Estudos de parâmetros ópticos em fases nemáticas liotrópicas e reológicos nas fases nemática, colestérica e "blue phases" termotrópicas

Abstract

In this work the transitions of reentrant isotropic (I RE) - discotic nematic (ND) - isotropic (I) and IRE - ND - biaxial nematic (BN) - calamitic nematic (CN) - I phases were initially studied through measurements of refractive indices, optical birefringence and analyses of digital image processing of textures (TDA) . The above mentioned nematic phases were characterized as well as the reentrant isotropic phase. As a consequence of the results for theses measurements and based on the mean field theory applied to the ND (NC) - NB phase transitions, three refractive indices of the NB phase were determined. This is an important result of this research since it allowed us to deepen the knowledge of the configuration dynamics of micellar aggregates, in special in the vicinities of the ND - NB - NC transitions. Next, the NC - I - hexagonal (hex) transitions were studied from the view point of the processing of the texture digital analysis. This made possible, when compared to the results obtained through the experiment of Nuclear Magnetic Ressonance (NMR), to completely determine these transition points. On a second moment, the nematic - isotropic and cholesteric (N *) - "blue phase" (BP) - isotropic (I) phase transitions were investigated using the TDA technique and viscosity measurements (η) in the thermotropic compounds - MBBA, 5CB and cholesteryl nonanoate/myristate. It was not only possible to assess aspects of texture dynamics between liotropic/thermotropic phases but to identify such transitions as well. The behavior of η as a function of temperature and "shear rate" in the vicinities of N * - "BP' - I phase transitions is studied. The transition points are marked by "peaks" of η that are well prominent. They were properly rescaled and, as a result of this process, a critical exponent which reflects the evolution of η towards its critical value was determined.