| Since the piezoelectricity of poly(vinylidene fluoride)(PVDF)was discovered in 1969,its applications in electronic components has attracted much attention from material scientists.PVDF have five kinds of crystal forms,and its piezoelectricity is mainly associated with the existence of polar ?-form crystals.Among the methods in obtaining the ? form crystals of PVDF,high pressure and high temperature crystallization are effective in the formation of PVDF ? crystals with few defects,and in the preparation of PVDF samples with high crystallinity and outstanding piezoelectric property.Therefore,it is of great significance for the investigation of the high-pressure crystallization behaviors of PVDF.Piezoelectric materials can achieve mutual conversion between mechanical energy and electrical energy,and they have been widely used in sensors,transducers,medicine and other fields.For piezoelectric polymeric materials,they have such advantages as good mechanical flexibility,low cost,easy processing and excellent electric insulation performance.Nevertheless,the applications of piezopolymers are still quite limited mainly due to their relatively low dielectric and piezoelectric coefficient.Moreover,for some piezopolymers,such as voided charged polymers,poling process is always an inevitable step in realizing their piezoelectricity for real applications.Undesirable problems arose from the rigorous poling process,such as substantial additional power consumption,and sometimes electrical breakdown failure occurred in polymer matrixes,resulting in the discard of the whole samples.Therefore,piezoelectric polymers with more excellent performance should be developed in order to achieve their wider range applications.In this work,two kinds of composite materials,PVDF/Nafion and PVDF/Nafion/GQDs,were prepared by solution casting.The as-fabricated composites were crystallized at high pressure by varying the temperature,pressure and time.And then,the crystallization behavior of the 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,piezoelectric generators,based on PVDF/Nafion PVDF/Nafion/GQDs composites,respectively,were fabricated,and their piezoelectric properties,as well as the piezoelectric mechanisms,were analyzed in detail.The main work and conclusions are listed as followings:(1)Research on the crystallization behaviors of PVDF/Nafion composites at high pressureSEM images showed that the holes,originally filled by Nafion,distributed uniformly in PVDF matrix after the etching process,which suggsted a relatively good Nafion dispersion was achieved in the binary composites by solution casting.Increasing crystallization temperature at high pressure favored the transformation of a to ? or y form crystals of PVDF.Only ? and y form crystals were formed and no a form crystal was detected in the samples on the condtion that the crystallization temperature was 260 ? or beyond.When crystallization pressure was 500MPa,the samples achived a higher degree of crystallinity,and more ? PVDF crystals were crystallized.Besides,? form extended-chain crystals were observed in the sample crystallized at 400MPa and 500MPa.(2)Investigation on piezoelectric properties of PVDF/Nafion compositesPVDF/Nafion dynamic piezoelectret generators(PNDPG)were fabricated with different compounding ratios,and large amounts of ? form crystals were formed in the binary composites crystallized at 400MPa,260? for 10 min.The working mechanism of the PNDPGs was attributed to the synergistic action of "intrinsic"polarity and orientation of molecular dipoles of PVDF cells with "artificial"macroscopic dipoles of Nafion fillers.Without any treatment of electrical poling,the open-circuit voltage output density of the PVDF/Nafion generator developed at the optimized conditions,stimulated under its dynamic deformation,reached 14.6Vcm-2,exceeding that of most of the state-of-the-art piezoelectric polymers reported in the open literature.Particularly,the controllable power output of such PNDPG was able to be realized by modulating the amplitude and frequency of the applied dynamic stimulation.Moreover,the PVDF-Nafion generators showed good stability and durability,and no decay of electrical output was observed for more than 100000 cycles continuous working.(3)Research on high pressure crystallization behaviors and piezoelectric properties of PVDF/Nafion/GQDs ternary compositesTEM showed that the GQDs achieved an overall good dispersion in PVDF/Nafion matrix,though there were still few aggregations observed within the size of 100nm.Large amounts of a form crystals were formed in the samples when crystallization temperature were 230 or 245 ?.However,only ? and y form crystals were crystallized in the composite samples when crystallization temperature was further increased to 260? or above.Moreover,the samples crystallized at 400 and 500MPa were founded to be endowed with relatively higher crystallinity.Particularly,large amounts of extended-chain crystals were observed in the ternary hybrid composites crystallized at 400MPa,260? for 10-60min.Interestingly,PVDF nanowire arrays were also observed in the pressure crystallized composites with 0.5%?1.0%and 3.0%GQDs loadings,respectively.With a force of 20N amplitude and 5 Hz frequency applied on,the PVDF/Nafion dynamic piezoelectret generator which has no GQDs created 50nA short-circuit current and 3.8V open-circuit voltage,and the PVDF/Nafion/GQDs dynamic piezoelectret generator(PNGDPG)with 3%GQDs generated 58nA short-circuit current and 5.0V open-circuit voltage.After soaking in distilled watet or NaCl solution with 10%concentration for 20h,the output of short-circuit current for such PNGDPG increased.The durability tests showed that short-circuit current output of the PNGDPG decreased with the increase of hit times,and the output performance became stable after 200000 repeat cycles. |
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