β-phase formation of poly(vinylidene fluoride) foam based on the porous morphology control via supercritical carbon dioxide

The increase of electroactive β phase of poly(vinylidene fluoride) (PVDF) is important in energy harvesting applications. In this work, a green method of supercritical CO2 physical foaming was applied to obtain the PVDF foams with high content of β-crystal by utilizing the biaxial stretching when the cells grew, which were applied in the field of piezoelectric sensors. By changing the foaming condition, the PVDF foams with various structures were obtained, and a comprehensive study on the crystal transformation of them was carried out. The PVDF foam with increased β crystals content was more accessible to obtain at a high saturation pressure, corresponding to the large expansion ratio and small cell size. The acquisition of β crystals mainly came from the α-β solid-solid phase transformation during decompression, rather than the annealing process. Due to the introduction of cell structure, the piezoelectric output performance of the sample was effectively amplified. The produced PVDF foam with the β-crystal content of 30%, exhibited the low density of 0.11 g/cm3 and the highest piezoelectric output of 4.1 V (corresponding to a specific voltage of 35 V‧cm3/g). This foaming strategy provides guidance for lightweight design and high-performance fabrication of piezoelectric materials. The β-phase content of PVDF was enhanced by environmentally friendly physical foaming. [Display omitted] •PVDF foams were prepared by hot pressing and environmentally friendly ScCO2 foaming.•The biaxial stretching during cell growth is beneficial to the increase of β crystal content.•The PVDF foam can achieve a maximum piezoelectric output of 4.1 V with a small size of 1 cm × 1 cm.