The effect of electric and magnetic fields on the melt organization of a nematic cyclic siloxane liquid crystal

Abstract

The presence of periodic diffuse meridional reflections in the X-ray diffraction pattern of the nematic phase of a pentamethylcyclosiloxane compound with pendant mesogenic groups is examined as a function of temperature and frequency of an applied electric field. These reflections are thought to be caused by a tendency of the mesogens to pack in columns of molecules that have no side-to-side registry (uncorrelated strings). Column lengths obtained from the intercepts of plots of δs ² versus s ², where s is the scattering vector and δs is the width of the reflection, indicate association of 3–6 mesogens depending 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 pseudo-layered packing arrangement with decreasing temperature is also discussed. 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. Layer correlation lengths are calculated and compared for the electrically (110 Å) and magnetically (180 Å) aligned samples.