Molecular configuration transitions of a nematic liquid crystal encapsulated in organically modified silicas

The nematic liquid crystal (LC) 4-n-pentyl-4'-cyanobiphenyl (5CB) was encapsulated as spherical microdroplets dispersed in solid organically modified silica matrices (ORMOSIL), obtained by the sol-gel method. The organic functional groups used to modify the matrix were methyl, ethyl and propyl. The amount of the organic groups in the confining matrix was found to affect the LC surface anchoring properties, and therefore the molecular configurations in the LC droplets. Changes in this molecular configuration by variation of the temperature were identified. The matrix chemical composition-temperature diagrams for the LC configurations were obtained for three sets of ORMOSIL matrices with methyl, ethyl and propyl functionalizations. A radial configuration of the LC droplets is promoted at lower temperatures and higher amounts of modifying groups in the matrix, while a bipolar configuration is obtained at low organic contents and high temperatures. An empirical model was developed to correlate the dependence of the surface anchoring transition phenomena with the chemical composition of the ORMOSIL matrices and the temperature, showing a good correlation with the experimental results.