Efeito de grupamentos éster e cinamato na obtenção de cristais líquidos termotrópicos.

Abstract

In this work, a homologous series of eight (08) liquid crystals derived from aryloxycinnamates with a wide range of mesomorphism will be presented. The molecules investigated in this study were synthesized from the Wittig olefination reaction, carried out by researchers at Quaid-i-Azam University in Pakistan. The characterization was carried out by infrared spectroscopic techniques, 1H and 13C NMR, which demonstrated the effectiveness of the reactions. These materials were designed with architecture suitable to exhibit liquid-crystalline properties. In our group, the objective was to study the structure-property relationship of the compounds of the series, and for that, the size of the terminal alkoxy chain was varied between three and twelve carbon atoms, to verify its influence on the mesomorphic properties of the compounds. Mesomorphic properties were investigated by MOLP, DSC, and XRD techniques. All molecules exhibited characteristic textures by MOLP as a result of the organizational defects of the liquid-crystalline phase, being, therefore, possible to prove the birefringence of the analyzed anisotropic materials. The different structural arrangements revealed the type of mesophase present, thus, the nematic phase was confirmed by the Schlieren-type texture and the smectic phase by the focal-conical and fan-shaped textures. The transition temperatures of the mesophases and the energies involved in the processes were determined by DSC, in the second heating and cooling cycle. These experiments confirmed that all compounds in the series exhibited a wide range of mesomorphism between 64 °C and 80 °C. The mesomorphism of the samples was shown to be dependent on the length of the alkoxy chain for the organization and stability of the liquid-crystalline phase, where smaller carbon chains exhibited the nematic phase, while larger chains presented exclusively the smectic phase. It was possible to observe that the increase in the chain length resulted in the appearance of more organized phases, as well as reflected in the reduction of melting points and lightening of the compounds. The confirmation of the molecular organization in the mesophase was made through XRD analysis, which showed interference sequences through the Bragg reflection peaks (d001 and d002). The relationship between the diffractions evidences the lamellar packing of the molecules, pointing to the presence of type A smectic mesophase, due to the inexistence of folding, interdigitalization, and inclination of the layers. The decomposition temperatures of the samples were investigated by TGA and the results denoted that all were thermally stable up to 230 ºC. The decomposition values were found to ensure that there is no decomposition of the compounds during heating and obtaining the mesophases.