Supramolecular Nanosheets Assembled From Poly(Ethylene Glycol)-B-Poly(N-(2-Phenylethyl)Glycine) Diblock Copolymer Containing Crystallizable Hydrophobic Polypeptoid: Crystallization Driven Assembly Transition From Filaments To Nanosheets

Supramolecular two-dimensional (2D) nanomaterials from block copolymers have received great interest for their unique structure and properties. Here, we report the formation of ultrathin nanosheets from self-assembly of amphiphilic poly(ethylene glycol)-b-poly(N-(2-phenylethyl)glycine) (PEG-b-PNPE) diblock copolymers, which contain rigid crystallizable polypeptoid segment. The PEG-b-PNPE copolymers were synthesized via ring opening polymerization (ROP). The obtained PEG-b-PNPE diblock copolymers can spontaneously form highly ordered structures in PEG selective solvents such as water and methanol. The copolymers with short PNPE segment can directly form nanosheets in water, and the obtained 2D nanosheets have a uniform thickness of 4-5 nm. In contrast, the copolymers with relatively long PNPE segment can only assemble into nanosheets with the assistance of methanol. It is proposed that the crystallization and pi-pi stacking of PNPE blocks play critical roles in the formation of the nanosheet as suggested by grazing incidence wide-angle X-ray scattering (GIWAXS). During the process of adding water into copolymer methanol solution, the nanosheets were observed to evolve from individual nanofibers, to parallel aligning nanofibers, and eventually to the nanosheet structures verified by transmission electron microscopy (TEM) and cryo-TEM characterizations. We demonstrated that the preassembled filament behave as a fundamental packing motif to align laterally and further fuse into platelet-like structures. The assembly structure evolution process was tracked by TEM, atomic force microscopy, and cryo-TEM techniques. The understandings of supramolecular 2D nanosheet formation offer a new opportunity to make hierarchical nanostructures from polypeptoids containing copolymers.