Discotic liquid crystal phases are classified into two main categories: discotic nematic mesophases (ND) and discotic columnar mesophases (Figure 4). The discotic nematic phase is the least ordered mesophase formed by disc-like molecules and is analogous to the nematic phase formed by rod-like molecules. It is characterized by possessing 1-dimensional long-range orientational and no long-range positional order (Figure 3). Discotic columnar mesophases are obtained when the disc-like molecules stack on top of each other. These phases have both long-range orientational and 2-dimensional positional order with the exception of the columnar nematic phase that has no positional order. The different columnar mesophases with 2-dimensional positional order are generally classified according to their packing symmetry. The symmetry is defined by the arrangement of the columns in space, the orientation of the molecular planes with respect to the columnar axis (Figure 3), and the degree of stacking order within the columns (Figure 6).2
Figure 3 Discotic liquid crystal phases: nematic discotic (ND), nematic columnar (NCol), columnar hexagonal (Colh), and columnar rectangular (Colr)3
The degree of order and molecular dynamics within a columnar stack has been a matter of debate for more than a decade. It is commonly accepted to distinguish between at least four states of intracolumnar order (Figure 6). The disordered (Cold) and ordered (Colo) stacks of a columnar phase have been distinguished between based on X-ray diffraction data, but the transition between the two is not strictly defined. A further increase in orientational and/or positional order leads to the plastic columnar phase (Colp,1D orientational and 3D positional order) and the helical columnar phase (H, 3D orientational and 3D positional order), which have been identified more recently.2
Figure 6 Columnar phases can be sub-classified according to the degree of order and the dynamics of the molecules within the columnar stacks: disordered columnar phase (Cold), ordered columnar phase (Colo), plastic columnar phase (Colp) and helical columnar phase (H).2
References:
References:
- S.Chandrasekhar;
Sadashiva, B.; Suresh, K. Pramana 1977, 7, 471-480. - Bushby, R. J.; Lozman, O. R. In Current Opinion in Colloid & Interface Science, 2002, 7, 343-354.