| The self-powered composite structures, able to capture trace amounts of energy from the ambient environments and transform it into electrical energy, have recently attracted considerable attention from the materials research community. Among the developed materials and technologies for kinetic energy harvesting, employing piezoelectric generator composites has shown more promise in the conversion of mechanical energy with variable amplitude and frequency into electricity, mainly due to their unique advantages, such as large power density, easy application, and so on. Various micro/nanostructured piezoelectric materials, such as ZnO, BaTiO3, alkaline niobate-based particles (KNLN), and (1-x)Pb(Mgi/3Nb2/3)O3-XPbTiO3 (PMN-PT), fabricated in a variety of forms, were incorporated in organic or inorganic matrixes for the realization of nanocomposites or macro-fiber composites (MFC) with enhanced piezoelectricity. Particularly, since a flexible polymeric matrix prevents the embedded piezoelectric fillers from breaking and cracking under mechanical stress, there have been attempts to combine piezoelectric macro/nanofillers with polymers, such as polyurethane, SU-8 polymer, polydimethylsiloxane (PDMS), and PVDF. This makes it possible to create novel piezoelectric composites that exhibit the properties of the piezoelectric macro/nanostructures while take advantages of the easy processability of polymers.PVDF are one of the limited known piezoelectric class of polymers that promise applicability in stretchable nanogenerators because of their chemical resistance, structural flexibility, processing simplicity, and large piezoelectric coefficients. The PVDF-based polymers exhibit a pronounced polymorphism, transforming between several crystal forms under certain conditions, and successful development of piezoelectric polymer devices depends on the effective fabrication of polar crystalline structures, such as (3 and y. Graphene quantum dots (GQDs), possessing one or a few layers of graphene with chemical groups connected on the edges, are anisotropic zero dimensional nanostructured carbon materials with lateral dimensions larger than their height. Owing to quantum confinement and edge effects, the GQDs have presented extraordinary properties, such as versatile photoluminescence, large surface area, high photostability, low cytotoxicity, and excellent biocompatibility. Hereby GQD-based materials have become the research focus in recent years, and they are being exploited for energy-related devices, composite materials, and environmental and biological applications.In this work, two kinds of composite materials, PVDF/GQDs and PVDF/GOQDs, were prepared by an environmentally friendly approach. The as-fabricated composites were crystallized at high pressure by varying the temperature, pressure and time. And then, the crystallization behavior of the recovered composites were investigated with wide-angle X-ray diffraction (WAXD), differential scanning calorimetry (DSC), Fourier transform infrared spectroscopy (ATR-FTIR) and scanning electron microscopy (SEM). Furthermore, the wettability and Piezoelectric Properties of the PVDF/GQDs composites were investigated with optical contact angle measuring device and periodic impacting test. The main work and conclusions are listed as followings:(1) Crystallization behaviors of PVDF/GQDs composites at high pressureFor the PVDF/GQDs (99.5/0.5, wt/wt) composite, fabricated by a solution casting process, the observation by transmission electron microscopy (TEM) showed that the GQDs generally achieved a good distribution in the PVDF matrix. DSC showed that both the melting point and crystallinity of the PVDF/GQDs samples first increased and then decreased with the crystallization temperature. Especially, the PVDF/GQDs samples crystallized at 400MPa,245℃ for Omin and 500MPa,230℃ for 30min were both endowed with a high crystallinity after the high pressure crystallization. The ratio of β phase increased with the increase of both the crystallization temperature and pressure, but decreased with the increase of crystallization time. A large amount of piezoelectric three dimensional (3D) micro/nano structures were oberserved in the PVDF/GQDs samples crystallized at 200-400MPa,230℃ for 30min, and relative lower temperature and pressure were found to be more appropriate for the growth of such unique structures. The formation of the 3D micro/nano structures was assigned to the self-assembly of PVDF molecular chains, induced by the catalysis of the GQDs at high pressure.(2) Crystallization behaviors of PVDF/GOQDs composites at high pressureTEM showed that generally the GOQDs achieved a size-distributed dispersion in the PVDF matrix after the solution casting process. DSC suggested that both the melting point and crystallinity of the PVDF/GOQDs samples increased with the increase of the crystallization temperature and time, and changed very little with the variation of the applied pressure. Also, the ratio of β phase crystals first increased and then decreased with the increase of crystallization temperature, pressure and time. Furthermore, more 3D micro/nano structures were revealed by SEM for the PVDF/GOQDs samples crystallized 200-400MPa,230℃ for 30 min. Compared with their counterparts in the PVDF/GQDs compsites, the 3D micro/nano structures in PVDF/GOQDs achieved a lower-density distribution in polymeric matrix. Nevetherless, their size was relative lager due to the stronger aggregation effect of the GOQDs.(3) Piezoelectric properties of pressure-crystallized PVDF/GQDs compositesThe GQDs generally achieved a distribution within nanoscale in the polymer matrix. Nevertheless, the GQDs intrinsically tend to aggregate, with the size of agglomerations distributed from a few nanometers to several tens of nanometers. The GQD/PVDF composites finalized their self-polarization at high pressure, through a size-distributed GQD-induced growth of a hierarchically structured network, totally composed of 1D nanowires and 3D micro/nanowire assemblies with crystalline β phase. The in situ formed macro/nanostructures with special morphologies remarkably enhanced the efficiency of mechanical-to-electrical conversion. Compared to the referenced pure PVDF, more than four times larger electrical output was revealed for a GQD/PVDF composite, without any treatment of electrical poling. Moreover, the pressure crystallized unusual structural features at the micro/nanometer scale enabled the achieving of controllable hydrophilic/hydrophobic surfaces for the GQD/PVDF composites, due to the competition effect between enhanced surface roughening and exposed micro/nanoscale hierarchical structures. |
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