Flexoelectric and ferroelectric properties of quenched polyvinylidene fluoride (PVDF) homopolymer films
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The flexoelectric and ferroelectric properties of quenched polyvinylidene fluoride (PVDF) homopolymer films have been investigated in this thesis. It is well known that PVDF homopolymer mainly has three phase structures, α phase (TGTG'), β phase (TTTT), and γ phase (TTTGTTTG'), depending on the chain conformations as the trans (T) or the gauche (G) linkages. However, the quenched PVDF homopolymer has different molecular chain conformations, which may render some unique mechanical and electric properties. Quenching is a mechanical process in which PVDF homopolymer is strengthened and hardened. This is done by heating PVDF solution at a certain temperature, where the polymerization cannot be formed, then rapidly cooling the polymer; in annealing process, only in certain local microscopic areas, polymerization can occur to let PVDF monomer molecules aggregate together to form the α, β, and γ phases. So the regions between those areas in the polymer have amorphous structures. Thus, the piezoelectric behavior of the quenched PVDF homopolymer might be altered since only β phase and γ phase in PVDF possess piezoelectric responses and the volume, in which both β phase and γ phase exist, in the quenched polymer is reduced. However, whether the direct flexoelectric effect is influenced or not in the quenched PVDF homopolymer is still unknown. This is because the direct flexoelectric effect can exist in polymers of any phase; the inhomogeneous deformation can break the inversion symmetry and induce electric polarization even in non-piezoelectric phase. In our experimental studies, the quenched PVDF homopolymer films were initially synthesized using solution polymerization process and then were quenched in ice-water. We further characterized our samples 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 quenched polymer films. Our studies have shown that both the effect and the behavior largely depend on the molecular chain configurations of the quenched PVDF films. Our experimental results also demonstrate that the ferroelectric-like behavior in the quenched PVDF film might be induced by its spatially distributed space electric charges. The measured flexoelectric coefficient (μ 13 ) may also give us valuable information about how aging phenomena would be induced in polymer films.