Hierarchical Self-Assembly of Halogen-Bonded Block Copolymer Complexes into Upright Cylindrical Domains

Summary Self-assembly of block copolymers into well-defined, ordered arrangements of chemically distinct domains is a reliable strategy for preparing tailored nanostructures. Microphase separation results from the system, minimizing repulsive interactions between dissimilar blocks and maximizing attractive interactions between similar blocks. Supramolecular methods have also achieved this separation by introducing small-molecule additives binding specifically to one block by noncovalent interactions. Here, we use halogen bonding as a supramolecular tool that directs the hierarchical self-assembly of low-molecular-weight perfluorinated molecules and diblock copolymers. Microphase separation results in a lamellar-within-cylindrical arrangement and promotes upright cylindrical alignment in films upon rapid casting and without further annealing. Such cylindrical domains with internal lamellar self-assemblies can be cleaved by solvent treatment of bulk films, resulting in separated and segmented cylindrical micelles stabilized by halogen-bond-based supramolecular crosslinks. These features, alongside the reversible nature of halogen bonding, provide a robust modular approach for nanofabrication.
Graphical Abstract
Highlights • Halogen bonding small molecules to block copolymers yields hierarchical self-assembly • Fluorinated additives increase block repulsion χ and enhance microphase separation • Upright cylinder structure alignment is favored by fluorocarbon low surface energy • Segmented cylindrical micelles can be obtained by solvent treatment of bulk films
The Bigger Picture Directed self-assembly (DSA) is probably the most promising strategy for high-volume cost-effective manufacturing at the nanoscale to support the ever-growing needs of the semiconductor market. DSA exploits the spontaneous self-assembly of block copolymers onto lithographically nanopatterned surfaces, resulting in large-area fabrication of periodic nanostructures on the scale of a few tens of nanometers. Tailoring the structure, periodicity, and orientation of the block copolymer is crucial and requires control over features such as block volume fraction, immiscibility, and surface energy. Here, we use fluorocarbon additives that specifically form halogen bonds with only one of the blocks, promoting microphase separation even upon rapid casting and without subsequent solvent annealing, which, along with the reversible nature of the halogen bond, could pave the way for new self-assembly routes toward the alignment and periodicity of block copolymers and nanostructures.
Halogen bonding drives the hierarchical self-assembly of low-molecular-weight perfluorinated molecules and diblock copolymers. Specifically, binding of the ditopic halogen-bond donor 1,8-diiodoperfluorooctane to pyridine nitrogens of poly(styrene)-block-poly(4-vinylpyridine) results in lamellar-within-cylindrical self-assembly and promotes upright cylindrical alignment in films. This feature, along with the reversible nature of the halogen bond, provides a robust and modular approach for nanofabrication. Thanks to the robust halogen-bond-based crosslinks, separated segmented cylindrical micelles can be obtained by solvent treatment of bulk films.