Electric Poling-assisted Additive Manufacturing Technique for Piezoelectric Active Poly(vinylidene fluoride) Films: Towards Fully Three-dimensional Printed Functional Materials

The development of science and technology simplifies the fabrication of functional materials and devices. Polymeric piezoelectric materials have attracted interest for actuation, sensing, energy harvesting, and storage applications due to their capability to sustain larger strains than ceramic piezoelectric materials. In this work, a novel electric poling-assisted additive manufacturing (EPAM) technology that integrates a fused deposition modeling (FDM) three-dimensional (3D) printer with a corona electric poling setup is demonstrated to directly print piezoelectric active materials with improved piezoelectric characteristics. The developed EPAM technology overcomes two main shortcomings of the conventional manufacturing methods for poly(vinylidene fluoride) (PVdF) materials: (1) their shapes are constrained to planar or fiber-like geometries, and (2) the need for additional electric poling as a post-processing treatment. Compared with the contact poling method, the EPAM method eliminates the need to apply electrodes for poling. The EPAM process can accomplish stretching and poling simultaneously which are necessary conditions for the polarization. During the EPAM process, stretching the molten PVdF rod rearranges the amorphous strands in the film plane, and the applied electric field aligns dipoles towards the same direction. The EPAM process can print free-form PVdF structures and induce the formation of beta-phase, which is primarily responsible for the piezoelectric response. The Fourier-transform infrared spectroscopy (FTIR) results indicate the beta-phase content increased from 15.38% to 17.14% corre-sponding to unpoled printed and EPAM printed samples, respectively. PVdF force sensors were successfully printed, and the piezoelectric activity (pC/N) was calculated based on the piezoelectric output voltage. The piezoelectric activity has a positive correlation with the piezoelectric coefficient that is explained in detail in section 3.2. The results demonstrate that the average piezoelectric activity of EPAM printed PVdF films was 47.76 pC/N, or about five times higher than unpoled 3D printed films, at 9.0 pC/N. The piezoelectric activity of unpoled 3D printed PVdF films indicates that 3D printing in the absence of an electric field does not result in dipole alignment. The 3D printed samples show an anisotropic mechanical behavior, and the bonding surface strength is sensitive to the printing speed. The maximum Young's modulus (534.63 MPa) occurred at 3 mm/s and 0 degrees infill, and the highest ultimate tensile strength (UTS) (25.35 MPa) was achieved at 20 mm/s, and 90 degrees infill. Both the Young's modulus and UTS from the EPAM printed samples are lower than unpoled samples. A fully 3D printed device was demonstrated to identify the position and magnitude of forces and results were visualized via structure-embedded integrated Light emitting diodes (LEDs). Overall, with the advantage of mechanical struc-tural design flexibility and ease of printing of piezoelectric materials, the developed EPAM technology opens a new avenue to simplify device design and fabrication methodologies along with low-cost implementations.