Competition and Promotion between Two Different Liquid-Crystalline Building Blocks: Mesogen-Jacketed Liquid-Crystalline Polymers and Triphenylene Discotic Liquid Crystals

Mesogen-jacketed liquid-crystalline polymers (MJLCPs) containing two triphenylene (Tp) units in the side chains, denoted as PPnV (n = 3 or 6, which is the number of the methylene units between the terephthalate core and Tp moieties in the side chains), were synthesized through conventional free radical polymerization, and the phase behaviors of these new combined main-chain/side-chain liquid-crystalline (LC) polymers were investigated. The chemical structures of the monomers were confirmed by elemental analysis, high-resolution mass spectrometry, and 1H/13C NMR. The molecular characterizations of the polymers were performed with 1H NMR, gel permeation chromatography, and thermogravimetric analysis. LC behaviors of the polymers were investigated by differential scanning calorimetry (DSC), polarized light microscopy, and wide-angle X-ray diffraction (WAXD). Both polymers exhibit excellent thermal stabilities. DSC and WAXD results indicate that covalently incorporating Tp discotic liquid crystals has a tremendous effect on the LC behaviors of MJLCPs. As expected, the glass-transition temperature decreases as the spacer length increases. Both polymers form rectangular columnar (ΦR) phases at relatively high temperatures. At low temperatures, however, Tp moieties in the side chains form a discotic nematic (ND) phase in conjunction with the ΦR phase developed by the rod-like supramolecular mesogen – the MJLCP chain as a whole, owing to the self-organization of the Tp moieties. For PP6V in particular, a higher symmetry hexagonal columnar (ΦH) phase forms when temperature exceeds 225 °C. Individual ordered structures developed by these two LC building blocks are not only competitive but also promotive to each other.