Short-Range Helical Ordering of Isotactic Vinyl Polymers in the Liquid State

Wide-angle x-ray scattering measurements on various vinyl polymer melts show that the main amorphous peak (at k$\sim $1.5 A$^{-1})$ in the structure factor initially broadens, and then forms a ``pre-peak'' that shifts to lower k as the size of the pendant group increases. To investigate this behavior we performed self-consistent PRISM calculations on isotactic polypropylene and polystyrene liquids. Good qualitative agreement was seen for the theoretical structure factors with scattering data. Analysis of the torsional angle distribution shows a significant amount of short-range helical content in the iPP and iPS melts. At 450$^{o}$K the average number of consecutive trans/gauche pairs along the chain backbone was significantly higher than for a random distribution of torsional angles. The theory indicates that the location of the pre-peak is a measure of the helix-helix correlation distance or helix ``thickness''.