Aqueous Self-Assembly of Cylindrical and Tapered Bottlebrush Block Copolymers

The self-assembly of amphiphilic bottlebrush block copolymers (BCPs), featuring backbones densely grafted with two types of side chains, is less well understood compared to linear BCPs. In particular, the solution self-assembly of tapered bottlebrush BCPs—cone-shaped BCPs with hydrophilic or hydrophobic tips—remains unexplored. This study investigates eight tapered and four cylindrical bottlebrush BCPs with varied ratios of hydrophobic polystyrene (PS) and hydrophilic poly(acrylic acid) (PAA) side chains, synthesized via sequential addition of macromonomers using ring-opening metathesis polymerization (SAM-ROMP). Self-assembled nanostructures formed in water were analyzed using cryogenic transmission electron microscopy, small-angle neutron scattering, and dynamic light scattering. Most BCPs generated multiple nanostructures with surface protrusions, including spherical micelles, cylindrical micelles, and vesicles, alongside transitional forms like ellipsoids and semi-vesicles. Coarse-grained molecular dynamics simulations supported the experimental findings, which revealed two distinct self-assembly pathways. The first involved micelle fusion, producing elliptical and cylindrical aggregates, sometimes forming Y-junctions. The second pathway featured micelle maturation into semivesicles, which developed into vesicles or large compound vesicles. This work provides the first experimental evidence of vesicle formation via semivesicles in bottlebrush BCPs and demonstrates the significant influence of cone directionality on self-assembly behavior in these cone-shaped polymeric amphiphiles.