Side chain length dependence on supra-molecular structures in a series of aromatic polyimides having terminal 4-cyanobiphenyl liquid crystalline side chains

A series of newly designed polyimides, composed of aromatic polyimide backbones and methylene side chains with terminal 4-cyanobiphenyl groups, were synthesized based on the polycondensation of 3,3′,4,4′-biphenyl tetracarboxylic dianhydride (BPDA) with 2,2′-bis{ω-[4-(4-cyanophenyl)phenyoxy-n-alkoxycarbonyl]}-4,4′-biphenyl diamine (nCBBP, where n is the number of methylene units in the side chains). We report our structural and morphological studies on this series of BPDA-nCBBP, which possesses n=7, 9, and 11 methylene units in the side chains. For these three polyimides, a nematic (N) phase was first formed at high-temperatures during cooling from the isotropic melt. The transition temperatures and enthalpies were cooling-rate independent as observed in differential scanning calorimetry. This N phase was further confirmed by the results of wide angle X-ray diffraction (WAXD) and polarized light microscopic experiments. At lower temperatures, ordered structures were formed. It was surprising that in the cases of BPDA-nCBBP (n=7 and 9), triclinic lattices were observed; while in the case of BPDA-11CBBP, a hexagonal lattice was evident, as determined by 2D WAXD experiments. This indicated that by increasing the number of methylene units in the LC side chains, the individual chains (base unit) used in constructing these supra-molecular structures changed their packing symmetry. Namely, when the number of methylene units in the side chains was relatively low (i.e. the length is short), the individual chains were packed into a ribbon-like structure. However, when the side chain length is long enough, the individual chains exhibit cylindrical symmetry. Regardless of the lattice formed by the supra-molecular structures, they are all on the nanometer length scale.