Pseudo-solid echo

It has been known for some time that the FIDs of such systems often decay in a way that is well represented for the most part by a Weibullian [60,61]. Arguments based on consideration of correlation functions suggested that the FID of high molecular weight polydimethyl siloxane in the melt should decay for the most part with a Weibullian power of between 1.25 and 1.5 [62], and the existence of residual static dipolar interactions in these systems was confirmed by the existence of the pseudo-solid echo [63]. This reference forms part of a much larger body of work on such systems by Cohen-Addad and co-workers which it is beyond the scope of this chapter to cover in any detail, but interested readers are directed to literature such as [64] and [65]. 

In isotropic liquid systems, this component obeys an exponential relaxation, corresponding when Fourier transformed to a unique Lorentzian resonance line (line width a few H ). When molecular motions are anisotropic (in the range 10 to 10 Hz) the resulting average interaction leads to a structure in the resonance spectrum, reflecting the distribution of anisotropies in the system . This non-zero average interaction may be checked clearly by the presence of a pseudo-solid echo after a convenient pulse sequence . The following points may be emphasized respectively in the absence presence of an external constraint imposed on the network. 

NMR Evidence for Networks Pseudo-solid Spin-echoes... 

The NMR characterisation of polymeric systems requires first the search for the existence of networks. The observation of a time reversal effect, specific to residual spin-spin interactions, gives evidence for the presence of polymeric networks [3]. This property is reflected by so-called pseudo-solid spin-echoes formed by applying a suitable radiofrequency pulse sequence that results in a rotation of the spin operators (Figure 8.3). 

Figure 8.3 Normalised proton transverse relaxation curve recorded from end-linked calibrated chains. Eight pseudo-solid spin-echoes are recorded to illustrate the time reversal effect specific to the presence of any polymeric network...

Figure 2. Pseudo-solid spin-echoes recorded from a polybutadiene solution (0.88 w/w) in toluene at 254 K relaxation ftmction (+) echo origins 0.8 ( ), 1.6 (O), 2.4 ( ) ms.

Characteristic pseudo-solid spin-echoes recorded from polybutadiene are shown in Fig.2 the polymer concentration in deuterated toluene was 0.88 (w/w). The fraction of monomeric units in the ds conformation was 0.96 (M =110,000 g/mol.). 

J. P. Cohen-Addad and R. Vogin, "Molecular motion anisotropy as reflected by a pseudo solid nuclear spin echo Observation of chain constraints in molten cis-1,4-polybutadiene," Phys. Rev. Letters 33, 940-943 (1974). 

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