THE EFFECT OF ELECTRIC AND MAGNETIC-FIELDS ON THE MELT ORGANIZATION OF A NEMATIC CYCLIC SILOXANE LIQUID-CRYSTAL

The presence of periodic diffuse meridional reflections in the X-ray d iffraction pattern of the nematic phase of a pentamethylcyclosiloxane compound with pendant mesogenic groups is examined as a function of te mperature and frequency of an applied electric field. These reflection s are thought to be caused by a tendency of the mesogens to pack in co lumns of molecules that have no side-to-side registry (uncorrelated st rings). Column lengths obtained from the intercepts of plots of Delta s(2) versus s(2), where s is the scattering vector and Delta s is the width of the reflection, indicate association of 3-6 mesogens dependin g on temperature and frequency. The length over which the strings are correlated are compared to values obtained from a magnetically aligned sample quenched into a glassy nematic phase. The development of a pse udo-layered packing arrangement with decreasing temperature is also di scussed. The appearance of a symmetric 4-spot pattern at temperatures slightly above the crystallization temperature suggests the formation of layered domains with local ordering similar to smectic C packing. L ayer correlation lengths are calculated and compared for the electrica lly (110 Angstrom) and magnetically (180 Angstrom) aligned samples.