Piezoelectric Polyethylene Terephthalate (PETP) Foams – Specifically Designed and Prepared Ferroelectret Films

Abstract

Detailed knowledge about the essential properties required for piezoelectric polymer-electret foams (or ferroelectrets) [1,2] is exploited in the design and the preparation of cellular polyethylene terephthalate (PETP) electret films with piezoelectric properties. The relevant relationships between structural, elastic and piezoelectric properties had been established previously for polypropylene (PP) ferroelectrets and could be successfully employed in the present demonstration of the novel ferroelectret polymer PETP.The piezoelectric properties of ferroelectrets are based on the quasi-permanent internal electric charging of the cellular structure with charge layers of opposite polarity on void surfaces that face each other. Due to the bipolar charging, the centrosymmetry of the polymer foam is broken and macroscopic electric dipoles are formed. In addition, the voids must deform and the effective dipole moments change under mechanical stress. So far, because of the availability of highquality cellular PP films, PP has been the ºworkhorseº of ferroelectret research and development.In PP, the combination of a relatively soft structure (low elastic stiffness) with internally trapped bipolar charges yields a high piezoelectric coefficient in the film-thickness direction. Typically, PP ferroelectret films show direct and inverse piezoelectric d 33 coefficients of several hundred pC/N (or pm/V). For cellular PP films, the preparation, the optimization of the cellular structure by means of controlled inflation, the electrical charging through internal micro-plasma discharges, and the charge storage in deep traps are well understood. [3±10] Furthermore, piezoelectric cellular PP films are already implemented in several applications. [2] There is, however, an essential requirement that PP ferroelectrets do not meet: The thermal stability of the currently available piezoelectric cellular PP films is limited to around 60 to 70 C, depending on the chosen annealing treatment. [11,12] Here, more stable materials are clearly needed for most applications. Furthermore, the ferroelectret concept should be confirmed with independently developed polymer foams that exhibit similar piezoelectric behavior.Here, we investigate PETP films that are already produced in industry on a large scale and that offer chances to prepare ferroelectret films of high quality and to achieve slightly better thermal (and temporal) stabilities than the existing PP ferroelectrets.Recently, the dependence of the piezoelectric response on the elastic stiffness and on the void structure was established for cellular PP films. [7,13] In summary, the inversely U-shaped behavior schematically shown in Figure 1 is observed. Films with small (or low) voids are relatively stiff and therefore show only low piezoelectricity. The controlled increase of the void height by means of inflation [7±9] decreases the elastic stiffness and increases the piezoelectric activity until an optimal foam structure is obtained. For cellular PP films, a minimal elastic COMMUNICATI...

Template-based fluoroethylenepropylene piezoelectrets with tubular channels for transducer applications

“…9 Consequently, cellular-foam ferroelectrets with higher thermal stability have been developed from polyethylene terephthalate ͑PET͒ and polyethylene naphthalate ͑PEN͒ by means of physical foaming with supercritical carbon dioxide. 10,11 The polyester ferroelectrets exhibit slightly higher thermal and long-term stabilities that are, however, still not sufficient for many practical applications.…”

Optimized Preparation of Elastically Soft, Highly Piezoelectric, Cellular Ferroelectrets from Nonvoided Poly(ethylene Terephthalate) Films

“…Figure 1 demonstrates that the sample densities cover a relatively broad range, which includes the values previously found to be optimal for ferroelectrets prepared from commercial PETP foams. [7] The microcellular films with the lowest density of approximately 0.32 g cm -3 have also been used in our subsequent stretching experiments. Considering the still rather high density after a CO 2 treatment of only 10 min, significant voiding cannot be expected in this case.…”

“…The success of the present optimized sequence of preparation steps can be seen from the fact that the highest piezoelectric activity and the lowest elastic stiffness of the PETP ferroelectrets that we had prepared earlier by means of structural optimization of extruded, filler-loaded films were 23 pC N -1 and 7.2 MPa, respectively. [7] Now, we have prepared PETP ferroelectrets that are more than 20 times softer and thus also exhibit approximately 20 times higher piezoelectric activities. One reason for the very low elastic moduli of the present PETP films is probably a partial microporation of the cell walls during processing so that the resulting foam must be considered as partially open-porous.…”

“…[6] Consequently, several novel or improved ferroelectret materials have been proposed and investigated during the last few years. Among the more promising ferroelectret polymers are suitably foamed poly(ethylene terephthalate) (PETP) films [7,8] as well as blends and copolymers of cyclo-olefin polymers (COPs). [9,10] In this context, we have recently demonstrated that cellular ferroelectret films may be prepared from commercially available PETP foams and that the fundamental concepts developed on cellular PP ferroelectrets [4] may be used as guidelines for the optimization of cellular PETP as well.…”

“…[9,10] In this context, we have recently demonstrated that cellular ferroelectret films may be prepared from commercially available PETP foams and that the fundamental concepts developed on cellular PP ferroelectrets [4] may be used as guidelines for the optimization of cellular PETP as well. [7] However, apart from the first successful demonstration of new ferroelectret materials, the recently reported preparation procedures for PETP [7,8] and for COPs or cyclo-olefin copolymers (COCs) [9,10] have two major disadvantages: i) The proposed routes are probably not very suitable for industrial processing because of the fine-tuned material combinations required for the COC and COP foams and the well-controlled expansion steps necessary for preparing ferroelectrets from commercial PETP foams, Electrically charged cellular polymer films can exhibit very high piezoelectric activity and are therefore more and more often employed in advanced electromechanical and electro-acoustical transducers. In this paper, we report an optimized sequence of steps for preparing such ferroelectrets from commercial nonvoided poly(ethylene terephthalate) (PETP) films by means of foaming with CO 2 , biaxial mechanical stretching, controlled void inflation, and bipolar electric charging.…”