Diffusion Measurements in Chiral Liquid Crystals
Pioneering NMR measurements of the molecular diffusion along the helical axis in twisted nematics have been performed by Doane and coworkers 1).
In contrast to common NMR diffusion measurement techniques, no external magnetic field gradient has to be applied to the chiral LC-sample, because molecular diffusion along the helical axis is connected with a reorientation of the di-rector and leads to specific changes in the NMR line shape.
We have performed 13C- and 1H-NMR measurements in twisted nematics to study the dependence of the diffusion coefficient Dp along the helical axis on the pitch and on the time of observation.
Variation of the pitch (in the range from 2 μm to 10 μm) were obtained by addition of small amounts of chiral molecules (up to 4 wt %) to the nematic material.
The measured values of Dp depend on the time of measurement tB, firstly observed bypBlinc et al 2). Using a novel 7H-NMR method we were able to study Dp in the time interval from 0.5… 50 ms.
The value of Dp increases for small values of p nearly quadratically with pp and should approach the value of D1 in nematic phase for large p.
For short times tB the value of Dp is significantly higher than it’s long time limit (t>100 ms).
To explain the observed behaviour of D a theoretical model is presented 3), where there exists ap coupling between rotational motion and translation along the helical axis. Qualitative agreement with experimental data is obtained g assuming collective motions of dusters containing some 106 molecules.