Charged cellular polymers with "ferroelectretic" behavior

Cellular space-charge electrets have recently emerged as a new class of materials for electromechanical devices, offering chances for a wide range of applications and challenges for materials optimization. However, many fundamental and applied aspects of the physics of these novel materials are not yet explored. Here we summarize our present understanding of the (quasi)-piezo- and -pyroelectricity in such materials. In contrast to the dipole-density piezoelectricity in ferroelectric polymers, the piezoelectric-like response of cellular polymers is intrinsic, with positive d/sub 33/ and negative d/sub 31/ and d/sub 32/ piezoelectric-like coefficients. Similarities with ferroelectric materials are outlined, especially switching of "polarization" and (quasi)-piezoelectricity. First steps towards patterned charging of cellular polymers are reported, an immediate consequence of the possibility for "polarization"-switchingin cellular materials. The results on cellular space-charge electrets suggest that well-known electret devices like microphones may be seen in a new light. Examples include dielectric and electromechanical hysteresis loops obtained with a commercially available electret microphone. In view of the results, cellular polymers may henceforth be called "ferroelectrets" and their material behavior "ferroelectretic". From an applied point of view, the performance of a Fresnel zone plate for focussing ultrasound is shown. Such a device may pave the way for a simple tool in nondestructive materials inspection, and demonstrates the large potential of cellular polymers for applications.     

Nonlinear optical polymer electrets current practice

Amorphous polymers with strong second-order optical nonlinearities contain molecular chromophore dipoles as guest molecules, as side groups, or as main-chain segments. In order to break the inherent centrosymmetry of the initially isotropic dipole orientation in these materials and to render them nonlinear optically active (as well as piezo- and pyroelectric), preferential dipole alignment by means of electrical poling is necessary. After poling the nonlinear optical polymer films are molecular dipole electrets so that the full range of techniques for the preparation and investigation of polar electrets may be employed, together with nonlinear optical techniques, originally developed for the investigation of ferroelectric polymers, such as poly(vinylidene fluoride). The field of nonlinear optical polymers developed from very enthusiastic beginnings in the early eighties to the present and more realistic approach, with real applications just about to appear     

Preparation and characterization of novel piezoelectric andpyroelectric polymer electrets

The preparation and characterization of novel piezoelectric and pyroelectric electrets is discussed with respect to their electret properties. Charged heterogeneous electrets, like closed-cell microporous polypropylene (pp) foams and `soft-hard' fluoropolymer hybrids are shown to yield large quasi-static and dynamic piezoelectric responses, indistinguishable from true piezoelectricity. Piezoelectric coefficients comparable to that of ceramic counterparts have been achieved. For the investigation of the piezoelectric and pyroelectric properties of the electret materials, the thermal-pulse technique has been extended towards temperature-dependent investigations and measurements on planar, cylindrical or spherical samples. The potential of the thermal-pulse technique not only for the measurement of electric-field and polarization distributions, but also for acoustical, electromechanical and pyroelectric properties is outlined     

Current practice in space charge and polarization profile measurements using thermal techniques

Thermal techniques for probing space charge and electric field distributions in dielectric materials became available approximately 30 years ago. The techniques have reached maturity and they have been employed not only for the primary purpose of electric field or polarization profiling, but also in a wide range of problems posed by materials research. The present survey provides an overview of the historical development, the experimental implementation of the different techniques, the theoretical foundation, methods for the data analysis and a comparison of thermal and acoustic techniques. The thermal wave technique LIMM is used as an example among the thermal techniques, for a discussion of data analysis techniques and for the spatial resolution that can be achieved with thermal wave techniques. A tour d'horizon is provided through recent applications of thermal techniques, in order to demonstrate their capabilities for dielectric material characterisation.     

Dielectric investigation of thermally-induced chromophoredegradation in nonlinear optical polymer electrets

One of the critical issues with poled photonic polymers is the thermal stability of the nonlinear optical effect in these dipolar electrets. The temporal decrease of the nonlinear optical coefficients is caused by dipolar-relaxation and molecular-degradation processes. In this paper, it is demonstrated that dielectric measurements allow for an in-depth investigation of dipole-degradation processes in a typical side-chain photonic polymer. The results yield an estimate of effective device lifetimes, which are in some cases dominated by irreversible chemical degradation rather than by reversible dipolar relaxation     

Dielectric investigation of photoinduced chromophore degradation in nonlinear optical side-chain polymer electrets

Organic materials with noncentrosymmetric chromophores are known to be susceptible to a number of photochemical processes, including reversible isomerization reactions as well as irreversible photooxidation or photoreduction reactions. Reversible isomerization is the basis for a variety of applications, such as photoinduced poling, optical data storage and optical grating formation. The irreversible processes that involve the destruction of the chromophores have been found useful for the fabrication of optical waveguides, but they also limit the life times of polymeric photonic devices. In this paper, it is demonstrated that dielectric measurements allow for an in-depth investigation of nonreversible chromophore degradation processes in a typical side-chain polymer. The time- and temperature-dependent dielectric function of the polymer at 1 kHz enables us to follow the chromophore-degradation kinetics and to monitor the bleaching depth as a function of time at room and elevated temperatures.