Orientational ordering spin-labeled polymer orientation

Analysis of the shape of the orienting potential, which constrains the preferred orientation of the effective axis of internal rotation of the tethered nitroxide, revealed the presence of two different conformations of the spin-label moiety. In order to explain these results, the quantum chemical procedure was utilized. The spin-label moiety in the polymer system was reduced to 4-acetamido-2,2,6,6-tetramethylpiperidine-l-yloxyl (II) (spin label plus tether). The molecule was subjected to a search for stable confonn-ers, which resulted in the two minima corresponding to the structures Da and nb." ... 

An example of typical ESR spectra, measured in the first derivative mode, is shown in Fig. 12. Just like NMR, ESR can be used to detect phase transitions and to study the orientation and dynamics of liquid crystals. The spectra shown in Fig. 12, for example, are from a study comparing the dynamics of the spin label at the end of the polymer chain and the freely dissolved spin probe in a liquid-crystalline polyether by continuous wave ESR (Fig. 12) and 2D Fourier transform ESR experiments [137]. The end label showed smaller ordering and larger reorientational rates than the dissolved spin probe. Furthermore, it was demonstrated that the advanced 2D FT ESR experiments (see below) on the end-labeled polymer chain could not be explained by the conventional Brownian model of reorientation, although this model could explain the ID spectra. This led to the development of a new motional model of a slowly relaxing local structure, which enabled differentiation between the local internal modes experienced by the end label and the collective reorganization of the polymer molecules around the label. The latter was shown to be slower by two orders of magnitude. 

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. 

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