Controlling Of Crystalline Structure And Piezoelectric Properties Of Polyvinylidene Fluoride Films

The high performance piezo-and ferro-electric polymer coating has widely applications in non-destructive testing,structural monitoring and vibration control etc.How to control the β-crystal of poly(vinylidene fluoride)(PVDF)to obtain high piezoelectricity and ferroelectricity is very significant in the various applications.In this thesis,the effects of coating methods,concentrations,solvents and blending on the crystal structure,morphology and piezoelectricity etc.of PVDF films were investigated,as well as the mechanism of droplet deposition patterns and film formation in the ultrasonic spraying process.The crystal structure,morphology and orientation of PVDF films were characterized by Fourier transform infrared spectroscopy(FTIR),twodimensional grazing incidence wide angle x-ray scattering(2D GIWAXS),atomic force microscopy(AFM)and piezoelectric force microscopy(PFM).In addition,the stretching machine,ultraviolet-visible spectrophotometer and optical contact angles and surface tension meter were used to characterize the mechanical properties,transparency and contact angles of PVDF films,respectively.The experimental results were showed following: 1)PVDF films prepared by ultrasonic spray coating method had relatively high content of ferroelectric phases and smaller crystal sizes in contrast with that by blade coating and solution casting methods.The formation of β-form crystals was affected by the interaction between solvent evaporation and fluid flow.Moreover,the ultrasonic spray coating PVDF films(USC-PVDF)were showed higher mechanical properties and transparency.2)Compared with DMF,DMAc and NMP solvents,the USC-PVDF films with DMSO solvent showed highest β-form crystal content,highest piezoelectric property and smallest contact angle.3)In the mixed solvent of DMSO/ethanol,the addition of small amount of ethanol was effective way to improve the β-crystal fraction of the USC-PVDF films.Highest β-crystal fraction was achieved when using the 8/2 v/v of DMSO/ethanol mixture solvents.4)The β-crystal fraction of PVDF films was increased firstly and then decreased with increasing concentration in the concentration range of 8-20 mg/ml,and the higher β-crystal fraction was obtained at concentration of 12 mg/ml.5)The β-form nucleation and crystal size of USC-PVDF films were significantly improved and reduced by adding small amount of PU(0-3 %),respectively.6)The coffee ring effect of a single droplet was reduced with increase the evaporation rate of solvent.The β-form crystals of PVDF were mainly distributed in the edge region of a droplet,and the α-crystals were mainly grown in the inner region of the droplet.7)Bulge-like β-form crystals and dendritic α-crystals were formed in a dry droplet with few millimeters of diameter in contrast with that with few tens micrometers of diameter.8)The crystal size and roughness of USC-PVDF films were dependent on the evaporation rate of the solvent and the distance between the nozzle and the substrate.The coffee ring effect and roughness were more obvious and increased as increasing the distance between nozzle and substrate,respectively,whereas the crystal size in the USC-PVDF film was gradually decreased.