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

ABSTRACT

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 ~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 δ(∆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.

GRAPHICAL ABSTRACT

KEYWORDS:

• Twist-bend nematic (N_TB) phase
• phase transition
• transitional entropy
• heat capacity
• birefringence
• tilt angle

Disclosure statement

No potential conflict of interest was reported by the authors.

Additional information

Funding

This work was financially supported by the University Assistance for Research Project [Project No: 2321/R–2022, Dated-17/05/2022]. AP is grateful to the University Grants Commission (New Delhi) for the award of an SRF [UGC/NFOBC/201920-19J6148547].