Multidimensional NMR Techniques for the Characterisation of Viscoelastic Materials

An important objective in materials science is the establishment of relationships between the microscopic structure or molecular dynamics and the resulting macroscopic properties. Once established, this knowledge then allows the design of improved materials. Thus, the availability of powerful analytical tools such as nuclear magnetic resonance (NMR) spectroscopy [1-6] is one of the key issues in polymer science. Its unique chemical selectivity and high flexibility allows one to study structure, chain conformation and molecular dynamics in much detail and depth. NMR in its different variants provides information from the molecular to the macroscopic length scale and on molecular motions from the 1 Hz to 1010 Hz. It can be applied to crystalline as well as to amorphous samples which is of particular importance for the study of polymers. Moreover, NMR can be conveniently applied to polymers since they contain predominantly nuclei that are NMR sensitive such as H and 13C. 

While well-established for liquid-like samples such as macromolecules in solutions [7], the applications of NMR to solid or solid-like polymers is more demanding because of the presence of anisotropic interactions that complicate the analysis of the results. Several techniques for the removal of these interactions have thus been developed and are nowadays in a state where they can be routinely applied. Non-averaged anisotropic interactions on the other hand provide valuable information that is lost in the solution state. Thus, while it is often necessary to remove the anisotropic interactions, in many cases one would simultaneously like to preserve them in order to exploit their information content. 

This is where two-dimensional (2D) spectroscopy comes into play, for example, by correlating one dimension where the anisotropic interactions are preserved with a (high-resolution) dimension where they are removed. That is, both objectives can be achieved within one two-dimensional experiment. Like conventional one-dimensional (ID) NMR techniques, 2D techniques were first applied to liquid or solutions, where they provided invaluable information for the structure assignments in biomolecules [7]. Later, after the 

After a brief introduction of the basic tools of NMR in Section 14.2, the 2D techniques that have been already applied to rubbers or viscoelastic materials (Section 14.3) will be reviewed. After briefly introducing each of the techniques, a more detailed overview of the applications and a discussion of some of the highlights will be given. This structure, where all information on a given method is presented within one section enables the interested reader to decide more easily which of the techniques might be most useful to them. NMR imaging which can be considered as a special form of 2D NMR will not be discussed but the interested reader is referred to the corresponding chapter in this book. 

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