Phase transitions, crystallization behaviors and structure of a nonracemic chiral main-chain liquid crystalline polyester and its macroscopic quasi-monodomains

Chirality is one of central features of chemical systems, which are involved in a wide variety of biologically and technologically important processes. With the development of asymmetric synthesis, different architectures of new chiral materials including chiral liquid crystals have been available. Introduction of chirality has led to new liquid crystalline structures and phase behaviors.; A nonracemic chiral main-chain liquid crystalline polyester (PET(R*)-7) was synthesized using condensation reaction of an AB type monomer: R-(−)4 ′-{lcub}(ω)-[2-(p-hydroxy-o-nitrophenyloxy)-1-propyloxy]-1-heptyloxy{rcub}-4-biphenyl carboxylic acid. Multiple phase transitions have been found during cooling and heating at different rates in differential scanning calorimetry (DSC) measurements. Wide-angle X-ray diffraction (WARD) experiments have proven the existence of the chiral smectic C (SmC*), chiral smectic A (SmA*), twist grain boundary A (TGBA) and high ordered tilted smectic phase.; Flat-elongated and twisted helical single crystals can be grown thermotropically from the liquid crystalline state. All helical crystals show a right-hand twist. Both types of crystals possess the identical monoclinic unit cell: a = 1.03 nm, b = 0.456 nm, c = 5.38 nm, α = β = 90° and γ = 84.2°. These single crystals also can be obtained by evaporating solvent from the nitrobenzene solution. The evaporation grown helical crystals have the same handedness as melt-grown crystals. PET(R*)-7 crystallizes much faster in oriented films. It has been found that the chain orientation of the crystalline phase is confined by the layer structure in the liquid crystalline phase from which crystallization takes place. The c axis of crystals is along the film drawing axis when crystallization occurs in the SmA* phase. When crystallization takes place in the SmC* phase, the tilted layer normal transforms into the tilted molecular chain axis in the crystals.; An important new observation is that macroscopic liquid crystal quasi-monodomains can be obtained by applying a mechanical external field. Detailed X-ray diffraction experiments proved these quasi-monodomains possess a tilted smectic monodomain-like structure. The liquid crystalline quasi-monodomain structure is retained in the crystals grown in the SmC* phase.