Structures and phase transformations of odd-numbered asymmetric main-chain liquid crystalline polyesters

Two new asymmetric odd-numbered main-chain liquid crystalline (LC) polyesters (BPE-Cn) were synthesized through the condensation polymerization of A-B type asymmetric α,ω-carboxylic acid-hydroxyl terminated monomers with odd-numbers of methylene units in the spacers (BPCA-Cn-PmOH, where n=7 and 9). The phase behaviors and structures of these odd-numbered BPE-Cn samples were characterized using the combined techniques of differential scanning calorimetry (DSC), wide angle X-ray diffraction (WAXD), selected area electron diffraction (SAED) and polarized light microscopy (PLM). One-dimensional (1D) powder WAXD results at different temperatures combined with DSC results revealed that during cooling these two polyesters enter a low-ordered LC phase before developing a crystalline phase at lower temperatures. Based on 2D WAXD of oriented fiber patterns, the low ordered LC phase of odd-numbered BPE-Cn was identified to be a smectic A (SmA) phase, which was constructed by each chemical repeating unit along the fiber drawn direction. The crystalline structures were determined to be monoclinic unit cells (K_M). However, BPE-C7 possessed a γ=87°, while BPE-C9 had a β=100°. These crystalline structure identifications were also confirmed by SAED from single crystals. Two-chain packing models of the K_M phases with four chemical repeating units were proposed on the basis of the experimental diffraction patterns. The different structures were attributed to the interplay between the mesogenic group ordering propensity and the chain constraints dictated by the methylene spacer and the meta-linkage at the end of mesogen. In the ordered phases of these two odd-numbered BPE-Cn polyesters, aromatic mesogenic groups are more or less parallel to each other along the chain with small angles of deviation from the c-axis of the crystal lattice, different from those large zigzag conformation structures in even-numbered BPE-Cn crystals. Crystallographic calculations of these two unit cell structures indicated that the neighboring chains in the crystals are translated along the c-axis (in the case of BPE-C9) or twisted away from the b-axis (in the case of BPE-C7) in order to incorporate both of the bend attributed to the odd-numbered methylene spacers and of the configurational meta-linkage at the end of mesogens. The simulated diffractions based on these calculations qualitatively agreed with the experimental observations. The phase identifications were also supported by the observed texture changes in PLM.