Electrooptic properties of liquid crystals filled with silica nanoparticles of different sorts

In this work we investigate electrooptic properties of nematic liquid crystal (LC) E7 from Merck filled with two types of silica nanoparticles (NPs). The particles of these types are of similar size and shape, however, they are made by different methods. The NPs of the first type, aerosil (A), are prepared by a high-temperature hydrolysis of chlorosilanes. The second type of NPs, based on hydrolysis and controlled precipitation of tetraethylorthosilicate in alcohol, yields monodispersed NP (MNPs) of silica. Transmittance versus applied voltage curves of the suspension layers are measured and analyzed. Our results show a strong dependence of the electrooptic properties on the particle preparation method. The contrast ratio (CR) 61:1 of LC-A suspension is achieved at relatively low A content (17 wt %). Unlike it, the value of the CR of LC-MNP suspensions is low (about 2:1) even at 30 wt % of MNPs in the mixture. Both types of suspensions show electrooptic memory. The maximum of the memory efficiency of LC-A suspensions is detected at 4 wt % of A, while for LC-MNP suspensions the maximum is achieved at 15 wt % of MNPs. The difference in the results for the LC-A and LC-MNP series of composites is caused by much stronger aggregation of MNPs compared with A in the LC matrix. Moreover, reactive particle fusion and formation of silica microcrystallites substantially enhances the effect caused by MNP aggregation. These processes deteriorating electrooptic performance may be stabilized by introduction of a polymer network in LC-MNP suspensions.