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Master's Dissertation
DOI
10.11606/D.43.2000.tde-27042009-113505
Document
Author
Full name
Evandro Luiz Duarte
E-mail
Institute/School/College
Knowledge Area
Date of Defense
Published
São Paulo, 2000
Supervisor
Committee
Itri, Rosangela (President)
Alcantara, Maria Regina
Oliveira, Elisabeth Andreoli de
Title in Portuguese
Estudo de Fases Termotrópicas por Microscopia Óptica, Medidas de Densidade, Entálpicas e Espalhamento de Raios X
Keywords in Portuguese
Calorimetria
Cristais Líquidos
Espalhamento de Raios X.
Microscopia óptica
Transições de fase
Abstract in Portuguese
Neste trabalho estudamos a influência do tamanho molecular médio sobre a natureza da transição de fase esmética A (SA)-colestérica (C) líquido-cristalina para os compostos termotrópicos de miristato de colesterila (C14), nonanoato de colesterila (C9) e misturas binárias de C14 e C9 e caproato de colesterila (C6) e C9. Medidas de microscopia óptica foram realizadas para determinar as temperaturas de transição de fase SA C e C-isotrópica (I). Isto possibilitou a escolha dos intervalos de temperatura próximos às transições de fases, como referências para as outras técnicas utilizadas no trabalho. A natureza das transições de fase (descontínua, também denominada de primeira ordem, ou contínua) foi inferida através das análises de medidas de densidade e entálpicas. Os resultados evidenciaram que a transição passa de primeira ordem para contínua quando o tamanho molecular médio e a temperatura reduzida, r (definida como a razão entre as temperaturas de transição de fases SA C e C I ), decrescem no sistema. A ocorrência de uma transição de fase SA C contínua foi observada para a concentração de 65% de C9 na mistura C6 C9, para o valor de r ~ 0, 92. Propriedades mesoscópicas dos compostos estudados, como a distância entre as camadas esméticas na fase SA, dSA, o tamanho molecular médio, , no domínio C, e o comportamento do tamanho de correlação, , ao longo das transições de fase, foram determinadas da análise dos picos de difração de raios X. Os dados revelaram que dSA diminui com o decréscimo do tamanho molecular médio, como era esperado. Além disso, os valores de obtidos na fase C, próximos à transição I C, concordam com os tamanhos moleculares médios calculados supondo que as moléculas estejam em conformação estendida. Mais ainda, foi observado um aumento no tamanho de correlação na fase C quando a temperatura é reduzida da fase I para a fase SA, representando um aumento no grau de ordem orientacional. Contudo, o parâmetro troca abruptamente na transição C SA. Tal descontinuidade diminui com o decréscimo do tamanho molecular médio (e assim com r). Para o sistema particular composto por 65% C9 : 35% C6, onde identificamos um transição de fase contínua SA C, observamos um comportamento de (T TSAC)0,5 no domínio da fase C, em bom acordo com a teoria de campo médio.
Title in English
Thermotropic phase study, by optical microscopy, density and calorimetric measurements and X-ray scattering.
Keywords in English
Calorimetry
Liquid Crystal
Optical microscopy
Phase transitions
X-ray scattering.
Abstract in English
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.
 
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Publishing Date
2009-04-27
 
WARNING: The material described below relates to works resulting from this thesis or dissertation. The contents of these works are the author's responsibility.
  • DUARTE, e L, et al. Smectic A-Cholesteric Liquid Crystal Phase Transition: a Density Study. European Physical Journal B, 1998, vol. 5, p. 835-838.
  • DUARTE, E. L., et al. Smectic A - Cholesteric Phase Transition Investigated by Small Angle X-Ray Diffraction and Viscosity Measurements. Brazilian Journal of Physics, 2002, vol. 32, p. 495-500.
  • DUARTE, E. L., ITRI, R., and PALANGANA, A. J. Study of Thermotropic Phases by Microscopy, Density, Enthalpic and X-ray Scattering Measurements. In 5th Ibero-American Workshop on Complex Fluids and Their Applications, Maringá, 2001. Abstracts do 5th Ibero-American Workshop on Complex Fluids and Their Applications., 2001. Abstract.
  • DUARTE, E. L., ITRI, R., e PALANGANA, A. J. Study of Termotropic Mesophases by SAXS, Enthalpic and Density Measurements. In VII Seminário Latino Latino Americano de Análises por Técnicas de Raios-X, São Pedro, 2000. Resumo.
  • ITRI, R., et al. Smetic A-Cholesteric Liquid Crystal Phase Transition:a density study. In 17th International Liquid Crystal Conference, Strasbourg, 1998. Abstracts., 1998. Resumo.
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