Flexible film-transducers based on polypropylene piezoelectrets: Fabrication, properties, and applications in wearable devices

In this work, piezoelectrets were prepared by the microstructure modification and corona charging of polypropylene (PP) foam sheets, and film-transducers based on such fabricated piezoelectrets were further developed. The electromechanical performances of the as-prepared transducers were investigated, including the complex dielectric spectra, frequency response, thermal stability, humidity sustainability, and fatigue resistance. The experimental results showed that the typical capacitance and impedance of the transducers were 43.7 pF and 3.6 MΩ at 1 kHz, respectively. Piezoelectric sensitivity up to 600 pC/g was achieved, but after annealing treatments at temperatures of 50, 70, and 90 °C for 24 h, only 54%, 30%, and 13% of the initial values were remained, respectively. The transducers had an appreciable sustainability in high-humid environments, and about 70% of the initial sensitivity was still retained in a humidity of 98% after 37 days. In fatigue tests, the sensitivity decreased to 70% of the initial value after 1.5 million cycles of operation with a dynamic pressure of 2 MPa that was applied perpendicularly onto the samples. The performance assessments demonstrated that the PP-piezoelectret film-transducers would have promising applications in wearable devices, which were also preliminarily exploited.