Thermally Induced Molecular Motion and Premelting in the Solid State of n-Hexatriacontane

Abstract

A combination of differential scanning calorimetry and solid-state CP/MAS 13 C NMR studies of pure n-C 36 H 74 (hexatriacontane) were carried out to evaluate its thermal behavior in the solid state with increasing temperature. Three different modes of crystallization were used, and particular attention was given to the NMR behavior relative to the pseudohexagonal transition. Peak sharpening due to increased molecular motion and chemical shift changes characteristic of increasing populations of gauche conformations near the chain ends were observed below the pseudohexagonal transition. These observations confirm a gradual increase in the conformational mobility of segments near the crystal surface with increasing temperature. The interior methylenes, however, maintain their all-trans conformation up to the transition to the pseudohexagonal phase at 72.6-72.7 °C. The combination of techniques employed clearly confirmed the occurrence of premelting (defined as the disordering of chain-end segments) at ca. 15-20 °C below the melting point, depending on sample treatment.