THE VARIATION OF THE D-SPACINGS WITH STRESS IN THE HEXAGONAL POLYMORPH OF POLYACRYLONITRILE

The normal X-ray fibre pattern of polyacrylonitrile (PAN) consists of just two sharp equatorial peaks (d(1) similar to 0.53 nm and d(2) simi lar to 0.30 nm), and very diffuse meridional and off-axis scattering. This has been attributed to a hexagonal packing of chains, with no per iodicity along the chain axis. In this work, we have investigated the effect of a tensile stress on this fibre pattern. It was found that bo th d, and d, decreased with increasing stress, but the ratio d(1)/d(2) remained constant within experimental error at root 3/1. This indicat ed that the 'molecular rods' decreased in diameter on application of t he stress, but their packing remained essentially hexagonal. The norma l meridional scattering from PAN shows a two-component broad peak; whe n stress was applied, there was no shift in the positions of either co mponent, but there was a transfer of intensity from high to low 2 thet a values. This was also consistent with a slight extension of the rod. By adopting a simple model of extending rods, it was possible to comp ute an X-ray stress-strain curve and a modulus from the change in the two equatorial peaks.