Effects of defects on the longitudinal acoustic mode of n-alkane chains

Abstract

Normal mode calculations show that a single conformational defect in an otherwise all-trans n-alkane molecule disrupts the longitudinal acoustic mode (LAM) associated with the all-trans molecule. Similar calculations for a chain in the conformation produced by smoothly twisting a planar zigzag through 180 °about the chain axis show that the LAM amplitudes and frequencies are unaffected by this gentle twist. These calculations indicate that the decrease in the LAM intensity observed in some polyethylene samples cannot be accounted for by smoothly twisted chains, but can be accounted for by defects which involve large localized departures from the all-trans conformation.