Chapter 13: Dipolar Based NMR Methods for Probing Intermediate Regime Motions in Polymers
Published:29 Jul 2019
J. G. Filgueiras, M. F. Cobo, G. C. Faria, T. B. Moraes, and E. R. de Azevedo, in NMR Methods for Characterization of Synthetic and Natural Polymers, ed. R. Zhang, T. Miyoshi, and P. Sun, The Royal Society of Chemistry, 2019, ch. 13, pp. 271-298.
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This chapter is dedicated to the discussion of some dipolar based solid-state NMR methods that can be used to probe molecular motions in polymers. The focus is on methods to study the so-called intermediate regime motions, i.e., occurring with motional rates from kHz to MHz. In the first part, 1H time domain NMR methods performed at low magnetic field are presented as a first approach to identify and characterize intermediate regime molecular motions. The use of methods such as Magic Sandwich Echoes (MSE) and Dipolar Filters (DF) to determine onset temperatures, motional rates and their distributions as well as activation energies of molecular motions is discussed in more detail. In the second part, 1H–13C Dipolar Chemical Shift Correlation (DIPSHIFT) MAS experiments are presented as an alternative to study intermediate regime motions with site-specific information, which is important when local molecular motion is of interest. As examples, the application of these methods to study many types of motion in different polymer systems is presented and discussed. Based on that, the combination of 1H TD-NMR at low magnetic field and 13C MAS NMR spectroscopy at high magnetic field is presented as an efficient way for characterizing molecular mobility and its temperature dependence.