THERMALLY-INDUCED MOLECULAR-MOTION AND PREMELTING IN HEXACONTANE

A combination of careful differential scanning calorimetry, solid-state NMR, small- and wide-angle X-ray diffraction, and selected area electron diffraction studies of pure C60H122 (hexacontane) were carried out to evaluate its thermal behavior in the crystalline state with increasing temperature. Solution-crystallized material showed only a single sharp endothermic peak at 99-degrees-C corresponding to the melting transition, while bulk crystallized material showed an additional small endothermic peak at 71-degrees-C. Variable-temperature C-13 CP/MAS spectra showed a loss of cross-polarization and peak intensities between 40 and 74-degrees-C which indicated increased molecular motion. A complete loss of signal occurred above 74-degrees-C, confirming relatively large-scale rotational, translational, and/or librational motion. Alternative use of high-power decoupling (C-13 HPD/MAS) spectroscopy allowed observation of the mobile components of the samples, especially at the higher temperatures from 77 to 99-degrees-C. Peak sharpening due to increased molecular motion plus chemical shift changes characteristic of increasing populations of gauche conformations near the chain ends were observed. These confirm a gradual increase in the conformational mobility of segments near the crystal surface with increasing temperature (probably related to translational movement at the alpha-transition) that generates a rough surface able to accommodate the end-group trans-gauche jumps. The interior methylene segments, however, maintain their all-trans conformation up to the melt temperature of 99-degrees-C. Wide-angle X-ray patterns of solution-crystallized samples confirmed orthorhombic structure, while bulk-crystallized material possessed the monoclinic structure. The combination of techniques employed clearly confirmed the occurrence of premelting (as described above) at ca. 30-degrees-C below the melting point for both samples that involves increased terminal group mobility coupled with chain translation. The lack of a DSC transition for the high-purity solution-crystallized sample plus the gradual change in the NMR behavior which occurs with increasing temperature is consistent with a nonconcerted process related to the concerted a transition observed for various molecular weight samples of polyethylene and impure n-alkanes.