Polymer ferroelectret based on polypropylene foam: piezoelectric properties prediction using dynamic mechanical analysis

Thin polypropylene (PP) foam films were produced by continuous extrusion using supercritical nitrogen (N-2) and then charged via corona discharge. The samples were characterized by dynamic mechanical analysis as a simple method to predict the piezoelectric properties of the cellular PP obtained. The results were then related to morphological analysis based on scanning electron microscopy and mechanical properties in tension. The results showed that the presence of a nucleating agent (CaCO3) substantially improved the morphology (in terms of cell size and cell density) of the produced foam. Also, an optimization of the extrusion (screw design, temperature profile, blowing agent, and nucleating agent content) and post-extrusion (calendering temperature and speed) conditions led to the development of a stretched eye-like cellular structure with uniform cell size distribution. This morphology produced higher storage and loss moduli in the machine (longitudinal) direction than for the transverse direction, as well as higher piezoelectric properties. The morphological and mechanical results showed that higher cell aspect ratio led to lower Young's modulus, which is suitable to achieve higher piezoelectric properties. Finally, the best quasi-static piezoelectric d(33) coefficient was 550pC/N for a cellular PP ferroelectret having a uniform eye-like cellular structure using N-2 as the ionizing gas inside the cells, while the highest value was only 250pC/N when air was used. Hence, the value of d(33) can be improved by more than 100% just by replacing air with N-2 as the ionizing gas. Copyright (c) 2016 John Wiley & Sons, Ltd.