a Nuclear Magnetic Resonance Study of Deuterated Poly(vinylidene Fluoride) and a Copolymer of Deuterated Vinylidene Fluoride and Tetrafluoroethylene.
Pulsed deuterium NMR experiments have been performed on deuterated samples of poly(vinylidene fluoride), PVF(,2), repeat unit CH(,2)CF(,2) and the copolymer poly(vinylidene fluoride)-tetrafluoroethylene (80-20), (PVF(,2)-F(,4)E). A deuterium line shape study has been employed to characterize the orientational distribution of the dipole moments in both poled and unpoled samples. In addition, the orientational distributions of chain axis alignment has been measured in stretched samples. PVF(,2) and PVF(,2)-F(,4)E exhibit unique piezoelectric and pyroelectric properties which are due to their crystalline components (approximately 50% in typical material). Four crystalline phases exist in the homopolymer PVF(,2), the (alpha), (beta), (gamma), and the (delta). The relatively electrically inactive, melt solidified (alpha) phase can be transformed into the ferroelectric (beta) phase by mechanical deformation (stretching) or by application of large electric fields (poling- 200 Mv/meter). The copolymer melt solidifies directly into the (beta) phase. The (beta) phase is characterized by a net dipole moment per unit cell due to the parallel alignment of the CF(,2) dipole moments in the unit cell. The orthorhombic unit cell dimensions of the (beta) phase render a nearly pseudohexagonal packing structure of the molecular chains and is thought to allow for dipole reorientations via 60(DEGREES) steps during the poling process. The orientationally dependent quadrupole interaction has been used to characterize the orientational distributions of the molecular dipole moments about the poling direction in both poled and unpoled samples of PVF(,2) and PVF(,2) -F(,4)E. No orientational dependence was found in either the poled or unpoled samples. This is in contrast to recent x-ray results which report substantial orientational anisotropy in rolled, poled protonated films. We have also measured the degree of chain axis alignment in stretched samples of both PVF(,2) and PVF(,2)-F(,4)E. Alignment in both samples is characterized by a gaussian distribution function about the stretch direction with a half width 1/e maximum of 22(DEGREES) and 18(DEGREES) respectively. The characterization of the experimental results was done by comparing computer simulated line shapes with the experimental line shapes.
- Pub Date:
- Physics: Condensed Matter