Exploring the nematic to twist-bend nematic phase transition: calorimetric and optical measurements

We investigate the nematic (N) to twist-bend nematic (N-TB) phase transition in binary mixtures of a symmetric liquid crystal dimer (CB7CB) and a calamitic bi-phenyl mesogen (7OCB). As the molar concentration of 7OCB increases, both the Iso-N and N-N-TB transition temperatures decrease gradually, consequently, the width of the nematic phase above the N-TB phase can suitably be varied from similar to 12 K to 46 K. Notably, as the nematic range expands, there is a remarkable reduction in transitional entropy, eventually approaching negligible or zero values. Additionally, the heat capacity peak and the phase shift disappear when transitional entropy reaches zero. This behaviour diverges from typical N-SmA and N-SmC phase transitions, where transitional entropy decreases near the tricritical point but the heat capacity peak persists. Our findings indicate a finite positive delta(Delta n) across all concentrations at the N-N-TB phase transition, suggesting a gradually weakening first-order nature. This contrasts with theoretical predictions of a second-order N-N-TB phase transition, as our experimental data consistently supports a first-order nature. The self-consistent mean-field Landau Model successfully validates our experimental data, establishing the universal first-order nature of the N-N-TB phase transition, with experimental entropy values in good agreement with theoretical prediction. [GRAPHICS] .