Concentration dependence of the scalar order parameter in liquid-crystalline systems with variable molecular shape

A theoretical model taking into account the photoisomerization influence on the nematic scalar order parameter is presented. An extension of the Maier-Saupe model is used to investigate the concentration dependence of the scalar order parameter. The evolution of the concentration of cis-trans isomers and of the scalar order parameter with the time of exposure to the illumination is investigated. Furthermore, it is shown that for sufficiently long exposition of the mesophase to the illumination the nematic phase can disappear. This phase transition takes place at a critical concentration of cis constituents.