Crystalline Three-Dimensional Polyhedron Nanoparticles from the Intramolecular Cyclization-Induced Self-Assembly of an Amorphous Poly(Amic Acid) in Water

The controlled preparation of crystalline three-dimensional (3D) nanoparticles is attractive in polymer science due to the diverse topological structure and complicated chain folding mechanism of polymers. In this study, an intramolecular cyclization-induced self-assembly (ICISA) strategy is proposed to prepare water-dispersible crystalline 3D polyhedron nanoparticles taking advantage of the amorphous to crystalline transition during self-assembly promoted by the thermally triggered intramolecular cyclization reaction of an amorphous poly(amic acid) (PAA). Upon thermal treatment of the PAA in water, the rigid and crystalline polyimide (PI) blocks are generated in the backbone due to the unique intramolecular cyclization reaction feature of PAA, leading to the in situ flexible to rod-like chain and the amorphous to crystalline transition simultaneously, as well as the formation of crystalline polyhedron nanoparticles. Benifiting from the excellent stability of PI, the chain packing patterns of the polymer can be clearly observed by high-resolution transmission electron spectroscopy, which is further proved by computer simulations, demonstrating that the arrangement of polymer chains has a tilt angle of about 30° with a d spacing of 0.34 nm. This finding brings new insights into understanding the crystallization behavior of polymers and the facile preparation of crystalline 3D nanoparticles in water.