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Morphological phase behaviour under pressure of polycatenar mesogens with a perfluorinated moiety

Two polycatenar materials composed of a four‐aromatic‐ring core with a perfluorinated moiety attached in one terminal position through either butylene‐ or pentylene spacer groups, and three tetradecyloxy chains at the other end (abbreviated as 14PC4F and 14PC5F), were investigated to study the effect of pressure on the phase transition behaviour. A polarizing optical microscope equipped with a high pressure optical hot stage, was used for the purpose. The T vs. P phase diagrams of 14PC4F and 14PC5F were constructed in the pressure region up to 100 MPa. 14PC4F showed the stable crystal (Cr1)–columnar tetragonal (Coltet)–smectic A (SmA)–columnar hexagonal (Colh)–isoropic liquid (I) phase transition sequence under all pressures. 14PC5F exhibited the phase sequence metastable crystal (Cr2)–cubic (Cub)–Coltet–SmA–I in a melt‐cooled sample on heating under pressure. But when the melt‐cooled Cr2 sample was annealed at 52–54°C for 2–3 h, the stable crystal (Cr1) was formed slowly, giving a stable Cr1–Cub–Coltet–SmA–I phase sequence. The temperature region of the stable cubic phase broadened with increasing pressure. Furthermore a new mesophase of 14PC5F was pressure‐induced between the I and SmA phases on cooling at pressures above about 16 MPa. Since the monotropic mesophase exhibited a texture very similar to that of the high temperature Colh phase of 14PC4F with planar orientation, the new phase was assigned at a high temperature columnar hexagonal phase of 14PC5F.