On the Influence of Director Fluctuations on NMR Relaxation in Liquid Crystals

(Received 16 October 1995, received in final form 12 February 1996, accepted 27 February 1996)

Abstract
We investigated and extended models describing the influence of director fluctuations (DF) on the orientational dynamics of individual (dissolved) molecules. The models are divides in two categories. On one hand the traditional models, relying on a seperation in time scales between the DF and the individual motion of the molecule. On the other high ordering models, which assume the motion of the individual molecule is very similar to the DF. The spectral densities calculated from all models were fitted to NMR relaxation data. It is found that virtually all result in acceptable fits for the frequency of J₁. Comparison to (sparsely available) J₂ data proves to be a much more severe test of the different models. From this we see high ordering models are inadequate. The traditional model gives a slightly better fit, although the calculated frequency dependence in J₂ is generally too small, which is actually caused by an overestimation of the effect of DF on J₁. Only the extended traditional model result in reasonable agreement between experimental and calculated curves.