Deuterium NMR is used to examine the molecular order exhibited by an organosiloxane tetrapode giving the first experimental evidence, using a bulk sample, for the existence of a biaxial nematic phase in this type of compounds. The temperature dependence of the averaged quadrupolar coupling constant and asymmetry parameter was determined in the compound’s nematic phase. Two distinct regimes could be identified, one with a vanishing asymmetry parameter corresponding to a uniaxial nematic phase and another with a significant temperature dependent asymmetry parameter, corresponding to a biaxial nematic phase. The high values obtained for the asymmetry parameter at the lower end of the nematic range are well above experimental error and constitute a definite proof of the biaxial nature of the nematic phase exhibited by the studied compound for those temperatures.

The biaxial nematic phase was recently observed in different thermotropic liquid crystals, namely bent-core compounds, side-chain polymers, bent-core dimers, and organosiloxane tetrapodes. In this work, a series of experiments with a nematic organosiloxane tetrapode where nuclear magnetic resonance (NMR) spectra are collected while the sample is continuously rotating around an axis perpendicular to the magnetic field, are discussed in conjunction with the analysis of a deuterium NMR experiment on the same system reported earlier. The sample used is a mixture of a deuterated probe with the tetrapode. The mixture exhibits a nematic range between $-40°\mathrm{C}$ and $37°\mathrm{C}$. The results of the two independent, but complementary deuterium NMR experiments confirm the existence of a biaxial nematic phase for temperatures below $0°\mathrm{C}$ with high values of the asymmetry parameter at low temperatures. The presence of slow movements of the tetrapode mesogenic units in the low-temperature regime could also be detected through the analysis of the NMR spectra. Simulations indicate that these movements are mainly slow molecular reorientations of the mesogenic units associated with the presence of collective modes in the nematic phases of this compound. In the case of tetrapodes, recent investigations attribute the origin of biaxiality to the hindering of reorientations of the laterally attached mesogenic units which constitute the tetrapode. This study relates the molecular movements with the nematic biaxial ordering of the system.

A novel type of PDLC display utilizing solid films obtained from the cellulose derivative hydroxypropylcellulose (HPC) with molecular weights of 60000 (HPC60000) and 100000 (HPC100000), cross-linked with two different di-isocyanates, 1,4-di-isocyanatobutane (BDI) and 1,6-di-isocyanatohexane (HDI), is presented. This kind of display (optical cell) is composed of a porous elastomeric film (20–60 μm thick) of HPC-BDI or HPC-HDI, impregnated with a nematic liquid crystal and contained between two transparent conducting substrates. The electro-optic behaviour of several optical cells is studied as a function of film thickness, the molecular weight of the polymer and the cross-linking agent. The preliminary studies presented in this work show that these systems present good contrast, high transmissivity in the on state and switching times in the range of some milliseconds to hundreds of milliseconds.