Exploiting methyl groups as motional labels for structure analysis in solid polymers.
We report results of magic-angle spinning (MAS) nuclear Overhauser experiments on solid polymers. Specifically, the focus of this work is to demonstrate that methyl groups provide unique motional labels in crystalline and amorphous materials that may be exploited to provide spatial information. The motivation for this work stems from the fact that recently developed techniques for distance determination in organic solids, e.g. REDOR and related spin-echo methods, typically require isotopic labelling. Comparison of heteronuclear MAS 13C-1H NOE growth rates for bis-phenol A (a crystalline organic solid), polycarbonate, chloral polycarbonate, polystyrene, poly(4-methylstyrene), and poly(alpha-methylstyrene) confirms that methyl groups are the only sources of cross-relaxation in these rigid polymers. The experiments are straightforward, and have the advantage that no isotopic labelling is required. In general, decreased growth rates and increased induction periods for carbons distant from methyl groups are observed. Critical analysis of the data suggests that the rates are sensitive to packing density, and we address the contribution of intrachain and interchain interactions to the observed rates. We expect that methyl-stimulated NOE's are general to all rigid solids, and that these experiments could prove useful for local structure characterization of polymers in cases where isotopic labelling is not practical.