Investigation of Mechanical and Thermal Post-Processing on Morphology and Electro-Mechanical Properties of Electrospun Piezoelectric Nanofibers
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Nanofibers are one dimensional nanomaterials with exceptional properties. Different types of materials and fabrication techniques could result in multifunctional nanofibers with broad applications. Ferroelectric polymers such as PVDF and its copolymers could be formed into nanofibers that exhibit piezoelectric and pyroelectric properties. Several fabrication methods are proposed to produce nanofibers. Electrospinning is a common nanofiber fabrication process with broad throughput from lab scale to industrial scale. Mechanical stretching and electric field poling could enhance the piezoelectric properties of piezoelectric materials. Therefore, electrospinning is a preferred technique for piezoelectric nanofibers fabrication, because a high stretching rate and strong electric field are applied to nanofibers during the processing due to the nature of the process. The electromechanical coupling factor is an important metric in measuring the performance of piezoelectric materials. This factor highly depends on piezoelectric coefficient and Young’s modulus of the materials. Post-processing techniques such as twisting and stretching as mechanical post-processing, and annealing as thermal treatment were used to enhance piezoelectric and mechanical properties. In this work, the conventional electrospinning method with rotating collector drum was used to produce aligned P(VDF-TrFE) nanofibers. Both mechanical and thermal post-processings were employed to enhance the electromechanical coupling factor through piezoelectric and Young’s modulus enhancement. Mechanical and piezoelectric properties of electrospun nanofibers were investigated in nano and macro scales through tensile and AFM-based nanoindentation tests. Piezoelectric response of nanofibers was studied by flexure test and PFM, in macro and nano scales, respectively. The evolution of molecular structure, orientation of polymer chains and crystalline phase transitions were investigated by spectroscopy, crystallography, and calorimetry methods, to explain the enhancements of material properties.