Rigid backbone polymers. XII. Lyotropic mesomorphicity of semiflexible macromolecules

It is shown that semiflexible polymers exhibit lyotropic liquid crystallinity once a threshhold solution concentration is surpassed. These polymers, namely polyisocyanides, polyorganophosphazenes, polyacenaphthylene, and cellulose derivatives, are characterized by persistence lengths or characteristic ratios experimentally determined to be intermediate between the very large ones belonging to extended rod polymers and the small ones belonging to flexible coil polymers. At the same time, the coefficient a in the Mark-Houwink equation of these polymers is of a magnitude typical of coiled molecules. Thus the semiflexible polymers adopt the configuration of expanded coils. The ability to pass through an anisotropic state in solution (and in the bulk below the melting point) is shown to be dependent on the ratio of the persistence length to the polymeric chain diameter, a relationship dependent on the flexibility of the chain and originating from Flory's theory of semiflexible chain molecules. Except for cellulose acetate all the studied polymers showed in the bulk only a lateral interchain ordering and no conventional crystallinity. This one-dimensional order was retained in the liquid crystalline solutions.