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 nF2BTFOIM7 (3-fluoro-4(1-methylheptyloxy)4'-(4"-alkoxy-2",3"-difluorobenzoyloxy)tolane]. These compounds exhibit smectic-C* (SmC*), twist-grain-boundary (TGBA for n = 10, TGBC for n = ii, 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 TGBC N* transition was expected. This suggests the existence of two thermodynamically distinct TGBC phases. R4sum4. Des exp6riences de calorim6trie haute r6solution ont 6t6 r6alis6es