A series of achiral 4-biphenyl carboxylic acid compounds (BPCA-Cn-PmOH) connected with alkoxyl chains having various carbon numbers (n ) 6-10) and terminated by phenyl groups with meta-positioned hydroxyl groups was synthesized. Different phase structures including nematic, smectic A (SmA), smectic C (SmC), and highly ordered smectic liquid-crystalline phases along with crystalline phases were identified based on wide-angle X-ray diffraction and electron diffraction experiments. It was found via infrared spectroscopy that the hydrogen (H)-bonds were formed between the carboxylic acids to construct headto-head dimers as the building blocks for these ordered structural formations. H-bonds formed via the meta-positioned hydroxyl groups also played an important role in forming ordered layers in these structures. The morphology of this series of BPCA-Cn-PmOH as observed under polarized light microscopy showed an oily streak (cylinder) texture with Myelin-figure in the SmA phase. When temperature cools to enter the SmC phase, these streaks (cylinders) started to twist into helical suprastructures, which were not only by the birefringence changes but also by the three-dimensional helical geometry observed in other microscopic techniques. The dynamic conformational changes of the aromatic and aliphatic parts in this series of BPCA-Cn-PmOH at different temperatures correspond well with the thermal transitions via solid-state carbon-13 nuclear magnetic resonance experiments. Computer simulation indicated that the head-to-head dimers possess a twisted rather than a bent conformation. It was deduced that the twisted conformation of the dimers and the terminal meta-substituted phenyl groups at both ends of the dimers are critically important in forming the helical suprastructures.
A series of new asymmetric bent main-chain liquid crystalline (LC) polyesters (BPE-Cn) were synthesized through the condensation polymerization of A-B type asymmetric R,ω-carboxylic acidhydroxyl-terminated monomers containing different even numbers of methylene spacers (BPCA-Cn-PmOH, where n ) 6, 8, and 10). Differential scanning calorimetry results showed three phase transition processes in this series of BPE-Cn samples. The phase change with the lowest transition temperature is cooling rate dependent, while the two higher temperature transitions are cooling rate independent. Onedimensional (1D) powder wide-angle X-ray diffraction (WAXD) results at different temperatures revealed that during cooling this series of polymers exhibits a low-ordered LC phase and a highly ordered smectic crystal phase before developing a crystalline phase at lower temperatures. The phase structures and symmetry were identified by 2D WAXD and selected area electron diffraction (SAED). On the basis of 2D fiber WAXD patterns, the low ordered LC phase was identified to be an anticlinically tilted SmC (SmC A) phase, which was constructed by an alternating synclinic SmC phase in each chemical repeating unit layer along the fiber drawn direction. The highly ordered smectic phase was an anticlinically tilted SmH (SmH A) phase, which was again constructed by an alternating synclinic SmH phase in neighboring layers. On the basis of the crystallographic point of view, this smectic crystal phase had an orthorhombic unit cell. The crystalline structure was determined to be an orthorhombic unit cell (K O) with Pna21 symmetry in these three polyesters. The crystalline structures and symmetry were also confirmed by SAED experiments. As the number of methylene units increased, the dimensions of the crystal structures also increased, yet they retained a zigzag conformation. A two-chain packing model of the KO phase with four chemical repeating units was proposed on the basis of the experimental diffraction patterns and qualitatively supported by simulated structural diffractions. Additionally, the phase identifications were also supported by the observation of texture changes in polarized light microscopy. Even though the zigzag molecular arrangement remains in the low ordered SmC A and the highly ordered SmHA phases which mimics the bent-core small molecules, chiral characteristics such as helical supramolecular structures are not observed due to the covalent bonding connections in this series of BPE-Cn polymers.
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