Pressure induced twist grain boundary phase

Abstract

We report high pressure studies on a binary system of 4-(2'-methyl butyl)phenyl 4'-n-octyl biphenyl-4-carboxylate (CE8 from BDH) and 4-n- dodecyloxy biphenyl-4'-(2'-methyl butyl)benzoate (C12). For the concentration range 0.32<X<0.62, where X is the weight fraction of C12, the system shows the following sequence of transitions at room pressure: Isotropic- Cholesteric (Ch)-Twist Grain Boundary (TGB)-Smectic A (A)Smectic C⁻ (C^∗ ). For X=0.25, the Ch phase transforms directly to the A phase at 1 bar, but there is an induced TGB phase between the Ch and A phases at elevated pressures. Analysis of the topology of the pressure-temperature diagram in the vicinity of the Ch-TGB-A meeting point indicates that is a critical end point rather than a bicritical point as expected from the mean field theory. At X=0.64 there is a direct TGB-C^∗ transition at 1 bar, but there appears a pressure induced A phase between the TGB and C^∗ phases. The topology of the phase diagram suggests that the TGB-A-C^∗ meeting point is a bicritical point.