principle is not valid in the thin films.

Design, Fabrication of Line-focus Lens-less Polyvinylidene Fluoride Transducers and Applications on Measuring Surface Acoustic Waves with V(f, z) Analytical Method

Line-focus-beam ultrasonic material property characterization system enables accurate measurements on the phase velocities of leaky surface acoustic waves(LSAW) or rayleigh-type leaky surface acoustic waves.But in view of the lens of the conventional acoustic microscopy is expensive and difficult to produce,new method of design and fabrication of two line-focus lens-less polyvinylidene fluoride(PVDF) transducers is presented for acoustic microscopy system utilizing a 40 ?m thick piezoelectric PVDF film to construct the active element.The transducers introduced here centre around 5 MHz and 11 MHz respectively.Different from high cost of equipments and measuring the oscillation period on an operation of tone-burst mode,an improved V(f,z) method based on tow-dimensional Fourier transform is developed for the broadband measurements.A defocusing system for measuring surface acoustic waves is established and a non-contact immersion experiment of a polished fused quartz is carried out.The LSAW velocity extracted by V(f,z) analytical technique is compared with the theoretical one and they both are in good agreement.

Measuring bending stress on an ice/aluminium composite beam interface using an embedded piezoelectric PVDF (polyvinylidene-fluoride) film sensor

This paper deals with the measurement of the interfacial stress of a composite beam using an embedded piezoelectric PVDF (polyvinylidene-fluoride) strip. The composite beam consists of an aluminium beam on which an ice layer is deposited. The ice layer is formed from sprayed supercooled water droplets, simulating atmospheric icing of exposed structures. The beam is submitted to sinusoidal stress at the ice/aluminium interface by an electromagnetic shaker clamped to one end of the beam. The use of PVDF strips as a stress measurement apparatus was validated by numerical simulations. Experimental results demonstrated that a 25 µm thick PVDF strip used as an embedded interfacial stress sensor is sensitive only to bending stress and does not allow the measurement of interfacial shear stress. That way, the adhesion strength of an atmospheric ice layer cannot be measured. However, the use of embedded PVDF strips provides a new and original method for measuring the ice adhesion strength on different substrates, due to high repeatability and sensitivity of the measurement.

Femtosecond laser micromachining of polyvinylidene fluoride (PVDF) based piezo films

Piezoelectric polymers have been known to exist for more than 40 years, but in recent years they have been recognized as smart materials for the fabrication of microsensors, microactuators and other micro-electro-mechanical systems (MEMS). In this work, femtosecond laser micromachining of a polyvinylidene fluoride (PVDF) film, coated with NiCu on both sides, has been studied to understand selective patterning mechanisms of NiCu layers and ablation characteristics of PVDF films. A detailed characterization of morphological changes of the laser-irradiated areas has been investigated using scanning electron microscopy. Through morphological analysis, the multiple shot damage thresholds of a 28 µm thick PVDF film and 40 nm thick NiCu layer have been determined. Surface morphology examination indicates that NiCu layers are removed from the PVDF film through a sequence of cracking–peeling off-curling. In addition, the NiCu layer on the rear side was also removed by the partially transmitted laser energy. The PVDF film was removed in forms of bundles of filaments and solid fragments by a combination of pure ablation and explosive removal of material by bursting of bubbles; the role of the explosive removal becomes more dominant with the increase of laser fluence. Optimal process conditions for cutting of the PVDF film and patterning of the NiCu coating without damaging the PVDF polymer have been established and applied to fabricate a vibration microsensor prototype that shows significant potential in using PVDF-based functional microdevices for telecommunications, transportation and biomedical applications.

An experimental teletaction system for sensing and teleperception of human pulse

Teletaction is the sensing of a remote object by transmitting tactile information from a remote tactile sensor to an operator’s skin through tactile interface devices. A tactile interface is used to reproduce the information such as force (static and dynamic), texture, roughness, temperature, and shape. This paper describes the design, modeling, simulation, fabrication and testing of an experimental teletaction system to detect and reproduce the human pulse. It consists of three major components: pulse sensing system, pulse teleperception system and the data processing system. Two 25 μm thick polyvinylidene fluoride (PVDF) film sensors had been fabricated and calibrated for pulse detection and force measurement. A computer controlled force feedback system was developed for achieving pulse teleperception. The simulation and experimental results demonstrated the close matching between the sensed and perceived pulse. A psychophysics test showed that the fingertip feeling at the pulse teleperception end closely matched the feeling of touching a human pulse directly.

