Self-Assembly of Bottlebrush Block Copolymers into Triply Periodic Nanostructures in a Dilute Solution

Bottlebrush block copolymers (BBCPs) exhibit a distinct brush-like topology, which extends their conformations and, consequently, limits chain-chain entanglement in bulk forms and in concentrated solutions. These characteristics make them attractive building blocks to create well-defined nanostructures with large periodicities by self-assembly. However, self-assembly of BBCPs into periodic nanostructures in a dilute solution has rarely been studied. Here, we report the solution self-assembly of amphiphilic BBCPs with poly(ethylene glycol) (PEG) and polystyrene (PS) as pendants attached to a polynorbornene backbone. Similar to the self-assembly of conventional linear block copolymers (BCPs), the BBCPs underwent a morphological transition from spherical micelles to vesicles as their brush ratio, defined by the weight fraction of hydrophilic PEG brushes compared to the weight of the block copolymer brushes (w(PEG)), decreased. We found that the BBCPs self-assembled into inverse mesophases when the value of w(P)(EG) was below 7%. In particular, highly symmetric icosahedral cubosomes of BBCPs with internal double diamond lattices were formed when allowed to undergo self-assembly for an extended time period. This condition led us to find that new morphologies from self-assembly of BBCPs in a dilute solution, such as nanotubes and tubular networks, were analogous to the reticulated cylindrical micelles observed from the nonergodic assembly of linear BCPs. Our results suggested that BBCPs could serve a class of macromolecular building blocks to create complex nanostructures with unusual morphologies.