Piezoelectric d33 coefficient of cellular polypropylene subjected to expansion by pressure treatment

The piezoelectric activity of charged cellular polypropylene films of originally 40 and $50\mu \mathrm{m}$ thickness can be significantly increased by thickness expansion due to an exposure to high pressure for time periods of the order of hours. After such a treatment, the $d_{33}$ coefficient, measured by quasistatic and interferometric methods, is found to be as high as $1200\mathrm{pC}∕\mathrm{N}$ at $0.001\mathrm{Hz}$ , decreasing to $350\mathrm{pC}∕\mathrm{N}$ just below resonance. At the resonance, which is located in the range of $150–400\mathrm{kHz}$ for differently treated samples, $d_{33}$ coefficients up to $1400\mathrm{pC}∕\mathrm{N}$ are found. The high $d_{33}$ coefficients result from a decrease of Young’s modulus $Y$ and an increase of the chargeability of the material due to the expansion, while the decrease of $d_{33}$ with frequency up to resonance is related to a corresponding increase of $Y$ . Static pressures up to $10\mathrm{kPa}$ have little influence on $d_{33}$ , but higher pressures result in a reversible decrease.