Estudo de Fases Termotrópicas por Microscopia Óptica, Medidas de Densidade, Entálpicas e Espalhamento de Raios X / Thermotropic phase study, by optical microscopy, density and calorimetric measurements and X-ray scattering.

AUTOR(ES)

Evandro Luiz Duarte

DATA DE PUBLICAÇÃO

2000

RESUMO

The influence of the mean molecular length on the nature of the smectic A (SA)- cholesteric (C) liquid crystal phase transition has been studied for cholesteryl myristate (C14), cholesteryl nonanoate (C9) and binary mixtures of C14 and C9 and cholesteryl caproate (C6) and C9. Optical microscopy was carried out to determine the temperatures of the SAC as weel as the C - isotropic (I) phase transitions. This allowed the choice of the optimum temperature interval near the phase transitions. Information concerning the nature of the transitions (descontinuous, also referred to as first order, or continuous) was evaluated through the analysis of the density and enthalpic measurements. The results have evidenced that the transition crosses over from first order to continuous when both the mean molecular length and the reduced temperature, r (ratio between SAC and CI phase transition temperatures), decrease in the system. The occurrence of a continuous SA C phase transition is observed for a concentration very near 63.1 molar percent of C9 in the C6 C9 mixture (65 wt% C9 : 35 wt% C6), at r ~ 0.92. Mesoscopic properties of the systems, as the distance between the smectic layers in the SA phase, dSA, the mean molecular length, , in the C domain, and the correlation length, , behavior along the phase transitons, were determined from the X-ray difraction peaks. The data have revealed that dSA decreases as the mean molecular length is reduced, as it was expected. Moreover, values obtained in the C phase near the I C phase transition agree with the mean molecular lengths calculated from the corresponding extended molecule lengths. In addition, an increase in the correlation length in the C phase is observed, as the temperature is reduced from I to SA phases, representing an increase in the orientational order. Nevertheless, the parameter jumps in the C SA phase transition. Such discontinuity decreases as the mean molecular length (and hence the reduced temperature) diminishes. For the particular system composed of 65 wt% C9 : 35 wt% C6, where a continuous SA C phase transition was identified, a behavior of (T TSAC)0,5 in the C domain was observed, in good agreement with the mean field theory.

ASSUNTO(S)

cristais líquidos espalhamento de raios x. transições de fase microscopia óptica phase transitions calorimetria calorimetry optical microscopy liquid crystal x-ray scattering.

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