Director disorder in liquid crystals : ESR investigations

Abstract

The first guest material employed to destroy the uniform director alignment is a clay.  On the application of a weak magnetic field inherent in the spectrometer, ESR found partial director alignment for clay concentrations less than 3.0wt%, but not for higher amounts.  We believe that the competition between the magnetic and the surface fields, with the elastic effect, accounts for these results.  Our results also suggest, indirectly, that the clay particles are also aligned as a consequence of surface coupling to the director alignment.

The investigation of the behaviour of liquid crystals in porous materials, namely controlled porous glasses, has produced some surprising results.  ESR has revealed a range of dynamic behaviour not observed in the bulk.  For example, the molecular reorientation is found to be partly in the slow motion regime and partly in the fast.  As the pore size increase so the fast motion fraction increases.

In a chiral nematic phase, the spectrometer magnetic field aligns the helix axis perpendicular to the field.  This produces a 2D director distribution which can be further perturbed by the field.  However, as the pitch of the helix decreases so the director distribution is observed, by ESR, to pass from 2D to 3D.  This offers another method of destroying the director alignment and hence determining the phase symmetry.

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