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

Abstract

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 (Φ_H) phase. As longer spacers are introduced, P-10 and P-14 can form a centered rectangular columnar (Φ_R) phase at low temperatures, which will change into the Φ_H 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 Φ_H–Φ_R 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 Φ_H-to-Φ_R transition of P-10 becomes very slow.