Photoresponsive Polymersomes Formed by Amphiphilic Linear-Dendritic Block Copolymers: Generation-Dependent Aggregation Behavior

Azobenzene-containing linear dendritic block copolymers (LDBC) with varied generation numbers were synthesized recently. This photosensitive LDBC consists of a linear solvophilic block (R) and solvophilic dendrons of which the periphery is attached with a solvophobic coil rod diblock (B-Y). The self-assembly and its photoresponsive transformation are explored by dissipative particle dynamics. Dependent on the generation number, polymer concentration, block lengths, and pi-pi interaction between rod blocks, the aggregate exhibits a rich variety of morphological conformations, including spherical micelle, worm-like micelle, cylindrical micelle, hamburger-like micelle, nanosheet, nanobowl, and vesicle. In general, polymersomes take shape for LDBCs with large generation number while nanofibers and nanosheets develop for LDBCs with small generation number. Upon UV illumination, the photoinduced trans-to-cis isomerization of azobenzene takes place and the membrane becomes distorted, wrinkled, and even ruptured. A significant increase in water permeation through the polymersome membrane is also observed. These simulation findings are consistent with experimental observations. By varying polymer concentration and lengths of R- and B-blocks, morphological phase diagrams and internal structures of the resulting aggregates are obtained. Transformation from nanosheet or nanobowl to polymersome is observed as polymer concentration grows. When the R-block length decreases, the generation number at which polymersomes start to form declines. On the other hand, nanosheets tend to form for long R-block length. This prediction is also consistent with experimental observations. Polymersomes can also be formed for long enough B-block length at lower generation number. The membrane resistance to water permeation grows with increasing B-block length because of the increment of the hydrophobic layer thickness. Finally, as the pi-pi strength is increased, the overall morphologies vary from polymersome, nanobowl, to nanosheet. Therefore, polymersomes are formed only when the pi-pi strength is weak enough due to the hindrance associated with high degree of alignment among azo-rods.