Effects of mPEG Grafts on Morphology and Cross-Linking of Thermally Induced Micellar Assemblies from PAAc-Based Graft Copolymers in Aqueous Phase

Two graft copolymers comprising acrylic acid (AAc) and 2-methacryloylethyl acrylate (MEA) units as the backbone and either poly(N-isopropylacrylamide) (PNIPAAm) alone or both PNIPAAm and monomethoxypoly(ethylene glycol) (mPEG) as the grafts were synthesized. These copolymers in the aqueous phase (pH 5.0) underwent thermally induced self-assembly into micelles. For the copolymer containing PNIPAAm grafts only, extensive interactions between un-ionized AAc residues and PNIPAAm segments occurred, thereby rendering polymer backbones partially embedded within the hydrophobic cores of thermally induced micelles. This then led to bulk (core/shell) cross-linking of micelles upon radical polymerization of the MEA units within the micellar assemblies in the aqueous phase. By contrast, with mPEG being incorporated into the copolymer, association of the backbones with PNIPAAm is greatly retarded. As a result, three-layer onion-like polymeric micelles consisting of hydrophobic PNIPAAm cores surrounded by AAc-rich shells and hydrophilic mPEG coronas were achieved. Shell cross-linked micelles were then produced via polymerization of the MEA units confined to the AAc/MEA-rich shell regions. The presence or absence of mPEG in the PNIPAAm-containing graft copolymer plays a crucial role in determining the morphological structure of micelles and the structural responses of the subsequently cross-linked micelles to pH and temperature changes.