New approaches to stimuli-responsive polymeric micelles and hollow spheres

Abstract

This article briefly describes some new approaches to stimuli-sensitive polymeric micelles and hollow spheres, which were developed in the authors  laboratory in recent years. (1) Self-assembly of component polymers to non-covalently connected micelles (NCCM) driven by specific interactions. For example, in water, PCL and PAA formed core-shell nanospheres due to interpolymer hydrogen bonding. After crosslinking the PAA shell and removing PCL core, nanocages  made of PAA network were obtained. This hollow structure shows perfect reversible size-pH dependence. (2) Simultaneous in-situ polymerization of monomers and self-assembly of the polymers. In this approach, PNIPAM network was formed by radical polymerization covering PCL particles. Hollow spheres of PNIPAM network were then obtained by biodegradation of PCL core. Both the core-shell spheres and hollow spheres show reversible size dependence on temperature change because of the phase transition of PNIPAM around 32ºC. (3) Complexation-induced micellization and transition between the micelles and hollow spheres. Graft copolymers hydroxylethyl cellulose (HEC) and PAA were prepared free radical polymerization. The copolymers showed pH dependent micellization, i.e., micelles formed when pH of graft copolymer solution decreased to around 3. The micellar structure could be locked by crosslinking the PAA grafts. The resultant cross-linked micelles undergo pH-dependent transition between the micelles and hollow spheres, which accompanies a remarkable particle size change. Both the micellization and the structure transition were found to be reversible and associated with H-bonding complexation between the main chain and grafts.