Molecular Motion in Crystalline and Mesomorphous Phases of Large Size Cycloalkanes as a Model for Polyethylene

Abstract

Cycloalkanes with 22 to 96 units crystallize in lamellae, where two adjacent stems are connected by two tight folds. Before melting, they form mesophases, which can be described as conformationally disordered crystals (Condis crystals). Conformational disorder and molecular mobility in the solid and in the mesomorphic state have been studied using MAS ¹³C-NMR spectroscopy and ²H-NMR spectroscopy. In the case cyclodoeicosane and cyclotetraeicosane, MAS ¹³C-NMR spectra show that slow jump type reptation results in an exchange of the carbon atoms between stems and folds already in the crystalline state. Conformational disorder and fast molecular motion is observed for the mesophase. The exchange of methylene groups can occur with rates which are larger than 10⁸s⁻¹. The molecular dynamics in the mesophase is described by fluctuation of additional gauche conformations. The passage of a gauche-trans-gauche defect along the chain leads to small translational motion of the chain segment and can thus explain the translational diffusion of methylene groups from the inner part of the lamellae to the folds and vice versa. This observation of fast chain reptation through the lamellae may help to understand the formation of extended chain crystals in polyethylene.