Chain Folding and Diffusion in Monodisperse Long n-Alkanes by Solid-State NMR

The size of the folded segment of the alkane chain in the integer-folded F2 form of C246H494 has been measured directly by NMR and shown to contain an average of 8 +/- 2 carbon atoms at 293 K. This tight fold is in contrast to the looser fold obtained indirectly for similar samples in earlier NMR work but in good agreement with the picture provided by SAXS and LAM Raman spectroscopy Studies. The recovery after saturation of the carbon-13 magnetization associated with the all-trans carbon-13 peak has also been measured for recovery times as long as 4000 s. The behavior as a function of recovery time has been compared with simple simulations which model the combined effects of chain diffusion and spin-lattice relaxation. These results suggest that at short times the mechanism for the magnetization recovery involves the alkane chain diffusing around the fold with the methyl groups constrained from entering the crystal through the opposite surface of the lamella. Hence, only small fluctuations in the length of the fold segment occur, and the two linked stems move in a loosely co-operative fashion. Despite the tight fold, the rate of magnetization recovery is compatible with a frequency for the underlying chain jump process similar to that responsible for the a-relaxation in polyethylene.