X-Ray Study of Phase Transition and Structures of Some Columnar Liquid Crystals

Three series of columnar discotic liquid crystals with different cores and lengths of side chains have been studied by use of X-ray diffraction. Overlapping diffraction peaks have been decomposed by use of a peak model. The measured d-spacings were indexed for the mesophases and some crystal phases. Radial distribution functions (RDF) along the columnar axes were calculated approximately. Octaether derivatives of tetrabenzocyclododecatetraene with n = 12, 13, 14, 15 carbon atoms in the side chains exhibit a hexagonal mesophase to centered rectangular mesophase transition without observable transition heat. It is found that the transitions take place over a wide temperature range (maximum 20^circC) n = 13 and 14. This transition range is really an intermediate phase which is neither hexagonal nor centred rectangular, but rather oblique. This series of specimens also shows crystal to crystal transitions with small transition heats. Detailed X-ray diffraction scans indicate that these transitions also occur over a temperature range (maximum 10 ^circC) and the transition is not a sudden transition as a function of temperature. Hexa-n-alcanoyloxy triphenylenes with side chain carbon atom number n = 6, 8, 10, 12 have been studied. HAT -C8 shows solid polymorphism between 80^ circC and 26^circC. HAT -C10 has two mesophases with small differences in their diffraction pattern. HAT-C12 had been reported to have three mesophases on cooling from 125^circ C to 70^circC, but the corresponding diffraction patterns are not stable. The stable patterns obtained above 70^circ C are those of the isotropic liquid. At 70 ^circC, the specimen does show a stable mesophase pattern corresponding to the D_0 phase. Benzene twins based on hexa-hydrobenzene hexa -n-heptanoate (R = C_6 H _{13}) also have been studied. These twins consists of two benzene rings joined by a long flexible spacer rm C_{n-2}H_ {2n-1}(COO)_2 with R as the side chains. TWINS-S6, 8 show a complex diffraction pattern in their mesophase temperature range. No mesophase is observed by X-rays. TWINS-S12, 14 show a simple mesophase diffraction pattern in the mesophase temperature range.