Aqueous self-assembly of cylindrical and tapered bottlebrush block copolymers

Self-assembly of amphiphilic bottlebrush block copolymers (BCPs), which have backbones with two types of densely grafted side chains, is less well understood compared to their linear counterparts. Specifically, the solution self-assembly of tapered bottlebrush BCPs, which are approximately cone-shaped with specific cone directionality (hydrophilic or hydrophobic tips), remains unexplored. This work describes a series of 8 tapered and 4 cylindrical bottlebrush BCPs with varying ratios of hydrophobic polystyrene (PS) and hydrophilic poly(acrylic acid) (PAA) side chains, synthesized by the sequential addition of macromonomers ring-opening metathesis polymerization (SAM-ROMP) method. The nanostructures formed by these BCPs in water were evaluated using cryogenic transmission electron microscopy, small-angle neutron scattering, and dynamic light scattering. Results showed that most BCPs formed multiple types of nanostructures, all with surface protrusions, including spherical micelles, cylindrical micelles, and vesicles, with transitional structures between each, such as ellipsoids and semi-vesicles. Coarse-grained molecular dynamics simulations aided interpretation of the experimental data. Collectively, results revealed two distinct self-assembly pathways through which the BCPs evolved from micelles into complex nanostructures. One pathway involved micelle fusion, resulting in elliptical and cylindrical aggregates that in some cases fused further to form Y-junctions. The second pathway entailed micelle maturation into semivesicles, which subsequently formed vesicles and, at times, large compound vesicles. This study provides the first experimental evidence supporting vesicle formation via semivesicles in bottlebrush BCPs. Collectively, these findings highlight how structural parameters such as cone directionality influence self-assembly in these large, cone-shaped polymeric amphiphiles.