The Dipolar-Correlation Effect on the Stimulated Echo. Application to Polymer Melts

Abstract

The quotient of the amplitudes of the stimulated and the primary echoes generated by a three-pulse sequence is strongly affected by dipolar coupling if motional averaging is incomplete. This effect can be experimentally discriminated from echo attenuation by relaxation and diffusion. A method is established on this basis which permits studies of molecular order and dynamics. The time scale is particularly long and is limited only by spin-lattice relaxation. A theory is presented which links the echo amplitudes with the dipolar-correlation function. The echo attenuation curves have been calculated for correlation functions decaying according to exponential and power laws. The results are compared with experimental data of entangled poly(ethylene oxide) and poly(dimethylsiloxane) melts. It is shown that power-law correlation functions are a suitable basis for the description of the observed phenomena. © 1995.