Although the 'crystal modulus' of most semicrystalline polymers has been estimated by measuring the displacement of meridional X-ray reflections on application of a stress to an oriented specimen, this method has not been applied to polyacrylonitrile (PAN). The X-ray diffraction behaviour and the long-range order in PAN are very controversial. Several reports state that the diffraction pattern of oriented PAN contains only equatorial peaks and therefore the material is only laterally ordered. However, there are a few papers that mention or imply sharp meridional and off-axis reflections. We have produced two samples of PAN which were oriented by different production methods. Both our specimens 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 specimens had a preferred double orientation of crystallographic axes and 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 hexagonal symmetry in a bulk sample of PAN. As we were unable to reproduce the 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 been done with other polymers. However, by considering the rod-like conformation adopted by the PAN chains, and comparing it with polymers which adopt a helical form, it was estimated that the maximum tensile modulus of atactic PAN would be about 55 GPa.
Посилання на статтю:
An investigation into the possibility of measuring an "X-ray modulus" and new evidence for hexagonal packing in polyacrylo n itri le / R. A. Allen and I. M. Ward // Polymer. – 1994. – Vol 35. – P. 2063-2071.