Solid State Nuclear Magnetic Resonance Spectroscopy

The Nuclear Magnetic Resonance (NMR) technique utilizes the fact that certain atomic nuclei have magnetic moments related to their spin and mass number. Isotopes of elements having odd mass numbers often have magnetic nuclei. When subjected to an external magnetic field, these magnetic nuclei absorb electromagnetic energy. Maximum energy is absorbed by the nuclei at their resonant frequency. When the eternal magnetic field is removed, the nuclei return to lower energy levels, emitting excess energy which is measured as absorption peaks in the NMR spectrum. The relative height of each peak correlates with the strength of absorption and area under the peaks is proportionate to the number of Hydrogen atoms contributing to that peak. This is the principle of NMR spectroscopy.

NMR experiments can provide valuable insights in studies of molecular structure and topology. Typically, organic compounds need field strengths ranging from about 1.4 to 18.0 teslas (T) and are subjected to electromagnetic radiation in the radio frequency range. NMR can be applied to substances in liquid solution or in solid state. When NMR spectroscopy is performed on solids, it is termed Solid State NMR.  

Solid state NMR spectroscopy of polymers is industrially significant, as most applications of polymers are in the solid state. Magnetic isotopes of elements such as ¹H, ¹³C, ¹⁹F, ²⁹Si, ¹⁵N, yield high-resolution NMR spectra at their resonant frequencies. These elements are important constituents of plastics, elastomers and adhesives.

When studying molecular structures in solid polymers, NMR spectroscopy  targets the magnetic ¹³C nucleus in the Carbon backbone. However only a small percentage of Carbon atoms in a polymer molecule comprises ¹³C isotope and they are surrounded by ¹H atoms (protons). The resulting dipolar interactions with magnetized protons distort the ¹³C signal. This is resolved by heteronuclear dipolar decoupling (DD) which involves sending a strong radiofrequency pulse at the ¹H frequency, during the period when the ¹³C signal is observed. Another necessary technique to be used in conjunction with Solid state NMR of polymers is magic angle spinning (MAS) which is used to deal with Chemical shift anisotropy that distorts signals. Chemical shift refers to the position of a peak relative to the peak of a reference chemical. Atoms in a molecule have different chemical shifts because they experience slightly different local magnetic fields owing to the presence of nearby electrons. The MAS procedure involves spinning the solid sample, at rates of a few thousand hertz and an angle of precisely 54.74° relative to the static magnetic field.

Polyethylene-co-vinyl acetate or EVA is an important elastomeric polymer. It is a random copolymer with   vinyl acetate percentage varying from 10 % to 40% depending on end use. The molecular structure of EVA copolymer is as below:

In this case study, solid state ¹³ C NMR with MAS was used to analyse a powdered sample of EVA co-polymer. Figure 1 shows the ¹³C MAS NMR spectrum. The peak at 30.2 ppm is the  CH₂ backbone and a smaller peak at 14.3 ppm shows the CH₃ radical of the acetate branch. Two small signals at about 21 and 25 ppm, are interpreted as CH₂ from the ethylene branching.

Figure 1: Solid-state MAS NMR ¹³C spectrum of EVA polymer in powder form showing peaks and relative quantities of Hydrocarbon functional groups.

Reference:

Maria Ines Bruno Tavares, Solid State NMR, DOI: 10.5772/intechopen.71004, December 6^th 2017, https://www.intechopen.com/chapters/57186