DSC studies on p-(n-alkyl)-p '-cyanobiphenyl (RCB's) and p-(n-alkoxy)-p '-cyanobiphenyl (ROCB's) liquid crystals

Solidification (supercooled nematic-to-solid or supercooled smectic-A-to-solid), melting (solid-to-nematic or solid-to smectic-A), smectic-A-to-nematic, and clearing (nematic-to-isotropic or smectic-A-to-isotropic) transition temperatures partial derivative(s), partial derivative(m), partial derivative(AN), and partial derivative(c), respectively, and enthalpies Delta H-s, Delta H-m, Delta H-AN, and Delta H-c, respectively, have been determined by differential scanning calorimetry (DSC) for members of the p-(n-alkyl)-p'-cyanobiphenyl (RCB) and the p-(n-alkoxy)-p'-cyanobiphenyl (ROCB) homologous series of liquid crystals, where 'R' is the number of carbon atoms in the iz-alkyl or n-alkyloxy tails. The study shows that the transition of the less ordered nematic phase to the mon ordered smetic-A phase, in liquid crystals that exhibit both phases (those with R = 8 or 9), involves a transient phase of intermediate order Broad or overlapping solidification exotherms indicate that two mesogenic aggregations supercool simultaneously. Finally, there is evidence for a transition between two solid forms that occurs at temperatures preceding partial derivative(m) or concurrent with it depending on 'R' and the heating rate. This refutes an earlier view that during melting the solid form with the higher melting point solidifies. (C) 2000 Elsevier Science B.V. All rights reserved.