pH-Responsive Self-assembly of Partially Hydrolyzed Polyacrylamide in Aqueous Solution

As a simulacrum of random copolymer poly(acrylamide-co-acrylic acid), partially hydrolyzed polyacrylamide (HPAM) self-assembled into polymeric micelles in pure aqueous solution in pH response. Spherical nanoparticles (D = 150 nm) were generated at pH 12, and giant semihexagonal nanoplates (450 nm x 250 nm x 50 nm) were obtained at pH 1.3, but no visible micelles were observed in pH 3-10. This is important for drug release considering the pH environment in human stomach and intestines. The giant semihexagonal micelles obtained at pH 1.3 are hierarchical multivesicular vesicles with the structure of (hydrophilic inner vesicle)@(hydrophobic continuous cyst wall)@(hydrophilic shell). These multivesicular vesicles were formed because the random copolymer has a random distribution of the hydrophilic units in the hydrophobic core. In addition, a new contrast enhancing strategy by in situ reduction gold doping that we established was efficient at illuminating the nano-structure of polymeric micelles with low contrast, and this strategy came with a way to obtain polymer nanoparticles smaller than 20 nm.