The Structure Control Of Ferroelectric Poly(vinylidene Fluoride)and Study Of Its Ferroelectricity

Poly(vinylidene fluoride)(PVDF)can be expected to be used in many high-tech fields,such as non-volatile memories,sensors,nano-generators,actuators,artificial muscle,artificial retina and so on due to its excellent electroactive properties(like ferroelectricity,piezoelectricity and pyroelectricity).All of those electroactive properties of PVDF can be attributed to its polar phase crystals:β crystal,y crystal and δ crystal.Therefore it is meaningful to study how to fabricate PVDF samples with high-content polar phase.In this thesis we have systematically studied the formation mechanism of β and γ crystal based on PVDF single layered film and PVDF/poly(methyl methacrylate)(PMMA)bilayered films respectively.The nearly 100%γ or β crystal sample was fabricated,and its ferroelectricity had also been characterized.The main results are presented below:1.PVDF/PMMA multi-layered films were prepared,and key factors that will influence formation of β crystal in the multi-layered films system were studied.It was found that the melt-quenching treatment favored the formation of β crystal.The crystallinity of(3 crystal was dependent on the thickness of PMMA layer when the thickness of PVDF was fixed.The modifications of samples after melt-quenching treatment changed from many kinds of modifications to pure β phase with thickening of PMMA layer,but the crystallinity was gradually decreased.Only when the thickness of PMMA layer was 90 nm,the final sample was 100%β-P VDF with relatively high crystallinity.The crystallinity of β crystal can also be enhanced by increasing the annealing temperature after melt-quenching treatment.Grazing incident infrared results indicated that there existed hydrogen bond interaction between PVDF and PMMA,and the direction of hydrogen bond interaction was perpendicular to the surface of sample.The formation of β crystal was the synergistic results of hydrogen bond interaction at the interface and diffusion extent of PMMA into PVDF layer.2.Original PVDF samples were prepared by spin coating method.When we choose the higher molecular weight PVDF to use,original PVDF film was a mixture of α,β and y crystals.The influences of heating rate,annealing temperature,cooling rate on the crystallization were systematically studied.It showed that a nearly 100%γ-PVDF sample can be fabricated when annealing temperature was 166℃.The results of in situ atomic force microscopy,X ray diffraction and infrared spectrum indicated that preexisting a and β crystal in original samples were melted,and the residual γ crystal can induce the formation of new y crystals as a crystal nuclei during cooling process(self-nucleation effect).The needle-like morphology of γ crystal was formed.When we choose the lower molecular weight PVDF to use,original PVDF film contained large amount of a crystal.A proper thermal treatment was designed.It showed that polar PVDF samples(y phase was the dominant polar phase)can be fabricated when annealing temperature was 174℃.It was evidenced that the formation of y crystal was attributed to melt memory effect of preexisting α crystal based on relevant characterization technigues,and the morphology was scroll-like.Ferroelectric capacitor and ferroelectric field effect transistor(FeFET)were prepared and the ferroelectricity of y crystal was also characterized.It showed that the remnant polarization value was 6.0 μC/cm2 and coercive electric field was 85.7 MV/m.The ON/OFF bistability of FeFET device at zero gate voltage is approximately 1.3xl03.3.To further explore the formation mechanism of y crystal due to memory effect of partly melted a crystal,pure a-PVDF original sample from non-isothermal crystallization of PVDF melt was studied.A proper thermal treatment was designed.It showed that samples containing large amount of a and y crystal can be fabricated when annealing temperature was 174℃.This part of y crystal was formed due to memory effect.After applying a second round thermal treatment to the sample treated after first round thermal treatment,the polar PVDF samples(y phase was the dominant polar phase)was fabricated.The formation of y crystal of final samples was the synergistic results of self-nucleation effect and memory effect.The preparation method of second round thermal treatment based on synergistic results will have guiding significance for the study of structure control of y-crystal PVDF.