Medidas de densidade e coeficiente de expansão térmica nas fases esmética C, nemática anômala e isotrópica

Abstract

In this work was the thermal behavior of the density of thermotropic liquid crystal systems, which show abnormal behaviors, was studied. The objective was aimed at trying to detect the possible existence of the nematic(1)-nematic(2) phase (N1 - N2 ), transition in the compounds from the 4-n-heptyloxybenzoic (7OBAC) and 4-n-octyloxybenzoic (8OBAC) acid class through densimetry technique. However, besides this technique, other experimental techniques were used to investigate these systems, such as: optic microscopy of polarized light and differential scanner calorimeter (DSC). The density measure results showed the existence of a smooth transition in both systems investigated indicating the possibility of a (N1 - N2 ), transition. Also, we were surprised by an anomalous density behavior in the smetic phase domain, shown in the system (7OBAC). To interpret the results, we considered the fact that the relative concentrations of different constituents forming the phase (closed dimmers, opened dimmers and monomers) can have an influence in the abnormal behavior shown by the systems. A theorical model existent in literature which considers the phase scale order parameter as a function of concentration of closed dimmers, opened dimmers and monomers was also discussed. From the density data we calculated the coefficient of thermal expansion and compared the results from the two investigated samples.