Freeze-Fractures in Cholesteric Mesophases of Polymers

Publisher Website
Google Scholar
Abstract
Freeze-fracture techniques were used to study the cholesteric liquid crystalline phases of three polymer solutions (PBLG, Xanthan, HPC). All fracture surfaces showed a regular stratification corresponding to the half-helical pitch, P/2. Electron microscopy permits very small helical pitches (P/2 < 300 Å) to be observed and defects and textures previously analyzed in the polarizing microscope can be easily recognized: edge dislocations, +π and −π disclinations etc. Fractures oblique with respect to the cholesteric stratification present a regular distribution of parallel series of nested arcs whose nature is directly related to the cholesteric organization but the shape of the arcs can be modified by the presence of steps in the fracture surface. The position of these steps is related to the fracture direction with respect to the cholesteric stratification. Fractures through polygonal fields are very different from the classical patterns observed in the polarizing microscope since we are observing the distribution of molecular orientations in a plane instead of a projection of the whole structure. In such fields, cholesteric layers are distorted to form domes and basins and fractures normal to the axes of these deformations produce spiral and saddle-shaped patterns.