Methyl groups as local probes for polymer dynamics as investigated by 1H double-quantum magic-angle spinning NMR spectroscopy

We illustrate the application of high-resolution homonuclear 1H double-quantum magic-angle spinning solid-state NMR recoupling experiments for an efficient measurement of segmental order parameters for polymers with methyl groups. A theoretical treatment of the spin dynamics of methyl groups during a corresponding pulse sequence including experimental imperfections is given. The applicability of the analytical result for the extraction of dipole–dipole coupling constants from experimental data is illustrated for a model substance. Experiments on entangled polymer melts indicate that double-quantum spectroscopy on methyl groups is well-suited for the extraction of very weak coupling constants, which permit the calculation of well-defined order parameters.