Synthesis and self-organization of azobenzene containing hemiphasmidic side-chain liquid-crystalline polymers with different spacer lengths

A series of new hemiphasmidic side-chain liquid crystalline (LC) polymers (P-n, where n is the number of methylene units in the spacer, n = 2, 6, 10, 14) were synthesized. The azobenzene containing hemiphasmidic mesogens are linked to a polymethacrylate main-chain through flexible spacers. The chemical structures of the monomers and polymers were confirmed by various characterization techniques. All of the P-ns exhibit enantiotropic LC phase behavior, which is influenced greatly by varying the spacer length. Specifically, P-2 and P-6 exhibit a hexagonal columnar (ΦH) phase. As longer spacers are introduced, P-10 and P-14 can form a centered rectangular columnar (ΦR) phase at low temperatures, which will change into the ΦH phase upon heating. The lattice dimensions of the columnar (Φ) phases are pretty large, approaching 9 nm when n increases to 14. This indicates that the column of the Φ phase of P-n is constructed from several chains laterally assembled together. For P-10 and P-14, the transition of ΦH–ΦR may be associated with the change in chain numbers of the supramolecular column. As the transition temperature is close to the glass transition temperature, the kinetics of the ΦH-to-ΦR transition of P-10 becomes very slow.