Myelin textures

Transitions with the participation of liquid crystals sometimes show characteristic phenomena. If a nematic modification turns to a smectic A or smectic C phase, transient stripes in the form of a myelinic texture (also called chevron texture or striated texture) are often visible. Typically for the polyester prepared from di-w-propyl-/ -terphenyl-4,4" carboxylate and tetramethylene glycol, the nematic phase separates from the isotropic liquid on cooling in droplets which coalesce and form large domains. Cooling of the threaded-schlieren texture produces a transition to the smectic A phase this change is characterized by transition phenomena, mostly stripes, which broaden into larger areas ( transition bars ). 

Stirring 0.7 wt% surfactant in water for several hours produced a turbid bluish-white system. A specimen under the microscope and between crossed polarizers revealed numerous birefringent spherulites 5 to lOOy in diameter, with characteristic Maltese crosses. Similar textures were also observed with 0.3 or 1.0 wt% NaCl present. Figure 1 is a photomicrograph of typical spherulites together with so-called myelinic figures (17b), which are caused by interference phenomena. 

Instabilities that manifest themselves in surfactant and polymeric systems have been considered in an attempted to elucidate the myelin instability. In polymer-like micelles (or wormlike micelles) instabilities have been observed in the directional growth of hexagonal phases in a temperature gradient (30), These instabilities are an example of the Mullins and Sekerloi type (31), In the case of polymer gels, instabilities appear during growth, which resemble a raspberry like texture at the surface. This instability is due to the elastic properties of the gel which is a network of chemically bonded polymers. As the gel swells at the surface it remains anchored to the rest of the unswollen gel and the surface buckles (50). 

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