The binary system of 4-octyl-4′-cyanobiphenyl (8CB) and 4-octyloxy-4′-cyanobiphenyl (8OCB) has been studied by means of differential scanning calorimetry (DSC) and ultraviolet absorption spectrophotometry (UV). The phase-transition temperatures, enthalpies, and entropies have been determined by using calorimetric methods on DSC. The results indicate clearly the existence of threephase regions across the crystalline-to-smectic A, smectic A-to-nematic, and nematic-to-isotropic transitions in the 8CB/8OCB mixtures. The obtained phase-transition temperatures of the 8CB/8OCB mixtures are between the data for 8CB and 8OCB. A few of the phase transitions cannot be observed at high heating rates. The phase-transition temperatures of the 8CB/8OCB mixtures rise with the heating rate between 2°C/min and 15°C/min. The activation energies were calculated by the Ozawa method for the phase transitions of 25% 8CB and 75% 8OCB liquid crystal mixtures. UV experiments were carried out to characterize the absorptivity constants of liquid crystal and their mixtures. The molar absorptivity and maximum absorption wavelengths were measured in chloroform solution by UV spectrophotometry. The maximum absorption wavelength of the 8CB/8OCB mixtures increases with decreasing percent weight of 8CB in 8OCB, a result associated with the different lengths of the alkyl chain.
The thermal and mesophase behaviour of ternary mixtures formed from hydrogen-bonded nematic liquid crystals 4-hexylbenzoic acid, 4-(octyloxy)benzoic acid and 4-(decyloxy)benzoic acid were investigated by differential scanning calorimetry (DSC) and phases identified by polarised optic microscopy (POM). The DSC and POM results clearly indicate that the produced mixtures display liquid crystalline properties. The phase transition temperature values obtained in the DSC experiments are in line with the POM observations. Investigation of the thermal properties of ternary mixtures reveals that the nematic range of the produced mixtures is affected in a way that is dependent on both the mixture ratio and the difference in the alkyloxy chain lengths. Furthermore, the phase transition temperature values of ternary mixture increase with increasing heating rate, and the calculated activation energy values show that the reaction arising during the phase transitions of the mixture is regular.
The hexylcyanobiphenyl (6CB) and octylcyanobiphenyl (8CB) mixtures have been investigated by means of differential scanning calorimeter and ultraviolet. Differential scanning calorimeter results indicate clearly the existence of four phase transitions in the 6CB/8CB mixtures. The phase diagram of mixture was obtained from differential scanning calorimeter and theoretical calculations. The experimental phase diagram is approximately similar to the calculated phase diagram. The phase transition temperatures of the 6CB/8CB mixtures rise with the heating rate between 2 ºC/min and 10 ºC/min. The activation energies were calculated for the phase transitions of 50 % 6CB and 50 % 8CB liquid crystal mixtures. UV experiments were carried out to characterize the absorptivity constants of liquid crystal and their mixtures. The molar absorptivity and maximum absorption wavelengths were measured in chloroform solution by UV spectrophotometry. The maximum absorption wavelength of the 6CB/8CB mixtures increases with decreasing percent weight of 8CB in 6CB, a result associated with the different lengths of the alkyl chain.
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