Structure of macroscopic monodomains and its soft confinements of chiral smectic phases on crystallization in a main-chain nonracemic liquid crystalline polyester

A main-chain nonracemic chiral liquid crystalline (LC) polyester was synthesized using an A-B type condensation reaction of a chiral monomer, R-(-)4′-{ω-[2-(p-hydroxy-o-nitrophenyloxy)-1-propyloxy]-1-heptyloxy} -4-biphenylcarboxylic acid, and was abbreviated as PET(R*-7). Wide-angle X-ray diffraction (WAXD) experiments combined with differential scanning calorimetry measurements revealed multiple phase transitions including a chiral twist grain boundary A (TGBA*) phase, a chiral smectic A (SmA*) phase, a chiral smectic C (SmC*) phase, and a highly ordered monoclinic crystalline (K _M) phase with decreasing temperature. Flat-elongated and right-handed double-twisted helical single crystals were grown thermotropically in the smectic LC states. Both crystal types possessed the identical monoclinic unit cell: a = 1.04 nm, b = 0.46 nm, c = 5.59 nm, a = β = 90°, and γ = 84.2° based on the WAXD and selected area electron diffraction results. Uniaxial elongation of centimeter-sized films (along the xz-plane in a three-dimensional, 3D, coordinate system) of submillimeter thickness (along the y-axis) resulted in macroscopic smectic monodomains. Analysis of the detailed 2D WAXD results indicated that in the SmA* phase the molecules are parallel to the layer normal in the direction of elongation. However, in the SmC* phase, the 3D macroscopic monodomain was constructed with the layers which possess a 13° synclinically tilted layer normal with respect to the elongation direction (the z-axis). The molecules in the 3D monodomains of SmC* phase were tilted ∼8° synclinically with respect to the elongation direction within the "in-plane" of the film (within the xz-plane), translating to a ∼21° tilt with respect to the layer normal. During the crystallization of the KM phase in the SmA* phase, the c-axis of the K _M crystal is along the elongation direction (the z-axis), yet the d-spacing of the (00l) (l = even number) planes were slightly larger than the layer spacing of the SmA* phase. When the crystallization occurred in the 3D monodomain of the SmC* phase, on the other hand, the synclinically tilted LC layers and the tilted layer normal orientation with respect to the elongation direction (the z-axis) were retained in the K _M crystal, although the d-spacing of the (00l) (l = even number) planes was larger than the layer spacing of the SmC* phase. The most striking observation was that in order to form the KM crystals in the SmC* phase the molecules must undergo a remarkable reorientation (∼21° tilting) to became a single-crystal-like monodomain while retaining the synclinically tilted layer structure.