Regulation of the self-assembly morphology of azobenzene-bearing double hydrophobic block copolymers in aqueous solution by shifting the dynamic host–guest complexation

Abstract

The double hydrophobic azobenzene-containing diblock copolymers, namely, poly(L-lactide)-b-poly[6-(4-(4-methoxyphenylazo)phenoxy)hexyl methacrylate] (PLLA-b-PMMAZO), were successfully prepared by sequential atom transfer radical polymerization (ATRP) and ring opening polymerization. These block copolymers (BCPs) can self-assemble into various morphologies in aqueous solution via host–guest interaction between the azobenzene (azo) groups and methyl-β-cyclodextrin (β-CD). The complexation equilibrium between azo and β-CD can be shifted by changing the length of the hydrophobic PLLA block, the micelle concentration or the β-CD/azo molar ratio; thus the micellar morphology and the size of the PLLA-b-PMMAZO/β-CD complexes are altered. A vesicle-to-sphere-to-entrapped vesicle transition was observed at β-CD/azo = 1 as the PLLA block length increases. In addition, a large β-CD/azo ratio or higher micelle concentration leads to a vesicle-like morphology, while entrapped vesicles tend to be formed at a smaller β-CD/azo ratio or a lower micelle concentration. It is found that prolongation of the UV irradiation time can also induce a vesicle-to-sphere-to-entrapped vesicle transition of the micellar morphology. Such a transition is reversible upon irradiation of visible light. In summary, the self-assembly behavior of PLLA-b-PMMAZO/β-CD complexes can be readily regulated by different methods. Moreover, the complexation between the azo groups and β-CD is inhomogeneous, which is responsible for the formation of entrapped vesicles and the microphase separation in the core of the spherical micelles and entrapped vesicles.