Knowledge Bank

The conformation and tactic structure of Polyacrylonitrile (PAN) have not been unambiguously determined. X-ray measurements provide some information as to the degree of order between chains. Phase transitions have been observed in which the lateral packing is altered upon annealing above $140\circ C$. Information concerning the amount of order along the chain has not yet been available. In the present work the neutron inelastic spectra have been obtained in several samples of PAN with different thermal histories and methods of polymerization. Frequency distributions in a range $30-800cm-1$ were derived from these measurements. Significant changes are observed in the intensity and width of the maxima at 560 and $265cm-1$, between the various samples and upon temperature cycling. These maxima are assigned, in agreement with I.R. data, to C-CN and C-C-N bending and C-C-C bending respectively. Since no change in tacticity can take place with change in temperature, these changes are associated with ordering along the chain and probably correspond to an increase in length of the segments of the chain in trans-planar conformation. The authors are guests at the Army Materials Research Agency, Watertown, Mass.