4 - Experimental methods

Summary

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.