Cellular Polypropylene Films Research Articles

Ultrathin, all-organic, fabric-based ferroelectret loudspeaker for wearable electronics.

All-organic, flexible, and body-compatible loudspeakers have become increasingly attractive for wearable electronics. Due to their remarkable piezoelectric response, ferroelectrets are suitable for loudspeakers. Two distinct kinds ofultrathin ferroelectrets, including cellular polypropylene films and expanded polytetrafluoroethylene (ePTFE) films, were combined with three different types of electrodes ((Poly(3,4-ethylenedioxythiophene):poly(styrene sulfonate) (PEDOT:PSS))-coated fabrics, PEDOT:PSS direct coating, and sputter-coated Au/Pd) for study regarding their frequency-dependent sound intensity and radiation directivity. Among the loudspeakers investigated, the all-fabric loudspeakers with ePTFE ferroelectret and PEDOT:PSS-coated spandex electrodes have a higher frequency dependency. Loudspeakers equipped with PEDOT:PSS-coated spandex electrodes are less angle dependent compared to other loudspeakers evaluated. Moreover, the soft loudspeaker constituted of an all-organic FEP(fluorinated-ethylene-propylene)-ePTFE-based ferroelectret and PEDOT:PSS-coated fabrics presented in this paper is easy to integrate with clothes and has a higher thermal stability. It is naturally compatible with the human body and a competitive candidate for future developments of all-organic loudspeakers for wearable electronic systems.

Tonometric Condition of Cellular Polypropylene Film Sensors in Measuring Arterial Pressure Waveform.

Tonometric continuous measurement of arterial pressure becomes feasible using a cellular polypropylene (Cellular PP) film sensor. A pulsatile arterial vascular phantom model was used to find the range of optimal tonometric conditions and the responsiveness to dynamic pressure changes. The optimal tonometric condition was assessed by the correlation coefficient between the hydraulic pressure and the Cellular PP output using two different types of tubes (the latex tube and the hydrogel tube) to simulate arteries. With a setting of the normal blood pressure range, the output of Cellular PP correlated strongly with the level of hydraulic pressure, 0.998 and 0.989 in the latex tube and the hydrogel tube, respectively. For maintaining the optimal tonometric condition, the depressed depths of the latex and the hydrogel tube were less than 1.2 and 0.6 mm, respectively. The phantom model also demonstrated that the Cellular PP sensor followed changes in a hydraulic pressure dynamically under the optimal tonometric conditions. The present results demonstrated the Cellular PP film sensor is applicable to the arterial tonometry in measuring the instantaneous blood pressure while the sensor is adjusted to maintain the minimal flatness of the underlying arterial wall.Clinical Relevance- To understand the physiological characteristics of blood pressure and arterial system, the instantaneous measurement of blood pressure is necessary. The present study suggests that Cellular PP films are applicable to peripheral arteries tonometrically to obtain simultaneously the respective blood pressure waveforms.

Charging of Piezoelectric Cellular Polypropylene Film by Means of a Series Dielectric Layer.

Piezoelectric polymer cellular films have been developed and improved in the past decades. These piezoelectric materials are based on the polarization of the internal cells by means of induced discharges in the gas inside the cells. Internal discharges are driven by an external applied electric field. With this polarization method, cellular polypropylene (PP) polymers exhibit a high piezoelectric coefficient d33 and have been investigated because of their low dielectric polarization, high resistivity, and flexibility. Charging polymers foams is normally obtained by applying a corona discharge to the surface with a single tip electrode-plane arrangement or a triode electrode, which consists of a tip electrode-plane structure with a controlled potential intermediate mesh. Corona charging allows the surface potential of the sample to rise without breakdown or surface flashover. A charging method has been developed without corona discharge, and this has provided good results. In our work, a method has been developed to polarize polypropylene foams by applying an insulated high-voltage electrode on the surface of the sample. The dielectric layer in series with the sample allows for a high internal electric field to be reached in the sample but avoids dielectric breakdown of the sample. The distribution of the electric field between the sample and the dielectric barrier has been calculated. Experimental results with three different electrodes present good outcome in agreement with the calculations. High d33 constants of about 880 pC/N have been obtained. Mapping of the d33 constant on the surface has also been carried out showing good homogeneity on the area under the electrode.

