Shape side-group orientation

The actual shape and size of the molecules depend on the substitution pattern of the arylene units. Although pure para-substituted cyclic phenylene-arylenes have been investigated, e.g. 1, most PAMs contain meta- or ortho-substituted aromatics at their corners and para-substituted aromatics for size expansion, respectively (2, Fig. 6.2) [11], The position where the side-groups are attached to the backbone determines if they point to the inside (intra-annular substituents, I, I ), to the outside (extra-annular substituents, E, E ) or if they can change their orientation according to an external parameter (adaptable substituents, A, A ). 

Long flexible pol3nners have a large number of Internal degrees of freedom. The typical primary structure of such molecules Is a linear chain of atoms (often carbon) connected by chemical bonds ("backbone"). Usually, every other backbone atom carries a side group. By rotation about the bonds In the backbone the molecule changes Its shape, and since there are many of these bonds In a polymer. a wide spectrum of conformations Is available. The rotation is hindered by the side groups, so that some of these conformations may be rather unfavourable. In some macromolecules (for instance proteins) sequences of preferred bond orientations show up as helical or folded sections In the molecules (secondary structure). 

At the same time, weakening of the orientational correlation between the main chain and mesogenic segments of a comb-shaped molecule in the bulk of a polymer facilitates the appearance of orientational order between the side groups of the different molecules and thus causes the formation of a mesomorphic structure in the polymer [36]. 

On the whole, it is possible to state that the tendency toward orientation of the side branches of the macromolecules forming the LC phase with the corresponding dielectric and diamagnetic anisotropy is the driving force of orientation processes in comb-shaped polymers. The interaction of the side groups with the external field sets the main chains in motion, which also determines certain specific features of polymer liquid crystals wtih respect to the effect of external fields. 

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