Methyl Groups as Local Probes for Polymer Dynamics as Investigated
1H Double-Quantum Magic-Angle Spinning
Chem. Phys. Lett. 362, 331-340 (2002).
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.