Hierarchical supramolecular assembly of a single peptoid polymer into a planar nanobrush with two distinct molecular packing motifs

Hierarchical nanomaterials have received increasing interest for many applications. Here, we report a facile programmable strategy based on an embedded segmental crystallinity design to prepare unprecedented supramolecular planar nanobrush-like structures composed of two distinct molecular packing motifs, by the self-assembly of one particular diblock copolymer poly(ethylene glycol)-block-poly(N-octylglycine) in a one-pot preparation. We demonstrate that the superstructures result from the temperature-controlled hierarchical self-assembly of preformed spherical micelles by optimizing the crystallization−solvophobicity balance. Particularly remarkable is that these micelles first assemble into linear arrays at elevated temperatures, which, upon cooling, subsequently template further lateral, crystallization-driven assembly in a living manner. Addition of the diblock copolymer chains to the growing nanostructure occurs via a loosely organized micellar intermediate state, which undergoes an unfolding transition to the final crystalline state in the nanobrush. This assembly mechanism is distinct from previous crystallization-driven approaches which occur via unimer addition, and is more akin to protein crystallization. Interestingly, nanobrush formation is conserved over a variety of preparation pathways. The precise control ability over the superstructure, combined with the excellent biocompatibility of polypeptoids, offers great potential for nanomaterials inaccessible previously for a broad range of advanced applications.

Biomacromolecules fold and assemble into complex, well-organized hierarchical structures by a network of noncovalent interactions, which enable tremendous architectural diversity in nature (1, 2). For example, polypeptide chains encoded with assembly information in their monomer sequence can fold into highly ordered conformations, which give rise to their biological functionality (3). Inspired by these intricate natural designs, numerous efforts have been devoted to fabricating hierarchical nanostructures using synthetic polymeric materials (4–11). However, the precision control over the assembly process and structure across many length scales, as commonly seen in biomacromolecules, remains a challenging task (12). This is because the assembly information content encoded within synthetic macromolecules is considerably lower.Synthetic chemists have looked to develop polymer systems that retain many of the structural features found in natural biopolymers. Bioinspired synthetic polymers have attracted growing attention, because of their inherent structural advantages, facile synthetic approaches, and improved stability, to serve as promising materials for the de novo design and assembly of hierarchical nanostructures. In particular, polypeptoids are a class of peptidomimetic polymers based on a polar amide backbone (13–15). This differs substantially from carbon-chain polymers such as polyethylene and polypropylene. The amide groups impart higher water solubility, excellent biocompatibility, the opportunity for multiple backbone−backbone interactions, and enable a wide range of bioactivities. Polypeptoids are devoid of hydrogen bond donating sites and chirality on the backbone due to the N substitution. This simplifies the complex molecular interactions inherent in peptidomimetic materials, resulting in efficient engineering and controllable architecture construction. For example, polypeptoids with alkyl side chain groups are semicrystalline with inherent characteristic packing domains, which is in sharp contrast to polypeptides (16–19).Macromolecular crystallization is an essential process in both nature and materials manufacturing. The self-assembly of block copolymers containing crystalline blocks generally enables the formation of multiscale architectures with a high level of control over morphology and dimension (20, 21). Recently, living crystallization-driven self-assembly has been demonstrated to be an effective strategy to precisely control the nanoscale morphology (22–30). Inspired by the natural encoded information-guided self-assembly, we based our design on a hydrophobic poly(N-alkylglycine) peptoid block that is known to form a rectangular crystalline lattice with controllable dimension and two melting transitions (31). It is also known that solvophobic interaction is the predominant driving force for assembly of systems with solvophobic segments (5). The delicate interplay between crystallization and solvophobicity is essential for biomolecule self-assembly (32), which potentially serves as a powerful strategy for self-assembly of block copolymers. Thus, we embarked on a study of block copolymers, where the relative strength of these two factors could be systematically adjusted. By optimizing the balance between these two factors, we discovered that poly(ethylene glycol)-block-poly(N-octylglycine) (PEG-b-PNOG) formed unique supramolecular planar nanobrush architectures in high yield. We developed a simple temperature-controlled assembly strategy to create superbrushes consisting of two distinct packing domains: a long core fiber, or “spine,” with lengths up to ∼2.0 µm, and laterally splayed shorter fibers of ∼400 nm in length on apposed side surfaces of the spine. We further demonstrated that this lateral growth of the brush exhibits living growth manner via a micelle intermediate, fairly distinct from known living crystallization-driven self-assembly approaches (16, 23, 33), where assemblies grow via the direct attachment of block unimers. Our results coincide with the reported “particle attachment” strategy observed in a range of biomacromolecules and small molecules, where intermediate higher-ordered species form in solution prior to their attachment to the crystal lattice, in contrast to monomer-by-monomer crystal growth (1, 32, 34, 35). Such pathways allow for the optimization of interactions to facilitate thermodynamic favored transition from the initial species to hierarchical assemblies.