Para-azoxyanisole

Para-azoxyanisole is the terminally substituted homologue. It melts at 118 °C and forms a nematic phase. The clearing temperature is 135 °C. However, its laterally substituted meta- and ortho-forms both are nonliquid crystals. The meta-form melts at 51 °C, the ortho-form melts... 

DETERMINATION OF INTERPHASE ENERGY IN THE SOLID-LIQUID CRYSTALLINE PHASE BOUNDARY OF PARA-AZOXYANISOLE AND CHOLESTERYL CAPRINATE. 

Leslie, F.M. Introduction to nematodynamics. In Dunmur, D., Fukuda, A., Luckhurst, G., INSPEC (eds.) Physical Properties of Liquid crystals Nematics, pp. 377-386, London (2001). Parodi, O. Stress tensor for nematic liquid crystals. J. Phys. (Paris) 31, 581-584 (1970) Miesowicz, M. The three coefficients of viscosity of anisotropic liquids. Nature 158, 27 (1946) Influence of the magnetic field on the viscosity of para-azoxyanisole. Nature 136, 261 (1936). 

Figure 18. Calculated relaxation frequencies in reduced units plotted as a function of order parameter for para-azoxyanisole [36].

Quasi-elastic scattering from a homologous series of molecules based on PAA (para-azoxyanisole) in the nematic phase has been analyzed by fitting a rotational diffusion model with an extra parameter to adjust the proportion of delta function in the model. Values close to zero for this excess of elasticity were taken to indicate a good model for the molecular motion. Initially, uniaxial rotation of the whole molecule about its principle axis was assumed, but this gave large negative values of the excess... 

In Fig. 2 we display (schematically) the comparison of several experiments with the mean field theory, including only the term in 1/2. We use T — Tp as the abscissa, so that the two materials depicted can be conveniently compared on the same graph. The dashed line is the theoretical curve computed from the mean field theory using only the term in t/o We note that there is a striking difference in the properties of the two materials, PAA (para-azoxyanisole) and PAP (para-azoxyphenetol). We note further that the simple theory cannot account for the order parameter of either material. Fig. 2 is, in fact, clear evidence for the necessity of higher-order terms in the potential. That the two substances have different values of T. must mean that... 

It is also interesting to mention that the compound (3.11) is the first published example that has lateral but no terminal substitutions and yet forms a liquid crystal phase. By and large, the studies on substitution effect have shown that while the terminal substitutions are favorable to the thermal stability and the formation of a liquid crystal phase, the lateral substitutions are not. The larger the lateral substitution, the more harm it can do to the thermal stability of the liquid crystalline phase. A typical and very convincing example for this idea is given by homologues of the well known para-substituted azoxyanisole (3.12) ... 

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