NMR OF SOLID POLYMERS

Complex solid sample NMR techniques used in other chapters of this text are discussed. These techniques are presented using conceptual arguments, rather than mathematical equations, so that those unfamiliar with NMR spectroscopy might get a quick grasp of the nature of the experiment. 

The papers presented in this volume represent a fraction of the applications developed from new techniques in NMR spectroscopy over the past decade. This flood of new methods has generated new terms which, though mathematically well defined, are difficult to visualize physically. As a result, the advantages of the newer methods (and the information to be gleaned from them) are often lost to the scientist not intimately familiar with NMR spectroscopy. 

The objective of this chapter is to provide a guide for the NMR layman to the methods used in the following chapters. Mathematical derivations have been avoided in favor of descriptions and diagrams to provide conceptual definitions. The discussion will concentrate on the techniques used in the following chapters, and therefore will not attempt to be comprehensive. Readers are referred to several texts and articles for detailed treatments of the subjects (1-5). 

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Schaefer J, Stejskal EO (1979) High-resolution 13C NMR of solid polymers, in Levy GC (ed) Topics in Carbon-13 NMR spectroscopy, Wiley, New York, vol 3... 

Multiple-Pulse NMR of Solid Polymers Dynamics of Polytetrafluoroethylene... 

It is obvious that the special characteristic of fluoropolymers for NMR is the incorporation of the F nucleus, which is in many ways particularly suitable for study and can act as a unique probe to examine the chemical microstructure, domain structure and mobility at the molecular level for these materials. However, some special techniques are frequently required for the best results from F NMR of solid polymers. These are fully described in Section 6.6. The net result of the difficulties encountered in obtaining high-resolution solid-state F spectra is that relatively few research papers to date have reported spectra with optimised resolution. Therefore, most reported F... 

J. Schaefer and E. 0. Stejskal, "High Resolution C-13 NMR of Solid Polymers", in Topics In Carbon-13 NMR Spectroscopy,... 

Lutnnger, G, Meurer, B, Weill, G Makromol Chem 1989,190, 2815 Majumdar, R N, Harwood, H J Appl Polym Anal Charact 1987,423 A single issue of Magnetic Resonance in Chemistry (1990,28) is devoted exclusively to the NMR of solids, although there are no specific arucles on solid-state fluonne NMR... 

D.E. Axelson, Carbon—13 Solid State NMR of Crystalline Polymers, In R.A. Komoroski (Ed.), High Resolution NMR Spectroscopy of Synthetic Polymers in Bulk, VCH, E>eerfield Beach, Florida, USA, 1986. 

Schaefer, J., High Resolution c nmr Studies of Solid Polymers, "Molecular Basis of Transition Relaxation",... 

In recent years new NMR techniques offering broad applications in stereochemical analysis have come into use. A prominent example is 2D-NMR (both 2D-resolved and 2D-correlated spectroscopy), which has been extensively applied to biopolymers (149-151). Its use with synthetic polymers has, until now, been limited to but a few cases (152, 153). A further technique, cross-polarization magic-angle spinning spectroscopy (CP-MAS NMR) will be discussed in the section on conformational analysis of solid polymers. 

Solids—Many polymers are either soluble or insoluble. NMR of solids generally give broad lines because of the effects of dipolar coupling between nuclei and the effect of chemical shift anisotropy (CSA). Both of these effects are greatly reduced for polymers in solution and allow for decent spectra of soluble polymers in solution. 

In NMR experiments, molecular mobility leads to narrowing of the resonance lines. Conversely, restricted molecular motion, as occurs in crystalline phases, causes line-broadening. Until the advent of magic angle spinning and related techniques, tliis was a hindrance to NMR studies of solid polymers. We have used it to advantage in following the orientation effects in solid PTFE. 

If — [MeHSiNH] — is used in place of — [F SiNMe] — in reaction 14, the resulting precursor is similar to the product of reaction 13. The solid state 29Si NMR of this polymer heated to selected temperatures is shown in Figure 221. The broad envelope of Si magnetic environments results from the series of SiNxC4 x structures, as seen previously43. 

Variable-Temperature Magic-Angle Spinning Carbon-13 NMR Studies of Solid Polymers... 

Methods of line narrowing in the NMR spectroscopy of solid polymers... 

Bliimich, B. Spiess, H.W. Two-dimensional solid state NMR spectroscopy new possibilities for the investigation of the structure and dynamics of solid polymers. Angew. Chem. Int. Ed. 1988, 27 (12), 1655-1672. 

High-resolution solid-state NMR spectroscopy, combined with quantum chemistry, provides detailed information on the structure and electronic structures of solid polymers through the observation of the NMR chemical shift [11-... 

One of the more recent applications of proton NMR in solid polymers has been the use of multiple quantum coherence (MQCOH), which is developed among clusters of dipolar coupled protons vide infra) to infer details of chain dynamics [20], and (which is another way of making the same statement) local dipolar couplings [21]. 

It has been demonstrated that solid state NMR spectroscopy provides useful information about the structure and dynamics of polymers in the bulk. At present, in polymer science, solid state NMR is recognized as one of the most powerful means for elucidating the structure and the dynamics of solid polymers in addition to X-ray diffraction. The history of solid state NMR, which has been used in polymer science, is very old. The appearance of new techniques in solid state NMR has certainly contributed to the development of polymer science and technology. 

C.R.C. Press, 1983 (b) R.A. Komoroski (Ed), High Resolution NMR of Synthetic Polymers in Bulk, VCH Publishers, 1986 (c) V.J. McBriety and K.J. Packer, Nuclear Magnetic Resonance in Solid Polymers, Cambridge University Press, Cambridge, 1993 (d) K. Schmidt-Rohr and H.W. Spiess, Multidimensional Solid State NMR and Polymers, Academic Press, London, 1994 (e) G.A. Webb and I.Ando (Eds), Ann. Repts. NMR Spectroscopy (Special Issue NMR in Polymer Science), Vol. 34, Academic Press, London, 1997. 

Cross Polarization Intensity as a Function of Contact Time. Two-component decays are often seen in the proton NMR of solid block polymers (20,21,22), and have been associated with the presence of two phases with different mobilities. Shafer, et. al. (23) have pointed out that the decay of the cross polarization signal as a function of contact time (which reflects the proton Tjp) can be used to characterize the polymer homogeneity. In the absence of phase separation the proton T]p values for each carbon should be the same. 

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