Isotropic Liquid Crystal Research Articles

Pretransition Effects in Nematic Liquid Crystals: Model Calculations

Abstract Calculations of the properties of some lattice models and continuum models of a hard rod liquid with and without attractive energies are presented. The theory is of the “molecular field” variety in that short-range-order is neglected. Both the long-range-order parameter and the density are varied to obtain the state of minimum Gibbs free energy at given pressures and temperatures. Among the properties reported are expansivity, compressibility and specific heat. Each of these displays pretransition effects qualitatively similar to those seen on the low temperature side of nematic liquid crystal-isotropic liquid phase transitions. There are important quantitative discrepancies however. It appears that the discrepancies are due more to the approximate nature of solutions than to the idealizations of the models. The “near critical” nature of liquid crystal phase transitions cannot be well described in terms of long-range-order alone. One gross error of the hard-rod models can, however, be remedied. T...

Attenuation of Sound in Para-Azoxyanisole

The attenuation of 10 megacycle sound pulses in the liquid crystal para-azoxyanisole is presented as a function of temperature in the region of the liquid crystal—isotropic liquid phase change. An absorption peak of optical sharpness is encountered at a temperature Tc just above the phase transition temperature. As one might suspect, Tc is dependent upon external influences. The dependence on externally applied magnetic fields is presented and the significance of these measurements is discussed. This work was supported in part by the Office of Naval Research.