Design and fabrication of a 40-MHz annular array transducer

This paper investigates the feasibility of fabricating a five-ring, focused annular array transducer operating at 40 MHz. The active piezoelectric material of the transducer was a 9-microm thick polyvinylidene fluoride (PVDF) film. One side of the PVDF was metallized with gold and forms the ground plane of the transducer. The array pattern of the transducer and electrical traces to each annulus were formed on a copper-clad polyimide film. The PVDF and polyimide were bonded with a thin layer of epoxy, pressed into a spherically curved shape, then back filled with epoxy. A five-ring transducer with equal area elements and 100-microm kerfs between annuli was fabricated and tested. The transducer had a total aperture of 6 mm and a geometric focus of 12 mm. The pulse/echo response from a quartz plate located at the geometric focus, two-way insertion loss (IL), complex impedance, electrical crosstalk, and lateral beamwidth all were measured for each annulus. The complex impedance data from each element were used to perform electrical matching, and the measurements were repeated. After impedance matching; fc approximately equal to 36 MHz and -6-dB bandwidths ranged from 31 to 39%. The ILs for the matched annuli ranged from -28 to -38 dB.

Focusing performance of epoxy- and air-backed polyvinylidene fluoride Fresnel zone plates

In this paper the focusing performance of epoxy- and air-backed polyvinylidene fluoride (PVDF) Fresnel zone plate (FZP) transducers are investigated. This work is motivated by an interest in constructing integrated, polymer FZP transducers for imaging applications on a silicon substrate using integrated circuit fabrication techniques. While it is well known that using an impedance matched backing facilitates broadband transducer performance, to the authors’ knowledge the effects on the acoustic beam patterns of polymer FZPs have not been reported. A model for the ideal zone plate is also presented, and a general pressure transfer function is derived in order to generate theoretical beam profiles. Epoxy- and air-backed, three-zone FZPs were constructed using 111 μm thick PVDF. The devices were tested in an ultrasound scanning tank while operating at 5 MHz. The focal plane profile for the epoxy-backed device was in good agreement with that predicted for the ideal zone plate. In addition, the epoxy backing was found to dampen higher order radial mode excitation. The radial modes are believed to be responsible for the narrow main lobe and elevated side lobes found in the focal plane profile for the air-backed device.

Polyvinylidene fluoride (PVDF) elastic, piezoelectric, pyroelectric, and dielectric coefficients and their non-linearities

This work presents a set of material coefficients for a uniaxially 4x stretched 25 μm thick PVDF film. Calculations of the type d = se, where d and e are piezoelectric coefficients and s is the compliance, are used to compare independent measurements of piezoelectric coefficients. The piezoelectric coefficient e333 is found to be constant to strains over 25%. The non-linearities in the dijk coefficients are due to non-linearities in the elastic moduli that are typical of polymers above their glass transition temperature, i.e. strain rate dependent, viscoelastic, temperature dependent.

Cosmic dust investigations

Our earlier laboratory investigations of polyvinylidene fluoride (PVDF) dust detectors — which we developed for cometary and interplanetary dust studies — were limited to dust mass and velocity ranges of 10−13–10−10 g, and 1–12 km/s, respectively. These measurements established a unique dependence of detector signal amplitude on particle mass and velocity, namely mavb, where a = 1.3 and b = 3.0, respectively, for particles that stopped in thick (e.g. 28 μm) PVDF detectors. We have now extended our dust accelerator investigations to higher dust masses (∼10−9–10−6 g) in the same velocity range so that the dust particles fully penetrate a wide range of PVDF detector thicknesses, including the 28 μm thick detectors we employed for our Comet Halley dust coma measurements. For these penetrating particles we show that the values of a and b are 0.90±0.05 and 1.05±0.05, respectively. We also report, for the first time, the measurement of crater sizes in the detector for these penetrating particles and the correlated detector signal amplitude and pulse shape. From these simultaneous measurements we have proved that the basic response mechanism is irreversible depolarization in the PVDF detector, as we had proposed earlier. These new laboratory investigations also were arranged to determine particle fragmentation and particle (and fragment) velocity after detector penetration by time-of-flight measurements. We discuss the bearing of these studies on our interpretation of measured Comet Halley dust coma mass spectra.

