Direct observation of two-dimensional nematic and smectic ordering in freely suspended films of a bolaamphiphilic liquid crystal

The T-shaped bolaamphiphile CT2, a liquid crystal with a hydroxyl-terminated, rod-shaped core and semifluorinated side chains, forms the Lam-Iso, Lam-Nem, and Lam-Sm lamellar phases, which are reported to exhibit respectively smectic layering of the side chains with additional in-layer isotropic (Iso), nematic (Nem) and smectic-like (Sm) ordering of the rigid molecular cores. Optical observation of 2–16 layer thick freely suspended films enables visualization and quantitative study of these transitions in two dimensions (2D). The 2D-Iso to 2D-Nem transition is found to be first order, occurring first in the film surface layers and then in the film interior on further cooling. The second order 2D-Nem to 2D-Sm transition is marked by the continuous evolution of the orientational texture from 2D nematic-like to 2D focal conic. The film geometry stabilizes the lamellar ordering, significantly increasing the birefringence associated with the in-plane ordering in the films.