High-pressure mesomorphic behavior of a chiral polyacrylate by X-ray diffraction in situ.

he high-pressure mesomorphic behavior of a chiral liquid crystalline polyacrylate has been studied by x-ray diffraction in situ. The results show the existence of monolayer smectic-A* and bilayer smectic-C* mesophases that coexist in the thermally untreated sample over a well-defined range of temperatures, from ambient pressure up to 133 MPa. Hydrostatic pressures of the order of 100-220 MPa are found to affect strongly the polymorphic behavior of the sample. The smectic-A* mesophase, which at ambient pressure coexists with an interdigitated smectic-C* structure, is stabilized over a temperature range which increases with pressure, whereas the formation of the smectic-C* phase is inhibited above 220 MPa. The pressure increment is found to introduce static positional disorder, and to reduce the long-range positional order in the smectic structure strongly. A transition sequence from the isotropic melt through the smectic-A* phase to a new crystalline structure is found by cooling isobarically down to room temperature. The proposed pressure-temperature phase diagram suggests the occurrence of peculiar disorder-order-disorder transitions as pressure is reduced isothermally.