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.

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.

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.

Time-dependent change in charge trap of fluorinated cellular polypropylene (PP) films exposed to air is found by thermally stimulated discharge current measurements. It is clarified that the change is due to a trace of oxygen in the reactive gas mixture, which leads to the formations of peroxy RO2* radicals as well as C=O-containing groups in fluorination as indicated by attenuated total reflection infrared analyses. However, time-invariant charge traps of fluorinated PP films can be formed by the post-treatment of fluorinated PP films with nitrous oxide which can effectively terminate the peroxy RO2* radicals. And the combined post-treatments of the fluorinated films with nitrous oxide and by isothermal crystallization generate the time-invariant deep charge traps.

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 7h 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.

An extremely effective and practical approach to improving the thermal stability of charge and thus piezoelectricity of cellular polypropylene ferroelectrets is proposed and its effectiveness is experimentally validated in this report. The approach is fluorination and subsequent isothermal crystallization treatment. Astonishing charge stability is discovered by the measurements of open-circuit TSD current, charge TSD and isothermal charge decay. For instance, the charge for samples treated by the approach remains about 72% of the initial value, whereas the corresponding value is only 30% for the virginal sample after 6.5 h of isothermal charge decay at 90degC. This is attributed to the changes in composition and structure by fluorination and isothermal crystallization as indicated by IR analysis and XRD because the treatments almost completely eliminate shallow charge traps and significantly deepen deep traps.