Crystallization-Driven Self-Assembly of Coil-Comb-Shaped Polypeptoid Block Copolymers: Solution Morphology and Self-Assembly Pathways

Crystallization-driven self-assembly (CDSA) of amphiphilic polymers into well-defined nanoscopic structures with different morphologies and functionalities has attracted increasing attention. Here, we investigate the CDSA of coil-comb-shaped diblock copolypeptoids, namely, poly(N-methyl glycine)-b-poly(N-decyl glycine) (PNMG-b-PNDG), in dilute methanol solution using X-ray/neutron solution scattering in conjunction with cryogenic transmission electron microscopy techniques. A series of PNMG-b-PNDGs were synthesized by sequential benzyl amine-initiated ring-opening polymerizations of the corresponding N-substituted N-carboxyanhydrides, in which the degree of polymerization and the length of the blocks were varied. The PNMG-b-PNDG polymers with a lower volume fraction of the crystalline PNDG blocks (f_PNDG = 0.44) were found to slowly self-assemble into one-dimensional long wormlike nanofibrils in methanol. The nanofibrils bear an anisotropic crystalline core where the comb-shaped PNDG blocks were stacked in a face-to-face fashion along the long axis of the nanofibrils. Upon increasing f_PNDG to 0.61 and 0.68, the final morphology of PNMG-b-PNDG micelles changed from wormlike nanofibrils to rigid short nanorods and then two-dimensional nanosheets. The nanofibrils were formed by a self-seeding growth pathway that involves the initial formation of a few seeded crystals followed by the addition of soluble unimers to the preferred crystal facets resulting in the gradual elongation of the micelles. By contrast, the nanorods were formed by a two-stage process involving the formation of spherical micelles with an amorphous core in the first stage and rapid confined crystallization of the micellar core and their fusion into rodlike nanostructures at the second stage. Understanding the relationship between chemical composition, micellar morphology, and CDSA pathway of coil-comb-shaped diblock copolypeptoids is an important step toward the rational design of anisotropic polymeric nanostructures with tailorable morphology.