AN INVESTIGATION INTO THE POSSIBILITY OF MEASURING AN X-RAY MODULUS AND NEW EVIDENCE FOR HEXAGONAL PACKING IN POLYACRYLONITRILE

Although the 'crystal modulus' of most semicrystalline polymers has be en estimated by measuring the displacement of meridional X-ray reflect ions on application of a stress to an oriented specimen, this method h as not been applied to polyacrylonitrile (PAN). The X-ray diffraction behaviour and the long-range order in PAN are very controversial. Seve ral reports state that the diffraction pattern of oriented PAN contain s only equatorial peaks and therefore the material is only laterally o rdered. However, there are a few papers that mention or imply. sharp m eridional and off-axis reflections. We have produced two samples of PA N which were oriented by different production methods. Both our specim ens always produced the 'standard' fibre pattern of PAN which consists of just two equatorial peaks and weak, diffuse meridional scattering. The standard pattern has been assumed in the literature to arise from a hexagonal packing of chains with no chain-axis order. One of our sp ecimens had a preferred double orientation of crystallographic axes an d when the X-ray beam was passed along the fibre axis of this sample, a six-arc pattern was obtained. This result is new evidence for hexago nal symmetry in a bulk sample of PAN. As we were unable to reproduce t he claims made by a few authors regarding sharp meridional peaks, the crystal modulus of PAN could not be measured in the same way as has be en done with other polymers. However, by considering the rod-like conf ormation adopted by the PAN chains, and comparing it with polymers whi ch adopt a helical form, it was estimated that the maximum tensile mod ulus of atactic PAN would be about 55 GPa.