Cellular polypropylene electromechanical properties: exploring the nonlinear region

Many studies have been conducted in the last decades on cellular polypropylene (Cell-PP) films. Most of them focus on the optimisation of the material for sensor applications. Processed under Gas Diffusion Expansion (GDE), Cell-PP films show high piezoelectric activity and low stiffness/density, properties that make them ideal for sensors. GDE increases the height and decreases the length over height ratio (aspect ratio) of individual voids within the material. This change in void morphology, and eventually stiffness, results in a nonlinear piezoelectric response of these materials. In this study, a Cell-PP sample was tested under static, quasi-static and low-frequency compressive stress. The main aim is to evaluate its mechanical and piezoelectric properties in the nonlinear region of its response over strain. The load–deflection curves as well as the piezoelectric responses were obtained for stresses up to 270 kPa (engineering strain close to 0.26). It is shown that both the magnitude of the initial load and the strain rate have a critical effect on the creep/stress relaxation of the film and eventually on its piezoelectric response. Finally, it is shown that under dynamic conditions, and for the same engineering strain region, it is more relevant to present the piezoelectric response, in terms of strain rather than stress.

Fabrication, Structure Characterization, and Performance Testing of Piezoelectret-Film Sensors for Recording Body Motion

During muscle contractions, radial-force distributions are generated on muscle surfaces due to muscle-volume changes, from which the corresponding body motions can be recorded by means of so-called force myography (FMG). Piezo-or ferroelectrets are flexible piezoelectric materials with attractive materials and sensing properties. In addition to several other applications, they are suitable for detecting force variations by means of wearable devices. In this paper, we prepared piezoelectrets from cellular polypropylene films by optimizing the fabrication procedures, and developed an FMG-recording system based on piezoelectret sensors. Different hand and wrist movements were successfully detected on able-bodied subjects with the FMG system. The FMG patterns were evaluated and identified by means of linear discriminant analysis and artificial neural network algorithms, and average motion-classification accuracies of 96.1% and 94.8%, respectively, were obtained. This paper demonstrates the feasibility of using piezoelectret-film sensors for FMG and may thus lead to alternative methods for detecting body motion and to related applications, e.g., in biomedical engineering or structural-health monitoring.

Characterisation and optimisation of the d 33 coefficient of cellular PP films

Piezoelectric polymer cellular films have been studied since 1986. In recent years, they have improved the potential applications due to materials progress. Cellular polypropylene (PP) polymers are characterised by its high piezoelectric coefficient d 33 in comparison with other piezoelectric materials such as ceramics. One of the advantages of these films is their elasticity and flexibility. This study is based on the optimisation of the activation process and characterisation results of polymer cellular films. Samples were made of a commercially available PP film which was modified by a thermal biaxial stretching. Coefficient d 33 was measured by a quasi-static method and the frequency characterisation was made for validating all characterisations and their relations. Transducer coefficients around 700 pC/N were obtained with polymer cellular PP films with silver coating electrodes, activated by corona discharge. An approximation of the surface charge density in the polymer voids was obtained with a mathematical model. Thermal stability was checked by thermally stimulated discharge currents (TSDCs) showing a relationship between the TSDC and the coefficient d 33. Time stability was determined after 3000 h ageing, and finally was obtained a resonance frequency by interferometry measurement.

Effect of Biaxial Stretching Temperature and Gas Diffusion Expansion on Cellular Morphology of PP/CaCO3and PP/talc Cellular Films

To evaluate the impact of the stretching temperature on the morphology of cellular polypropylene (PP) films, a series of PP films filled with spherical particles of calcium carbonate (CaCO3) and platy talc particles with weight concentrations varying from 15 to 35 wt% were prepared by twin-screw extrusion then biaxially stretched at three temperatures (152, 155 and 158°C). Their corresponding cellular morphology was analyzed from scanning electron microscopy (SEM) micrographs. It was shown that film stretching at the lower temperature (152°C) led to delamination of PP/CaCO3or PP/talc interface, while it was not the case when these films were stretched at 158°C. It was also observed that, when films were stretched at 152°C, their corresponding developed cells had an average cell wall thickness t of around 3.6 μm, an average cell height b of around 13.6 μm, and an average cell aspect ratio a/b of around 5.6, where a (mm) is the average cell width. These values were within the aimed cell geometrical specifications needed for cellular piezoelectric films.

