A synchrotron X-ray scattering study of the rotator phases of the normal alkanes

Abstract

The authors present results of an X-ray scattering study on the rotator phases of normal alkanes: CH3-(CH2)n-2-CH3 (20<or=n<or=33). These are layered plastic-crystal phases in which the long axes of the molecules are all parallel and can be either parallel to or tilted from the layer normal. They have characterized a new tilted rotator phase and measured the distortion, area per molecule and tilt parameters in all five rotator phases. There is no strong even-odd chain effect within the rotator phases, but there is a significant change in phase sequence in the C26-C27 carbon number region. A typical phase sequence for the shorter chains is L-RII-RI-RV-crystal. For longer chains the sequence is L-RIV-RIII-crystal. In the RV, RIV and RIII phases the molecules are tilted. There is a general trend towards decreasing distortion and increasing magnitude of tilt with increasing carbon number. An unusual decrease in area per molecule with increasing temperature for the high-carbon-number phases suggests that defects may play a role in the RIV phases.