An E.P.R. Investigation of the Alignment of Two Smectic a Liquid Crystals

Abstract

The purpose of this work is to extend our studies of liquid crystals that display both nematic and smectic A phases, using the technique of electron paramagnetic resonance (epr). When the liquid crystal is in the nematic phase, an application of a magnetic field of the magnitude required for our X-band spectrometer (3.3 KG) is more than sufficient to uniformly align the director throughout the sample. In other words the preferred direction is the same throughout the entire sample. When the sample temperature is lowered through the nematic→smectic transition, the picture is somewhat more complicated. The forces between the liquid crystal molecules and the surface of the sample tube are now considerably greater. In addition, the lateral forces between the smectic molecules are also fairly large, so that any surface alignment that does take place may penetrate deep into the bulk of the sample. These effects oppose the efforts of the magnetic field to align the sample. In this investigation we study the smectic A alignment as a function of both the cooling rate through the transition point and the size of the aligning magnetic field.