NMR spectroscopy

Nuclear magnetic resonance (NMR) is the primary spectroscopic technique utilized in the study of polymer dynamics. NMR is in many ways complementary to the scattering (neutron) and relaxation (mechanical and dielectric) techniques described in Chapters 1 and 2. The major advantage of NMR over other methods of characterizing polymer dynamics is its selectivity. Firstly, the precession (Larmor) frequencies of different nuclei, and even different isotopes of the same element, differ dramatically. Secondly, the resonance of a given nucleus depends upon its surroundings due to internal coupling, making NMR sensitive to the details of the chemical structure of the polymer. Finally, the dramatic differences in the natural abundances of different isotopes provide the opportunity to increase selectivity through isotopic labeling. NMR provides information on the dynamics of local motions. Many NMR parameters are sensitive to local molecular motions and the NMR methods that have been applied in probing polymer dynamics are varied and numerous [1, 2]. NMR parameters that are sensitive to local molecular motions include relaxation times, spectrum line shape, the strength of dipolar coupling, and chemical-shift anisotropy. The accessible spectral windows depend upon the type of measurement performed, ranging from 10 −1 Hz for measurements sensitive to slow motions to several hundred megahertz to those sensitive to fast motions. In the case of polymers at temperatures well above the glass transition temperature local motions are fast processes that average chemical-shift anisotropy, homonuclear and heteronuclear dipolar couplings, and quadrupolar couplings.