THE SYMMETRY OF THE NEMATIC PHASE OF A THERMOTROPIC LIQUID-CRYSTAL - BIAXIAL OR UNIAXIAL

The symmetry of the thermotropic nematic phase of 2,3,4-tri-n-hexyloxycinnamic acid is investigated using deuterium NMR spectroscopy of the nematogen selectively deuterated in the ethylenic bond. In our experiments the sample was spun about an axis orthogonal to the magnetic field in order to produce a random distribution of the director in two dimensions. The resultant NMR powder pattern is characteristic of a partially averaged quadrupolar tensor with cylindrical symmetry and hence of a uniaxial nematic phase. Simulation of the powder patterns reveals that the upper limit to the biaxiality parameter is approximately 0.1 which is in marked contrast to the large values found for lyotropic biaxial nematics. Our result is not, however, necessarily inconsistent with conoscopic measurements which indicated a small optical biaxiality for this thermotropic nematic.