Dipolar correlation effect

Nuclear magnetic resonance (NMR) spectroscopy has great relevance in the area of investigation of molecular structures. Many studies using NMR spectroscopy and microscopy have been reported in elastomer based systems. Due to the possibihty of reuse of samples after the analysis (NMR is nondestructive) NMR attracts many material scientists to select this technique for characterization. In the case of polymers NMR is specifically useful in finding out the crosslink density. Since the crosslink density is related to the size of pores or cavities inside solid polymers, this method points towards the structural elucidation of polymers. From the parameters such as magnetic relaxation and the dipolar correlation effect obtained from the NMR spectrum, crosslink density can be calculated. In addition to the crosslink density, the behaviour of small particles inside the polymer matrices can also be... 

Garbarczyk, M. Grinberg, F. Nestle, N. Kuhn, W., A Novel Approach to the Determination of the Crosslink Density in Rubber Materials with the Dipolar Correlation Effect in Low Magnetic Fields. J. Polym. Sci., PartB Polym. Phys. 2001, 39, 2207-2216. 

One-dimensional (ID) NMR methods based on the dipolar correlation effect in combination with the Hahn and solid echoes (52,63), the stimulated echo (5), the magic echo (64), magnetization-exchange (65), and cross-relaxation dynamics (66) provide access only to the second van VIeck moment via a model which takes into accoimt the solid-like and liquid-like contributions to the spin system response (67). Model free access is given by the analysis of multiple-quantum builtup (68) and decay (69) curves recorded in the initial regime of the excitation/reconversion periods. 

Chain Orientation and Siow Dynamics by Dipolar Correlation Effect. 

Quite stronger dipolar correlation effects were found in swallow-tailed liquid crystals. Experimental data of the sample for which the chemical formula is given at the... 

A similar approach to ji decay in that it too relies on a combination of echoes is referred to as the dipolar correlation effect (DCE). This technique relies on a ratio of the stimulated and Hahn echoes and has also been applied to the study of silicone materials. Notably, the DCE has been invoked to study the aging of a filled silicone upon exposure to ionizing radiation [55]. These data yielded direct evidence of surface-adsorbed species and have shown the fraction of these chains to change as a function of radiation exposme. 

Some care must be taken with fringe field methods if motional averaging of dipolar interactions is incomplete. Echoes then tend to be modulated by the so-called dipolar correlation effect [15, 16]. One can account for this phenomenon by dividing the (normalized) echo amplitudes recorded with and without gradient at the same Larmor frequency and at the same pulse intervals [11,12]. Any influence by relaxation and the dipolar correlation effect can be eliminated in this way. 

The free-induction decay of transverse magnetization has been analyzed in terms of polymer dynamics [23-26]. A solid echo technique was employed for the same purpose [27, 28]. The so-called dipolar correlation effect on the stimulated echo turned out to be a particularly simple and robust tool in this context too [15, 29, 30]. Finally, double-quantum NMR spectroscopy was suggested [31, 32] as a means of probing features of chain dynamics. 

The brackets indicate ensemble averages. An analogous formalism for the dipolar correlation effect can be found in Refs. [2, 15, 16]. 

In this context, a study of the dipolar correlation effect in the mesophase of PDFS is of particular interest [168]. Using this technique, it was concluded that the sample consists of defect-enriched areas and ordered domains. The two dynamic states are connected with segments of different mobility, and they fluctuate temporally and spatially. The exchange times between the two mobihty states range between 0.1 and 1.0 s in the mesomorphic temperature range. 

The dipolar correlation effect is based on stimulated-echo signals [2] and therefore contains elements both of transverse and longitudinal relaxation. Flip-flop spin diffusion as well as material transport may contribute to the exchange mechanism identified this way. An interesting result of the dipolar correlation effect study is the temperature dependence of the fraction of the more mobile segments which was shown to obey the empirical law (see Fig. 39) [168]... 

Permanent or thermoreversible cross-links mediate the opposite effect on chain dynamics compared with dilution by a solvent instead of releasing topological constraints by dilution, additional hindrances to chain modes are established by the network. With respect to NMR measurands relatively large cross-link densities are needed to affect chain modes visible in the experimental time/frequency window, as demonstrated with proton spin-lattice relaxation dispersion of polyethylene cross-Unked by 10-Mrad irradiation with electron beams [123] and with styrene-butadiene rubbers [29]. However, there is a very strong effect on the dipolar correlation effect which probes much slower motions and can therefore be used favorably for the determination of the cross-Hnk density [29, 176, 177]. 

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