Magnetoelectric phenomena of insulator polymers after corona poling: Procedure and experiments

In this paper, cellular polypropylene (PP) and polyvinyl chloride (PVC) films have been negatively charged via a high voltage corona triode. In order to evaluate the ability of space charge retention in both dielectric materials, their decay tendencies of the surface potential after corona discharge have been analyzed and compared. Moreover, the magnetoelectric responses of cellular PP and PVC between, before and after corona discharge have also been discussed for the first time in this work. It indicates that cellular PP possesses better charge storage ability than PVC and their magnetoelectric coefficient and current can be increased after corona poling. For both electrets, the bias magnetic field and frequency dependences of the magnetically induced current revealed that it has an inductive origin via Lenz and Faraday Laws, as well as magnetoelectric effect.

Piezoelectricity and ferroelectricity of cellular polypropylene electrets films characterized by piezoresponse force microscopy

Cellular electrets polymer is a new ferroelectret material exhibiting large piezoelectricity and has attracted considerable attentions in researches and industries. Property characterization is very important for this material and current investigations are mostly on macroscopic properties. In this work, we conduct nanoscale piezoelectric and ferroelectric characterizations of cellular polypropylene (PP) films using piezoresponse force microscopy (PFM). First, both the single-frequency PFM and dual-frequency resonance-tracking PFM testings were conducted on the cellular PP film. The localized piezoelectric constant d33 is estimated to be 7–11pC/N by correcting the resonance magnification with quality factor and it is about one order lower than the macroscopic value. Next, using the switching spectroscopy PFM (SS-PFM), we studied polarization switching behavior of the cellular PP films. Results show that it exhibits the typical ferroelectric-like phase hysteresis loops and butterfly-shaped amplitude loops, which is similar to that of a poly(vinylidene fluoride) (PVDF) ferroelectric polymer film. However, both the phase and amplitude loops of the PP film are intensively asymmetric, which is thought to be caused by the nonzero remnant polarization after poling. Then, the D-E hysteresis loops of both the cellular PP film and PVDF film were measured by using the same wave form as that used in the SS-PFM, and the results show significant differences. Finally, we suggest that the ferroelectric-like behavior of cellular electrets films should be distinguished from that of typical ferroelectrics, both macroscopically and microscopically.

Significantly improved piezoelectric thermal stability of cellular polypropylene films by high pressure fluorination and post-treatments

Cellular polypropylene (PP) films were fluorinated under a high pressure of 13 bar of the F2/N2 mixture and were post-treated by nitrous oxide and isothermal crystallization. The fluorinated and post-treated PP films after being expanded and corona charged exhibit a significantly improved piezoelectric thermal stability. After annealing at 70 °C for 151 h or at 90 °C for 224 h, the piezoelectric d33 value of the fluorinated and post-treated piezoelectric sample still retains 58% or 45% of its initial d33 value, while the corresponding value of the virgin piezoelectric sample has decreased to 29% or 15% of the initial value. Chemical composition analysis of the cross section of the fluorinated and post-treated film by energy-dispersive x-ray spectroscopy indicates that the internal layers have been fluorinated, in spite of a lower degree of fluorination compared with the fluorinated surface layer. Short-circuit and open-circuit TSD current measurements reveal that the fluorinated internal layers, like the fluorinated surface layer, also have very deep charge traps, although there probably is a difference in density of the deep traps between them. The deeply trapped charge on the internal layers of the fluorinated and post-treated piezoelectric sample is responsible for its significantly improved piezoelectric thermal stability.

