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<p>A systematic study of the high-resolution specific heat capacity and optical birefringence data for a new binary system composed of 1′′,7′′-bis(4-cyanobiphenyl-4′-yl) heptane (CB7CB) and 4-cyanobiphenyl-4´ (4-heptylbenzoate) (7BCB) near the nematic (N) to twist-bend nematic (N<sub>tb</sub>) phase transition is presented. The introduction of 7BCB into CB7CB leads to a near-linear increase in the Iso–N transition temperatures, while the N–N<sub>tb</sub> transition temperatures decrease, broadening the nematic phase range (~ 12&#xa0;to 82&#xa0;K). As the nematic range expands, the transitional entropy decreases significantly, eventually becoming negligible for large nematic ranges. The latent heat variation with concentration fits a crossover function, compatible with a mean-field free-energy expression that includes a nonzero cubic term due to the Halperin, Lubensky and Ma (HLM) coupling effect. Furthermore, as the transitional entropy approaches zero, both the heat capacity peak and the corresponding phase shift vanish. This behavior deviates from typical N–SmA, N–SmC and SmA–SmC transitions, where the transitional entropy decreases near the tricritical point (TCP), but the heat capacity peak persists. This supports a first-order, rather than second-order, N–N<sub>tb</sub> phase transition. The mean-field Landau model and renormalization theory confirm the first-order nature of the N–N<sub>tb</sub> transition, with experimental entropy data aligning with theoretical predictions. Birefringence results confirm the helical structure of the N<sub>tb</sub> phase, provide temperature-dependent tilt angle <i>θ</i>, and yield a critical exponent (<i>β</i>′) consistent with the tricritical hypothesis (<i>β</i>′ ~ 0.25), further supporting the first-order nature of the transition.</p>

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Phase transitional behaviour of binary liquid crystalline mixtures exhibiting nematic and twist-bend nematic phases

  • 期刊论文

  • 发表日期：

    2026年7月11日

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  • Apsari Parvin,
  • Malay Kumar Das,
  • Przemyslaw Kula

摘要

A systematic study of the high-resolution specific heat capacity and optical birefringence data for a new binary system composed of 1′′,7′′-bis(4-cyanobiphenyl-4′-yl) heptane (CB7CB) and 4-cyanobiphenyl-4´ (4-heptylbenzoate) (7BCB) near the nematic (N) to twist-bend nematic (Ntb) phase transition is presented. The introduction of 7BCB into CB7CB leads to a near-linear increase in the Iso–N transition temperatures, while the N–Ntb transition temperatures decrease, broadening the nematic phase range (~ 12 to 82 K). As the nematic range expands, the transitional entropy decreases significantly, eventually becoming negligible for large nematic ranges. The latent heat variation with concentration fits a crossover function, compatible with a mean-field free-energy expression that includes a nonzero cubic term due to the Halperin, Lubensky and Ma (HLM) coupling effect. Furthermore, as the transitional entropy approaches zero, both the heat capacity peak and the corresponding phase shift vanish. This behavior deviates from typical N–SmA, N–SmC and SmA–SmC transitions, where the transitional entropy decreases near the tricritical point (TCP), but the heat capacity peak persists. This supports a first-order, rather than second-order, N–Ntb phase transition. The mean-field Landau model and renormalization theory confirm the first-order nature of the N–Ntb transition, with experimental entropy data aligning with theoretical predictions. Birefringence results confirm the helical structure of the Ntb phase, provide temperature-dependent tilt angle θ, and yield a critical exponent (β′) consistent with the tricritical hypothesis (β′ ~ 0.25), further supporting the first-order nature of the transition.