PDLC-like patterns at the isotropic to cholesteric transition entrapped byin situ photopolymerization

When investigated by optical microscopy between crossed polarizers, the iso tropic to cholesteric transition may appear like fingerprint-patterned drop lets embedded in a black isotropic matrix. In the present work, such PDLC-l ike (polymer dispersed liquid crystal) patterns, only occurring over 0.7 de grees C, have been entrapped and stored at ambient temperature in a polymer him. We used a UV polymerization process with different sequences in which illumination time and UV power progressively vary. From a conceptual viewp oint, these PDLC-like patterns come solely from liquid crystalline material , whereas all the conventional PDLCs are binary mixtures of a macromolecula r compound or 'prepolymer' with a conventional low molecular mass liquid cr ystal. The fact that isotropic matrix and cholesteric droplets differ only from the viewpoint of molecular order and not in their chemical nature, per mits comparisons with the usual case for which the choice of polymer-formin g material is crucial and the polymer/liquid crystal interface is an import ant factor for controlling PDLC electro-optic properties. The present syste m gives an opportunity to investigate by scanning electron microscopy (SEM) the droplet microstructure (isotropic-cholesteric interface, fingerprint p atterns or defects), whereas previous SEM studies were focused on the shape and size of empty cavities, since the fluid liquid crystal was inevitably removed from the PDLC system.