Reptation echo attenuation

In contrast to center-of-mass diffusion, segment diffusion is characterized by non-Gaussian propagators. The attenuation of spin echoes is then no longer governed by unspecific equations like Eqs. 13 or 14. However, in special cases such as the reptation model, a formalism for the evaluation of the echo attenuation can be set up [3]. 

Figure 44 shows typical echo attenuation curves recorded with PEO in a solid PHEMA matrix [186]. The good coincidence of the theoretical curves calculated on the basis of Eqs. 76-79 for all accessible values of the experimental parameters k and t for different molecular weights [11] corroborates the validity of the tube/reptation model for linear polymers confined to arti-... 

Fleischer A1) reported on a PGSE study of linear polystyrene and linear and branched polyethylene melts (M g 53000) at 200 °C. A rapidly-attenuated echo component at low x (see eq. 2) was also 32) attributed to low-molecular-weight traces some enhancement of D at the shortest t was thought to arise from gel-like fluctuations. The results follow eq. (7) with n = —2 since this slope holds below as well as above Mc, the reptation interpretation 26,27) is in doubt. 

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