Finite element modeling of temporal evolution of the quasi-piezoelectric d33 coefficient of cellular piezoelectret Polypropylene film

Voided charged polymer film, also called cellular piezoelectret, shows quasi-piezoelectric response macroscopically. The piezoelectric d33 coefficient can be as large as that of Lead Zirconate Titanate ceramics. Because of the inherent viscosity of polymer, the piezoelectric d33 coefficient of cellular piezoelectret is time-dependent. In this article, the effective piezoelectric d33 coefficient of cellular piezoelectret is simulated by means of the finite element method. The cellular piezoelectret film is regarded as the periodic composite material, where the closed flat voids are scattered periodically. The hollow oblate tetrakaidecahedron is used to model the void in a representative volume element. The polymer is taken to be isotropic and viscoelastic. The electrostatic field and the deformation are numerically solved. The piezoelectric d33 coefficient is obtained in terms of the converse piezoelectric effect. Qualitative analysis of temporal evolution of the effective piezoelectric d33 coefficient is presented with respect to various parameters, including the microstructural void parameters, charge density as well as the viscoelastic parameters. Finally, this finite element model is used to simulate the experimental results of the effective piezoelectric d33 coefficients of cellular piezoelectret Polypropylene film published in literature.

Influence of corona poling on the electrostrictive behavior of cellular polypropylene films

In this study, a quadratic electric field dependence of the strain on the −30kV corona-charged cellular polypropylene (PP) was observed, which was found to mainly originate from electrostriction after evaluation of Maxwell stress effect. The −30kV-charged sample presented a maximal strain twice as high as that of the non-charged sample for the same applied electric field. Both the dielectric constant and apparent electrostrictive coefficient of cellular PP were also seen to increase after corona poling treatment. We investigated such higher electrostrictive response from the behavior of surface potential decay, the TSC current and the thermal properties. From TSC current curves, it was found that a certain amount of charges became injected into the film. And then, the total charge within the PP was calculated by integrating the TSC current, which provided a convincing evidence for enhanced electrostriction of studied polymer. In addition, from DSC analysis, it was demonstrated that the corona treatment increased the crystallinity of the cellular PP, which was also highly desirable for enhancing the electrostrictive response. Increase in the permittivity and strain was explained experimentally and mathematically by using the space-charge theory and a simplified model.

Time dependence of piezoelectric d33 coefficient of cellular ferroelectret polypropylene film

Due to the inherent viscosity of polymer, piezoelectric response in the thickness direction (d33) of cellular ferroelectret films usually depends on the time of measurement. In this letter, the micromechanical theory of viscoelastic composite was extended to predict the time dependence of the overall piezoelectric d33 coefficient of voided charged polymer foam. Experiments were carried out to find the time spectra of piezoelectric d33 coefficient of voided charged polypropylene film. Theoretical simulation agrees well with experiment data.

Fluorinated cellular polypropylene films with time-invariant excellent surface electret properties by post-treatments

In this work, to improve the electret properties of cellular polypropylene films, they were fluorinated and post-treated with nitrous oxide and by isothermal crystallization. Surface electret properties of the samples were investigated by thermally stimulated discharge current measurements, and their compositions and structures were analysed by attenuated total reflection infrared spectroscopy and wide angle x-ray diffraction, respectively. Time-dependent deterioration of surface electret properties was observed for the fluorinated samples without the nitrous oxide post-treatment. However, deterioration did not occur for the fluorinated samples post-treated with nitrous oxide, and time-invariant excellent surface electret properties or deep surface charge traps were obtained by the combined post-treatments of the fluorinated samples with nitrous oxide and by isothermal crystallization. Based on the analyses of composition and structure of the treated samples, the deterioration was clarified to be due to a trace of oxygen in the reactive mixture, which led to the formation of peroxy radicals in the fluorinated surface layer. The time invariability of surface electret properties was owing to the rapid termination of the peroxy radicals by nitrous oxide. And the deep surface charge traps resulted from the isothermal crystallization treatment which led to an increase in the efficient charging interface between the crystallite and amorphous region and its property change.

Significantly improved charge stability of cellular polypropylene films by fluorination and subsequent annealing

An extremely effective and practical approach is proposed and used for improving charge stability of cellular polypropylene (PP) films. The approach is composed of fluorination and subsequent annealing of the PP films. Surface charge stability is significantly improved by the approach, as revealed by the measurements of open-circuit thermally stimulated discharge (TSD) current, charge TSD and isothermal charge decay at 90 °C. As an example, after 7 h at 90 °C, the amount of surface charge of the treated PP film by the approach still remains about 72% of its initial value, while the corresponding value is only 27% for the virgin PP film. The improvement of charge stability is attributed to chemical composition change and structure modification of the PP films due to the fluorination and annealing treatments, as indicated by attenuated total reflection infrared analyses and wide angle X-ray diffraction measurements. The treatments eliminate the shallow charge traps almost completely and produce much deeper ones.