Cosmic dust investigations

Abstract A series of experiments have been completed using accelerator dust particles in the mass range ∼10−9–10−6 g and velocity range ∼2–12 km/s to measure the velocity loss and degree of fragmentation for dust particles penetrating 6 and 28 μm thick polyvinylidene fluoride (PVDF) dust detectors. These measurements prove that even for a ratio of PVDF foil thickness to particle diameter as large as 0.6, the velocity loss and fragmentation is far less than expected from earlier reports in the literature. For example, for 28 μm thick PVDF foils the velocity loss is ∼ 20%, the fraction of particles suffering serious fragmentation is ∼ 50% and the angular dispersion or “spray angle” of the fragments from the incident particle direction is ≤ 3°. For 6 μm thick foils the velocity loss is ≤ 5%. These experiments are based on an extension of our earlier work which showed that two PVDF foils spaced a given distance apart could provide accurate time-of-flight (TOF) information due to the fast pulse rise time of PVDF detector response. We also report on our present state of development of PVDF position-sensing detectors which identify the x, y coordinates of particle impact, using detector and electronic pulse techniques adapted from our semiconductor position-sensing cosmic-ray detectors. Typical position errors of ∼ 1 mm are readily achieved. Finally, we have combined the above developments into a dust-particle telescope which accurately (∼ 1° angular accuracy) measures the trajectory of the incident particle as well as its mass and incident velocity, irrespective of whether it is a charged or neutral particle. We discuss how this practical dust telescope can be combined with dust capture cells for space flight and later recovery for laboratory determination of elemental and isotopic composition of captured dust. We also describe a simpler trajectory array based on discrete mosaics of thin detectors which would measure trajectories with a mean angular error of ∼ 4°. We discuss the application of these instruments for distinguishing between interplanetary dust of cometary and asteroidal origin, and for measurements on a space station, from near-Earth trapped dust of artificial origin.

Tensile and bending piezoelectricity of single-film PVDF monomorphs and bimorphs

Films of 22 mu m thick polyvinylidenefluoride (PVDF) are poled with partially penetrating monoenergetic electron beams, resulting in monomorph polarizations (zero polarization over part of the sample thickness and large polarization over the remaining part) or bimorph polarizations (polarization changing its sign within the sample) with corresponding distributions of piezoelectricity. Experimental data for the tensile piezoelectric strain constant d/sub 31/ and the bending piezoelectric stress constant beta /sub 31/ with values as high as 13 pC/N and 110 nC/m, respectively, are reported. Both constants decrease with increasing energy of the electron beam. Upon annealing, d/sub 31/ drops initially more rapidly than beta /sub 31/, whereas the opposite is true at later times. The mechanical resonance frequencies and the mechanical deflection under DC bias are discussed. >

Multisensor piezoelectric polymer insole for pedobarography

Abstract Analysis of the pressure distribution under the foot in walking and standing patients (pedobarography) can be used to detect orthopaedic as well as neurological defects. Sensorized insoles are increasingly used, at least for research purposes, because of the inherent advantages they offer compared to the force platforms presently available. We have developed and evaluated a piezoelectric polymer insole for pedobarography, using a 200 μm thick polyvinylidene fluoride (PVDF) film which had been uniaxially stretched and poled in selected areas. Sixteen circular aluminium disks, 6 mm diameter each, deposited onto the film by vacuum evaporation, served as the electrodes for the pressure sensors and were located so as to optimize the response of the five metatarsal transducers, which are particularly critical for a correct gait analysis. The 16 sensors were disposed on the ideal loading surface, the response of which identified incorrect foot conformation on loading. Sixteen conductive tracks, one for each transducer, converged toward a side of the insole, where 16 contact pads were provided to bond connecting wires, leading to an electronic unit conveniently attached to the leg of the patient. A system was designed to allow data acquisition and processing by a Digital MINC 11 minicomputer. This sensorized insole is currently evaluated for gait analysis, and tests on a normal subject are reported.

A study of structural and electrical properties of stretched polyvinylidene fluoride films

X-ray diffraction profiles and piezoelectric strain coefficient d 31 of 50μm thick polyvinylidene fluoride (PVF 2) films, originally containing non-polar Form 2 type crystallites, have been studied after (i) corona poling (ii) uniaxial stretching and (iii) uniaxial stretching and corona poling. The results suggest that piezoelectricity in this material is of dipolar origin. Furthermore a study of the changes in birefringence and small angle light scattering (SALS) patterns have also been made in the present work. The results indicate that the major contribution to the observed orientation on uniaxial stretching originates from the crystalline region of the polymer.