Mesogenos alquiloxicianobifenilosaspectos teoricos y experimentales en las transiciones de fase sma-n y n-i

Résumé

The work presented here has been focused, basically, on the study of the SmA-N and N-I phase transitions of a group of liquid crystals, the alkyloxycyanobiphenyls (nOCB), in which, for some of them, as it is the case of 9OCB, there was practically no information. This study has been performed for the pure compounds (6OCB, 7OCB, 8OCB, 9OCB, 10OCB) and also for certain binary mixtures (8OCB+10OCB, 7OCB+9OCB). From an experimental point of view, there were used various techniques, as MDSC, dielectric spectroscopy, X ray diffraction, optical microscopy, densitometry and pressure calorimetry. These techniques allow us to obtain the evolution of various physical magnitudes of the materials used throughout their different phase transitions. The theoretical study centered on the application of the different theories about the order of the phase transitions with the aim of evaluating their critical behavior, based, generally, on thermodynamic and structural considerations. At the same time, using purely thermodynamic considerations, there has been developed an extension of the Oonk's method, which, makes possible the calculation of reentrant nematic domains and the estimation of the tricritical temperatures.