Pressure-induced stabilization of the smectic A* phase in a chiral polyacrylate

The pressure-temperature phase diagram of a chiral liquid crystal polyacrylate has been studied in situ by X-ray diffraction. Hydrostatic pressures of the order of 100-200 MPa strongly influence the polymorphic behaviour 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. The formation of the smectic C* phase is inhibited at 220 MPa and occurs only upon the first heating process at 133 MPa. 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 phase diagram suggests the occurrence of peculiar disorder-order-disorder transitions as pressure is reduced isothermally.