Nuclear Magnetic Resonance Spectroscopy

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Nuclear magnetic resonance spectroscopy (NMR spectroscopy)

Definition

Nuclear magnetic resonance (NMR) spectroscopy is one of the most important spectroscopic methods; due to the multitude of interactions between nuclear spins and with their surroundings, it provides precise information from the very heart of matter. NMR spectroscopy can be used to investigate the structure and dynamics of plastics.

The method has become of paramount importance as an analytical tool in chemistry, biology, polymer science and many other fields for the detection of constituents in a test piece, the determination of molecular structures and the investigation of interactions between molecules (see also: in-situ tensile testing in NMR). A distinction is made between high-resolution and spatially resolved spectrometers in terms of instrumentation.

Fig. 1:

Superconducting magnet of the VARIAN Gemini high-resolution NMR spectrometer (300 MHz)

Superconducting magnet of the VARIAN Gemini high-resolution NMR spectrometer (300 MHz)

High-resolution NMR spectroscopy in solution generates very narrow resonance lines, the exact position and fine structure of which are influenced by the chemical environment of the nuclei. Its use in polymer analysis enables the determination of tacticity – no other method can resolve stereochemical microstructures with such precision.

In the case of homopolymers, the frequency of various configurational monomer triads can also be determined; for copolymers, the composition as well as the configurational and compositional sequence length distribution can be determined. Furthermore, the method enables the determination of short-chain branches and end groups, and can contribute to the characterisation of the polymerisation process and the development of bespoke plastics.

A typical frequency range for a high-resolution NMR spectrometer is 15–300 MHz. The required sample quantity is 10–100 mg.

The following nuclei can be detected: ¹H (see Fig. 2), ²H, ¹³C, ¹⁵N, ¹⁷O, ¹⁹F, ²³Na, ²⁹Si, ³¹P.

Fig. 2:

Structural formula and 1H NMR spectrum of isotactic polystyrene (abbreviation: PS)

Spatially resolved nuclear magnetic resonance spectroscopy

Spatially resolved NMR spectroscopy is also known as NMR imaging or MRI (Magnetic Resonance Imaging) and is primarily used in medicine as a diagnostic imaging technique under the term MRT (Magnetic Resonance Tomography).

The advantages of the method – such as non-destructive analysis of the test piece, the freedom to choose any cross-sectional plane, and the wide range of contrast options available when generating images – also allow for a wide variety of applications in the field of materials research.

Spatially resolved investigations serve to visualise inhomogeneities in the structure and dynamics of polymeric materials. Material defects can be identified and quality assurance carried out using NMR imaging. Dynamic processes such as chemical reactions, curing behaviour, separation, and diffusion and swelling experiments can be monitored over time and space. Volume-selective investigations of physically and thermally aged test pieces, such as rubber-elastic materials (see: ageing elastomers), as well as biological tissues and specimens, provide insight into their morphology.

The following nuclei can be detected: 1H, 2H, 19F. The focus is primarily on elastomers and ‘soft’ (molecularly highly mobile) plastics, composite materials, fibres, films and biological materials.

See also

• In-situ tensile test in NMR
• Hybrid methods
• Hybrid methods, examples

References

[1]	Claridge, T. D. W.: High-Resolution NMR Techniques in Organic Chemistry. 3rd Edition, Elsevier Science, Oxford (2016) (ISBN 978-0-0809-9986-9; E-Book ISBN 978-0-0809-9993-7)
[2]	Friebolin, H.: Ein- und zweidimensionale NMR-Spektroskopie: Eine Einführung. 5. Auflage Wiley-VCH Verlag (2013) (ISBN 978-3-527-33492-6)
[3]	Blümich, B.: NMR Imaging of Materials. Clarendon Press, Oxford (2000) (ISBN 9780-1985-2676-6)

In-situ tensile test in NMR (own works):

[4]	Döhler, S., Heuert, U., Grellmann, W.: Material Properties Imaging zur Werkstoffdiagnostik. In: Forschungsbericht, Hochschule Merseburg (2011), pp. 119–125 (ISBN 978-3-942703-07-9)
[5]	Döhler, S., Heuert, U., Grellmann, W.: Material Properties Imaging zur Aufklärung von Materialeigenschaften. 13. Tagung Problemseminar „Deformation und Bruchverhalten von Kunststoffen“, Proceedings (2011), pp. 419–422
[6]	Döhler, S., Glatz, D., Heuert, U., Grellmann, W.: Entwicklung einer neuartigen in-situ-Zugeinrichtung für kernmagnetische Untersuchungen an Elastomeren. In: Honekamp, W., Schindler, P. (Hrsg.), 13. Nachwuchswissenschaftlerkonferenz Proceedings (2012), pp. 185–190 (ISBN 978-3-86870-436-5)
[7]	Döhler, S., Reincke, K., Heuert, U., Grellmann, W.: Entwicklung eines hybriden Prüfverfahrens zur strukturellen Aufklärung von Deformationseigenschaften elastomerer Werkstoffe. 14. Problemseminar "Deformation und Bruchverhalten von Kunststoffen", 25.–27. Juni 2014, Merseburg, Proceedings pp. 657–663 (ISBN 978-3-942703-30-7)
[8]	Döhler, S., Heuert, U., Reincke, K., Grellmann, W.: Entwicklung eines neuartigen hybriden Prüfverfahrens zur Kopplung von Zugversuch und strukturaufklärender NMR-Spektroskopie. Werkstoffprüfung 2015, Fortschritte in der Werkstoffprüfung für Forschung und Praxis, 3. und 4. Dezember 2015, Bad Neuenahr, Proceedings pp. 267–272 (Eds.: Borsutzki, M., Moninger, G.) (ISBN 978-3-514-00816-8); see AMK-Library under M 36)