Maximizing Polyacrylonitrile Nanofiber Piezoelectric Properties through the Optimization of Electrospinning and Post-thermal Treatment Processes

While high-performance piezoelectric polymeric nanofibers such as polyvinylidene fluoride and its derivatives have been extensively studied to various applications, limited works examined other functional piezoelectric organic polymers with different chemical functionalities. In this work, size- and conformation-dependent piezoelectric properties of polyacrylonitrile (PAN) nanofibers were systematically investigated. PAN nanofibers with diameters ranging from 40 to 600 nm were systematically synthesized by adjusting electrospinning solution conditions where their conformation was further tuned through post-thermal treatment. Through in situ poling and stretching of polymer chains, the electrospinning process allowed the alignment of polar functional groups along the nanofibers to form a greater fraction of the electroactive phase (i.e., zigzag) over 31 helical (nonelectroactive) conformation. Smaller fiber further increased the electroactive content by the dimensional confinement effect. Fourier-transform infrared spectroscopy analysis and X-ray diffraction analysis confirmed the enhancement of zigzag conformation over 31 helical by reducing the fiber diameter and postannealing. A piezoelectric charge constant of 39.0 pm/V was achieved via reducing the PAN nanofiber diameter down to 40 nm, followed by post-thermal treatment at 95 °C, which paved a way to develop a flexible high-performance nanogenerator with diverse chemical functionality.