Rui-Yang Wang

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

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.

Hierarchical self-assembly, photo-responsive phase behavior and variable tensile property of azobenzene-containing ABA triblock copolymers

A series of triblock copolymers with liquid crystalline (LC) poly{6-[4-(4-methoxyphenylazo)phenoxy]hexyl methacrylate} (PMMAZO) as the end blocks and rubbery poly(n-butyl acrylate) (PnBA) as the midblock were synthesized. The effect of the interplay between the LC ordering and microphase separation on the hierarchical assembly of the triblock copolymers was studied. It is found that microphase separation at a larger scale can affect the LC ordering at a smaller scale, such as the stacking of the LC moieties, LC temperature and the domain size of the LC phase. On the other hand, alteration of the LC ordering, such as isotropization and smetic-to-nematic transition, may also lead to an order–order transition (OOT) or change in the long period of the microphase-separated structure. UV light can trigger the isomerization of the azobenzene LC moieties, which can be further amplified and exerts an effect on the microphase separation behavior, including the regularity of the microphase-separated structure and the OOT. The triblock copolymers also exhibit light-variable tensile properties. The results reveal that the phase behavior and mechanical properties of this type of triblock copolymer can be readily regulated by light, thus it may be used as smart and functional thermoplastic elastomer.