Time-domain nuclear magnetic resonance
Time-domain NMR (TD-NMR) is an alternative to classical NMR, which is commonly used in structural analysis. TD-NMR works at low magnetic fields, which means that permanent magnets can be used. This is an advantage, as permanent magnets are cheap, simple and do not require extensive cooling by liquid gasses. However, the low magnetic field results in a low resolution, which is insufficient for obtaining Fourier-transformation frequency spectra. TD-NMR thus obtains information from the analysis of the dependence of signal intensity on time, i.e., form the “raw” data. The signal time-dependency is either directly fitted with relevant equations or relaxation time constants are extracted from it.
Figure 1: TD-NMR magnet
Figure 2: NMR test tubes (8 mm) with various samples
Experiments evaluating sample composition are usually based on direct fitting of the signal time-dependency. These experiments can distinguish between components (phases or compounds) that have different molecular mobilities. In polymer science, the crystallinity or copolymer content can be obtained from TD-NMR experiments. It is also possible to study semi-rigid interfaces, changes of composition with temperature or the effect of annealing. The relaxation time constants (T1 and T2) are physical properties of the sample and are sensitive, e.g., to molecular mass, viscosity of a liquid sample or “rigidness” of a solid sample. It is thus possible to obtain information on the sample properties from these constants. The relaxation times are also used to asses the moveability of polymer chains, which is hard to estimate by other methods. Relaxation time analysis can also provide information on the chain mobilities in individual phases of heterogeneous polymers.
Table 1: Approximate T2 values in orders of magnitude for various materials.
|Sample||State||T2 / µs|
|amorphous phase of PE||semi-crystalline solid||102|
|crystalline phase of PE
- FID – basic experiment, used for composition estimation
- Echo-based experiments – evaluation of relaxation constants
- Spin-diffusion experiments – evaluation of domain sizes
- Gradient experiments – estimation of diffusion coefficients in systems with no concentration gradients (pure liquids, systems in equilibrium)