Elucidation of the orientational order and the phase diagram of p-quinquephenyl.

The orientational order in the nematic phase of p-quinquephenyl, the pentamer of p-phenylene, has been determined by means of birefringence measurements and by wide-angle X-ray scattering (WAXS). The experimentally determined order parameters <P(2)> are compared with the temperature-dependent order parameter predicted by the Maier-Saupe theory. The order parameters derived from the birefringence at different temperatures in the nematic phase of p-quinquephenyl were in excellent agreement with the Maier-Saupe predictions. The <P(2)> values calculated from the azimuthal profiles derived from WAXS measurements were significantly lower than those determined by the birefringence measurements, especially at higher temperatures. For the birefringence measurements, alignment of the director is achieved by using polyimide alignment layers, whereas director alignment for the WAXS experiments was achieved by a magnetic field. We assign the low overall order parameters that were measured by WAXS to a lower macroscopic orientational order. In addition, upon reinvestigating the mesophase behavior of p-quinquephenyl, a monotropic smectic A phase has been observed upon cooling at 390 °C, just before the crystallization temperature is reached.