An approach for creating complex structures with embedded actuation in planar manufacturing steps is presented. Self-organization and energy minimization are central to this approach, illustrated with a model based on minimization of the hyperelastic free energy strain function of a stretched elastomer and the bending elastic energy of a plastic frame. A tulip-shaped gripper structure illustrates the technological potential of the approach. Advantages are simplicity of manufacture, complexity of final structures, and the ease with which any electroactive material can be exploited as means of actuation.

When exposed to sufficiently high electric fields, polymer-foam electret materials with closed cells exhibit ferroelectric-like behavior and may therefore be called ferroelectrets. In cellular ferroelectrets, the influence of the cell size and shape distributions on the application-relevant properties is not yet understood. Therefore, controlled inflation experiments were carried out on cellular polypropylene films, and the resulting elastical and electromechanical parameters were determined. The elastic modulus in the thickness direction shows a minimum with a corresponding maximum in the electromechanical transducer coefficient. The resonance frequency shifts as a function of the elastic modulus and the relative density of the inflated cellular films. Therefore, the transducer properties of cellular ferroelectrets can be optimized by means of controlled inflation.

Polarization switching at the nanoscale in ferroelectric copolymer thin films Appl. Phys. Lett. 99, 142904 (2011) Tunable temperature dependence of electrocaloric effect in ferroelectric relaxor poly(vinylidene fluoridetrifluoroethylene-chlorofluoroethylene terpolymer Appl. Phys. Lett. 99, 052907 (2011) Beneficial and detrimental fatigue effects of dielectric barrier discharges on the piezoelectricity of polypropylene ferroelectrets J. Appl. Phys. 110, 024108 (2011) Energy harvesting by nonlinear capacitance variation for a relaxor ferroelectric poly(vinylidene fluoridetrifluoroethylene-chlorofluoroethylene) terpolymer Appl. Phys. Lett. 98, 222901 (2011) Photocrosslinking of ferroelectric polymers and its application in three-dimensional memory arrays Appl. Phys. Lett. 98, 183302 (2011) Additional information on J. Appl. Phys. We describe the concept, the fabrication, and the most relevant properties of a piezoelectric-polymer system: Two fluoroethylenepropylene ͑FEP͒ films with good electret properties are laminated around a specifically designed and prepared polytetrafluoroethylene ͑PTFE͒ template at 300°C. After removing the PTFE template, a two-layer FEP film with open tubular channels is obtained. For electric charging, the two-layer FEP system is subjected to a high electric field. The resulting dielectric barrier discharges inside the tubular channels yield a ferroelectret with high piezoelectricity. d 33 coefficients of up to 160 pC/N have already been achieved on the ferroelectret films. After charging at suitable elevated temperatures, the piezoelectricity is stable at temperatures of at least 130°C. Advantages of the transducer films include ease of fabrication at laboratory or industrial scales, a wide range of possible geometrical and processing parameters, straightforward control of the uniformity of the polymer system, flexibility, and versatility of the soft ferroelectrets, and a large potential for device applications e.g., in the areas of biomedicine, communications, production engineering, sensor systems, environmental monitoring, etc.

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...

ABSTRACT:The influence of the solvent-evaporation rate on the formation of a and b crystalline phases in solution-cast poly(vinylidene fluoride) (PVDF) films was systematically investigated. Films were crystallized from PVDF/N,N-dimethylformamide solutions with concentrations of 2.5, 5.0, 10, and 20 wt % at different temperatures. During crystallization, the solvent evaporation rate was monitored in situ by means of a semianalytic balance. With this system, it was possible to determine the evaporation rate for different concentrations and temperatures of the solution under specific ambient conditions (pressure, temperature, and humidity). Fourier-Transform InfraRed spectroscopy with Attenuated Total Reflectance revealed the b-phase content in the PVDF films and its dependence on previous evaporation rates. Based on the relation between the evaporation rate and the PVDF phase composition, a consistent explanation for the different amounts of b phase observed at the upper and lower sample surfaces is achieved. Furthermore, the role of the sample thickness has also been studied. The experimental results show that not only the temperature but also the evaporation rate have to be controlled to obtain the desired crystalline phases in solution-cast PVDF films.

The zeta potential of the motile spores of the green alga (seaweed) Ulva linza was quantified by video microscopy in combination with optical tweezers and determined to be -19.3+/-1.1 mV. The electrostatic component involved in the settlement and adhesion of spores was studied using electret surfaces consisting of PTFE and bearing different net charges. As the surface chemistry remains the same for differently charged surfaces, the experimental results isolate the influence of surface charge and thus electrostatic interactions. Ulva spores were demonstrated to have a reduced tendency to settle on negatively charged surfaces and when they did settle the adhesion strength of settled spores was lower than with neutral or positively charged surfaces. These observations can be ascribed to electrostatic interactions.

In cellular, electromechanically active polymer films, the so-called ferroelectrets, the cell size and shape distributions can be varied through a controlled inflation process. Up to now, high-pressure treatments were usually performed at elevated temperatures. There are, however, significant experimental limitations and complications if the pressure and temperature treatments are performed at the same time. Here, we demonstrate the controlled inflation of cellular polypropylene films by means of separate pressure and temperature treatments. Separate procedures are much easier to implement. Excellent electromechanical properties were achieved with such a two-step inflation process. The technique has significant potential for inflating large-area transducer films for electromechanical and electroacoustical applications.

Ferroelectrets (i.e., charged cellular polymers) are rendered piezoelectric by means of barrier discharges inside the air-filled voids. The light emission from barrier discharges in cellular polypropylene ferroelectrets was quantitatively studied. Light emission typically occurs above a threshold voltage of 3 kV and then significantly increases with the applied voltage. Time-resolved images reveal discharge processes in individual voids. In addition, a second "back discharge" emission is observed when the voltage is reduced to zero. The buildup of the "effective polarization" in cellular PP ferroelectrets was studied by an acoustic method and dielectric resonance spectroscopy. A polarization-voltage (P-V) hysteresis loop was obtained by analyzing the data with an existing model for the piezoelectric d33 coefficient of ferroelectrets, from which a threshold charging voltage of 3 kV and the back barrier discharges were confirmed and a zero-field "effective polarization" o f 0.5 mC/ m2 was determined. However, charge densities of up to 2 mC/ m2 were measured under an applied bias voltage, leading to the conclusion that the observed back discharges destroy a significant fraction of the effective charge density