Orientational properties and dynamics of nematic liquid crystals mixed with dendrimers for electro-optical switches

Abstract

Novel two-phase materials based on nematic liquid crystals (LC E7) filled with poly(propylene imine) dendrimers of 2–3 nm diameter were investigated for application in light-scattering, electro-optical switches. Polarizing optical microscopy, differential scanning calorimetry and rheological experiments all indicate the presence of a pronounced polydomain structure within these LC mixtures. The molecular ordering and dynamics of the dendrimer-filled nematics were studied in more detail by dielectric relaxation spectroscopy. The director order parameter S _d was derived from the strength of the two principal molecular relaxations, α and δ, in the presence and absence of an electric field. In addition, a three-phase model was applied for determining the size of the different LC populations in the mixture. A substantial decrease in the fraction of randomly oriented LC was found upon application of a d.c. field. Over a broad temperature range the LC molecular dynamics were assessed from the variation in the strength of the dielectric relaxations as a function of the d.c. bias. A fast response was found at room temperature. Electro-optical switching experiments showed that the dendrimer-filled LCs can be switched rapidly from a light-scattering, opaque state to a more transparent state.

Notes

^†The modified dendrimers were kindly supplied by Bas de Waal and Maurice Baars, of the Macromolecular and Organic Chemistry group (SMO) headed by Prof. Dr E.W. Meijer, Eindhoven University of Technology.