XRD study of the new rigid-rod polymer fibre PIPD

Abstract

The new rigid-rod polymer fibre PIPD was studied using wide angle X-ray scattering. The crystal structure of as-spun PIPD fibre can be described as a two-dimensionally ordered crystal hydrate. Its equatorial X-ray diffraction pattern was indexed to a non-primitive rectangular unit cell with parameters 16.85, 3.38 Å, containing two polymer chains. Heat treatment initially (i.e. up to 300°C) leads to the loss of water molecules from the structure as was shown by high temperature X-ray diffraction. At sufficiently high temperatures three-dimensional crystalline order is developed, as evidenced by the presence of strong off-axis reflections in the diffraction pattern of the heat-treated fibre. A procedure of unit cell determination was introduced that made it possible to index the latter diffraction pattern and elucidate the three-dimensional packing. The crystal structure of heat-treated PIPD fibre has monoclinic symmetry described by the space group P2 1/ a. Its cell dimensions are 12.60, 3.48, 12.01 Å, 90.0, 108.6, 90.0° with ρ x =1.77 g cm −3. The unit cell is non-primitive ( Z=2), with chains located at the centre and corners of the rectangular projection cell describing the lateral packing. Neighbouring chains packed along the diagonals of the projection cell are shifted relative to each other in the direction of the c-axis (chain axis) by 2.0 Å. This regular c-axis shift explains the observed absence of significant meridional scattering from the lower order layer lines of the diffraction pattern. A hydrogen bonding scheme is proposed consisting of intermolecular N–H—O bonds and intramolecular O–H—N bonds. The intermolecular hydrogen bonds form a bi-directional hydrogen bonding network linking each polymer chain to its four axially shifted neighbours. Departures of the latter hydrogen bonding scheme may be present in the form of hydrogen bonded sheets. The proposed hydrogen bonding scheme is consistent with thermal expansion data provided by high temperature XRD. The monoclinic crystal structure with its bi-directional hydrogen bonding network provides a satisfactory explanation for the exceptionally good compression performance of heat-treated PIPD fibre.