PVDF and ? phases and T3GT3G? for ?

PVDF and its copolymer are the most commonly studied ferroelectric polymers and most widely used. This semi-crystalline polymer shows five distinct crystalline phases related to different chain conformations designed as all trans (TTT) for the ?-phase, TGTG? (trans-gauche–trans-gauche) for the ? and ? phases and T3GT3G? for ? and ? phases.20-21 The thermodynamically stable phase at ambient conditions is ? phase. Figure 2.6 shows the most investigated PVDF phases ?, ? and ?-phases.
Many of the interesting properties of PVDF are due to the presence of strong electrical dipole moment of the PVDF monomer unit (5–8 × 10?30 Cm). The strong dipole moment originates from the electronegativity of fluorine atoms as compared to those of hydrogen and carbon atoms.21-23 The highest net dipole moment per unit cell and hence most electroactive is the ? phase (8 × 10?30 C m)24 due to the optimal alignment of the strong electronegative fluorine atoms with respect to hydrogen and carbon atoms, forming a net dipole moment perpendicular to the polymer chain.21 Moreover, the ? phase is non-polar because of antiparallel packing of the dipoles within the unit cell and centro-symmetric symmetry of the unit cell.22, 25-26 Thick paraelectric ?-PVDF films can be converted to ferroelectric ?-PVDF films by biaxial stretching.21, 27-28 Another way of inducing ?-phase formation is to blend PVDF with a second polymer, like poly(methyl metacrylate) (PMMA).29-31 The conformation of the polymer chains in ?-PVDF is in between that of the ? and ?-phases. The ?-phase is ferroelectric but experimentally hardly accessible. Formation typically requires extreme temperature control and high pressure.32 The ?-phase is a polar version of the ?-phase. It can be formed by electro-forming from an originally ?-phase bulk sample in a high electric field of about 250 MV/m.33

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