pH Responsive Self-Assembly of HPAM-HEC in Aqueous Solution

A low-molecular-weight partially hydrolyzed polyacrylamide (HPAM) was synthesized because of its similar segment distribution to that of the random copolymer of acrylamide and acrylic acid P(AM-co-AA) to investigate the self-assembly of random copolymers in aqueous phase. The pH responsive self-assembly of HPAM with hydroxyethyl cellulose (HEC) in aqueous solution was investigated by transmission electron microscopy (TEM) and various polymeric micelle morphologies were observed in different pH ranges such as 100 nm cube-like micelles, 200 nmx100 nm pseudo ellipsoidal micelles, 100 nm string beads-like micelles, and 500 nmx300 nmx50 nm half-moon micelles. A method to couple the in situ reduction of gold at the polymeric micelle surface and TEM characterization was established and used to detect the nanoscale details of the low contrast polymer micelles. We confirmed by the above method and with the assistance of electron probe X-ray microanalysis (EPMA) and scanning electron microscopy (SEM) that the delicate conformation of the half-moon polymeric micelle was a multivesicular vesicle (MVV) which possessed the following hierarchical structure: hydrophilic inner vesicle @ hydrophobic continuous cystwall @ hydrophilic shell. This structure collapsed at pH 0.9 and it disintegrated into 10 nm pseudo spherical polymeric micelles with the following structure: hydrophobic core @ hydrophilic shell. The respective compositions of the core and the shell of the micelles were interpreted based on an understanding of the pH responsive degree of protonation of the various chain units and the experimentally obtained zeta potential and light absorbance. Therefore, we obtained new information about the driving force and morphology of HPAM self-assembly in the aqueous phase.