Molecular Motion of Solid Cycloalkanes

Abstract

Cycloalkanes with 12 to 96 methylene groups show solid-solid transitions of high cooperativity. Although the molecules are highly flexible and do not contain mesogenic units as it is typical for liquid crystals, they form a mesomorphic phase. In this mesomorphic state, fast molecular motion with correlation times smaller than 10⁻⁸ s is observed by 2H-NMR. The molecular structure is characterized by dynamic conformational disorder. In addition slow conformational interconversion is evident below the phase transition. Thus, jump type interconversion processes which do not result in disordering occur in the fully ordered crystal. At the phase transition, the increase in the specific volume and the onset of additional molecular mobility results in the formation of a partially disordered, fluid structure.