Melting biaxial nematics

Fig. 5 Atomistic and coarse-grained representation of a P3HT chain (left), including coarse-grained angular (0) and dihedral (tf) degrees of freedom (center). Biaxial nematic alignment in a melt of P3HT chains (right). Adapted with permission from Gemiinden et al. [64]. Copyright (2013) American Chemical Society...

The mono-substituted PEIs melt from a saniditic layered structure into a normal nematic phase whereas the disubstituted presumably yield a biaxially oriented nematic melt. The disubstituted PEIs isotropise from the melt around 200 °C lower than the monosubstituted of equal chain length. As the second substituent should increase chain stiffness rather than reduce it, Kricheldorf suggests that interactions between temporarily coplanar aromatic n systems, particularly donor-acceptor interactions, can make an efficient contribution to the stabilisation of nematic phases. If this hypothesis is correct then a greater variety of non-linear monomers may be regarded as building blocks for liquid crystal polymers. 

Figure 3. Plot of the transition temperatures (y-ax-is) versus the number of methylene units (j -axis) in the alkyl bridge of the twin ethers (compound 9). C crystal Nu t, discotic nematic biaxial I isotropic x melting points clearing points (from [18]).

---