Mixtures of NTB and smectogenic liquid crystals

Abir Aouini (1) , Christophe Blanc (1) , Ivan Dozov (2) , Daniel Stoenescu (3) , Maurizio Nobili (1)

1 L2C - Laboratoire Charles Coulomb
2 LPS - Laboratoire de Physique des Solides
3 IMT Atlantique - IMT Atlantique

Heliconal nematic twist bend (NTB) phases are frequently found at high temperatures. In the last years the search of stable room-temperatures NTB phases has led to the synthesis of new compounds [1] and to the design of multicomponent systems. Mixtures of NTB compounds with nematogens ,such as 4-Cyano-4'-pentylbiphenyl (5CB) dimeric liquid crystal 1,7-bis(4-cyanobiphenyl-4-yl) heptane (CB7CB) decrease strongly the temperature range of the NTB phase and tune the optical, thermal and dielectric properties of NTB [2]. In this work we have explored the physical properties of mixtures of CB7CB with several smectogens with a particular focus on 4′-octyl-4-biphenylcarbonitrile (8CB). The 8CB/CB7CB phase diagram reveals striking features compared to the 5CB/CB7CB one. Despite their macroscopic resemblance, SmA and NTB domains appear incompatible and remain separated by a nematic phase that extends to very low temperature (-20°C) at a CB7CB fraction of ϕ_c≈20%. We have characterized the optical, thermal, dielectric and anchoring properties of the N and NTB phases of the mixtures. Compared to the pure CB7CB system, the N phase shows unusual behavior over a large range of composition and reveals strong pre-transitional effects when approaching ϕ_c. We also found that, for specific aligning substrates, adding 8CB to CB7CB drastically changes the anchoring properties (fig1) facilitating homeotropic alignment.
[1] Yuan Wang, Quan Li; Room temperature heliconical twist-bend nematic liquid crystal; Cryst. Eng. Comm; 17; 2778-2782; 2015.
[2] Nina Trbojevic and Mamatha Nagaraj; Dielectric properties of liquid crystalline dimer mixtures exhibiting the nematic and twist-bend nematic phases; Phys. Rev. E ;96; 052703; 2017.

Domaines: Matière Molle [cond-mat.soft]; Matériaux

Abir Aouini, Christophe Blanc, Ivan Dozov, Daniel Stoenescu, Maurizio Nobili. Mixtures of NTB and smectogenic liquid crystals. 15th European Conference on Liquid Crystals, Jun 2019, Wroclaw, Poland. ⟨hal-02179552⟩