The Thermal and Dielectric Properties of Crystalline Long-Chain Acetates

Abstract

This investigation deals with the normal long‐chain acetates with 18, 20, 24, and 28 carbon atoms. When crystallized from the melt, these compounds first give the transparent α‐phase which gradually changes to the white, opaque β‐phase on standing or on cooling below a certain temperature. It is found that the acetates in the α‐phase all give large dielectric absorption, whereas, in the β‐phase the absorption is small, or, in some cases, negligible. Of the long‐chain compounds so far investigated, the acetates in the α‐phase are unique in giving two absorption regions which appear to be the result of the rotational transitions of complete molecules. The maxima for the two regions are separated by about 7 decades in frequency. Measurements of the lower frequency absorption at various temperatures show that the associated energy barrier is very large compared with previous values for compounds of similar chain‐length. To explain these results it is suggested that a molecule rotating in the crystal lattice of a long‐chain compound possesses four positions of equilibrium instead of two, as previously proposed. In addition to the two absorption regions mentioned above, the long‐chain acetates in the α‐phase give further absorption at still higher frequencies, which is probably caused by independent orientation of the polar groups near the ends of the molecular chains.