Abstract
Nematic twist-bend phases (NTB) are new types of nematic liquid crystalline phases with attractive properties for future electro-optic applications. However, most of these states are monotropic or are stable only in a narrow high temperature range. They are often destabilized under moderate cooling, and only a few single compounds have shown to give room temperature NTB phases. Mixtures of twist-bend nematic liquid crystals with simple nematogens have shown to strongly lower the nematic to NTB phase transition temperature. Here, we examined the behaviour of new types of mixtures with the dimeric liquid crystal [4′,4′-(heptane-1,7-diyl)bis(([1′,1″-biphenyl]4″-carbo-nitrile))] (CB7CB). This now well-known twist-bend nematic liquid crystal presents a nematic twist-bend phase below T ≈ 104 °C. Mixtures with other monomeric alkyl or alkoxy -biphenylcarbonitriles liquid crystals that display a smectic A (SmA) phase also strongly reduce this temperature. The most interesting smectogen is 4′-Octyl-4-biphenylcarbonitrile (8CB), for which a long-term metastable NTB phase is found at room and lower temperatures. This paper presents the complete phase diagram of the corresponding binary system and a detailed investigation of its thermal, optical, dielectric, and elastic properties.
Highlights
The nematic phase, N, where mesogenic molecules possess long-range orientational order along a direction is one of the simplest liquid crystalline states and is the most used one for electro-optic applications
Mixing a smectogen and a twist-bend nematic liquid crystal could be a simple way to enforce the coexistence of smectic A (SmA) and Nematic twist-bend phases (NTB) phases in a binary system and induce the elusive phase transition between them
Contact experiments performed in liquid crystals cells between CB7CB and one of these mesogens showed the simultaneous presence of SmA and NTB phases separated by a nematic domain, even at room temperature
Summary
The nematic phase, N, where mesogenic molecules possess long-range orientational order along a direction (called the director n) is one of the simplest liquid crystalline states and is the most used one for electro-optic applications. Long-lived metastable NTB phases at room temperature are quite rare, even if recent synthesized compounds have shown to exhibit a wider NTB temperature range [14] or even to form a glassy NTB phase rather than to crystallize [15] Using mixtures is another way to lower the NTB temperature range. Mixing a smectogen and a twist-bend nematic liquid crystal could be a simple way to enforce the coexistence of SmA and NTB phases in a binary system and induce the elusive phase transition between them. CB7CB is the first dimeric compound where the nematic twist-bend phase was unambiguously identified [2] It has been studied in detail and its properties thoroughly investigated [4]. K33 when approaching the NTB phase in pure compounds [3,29,34]
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