Entangled polymer chain solutions. Interpretation of the dipolar spin coupling modulation effect observed on NMR spectra

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Abstract
Concentrated entangled polymer chain solutions are known to exhibit a NMR spectrum narrowing effect upon sample rotation. However, the expected side bands normally induced by dipolar coupling modulation are not observed. To explain this property, the spin-system response to a driving field is calculated to fourth order in the dipolar spin interaction, taking the spin coupling modulation into consideration. The fourth moment of the whole spectrum is exactly expressed. It is shown that the spin interaction modulation effect depends upon the resonance line shape observed without modulation ; i) the side bands should be well observable when the resonance line has a gaussian shape; ii) the side band amplitudes become negligible when the resonance line has a nearly lorentzian shape