The
crystal structures of the two types of iodine complexes (complex I
and II) of poly(vinyl alcohol) (PVA), which were prepared respectively
from the KI/I2 aqueous solution of middle (0.1–0.5
mol/L) and high (1–3 mol/L) concentration of I2,
have been analyzed quantitatively on the basis of 2-dimensional X-ray
diffraction diagrams and Raman spectral data in addition to the computer
simulation method. In the crystal lattice of the original PVA, the
PVA chains of zigzag conformation are packed together by intermolecular
hydrogen bonds. Once the I3
– ions migrate
into the crystal lattice of PVA, these hydrogen bonds are deconstructed
and the PVA chains are displaced from the original positions so that
the I3
– ions can be coupled together
with the PVA chains through the charge-transfer between the I3
– and hydroxyl groups, resulting in the
formation of complex I. In this complex, the averaged occupancy of
I3
– ions is about 0.2, and the PVA–iodine
pairs and the original PVA chain pairs coexist to form a kind of super
lattice with the disordered packing structure of PVA chains and iodine
ion columns. As the iodine concentration is increased furthermore,
most of the PVA chains are combined with I3
– ions at the occupancy of about 0.7 to create the complex II. The
comparison of X-ray end diffraction patterns of the doubly oriented
samples clarified the spatial relation of the crystal lattices between
PVA, complex I and complex II. In the original PVA sample, the planar–zigzag
planes of PVA chains are parallel to the rolled plane of the doubly
oriented sample. This geometrical relation is kept in the complex
I sample. However, in the complex II, the PVA chains have been found
to rotate by 38° around the chain axis in the transition process
from complex I to complex II. In this way, the change in the packing
mode of PVA chains and iodine ions in the complex formation process
has been successfully revealed for the first time from the atomic
level.
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