In this chapter, a number of examples are chosen to illustrate further the diversity of application of NMR in the field of solid polymers. The treatment is necessarily cursory since each topic forms a major area of research in its own right. The particular selection also serves to illustrate the point that useful information can be obtained with relatively unsophisticated and routine NMR procedures without the need for absolute recourse to state-of-the-art facilities.

Network systems

This discussion on network and interpenetrating network (IPN) systems affords the opportunity to comment further on the molecular motional regime between the extremes of solid and liquid behaviour, of which the interfacial effects discussed in Section 6.3 are examples. Generally, topological constraints such as entanglements and cross links induce significant deviations from liquid-like behaviour which are usually reflected in complex NMR relaxation decay (Kimmich, 1988). Motion is inevitably highly anisotropic and this leads to a residual dipolar coupling that typically defines the shape of the transverse relaxation function. Conversely, the shape of the function contains valuable information on the underlying complex motions. These effects were implicit in early measurements of the influence of molecular weight on NMR relaxation (McCall et al., 1962) where the Rouse-like dynamics for low-molecular-weight polymers developed into the more complex behaviour of entangled chains as the molecular weight increased.

Introduction

Application of NMR principles and experiments to solid polymers described thus far will now be illustrated in this and subsequent chapters. It is not intended to present comprehensive reviews since these appear in the literature on a regular basis and will be referred to as appropriate. Rather, the intention is to cover the main features of the role of NMR in illuminating relevant polymer properties in order to give the reader a clear view of the possibilities and diversity of options. The choices of how to organise the presentation of information are many. Here the interests of the polymer scientist rather than of the NMR specialist are emphasised.

These interests will vary considerably. Those who primarily wish to synthesise new polymers will want to know what polymer has been made, largely in terms of its chemistry. Those more interested in polymer properties and processing will want to use NMR to develop further their understanding of the relationship of mechanical and other properties to chain organisation, orientation, dynamics and so on. NMR of polymer solutions and melts has been of great utility, particularly since the introduction of pulsed Fourier transform methods and the routine availability of high-resolution 13C spectra. Such liquid-state high-resolution spectra have given the synthetic polymer chemist greater insight into the effects of synthesis conditions on microstructure, molecular weight distributions and so forth.

The study of solid polymers by NMR requires spectrometers with rather special characteristics. In this chapter we outline their principal features and describe a number of experiments that explore different aspects of the way in which resonant nuclei behave: measurements on 1H, 2H and 13C predominate. The low natural abundance of 13C and the unusually broad deuterium linewidths pose special technical problems.

Although the emphasis throughout is on pulse methods of excitation, it is important to recall that, for the determination of spectra, the only method widely used for many years was the field or frequency sweep technique, often referred to as continuous wave (CW) excitation. In this method an rf field of small amplitude is applied continuously and either the B0 field or the rf frequency is swept across the resonance absorption. Because of the very different requirements of high-resolution measurements in liquids and the broad, low-resolution spectra characteristic of abundant spins in solids (for example 1H and 19F), quite different spectrometer designs evolved for these two areas. The so-called ‘broadline’ CW spectrometer used field modulation techniques which generate the derivative of the absorption lineshape. Corrections were often necessary to account for the fact that component lines of the spectrum with substantially different spin—lattice relaxation characteristics and linewidths responded differently to the imposed modulation.

The nature of polymers

One of the remarkable features of current progress in materials technology is the ever-increasing ability to control and design the physical and chemical responses of synthetic polymers. An unprecedented range of properties is now routinely accessible through a variety of processing techniques which include chemical synthesis and substitution, thermal and electrical treatment, addition of fillers, blending of dissimilar polymers and mechanical deformation to name but a few.

Polymers are long chain molecules comprising large numbers of basic repeat units: the chemical structures of some of the more common types are furnished in table A1.1 (Appendix 1). For the most part, they are organic materials of high molecular weight in crystalline, glassy or rubbery states, one or more of which may be simultaneously present. Attempts to understand more fully the underlying reasons behind the properties of polymers are bedevilled by a characteristically complex morphology which defies precise description even in the chemically simplest cases. There is concomitant complexity too in the way in which polymer molecules move. Motions of flexible backbone chains, for example, derive from many coupled degrees of freedom and there is often a broad distribution of spectral frequencies associated with a given motional event. Two or more discrete motions may be active at the same time.

Introductory remarks

Preferred orientation can arise, either by accident or design, when polymers are subjected to particular fabrication procedures (Ward, 1982, 1985). Mechanical deformation typically induced by drawing, compression or rolling, can lead to anisotropy at the macroscopic level where, in effect, the orientational dependence that characterises the structural units at the molecular level is retrieved in part in the bulk: ultra-high-modulus fibres are notable examples. In other instances, molecular orientation can be achieved by suitable application of electric and magnetic fields and preferred orientation can occur at inter-phase surfaces in heterogeneous polymer systems, as discussed in Section 6.3. On the other hand, unwanted orientation may be induced by rapid spinning as in MAS (Maciel et al., 1985).

In materials as morphologically complex as polymers, one is inevitably concerned with macroscopic averages, or moments, of the distribution of structural units. Clearly it is important to determine such moments if the overall properties of the polymer are to be understood and quantified. The natural inclination is to resort to X-ray analysis which, indeed, has been predictably successful. However, a number of other theoretical and experimental tools have collectively given greater insight into the structure of non-isotropic polymers. Results from a variety of NMR experiments correlate well with each other (Kretz et al., 1988) and with alternative approaches including X-ray analysis (McBrierty et al., 1971b; Windle, 1982; Pietralla et al., 1985; Ward, 1985).