Synthesis and phase structures of mesogen-jacketed liquid crystalline polymers containing 1,3,4-oxadiazole based side chains

A series of new monomers of 2,5-bis[(4-tertbutyl-phenyl)-1,3,4-oxadiazole] styrene (M-Ct) and 2,5-bis[(4-alkoxy-phenyl)-1,3,4-oxadiazole]styrene (M - OCm, m is the number of the carbons in the alkoxy groups, m = 8, 10, 12, 14) were synthesized. Conventional radical polymerization of the monomers resulted in a series of new mesogen-jacketed liquid crystalline polymer (MJLCP) containing the 1,3,4-oxadiazole unit. The chemical structures of the monomers were confirmed by elemental analysis, mass spectrometry, ¹H NMR, and IR. The molecular characterization of the corresponding polymers of P-Ct and P-OCms was performed with ¹H NMR, gel permeation chromatography, and thermogravimetric analysis. Their phase structures and transitions were investigated by differential scanning calorimetry, wide-angle X-ray diffraction, and polarized light microscopy experiments. The P-Ct formed the hexatic columnar nematic (Φ_HN) phase that is typical for MJLCPs, wherein the chain molecules were rodlike. The P-OCms exhibited a well-defined smectic A (S_A) phase. As the mesogenic group is laterally jacketed to the polyethylene backbone through a single carbon-carbon bond, the P-OCm molecule in the SA phase should be more or less ribbonlike with the backbone squeezed by the parallel aligned side chains on both sides. The transition of the four P-OCms follows the sequence of S_A ↔ N ↔- I. The comparison between P-Ct and P-OCms indicates that the flexibility of the side-chain tails is crucial to determine the LC structures. Namely, simply changing the chemical structures of small portion of the MJLCP may greatly vary the molecular packing behavior and thus the molecular shape in LC phase structures.