Crystal Structures of Three Crystalline Forms of Poly(vinylidene fluoride)

The crystal structures of three forms of poly(vinylidene fluoride) were studied by X-ray diffraction method. Although the structure of form I has been determined to be a fully extended planar zigzag by Lando, et al. [orthorhombic; a=8.58 Å, b=4.91 Å, and c(fiber axis)=2.56 Å; space group Cm2m(C2v14)], an alternately-deflected molecular structure was postulated in order to release the steric hindrance between the fluorine atoms along the chain. A satistically disordered packing of such deflected chains satisfies the observed fiber period and improves appreciably the structure factor agreement. Form II is monoclinic [pseudo-orthorhombic; a=4.96 Å, b=9.64 Å, c(fiber axis)=4.62 Å, and β=90°; space group P21/c(C2h5)], and its cell contains two molecular chains. The molecular conformation is essentially the TGTG type (internal rotation angles, 179° and 45°), and the glide plane of the molecular chain coincides with the c glide plane of the lattice. It is suggested that form III is monoclinic [a=8.66 Å, b=4.93 Å, c(fiber axis)=2.58 Å, and β=97°; space group C121(C23)], and the structural features similar to that of form I.