The flexibility of various molecular swivels used to control the rigidity and tractability of aromatic heterocyclic polymers

Some aromatic heterocyclic polymers are rigid or rodlike and form liquid-crystalline phases which can be used to prepare films and fibers having unusual strength and thermal stability. They are, however, very nearly intractable unless some atoms or groups of high flexibility are introduced along the chains. The present theoretical investigation employs semiempirical methods to calculate intramolecular energies of various conformations about such “molecular swivels” in order to characterize their flexibility. In addition, simple geometric arguments are used to determine which swivels have one or more conformations exhibiting the parallel or collinear chain continuation which is characteristic of liquid-crystalline phases and apparently essential to the unusual mechanical properties of these materials.