Wideline Separation Experiments

A study of maltodextrin and inulin gels was carried out by cross-relaxation spectroscopy, TIP (H) measurements, and a wideline separation experiment (WISE).132 During ageing, the amount of immobilized polymer increases owing to the formation of crystalline regions, but it is not associated with change in molecular mobility. Different kinetics were found for maltodextrin and inulin gels.132... 

Solid-state NMR methods allow the investigation of local orientation, dynamics and conformational order of polymer chain segments. Si chemical shifts contain information about molecular moieties and about the conformation of chain segments. The line width of a wideline spectrum characterizes the strength of the dipolar couplings among protons and, therefore, the molecular mobility. H spin diffusion, mediated by the homonuclear dipolar couplings, is a powerful technique to obtain information about the spatial proximity of molecular moieties. These concepts have recently been combined in a two-dimensional Wideline Separation Experiment (WISE-NMR spectroscopy)... 

Among several indicators of motional state of organic solids H linewidths has been used from very beginning of solid-state NMR. However, inherently low resolution of H MAS NMR spectra caused by the very strong homonuclear interactions and relatively low MAS speed lead to a very limited application of H MAS NMR for studies of mobility.1 A 2D wideline separation (WISE) experiment correlates carbon chemical shifts recorded under MAS with broad H lines.1,24 As a result a broad ll lines are separated in the components corresponding to the 13C sites in their close proximity. For mobile domains in a sample, the H - ll coupling are relatively weak which leads to relatively narrow 11 static lines. The opposite is observed for the rigid domains. 

A series of ID and 2D NMR experiments performed on nanocomposites of hectorite, a smectite clay mineral, showed new opportunities for studying large assemblies of layered minerals with polymeric materials.In particular. rotational-echo double-resonance (REDOR) experiments afforded H- Si distances with precision, and a mobility gradient of intercalated polyethyleneoxide (PEG) segments was demonstrated in H-" Si wideline separation (WISE) experiments. 

A second group of techniques may be called lineshape analysis. Simple methods entail the measurements of linewidths or second moments as a function of temperature. More sophisticated methods involve the analysis or the model fitting of spectral lineshapes. A prominent method is ID lineshape analysis for deuterium-labeled polymers, which is sensitive to motions in the frequency range of lO -lO s (149). The 2D wideline separation NMR (WISE) experiment permits correlation of the high resolution spectrum with the wideline spectrum, which provides dipolar information (11,150). The linewidth is a function of the frequency of the polymer motion relative to the time scale of dipolar couplings. 

Multidimensional NMR provides especially interesting information about polymer dynamics. A long-standing method for qualitative characterization of molecular mobility is H wideline NMR spectroscopy. There, large-amplitude motions are detected through the reduction of the dipolar line width. However, ID proton lineshapes leave many questions open, as they typically represent superpositions of broad and narrow lines, and their relation to different structural units is often not obvious. In a straightforward combination of wideline NMR, cross polarization (CP) and MAS spectroscopy in a 2D experiment [23], it is possible to separate the dipolar patterns for the different structural units (Wideline SEparation, WISE). This is demonstrated in Figure 5.4, where a WISE... 

Spectroscopic studies, such as solution NMR and infrared spectroscopy, are often used in order to confirm the PEBA chemical composition [4,7] and to quantify the PA/PE ratio. Few authors have used spectroscopic studies to characterize the PEBA solid-state organization [7]. Figure 11 illustrates an NMR solid-state H-Wideline Separation (WISE) experiment with Sample H at room temperature, where the first dimension corresponds to the chemical shifts and provides information about the chemical structure. The second dimension corresponds to the broadband lines and gives information about local fields... 

Fig. 10. The pulse sequence for the WISE experiment.21 This two-dimensional experiment separates H wideline spectra according to the isotropic l3C chemical shift of the 13C each H is bonded to. An initial H 90° pulse creates transverse H magnetization that is allowed to evolve in t. A short cross-polarization step then transfers the remaining H magnetization to the nearest 13C spin, i.e. the bonded one. The resulting 13C transverse magnetization is then allowed to evolve in ti under magic-angle spinning, where an FID is recorded.

The NMR method makes use of measurements of the dependence of to Si cross polarisation efficiency as a function of CP contact time to determine the distances between different silicon atoms in the ordered complex to H atoms in /luru-xylene molecules in the pores. The experiment has to be performed where the molecules show little translational motion, and under conditions where any molecular re-orientation is well understood (from wideline NMR, for example see Section 7.2.3). To give additional information, the experiments were performed separately on selectively deuterated and dg- i-xylene molecules. Plots of the Si signal intensity from different crystallographic T-sites against the contact time show an increase to a maximum value. 

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