Phase behaviors, molecular and supramolecular structures in polymers containing rigid-rod backbones with cyanobiphenyl side chains

One of the most important and challenging topics in materials chemistry involves designing nano-structures in synthetic materials via self-assembly for various highly technical applications. A specially designed combined liquid crystalline polymer containing a polyester backbone with cyanobiphenyl side chains has been studied in aspects of phase behaviors and crystal structures. The triclinic crystal phases identified in this series of polymer are all found to be constricted by 4-monomer unit cells. This discovery of 4-monomer triclinic unit cells motivates a search for the existence of supramolecular phases and understanding the possible molecular packing. A series of newly designed polyimides, which are composed of aromatic polyimide backbones with 4-cyanobiphenyl mesogens on the side chains has been synthesized. This series of polymers possesses a lesser degree of coupling between the backbones and side chains, which indicates the possibility of microphase separation between them. The representative polyimides of BPDA-7CBBP and BPDA-11CBBP in this series, in which 4-cyanobiphenyl side chains are connected onto the backbones through seven and eleven methylene units respectively have systematically studied in this research.; Two crystal forms were recognized in BPDA-11CBBA, and one of them possesses six repeating units in one monoclinic unit cell. Moreover, the existence of a supramolecular phase has been proposed based on 2D WAXD fiber patterns. In the case of BPDA-7CBBP, three crystal forms were identified: two of them are constructed by triclinic lattices with large unit cells. The numbers of repeating units in those unit cells are seven and eight, respectively. Complicated phase behaviors including a second-order transition between the supramolecular phase and a high-order liquid crystal phase have been explored. The fact that large unit cells in both polymers with the numbers of repeating units in unit cells being 6, 7, and 8 leads to an important issue for our study: how are these molecules packed into these large unit cells?; For understanding the various numbers of repeating units found in those low-symmetry unit cells, a waved-layer type of motif has been considered responsible for constructing the crystal phases with big unit cells. The distributions of order domains in the supramolecular phase are found less likely to possess cylindrical symmetry. Furthermore, the waviness of the layer-type motif has been realized to be a necessary element as long as the layer-type motif is the packing unit in crystal phases with large unit cell. (Abstract shortened by UMI.)...