From nematic emulsions to polymer dispersed liquid crystals

Nematic emulsions and polymer dispersed liquid crystals (PDLCs) are composite materials formed by micron-sized droplets of mesogenic molecules, which are embedded in fluid or solid matrices, respectively. Both systems can be switched from an opaque to a trasparent state by application of a suitable electric field. The main differences reside in the switching field and relaxation time values. Polymer dispersed liquid crystal films are generally characterized by higher switching fields and faster relaxation times. On the contrary, nematic emulsions show lower switching fields and longer decay times. Recently, we have obtained PDLC films by photopolymerization of nematic emulsions. Such films keep the advantages of conventional PDLCs, but have the opportunity of a finer control of liquid crystal droplet size. In this article we have focused our attention to the polymerization process, which converts nematic emulsions in polymer dispersed liquid crystals. In particular, we have investigated the electro-optical changes induced in nematic emulsions by the polymerization of the fluid matrix as a function of the irradiation time. A progressive increase in the switching fields is observed, while the decay times decrease in a significant way. Such results can be explained by assuming a decrease of liquid crystal droplet viscosity and an increase of boundary restoring forces acting on liquid crystal due to the polymerization process. (C) 2002 American Institute of Physics.