Calorimetric and neutron scattering studies of plastically crystalline cyclooctanol*

Abstract

The heat capacity of cyclooctanol was measured with an adiabatic calorimeter in the temperature range 5–340 K. Liquid cyclooctanol crystallized into crystal I, a plastic (orientationally disordered) phase. Crystal I was supercooled readily and underwent a glass transition at 160 K. Crystal II, obtained by annealing crystal I at about 200 K, also underwent a glass transition at 160 K, indicating that crystal II is also an orientationally disordered phase. On heating, crystal II transformed to crystal I at 261.7 K with a transition entropy of 8.06 J K−1 mol−1 and crystal I fused at 295.3 K with a fusion entropy of 7.00 J K−1 mol−1. Neutron scattering of cyclooctanol was measured in the temperature range 20–335 K, energy range 0.1–20 meV and momentum transfer range 0.23–2.7 Å−1. A clear boson peak was found around 2.5 meV in both orientational glasses of crystal I and II. Quasielastic scattering appeared at temperatures as low as the glass transition temperature. This may be due to a fast β process which has been observed in most glass-forming liquids. The present results indicate that glass-forming plastic crystals are similar to glass-forming liquids in their dynamical properties in terahertz region.