An Overview on Piezoelectric Cellular Polymers: Properties and Preparation

Piezoelectricity can be described as a capability of some particular materials which convert mechanical energy to electrical and vice versa. In addition to ceramics (such as PZT) and polar polymers (PVDF and its copolymers), non-polar cellular polymers have received much attention in the last two decades due to their reasonable price, light weight, flexibility, and piezoelectric coefficient (d33). These polymers are used for a variety of applications, such as energy absorbers, sensors, and the medical sciences. PP, PET, PEN, PE and COP are thermoplastic materials which show high piezoelectric coefficient. By placing these cellular films in a strong electric field (corona discharge or between two electrodes) and in the vicinity of ionizing gas, the gas is ionized by electric field and remains inside the cells, indicating a bipolar moment. Due to the quasi-piezoelectric behavior of these materials, they are called ‘piezoelectrets’ and their properties are called ‘ferroelectricity’. Important factors such as cell structure (morphology, size and density), ionizing gas type, and Young's modulus have direct effect on the piezoelectric coefficient. Furthermore, some post-processing treatments such as chemical and stretching processes improve the piezoelectric properties. Polypropylene is one of the most widely used cellular polymers in the field of piezoelectrics due to features such as reasonable price, good fatigue resistance, and good charge trapping in cells, which has provided a study platform for other polymers. In this review article, we discuss on recent developments in improving piezoelectric coefficient and the work process. The effects of different parameters like electrical breakdown strength of various gases, additives, servicing temperature of polymers and Young's modulus on piezoelectricity in cellular polymers are investigated.