Experimental studies of flexoelectricity in polyvinylidene fluoride (PVDF) homopolymer thin films
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The electric polarization induced by the non-uniform deformation, i.e. the direct flexoelectric effect, in polyvinylidene fluoride (PVDF) has been investigated based on a beam bending method in this thesis. The primary goal of my research is to understand the properties of both ferroelectricity and flexoelectricity of PVDF homopolymer films under the unaligned state. Generally speaking, bulk PVDF materials exists in three phase structures, &agr; phase (TGTG'), β phase (TTTT), and γ phase (TTTGTTTG'), depending on the chain conformations as the trans (T) or the gauche (G) linkages. Only β phase and γ phase possess piezoelectric responses; however the direct flexoelectric effect can exist in any phase. This is because the inhomogeneous deformation can break the inversion symmetry and induce electric polarization even in non-piezoelectric crystalline materials. In my experimental study, the unaligned PVDF homopolymer films were initially synthesized using solution polymerization process and then characterized by using X-ray Diffraction (XRD) and Scanning Electron Microscope (SEM). The flexoelectric coefficients of those polymer films were then measured. Both the enhanced direct flexoelectric effect and the ferroelectric-like behavior were observed in the polymer films. My studies have shown that both the effect and the behavior largely depend on the molecular chain configuration of PVDF films. Compared with certain aligned PVDF films, the flexoelectric properties of the unaligned PVDF film could be significantly different. My experimental results also demonstrate that the ferroelectric-like behavior in the unaligned PVDF film might be induced by its spatially distributed space electric charges. The measured flexoelectric coefficient (&mgr;13) and the piezoelectric coefficient (d13) can also give us valuable information about the property degradation, or aging phenomena, in polymer films and the related polymer devices. Finally I summarize my research and discuss some possible future research topics which could follow my thesis.