The thermal stability of positive charge has been investigated in chemically and physically treated polytetrafluoroethylene (PTFE) films. It has been found that virgin films, oriented by the manufacturer, display an increase in thermal stability of positive charge with an increase of the initial value of surface potential. Such an anomalous behavior is explained by the influence of a negative tribocharge, trapped some small distance below the surface. In PTFE samples treated with orthophosphoric acid and with tetraethoxysilane, a considerable improvement of positive charge stability has been achieved, but no influence of the initial value of surface potential has been observed. However, this influence should be kept in mind when comparing charge stability in virgin and modified samples. In nonoriented PTFE films, no influence of the initial value of surface potential on charge stability has been observed. This could be due to the fact that these films did not possess a noticeable negative tribocharge. After the treatment in glow-discharge defluorination, oxidation and appearance of polar groups have been detected on the surface. These changes in chemical composition of a PTFE surface resulted in a noticeable improvement in thermal stability of positively charged electrets. This improvement is attributed to the formation of deeper traps on the modified surface.

Chemical and physical surface modification of PTFE films—an approach to produce stable electrets

In connection with the development of nanotechnology in the last 15—20 years, the method of molecular layering (ML), created in the middle of the last century in the USSR, has attracted increasing attention. The features of structural-dimensional effects in the products obtained by new chemical nanotechnology and promising directions of their practical application in "core—nanoshell" compositions are considered in the review. In accordance with the synthetic capabilities of the ML method, the functional properties of the "core—nanoshell" type materials are affected by the "monolayer effect" and the substrate shielding effect, as well as the multicomponent nature of the system, and mutual structural coordination effect of the substrate and the build-up nanolayer. The review presents theoretical and experimental data on the application of the observed effects in the creation of shell pigments and fillers, modified sorbents, catalytic membrane reactors, nanodoped ceramic materials, polymers, quartz fibers with adjustable optical characteristics, electrets, etc.

Structural-dimensional effects and their application in the "core—nanoshell" systems synthesized by the molecular layering

Significant amounts of localised or trapped charges, as well as large electric dipoles from pairs of such charges, are usually detrimental for electrical insulation. The same charges can, however, lead to useful electro-mechanical and mechano-electrical effects (or inverse and direct piezoelectricity, respectively) if they are properly arranged in dielectric materials with extremely low conductivities. The resulting space-charge and dipole electrets are widely applied, e.g. in microphones, air filters, radiation dosimeters, ultrasonic transducers, etc. Furthermore, the coupling between localised charges and anisotropic mechanical properties may also give rise to pyroelectric and electrocaloric effects, as well as to soft and flexible or stretchable sensors and actuators with large figures of merit. Basically, the performance of all such transducer materials relies on the attraction (and repulsion) of electric charges and thus directly on the electromagnetic interaction, i.e. on one of the fundamental interactions that Nature provides. The beneficial transducer effects of localised excess charges of both polarities in dielectric polymer materials are being exploited in recent developments in the areas of anisotropic polymer ferroelectrets with quasi-ferroelectric behavior, of soft and basically incompressible electro-electrets (dielectric elastomers) with extremely large strains (used in sensors, energy harvesters and actuators), of molecular-dipole electrets with significant ferro-, pyro- and piezo-electricity, and of space-charge polymer electrets with locally stabilised charges.

A matter of attraction: Electric charges localised on dielectric polymers enable electromechanical transduction

Polyethylene terephthalate film 12 ����m thick with vacuum-deposited AlOx coating was studied. Optical and atomic force microscopy was used to investigate surface topology. Film haze, optical density, surface tension, oxygen and vapor transmission rates were measured to estimate barrier performances of the film. To study effect of the coating on electret properties of the film surface potential, electric field strength and efficient surface charge density and thermally stimulated surface potential relaxation were measured. Optimal charging conditions for coated and uncoated polyethylene terephthalate films were found. Interaction of the film with different model mediums simulating foodstuff was studied. Coated electret film can be used as active packaging material with high barrier properties.

Effect of aluminum oxide coating on structural, barrier and electret properties of polyethylene terephthalate films

We report first investigations on a ferroelectret transducer material that is based on the same layer sequence as a simple electret microphone. Only one electrically charged polymer film is employed instead of two as in previous tubular-channel ferroelectrets. The new materials design has major advantages such as a reduced number of polymer films to be processed and a reduced amount of expensive electret material, as well as the possibility to choose the polarity of the charged layer for optimal stability. A drawback seems to be a lower piezoelectric coefficient, since the values obtained in our preliminary experiments amount to roughly half of those found on bipolar ferroelectrets made from two polymer-electret films. Nevertheless, the d33 values observed on our samples are still twice as high as those of polyvinylidene fluoride (PVDF) and can be further improved, e.g. through modifications of the ferroelectret geometry, the layer sequence and the electret properties or via multi-layer stacking as already demonstrated for polypropylene piezoelectrets.

Unipolar ferroelectrets - Following the example of the electret microphone more closely

Tetrafluoroethylene–hexafluoropropylene copolymer (FEP) films were treated with titanium-tetrachloride vapor in a molecular-layer deposition process. As a result of the surface treatment, significant improvements of the thermal and temporal charge stability were observed. Charge-decay measurements revealed enhancements of the half-value temperatures and the relaxation times of positively charged FEP electrets by at least 120 °C and two orders of magnitude, respectively. Beyond previous publications on fluoropolymer electrets with surface modification, we here report enhanced charge stabilities of the FEP films charged in negative as well as in positive corona discharges. Even though the improvement for negatively charged FEP films is moderate (half-value temperature about 20 °C higher), our experiments show that the asymmetry in positive and negative charge stability that is typical for FEP electrets can be overcome by means of chemical surface treatments. The results are discussed in the context of the formation of modified surface layers with enhanced charge-trapping properties.

Higher stabilities of positive and negative charge on tetrafluoroethylene–hexafluoropropylene copolymer (FEP) electrets treated with titanium-tetrachloride vapor

Thermal stability of the electret state of polytetrafluoroethylene films with element oxide structures on the surface was studied. The frequency factor and the activation energies of the newly formed groups of deep centers that capture the electret charge were determined.

Influence of Chemical Modification of the Surface on the Electret Properties of Polytetrafluoroethylene

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Higher stabilities of positive and negative charge on tetrafluoroethylene–hexafluoropropylene copolymer (FEP) electrets treated with titanium-tetrachloride vapor

Influence of Chemical Modification of the Surface on the Electret Properties of Polytetrafluoroethylene