2D exchange NMR

For PIB the apparent activation energy found for the structural relaxation time in the NSE window is almost twice that determined by NMR [136] (see Fig. 4.9 [125]). For aPP, the temperature dependence of NMR results [138] seems, however, to be quite compatible with that of the NSE data nevertheless, 2D exchange NMR studies on this polymer [139] reveal a steeper dependence. This can be seen in Fig. 4.11 [ 126]. 

The most recent developments in 2D NMR of solids are the heteronuclear chemical shift correlation spectroscopy (421), 2D exchange NMR, which enables very slow molecular reorientations to be monitored (422), and heteronuclear. /-resolved 2D NMR (423). 

The molecular motions underlying the dynamic mechanical and dielectric f3 transition in PMMA have been studied in detail [77] by using the 2D exchange NMR experiment. This detects slow reorientations that occur during a mixing time, fm, by measuring the angular-dependent NMR frequencies (expressed in ppm) before and after tm. The 2D frequency spectrum S( >i,... 

It is not clear, whether the experimentally observed random local freezing of the deuterons in the O-D—O bonds in deuteron glasses corresponds to a true thermodynamic phase transition or whether one deals with a dynamic phenomenon which only seems static because of the finite observation time of the experimental techniques. The recently observed42 splitting between the field-cooled and zero-field dielectric susceptibilities below an instability temperature Tf seems to speak for the occurrence of an Almeida-Thouless-like thermodynamic phase transition in deuteron glasses. It is well known that ID NMR and EPR allow a direct measurement of the Edwards-Anderson order parameter qEA only on time scales of 10 3-10 8 s and 2D exchange NMR possibly seems to be a better technique for such slow motions. 

Figure 26(a) shows the it- H 2D exchange NMR spectrum of LDS/PCS(8/2) with a mixing time of 100 ps. The cross peaks, which come from the existence of H- H spin diffusion between the phenyl protons and CH3 protons, and the phenyl protons and the CH2 protons appear clearly. For convenience, the expanded spectrum in the cross peak region is shown in Fig. 26(b). The dashed lines indicate the H chemical shift positions of unblended LDS and PCS. These values become reference data for understanding the observed cross peaks due to H- H spin diffusion. The peak top at around the H chemical shift position (XI, X2) = (—0.8 ppm, 6.9 ppm) may be assigned to intramolecular spin diffusion between the CH3 protons of LDS and the phenyl protons of LDS because of the most intense peak. The shoulder around the chemical shift position... 

In a miscible blend, a specific interaction would influence chemical shifts and/or lineshapes of component polymers (see Section 10.2.2.1). Molecular motion is also affected by the interpolymer interaction, and is investigated by spin-lattice relaxation, and lineshapes and 2D exchange NMR (see Section 10.2.2.2). 

The time-consuming 2D experiment can be replaced by ID selective inver-sion-recovery/saturation-transfer experiments [119]. Its application to study the heterogeneity of PS/PVME is mentioned afterwards (see Section 10.3.1.1). Furthermore, Campbell and VanderHart [109] showed that the selective techniques under multiple-pulse homonuclear decoupling are not necessary. They realized that at certain t value in a 2D exchange NMR experiment, polarization gradient necessary for spin diffusion can be achieved by the chemical shift difference. The optimum preparation period for the ID analogue of the 2D exchange NMR experiment was discussed. 

Instead of using H spins, spins may be useful, provided that inefficient spin diffusion is somehow enhanced. To facilitate spin diffusion, Henrichs et al. used enriched polymers [120, 121]. They applied the 1D/2D exchange NMR to study PC/PET = 25/75. Figure 10.15... 

Recently, it was shown that Form II of i-PlB exhibits significant slow conformational exchange above the grass transition by using 2D exchanged NMR under conditions of MAS [46]. Comparison with spectra of Form I proved that this motion occurs within the crystalline region. 

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