Piezoelectricity of Cellular Polypropylene Films Expanded by a Dichlormethane Evaporation Process

In this paper, a new method for expanding stretched cellular isotactic polypropylene (i‐PP) is presented. Firstly, the cellular i‐PP films with small cavities were saturated with dichlormethane (DCM) and then, the DCM was abruptly evaporated at elevated temperature to create large‐cavity cellular films. Finally, the expanded film samples were quenched in liquid nitrogen to fix the cell structure. After corona charging, the piezoelectric d33‐coefficients of such expanded films were measured by quasistatic and interferometric methods. The results show that the highest values of the quasistatic piezoelectric d33‐coefficient of up to 2500 pC/N at low frequencies were observed on samples twice expanded with DCM. A high thermal stability of the piezoelectric activity was also observed. The high d33‐values are due to the air escape sideways out of the large voids of the cellular films.