NMR characterisation of polymers

Nuclear magnetic resonance (NMR) spectroscopy is an important method for materials characterisation and for the study of polymer structure-property relationships. The importance of NMR as a technique arises in part because the signals can be assigned to specific atoms along the polymer backbone and side chains [1,2]. The properties of the NMR signals depend on the magnetic environment of the NMR active nuclei, and the local fields that they experience. Since the NMR spectrum is determined by local forces, this method provides valuable and unique information about polymers on an atomic-length scale. 

NMR spectrometers with fields strengths as high as 14.1 T (600 MHz for protons) are currently available, but the highest possible field strengths are not required for most applications. 

The NMR sensitivity depends on the magnetogyric ratio and the natural abundance of the NMR active nuclei. Table 3.1 lists the properties of some of the nuclei that are of particular interest to polymer chemists. The sensitivity is highest for protons and fluorines, but nuclei such as carbon, silicon and phosphorus can be routinely studied. Deuterium and nitrogen have poor sensitivities, but they provide valuable insight into the structure and dynamics of polymers when isotopically enriched samples are prepared. 

Isotope Abundance (%) Spin 10 Relative sensitivity Frequency at 2.35 T (MHz) 

The line widths and relaxation times are determined by the local atomic fluctuations in the polymer main- and side-chain atoms. The line widths can be directly determined from the spectrum, while the relaxation times are measured by monitoring the return of the spin system to equilibrium following some perturbation. The relaxation times are typically of the order of milliseconds to seconds in high polymers, but depend on the molecular motions that occur on the picosecond to microsecond time scale. 

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In this chapter we illustrate a direct method of characterisation of polymer/additive dissolutions by means of (500 MHz) NMR, which takes advantage of selective signal suppression allowing elimination of unwanted signals, such as the ca. 105 x more intensive PE signal. The most effective approach to solvent suppression is the destruction of the net solvent magnetisation by pulsed... 

Another polymer symposia was organised by IUPAC in 1947 in Liege. At this conference, the discussion included synthesis and technology of polymers like polyethylene, nylon, polyester. New characterisation methods such as x-ray scattering, x-rays, electron microscope, osmometry, nmr, IR, Raman spectroscopy, etc. were now available for characterisation of polymers. These methods become essential because of increasing complexity of new polymers. 

A particularly important application of NMR spectroscopy is the structural characterisation of polymers. While initial attempts were made using NMR, the wide... 

The determination of the microstructure of vinyl polymers is not merely a characterisation tool. Each polymer molecule is unique, and each polymer chain is a record of the history of its formation, including mis-insertions, rearrangements, the incorporation of co-monomers, and the mode of its termination. NMR analysis of polymers can therefore be used to provide detailed mechanistic and kinetic information. This approach has been applied particularly successfully to the microstructure, i. e. the sequence distribution of monomer insertions, of polypropylene, giving rise to a wealth of studies far too numerous to cover here. Progress in this area has recently been summarised in two excellent and very comprehensive review articles [122, 123[. Here we will cover only the most fundamental aspects of stereoselective polymerisations. 

The dominance of studies of the solid state NMR literature clearly reflects the central role played by carbon in organic polymers. The sensitivity of the C chemical shift allows detailed NMR characterisation of the different fragments present in organic polymers. The use of C NMR is widespread, despite its low sensitivity due to its natural abundance of only 1.1%, since the C signal is routinely enhanced by the use of CP. When CP cannot be used, C must be observed directly in a 1 pulse... 

An additional more complex type of static magnetic interaction is experienced by nuclei which have a spin quantum number greater than one half. Far from ruling out the studies of such species, application of quadrupolar techniques has added a new dimension to the characterisation of polymers. This is because the quadrupolar interaction is remarkably sensitive to order, orientation and local motion. With the necessary synthetic skills quadrupolar atoms such as deuterium can be inserted into a polymer at a chosen segmental site and can then be persuaded to report on their surroundings. The spectrometer is tuned to the specific nuclear frequency and the data are collected without unwanted responses from the rest of the sample. It is only the need for labelling that has restricted the more routine use of deuterium NMR for the study of polymers. Despite this, it has developed into an uniquely powerful research tool. 

A novel approach for solid-state NMR characterisation of cross-linking in polymer blends from the analysis of polarisation transfer dynamics is... 

Table 5.17 shows the use of NMR in polymer structural and dynamical studies. 13C 1-NMR is successfully applied in detailed characterisation of homo- and... 

Several modem analytical instruments are powerful tools for the characterisation of end groups. Molecular spectroscopic techniques are commonly employed for this purpose. Nuclear magnetic resonance (NMR) spectroscopy, Fourier transform infrared (FTIR) spectroscopy and mass spectrometry (MS), often in combination, can be used to elucidate the end group structures for many polymer systems more traditional chemical methods, such as titration, are still in wide use, but employed more for specific applications, for example, determining acid end group levels. Nowadays, NMR spectroscopy is usually the first technique employed, providing the polymer system is soluble in organic solvents, as quantification of the levels of... 

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