Calorimetric study of phase transitions involving twist-grain-boundary TGB(A) and TGB(C) phases

High-resolution calorimetry has been used to determine the heat capacity and latent heat associated with phase transitions in the homologous series of chiral liquid crystals nF(2)BTFO(1)M(7) [3-fluoro-4(1-methylheptyloxy)4'-(4 ''-alkoxy-2 '',3 ''-difluorobenzoyloxy)tolane]. These compounds exhibit smectic-C* (SmC*), twist-grain-boundary (TGB(A) for n = 10, TGB(C) for n = 11, 12) and cholesteric (N*) phases. All the phase transitions are first order with small to moderate latent heats. There is a large rounded excess heat capacity peak in the N* phase that is consistent with the predicted appearance of short-range TGB order (chiral line liquid character). This is analogous to the development of an Abrikosov flux vortex liquid in type-II superconductors. Both the n = 11 and 12 homologs exhibit two closely spaced transitions in the region where a single TGB(C)-N* transition was expected. This suggests the existence of two thermodynamically distinct TGB(C) phases.