Significant influence of isothermal crystallization conditions on charge stability of fluorinated cellular polypropylene films

On the basis of the measurement of open-circuit thermally stimulated discharge current and isothermal charge decay, the influence of isothermal crystallization conditions on charge stability of fluorinated cellular polypropylene (PP) film was systematically investigated. The results indicate that the time and temperature of isothermal crystallization of the fluorinated PP film have significant influences on its charge trap structure and charge stability. Charge traps and charge stability can be significantly deepened or improved even by isothermal crystallization at 90 ℃ for 0.5 h. And with the increase of crystallization temperature and time, charge traps are deepened further, presenting further improved charge stability, as observed in the case of isothermal crystallization at 130 ℃ for more than 2 h. Attenuated total reflection infrared analysis and wide angle X-ray diffraction measurement indicate that the improvement of charge stability results from the changes in chemical composition and structure of the PP film.

Piezoelectricity of Cellular Polypropylene Films Expanded by a Dichlormethane Evaporation Process

In this paper, a new method for expanding stretched cellular isotactic polypropylene (i-PP) is presented. Firstly, the cellular i-PP films with small cavities were saturated with dichlormethane (DCM) and then, the DCM was abruptly evaporated at elevated temperature to create large-cavity cellular films. Finally, the expanded film samples were quenched in liquid nitrogen to fix the cell structure. After corona charging, the piezoelectric d33-coefficients of such expanded films were measured by quasistatic and interferometric methods. The results show that the highest values of the quasistatic piezoelectric d33-coefficient of up to 2500 pC/N at low frequencies were observed on samples twice expanded with DCM. A high thermal stability of the piezoelectric activity was also observed. The high d33-values are due to the air escape sideways out of the large voids of the cellular films.

Significantly improved charge deposit properties of the cellular polypropylene film modified by gaseous fluorine

Fluorination modification and electret properties were studied on the cellular polypropylene (PP) films modified by gaseous fluorine by attenuated total reflection (ATR) infrared analysis and the measurment of open-circuit thermally stimulated discharge (TSD) current，in situ TSD charge and isothermal charge decay. The results showed that the PP cellular films，especially the preoxidized ones，can be effectively fluorinated by gaseous fluorine，and that charge storage thermal stability of the fluorinated films，especially the fluorinated preoxidized ones，is significantly improved compared with that of the virginal PP film，even under conditions of negative pressure with a low partial pressure of gaseous fluorine，low reaction temperature about 60℃ and short reaction time of 15min.

Piezoelectric activity and thermal stability of cellular fluorocarbon films

Two methods for preparing piezoelectric fluorocarbon polymers with cellular structure (ferroelectrets) are introduced. Piezoelectric activity of laminated fluorocarbon films made from polytetrafluoroethylene (PTFE) and fluoroethylenepropylene (FEP) is characterized by quasistatic measurement of piezoelectric d33 coefficient, while the thermal stability of d33 is investigated by their isothermal decay. The results show that the quasistatic d33-coefficients between 500 and 2200 pC/N are obtained for such fluorocarbon films; d33-coefficients are relatively independent of the static pressure in the range of 20 kPa; comparing to cellular polypropylene (PP) film the new fluorocarbon films show not only higher values of d33, but also much better thermal stability; moreover, thermal stability of the fluorocarbon film can be further improved by the process of pre-ageing.

Piezoelectricity of single-and multi-layer cellular polypropylene film electrets

The piezoelectricity of a pressure-treated cellular polypropylene (PP) (commercially available, trade name PQ50) film electret was studied by the measurement of direct-and inverse-piezoelectric d 33 coefficient. The sample expanded with optimal parameters has a quasi-static piezoelectric d 33 coefficient of more than 600 pC/N, which is about 40 times as high as that of polyvinylidene fluoride (PVDF). In addition, the hybrid multi-layer system, which properly combines single-layer cellular PP film electrets, shows a quasi-static piezoelectric sensitivity of as high as 2010 pC/N. This is around three times higher than that of well-known lead zirconate titanate (PZT) ceramics. The results are theoretically and technically helpful to promote the application of cellular PP film electrets.