Polymorphism of poly(vinylidene fluoride). III. The crystal structure of phase II

Abstract

In this study, X-ray diffraction has been used to determine the crystal structure of poly(vinylidene fluoride) phase II. Three principal chain conformations were considered, which included two 2₁ helical forms [degenerate (planar) and nondegenerate] and the trans-gauche-trans-gauche' form.

An X-ray crystal structure determination has shown that the trans-gauche-trans-gauche' (TGTG′) conformation is the correct one for phase II. Experimental X-ray intensity data were taken at both −145° and 20° C in order to study the effect of temperature on the resulting structure.

From the comparison between observed and calculated intensities, there are two possible crystal symmetries, P₂₁ and P₁, depending upon whether the two chains are taken to be antiparallel or parallel to one another. For both cases, the reliability indices for the chosen structure are almost identical. For either symmetry the unit cell is metrically orthorhombic (all angles equal to 90° within experimental error) with the following lattice constants at 20°C: a = 5.02 Å, b = 9.63 Å, and c (chain axis) = 4.62 Å.