Critical point for the blue-phase-III–isotropic phase transition in chiral liquid crystals

The highly chiral compound S,S-4{''}-(methylbutyl)phenyl-4{'}-(methylbutyl) biphenyl carboxylate (S,S-MBBPC) undergoes a continuous supercritical evolution from the isotropic (I) phase to the third blue phase (BPI III). Mixtures of S,S-MBBPC and its racemate have been studied with high-resolution calorimetry capable of quantitative latent heat determinations and with optical activity measurements. Both experiments indicate that the first-order BP III--I transition line ends at a critical point when the chiral mole fraction Xc{}0.45. Analysis of Cₚ and optical activity data for the near-critical mixture with X=0.45 indicates mean-field behavior instead of the theoretically predicted Ising fluctuation behavior, which would be analogous to that at the liquid-gas critical point of a simple fluid. It is speculated that the Ginzburg criterion can explain this mean-field behavior since the critical regime may be too small for experimental observation, as is the case for almost all Smectic-A--Smectic-C transitions. {} 1996 The American Physical Society.