Phase Transitions and Dielectric Anisotropy in Nematic Binary Mixtures: Comparing 7CB and MBBA Mixtures to 5CB and MBBA Mixtures

Saleh Satti

Nematic liquid crystals 4-n-heptyl-4'-cyanobiphenyl (7CB) and 4-methoxybenzylidene-4-butylanaline (MBBA) were mixed at various molar concentrations. The phase transition temperatures were measured using a phase transition set-up that recorded through a digital video camera any qualitative change in the liquid crystal phase as the temperature was altered. In addition to the isotropic-nematic phase transition, there was another phase transition that was observed and recorded for certain concentrations. By using the 'glass plate test', and by qualitatively comparing a sample picture of this new phase to others already documented, it was concluded that the two mesogens produced a smectic G phase for some concentrations.

The cells with various mixtures were then modeled as capacitors in parallel to a resistor in order to measure the epsilon perpendicular and parallel values. For both values it was found that there was a linear increase as the molar concentration of 7CB increased; however, the difference of the two values (delta epsilon) showed no linear properties. Both the phase transition values and the epsilon values of these mixtures were compared to those of 4-n-pentyl-4'-cyanobiphenyl (5CB) and 4-methoxybenzylidene-4-butylanaline (MBBA). It was found that the phase transition temperatures and epsilon perpendicular values of the 4-n-heptyl-4'-cyanobiphenyl and 4-methoxybenzylidene-4-butylanaline mixtures were larger, however their epsilon parallel, and thus the delta epsilon values were less.