REACTIVITY OF MESOGENIC DIACETYLENES COUPLED WITH PHASE-TRANSITIONS BETWEEN CRYSTAL AND LIQUID-CRYSTAL PHASES

Among mesogenic diacetylenes, droxypenta-1,3-diynyl)benzylidene]-4-alk oxyaniline (DA-Cn), in which n is the number of carbon atoms in the al koxy group, DA-CE( has been found to have four phases divided by three endothermic transition temperatures, The four phases have been assign ed to crystal phase I (T < 96 degrees C), crystal phase II (TCI-CII= 9 6 degrees C), crystal phase G (TCII-CG = 115 degrees C) and nematic ph ase (TCG-N = 123 degrees C), based on DCS measurements and observation under a microscope. The crystal structure of DA-C8 in phase I has bee n determined, Crystal data are triclinic, space group P (1) over bar, a = 14.580(3), b = 17.451(4), c = 10.219(3) Angstrom, alpha = 93.40(2) , beta = 103.83(2), gamma = 116.85(3)degrees and Z = 4. Four molecules in a unit cell are hydrogen-bonded to each other at the hydroxymethyl groups and/or the imino-nitrogens of the benzylideneaniline moieties to form a checker-like structure. The phase transitions have been moni tored by X-ray powder diffraction measurements. The checker-like struc ture in crystal I is transformed to a layered structure in crystal pha se II and in crystal phase G, The structural transformation may be cou pled with a change in the hydrogen-bonding scheme. Whereas crystal pha se I is inactive towards the solid state polymerization, crystal phase II polymerizes with a long induction time of 50 h at 110 degrees C, H owever, the induction time of the polymerization is shortened greatly in crystal phase G, and the polymerization in the nematic phase procee ds smoothly with almost no induction time, The results are interpreted in terms of the relative orientation of diacetylene molecules in each phase.