Role of Solvophobic Sequences in Amphiphilic Hyperbranched Polymers during Self-Assembly and Photoresponsiveness

Self-assembly of amphiphilic molecules is a simple and effective method for creating various nanoscale shapes such as spheres, rods, cylinders, and vesicles. The arrangement of amphiphilic molecules during self-assembly is influenced by various factors. These factors include the balance between hydrophobic and hydrophilic components, length of solvophobic segments, amphiphile structure, temperature, and concentration. This study explores how the sequence of grafted solvophobic segments influences the self-assembled morphology and stimuli-responsive behavior. Herein, the desired amphiphiles were prepared by reacting the peripheral alkyne-terminated hyperbranched polyester core with two distinct azide-functionalized amphiphiles using Cu-catalyzed click transformations. The solvophobic sequences of these two amphiphiles differed by azide functionality positioning. For the Set-1 amphiphile, the azide group presented at the hydrophilic chain ends, while for the Set-2 amphiphile, it was the opposite. Furthermore, the solvophobic segments of these two amphiphiles were connected via a photoresponsive azobenzene spacer. Despite the similar hydrophobic/hydrophilic balance, the self-assembly of these two HBP-amphiphiles exhibited disparities in morphologies and photoresponsive behavior. HBP-1 amphiphile (HBP-OEG-Azo-C12) is self-assembled into a vesicular structure, whereas the HBP-2 amphiphile (HBP-C12-Azo-OEG) is self-assembled into spherical micelles. The photoresponsiveness of these two amphiphiles was investigated by assessing the release of guest molecules. A photophysical study revealed that the vesicular morphology of HBP-1 exhibits controlled release of guest molecules, while self-assembled HBP-2 initiates burst releases. Spectroscopic and microscopic imaging techniques were employed to gain insight into the mechanism for these differences. The sequence of grafted solvophobic units appears to influence the geometry of the self-segregated segments and the stability of the self-assembled structure, resulting in different morphologies and responsive functions.