Aqueous Reversible Addition-Fragmentation Chain Transfer Dispersion Polymerization of Thermoresponsive Diblock Copolymer Assemblies: Temperature Directed Morphology Transformations

In this work, we demonstrate the formation of various 3D structures formed by a structural reorganization; a process not governed by self-assembly. First, we packaged well-defined diblocks, thermoresponsive poly(N-isopropylacrylamide-b-styrene) or P(NIPAM-b-STY), into spherical particles made in situ using a reversible addition-fragmentation chain transfer (RAFT) nanoreactor technique in water to obtain high polymer solids. The resultant spheres reorganized through a temperature stimulus to form equilibrium and kinetically trapped structures; we denote this process as temperature directed morphology transformation (TDMT). Cylinder and vesicle structures, other more unusual loop, buckled sphere and cauliflower (via ultrasound) structures were observed below the lower critical solution temperature (LCST) of PNIPAM. These structures were produced efficiently, rapidly, reproducibly at high polymer solids and can be stored for years in solution or be freeze-dried and rehydrated without a change in structure, which becomes important in biological applications. We generated the first phase diagram for the TDMT changes of thermoresponsive diblock micelles in concentrated solutions (> 7% polymer solids) produced from narrow molecular weight diblock copolymers (PDI similar to 1.1). The sphere/cylinder and cylinder/vesicle transition in the phase diagram were surprisingly found to be predominantly dictated by the length of the hydrophilic block, poly(N-isopropylacrylamide). (C) 2012 Wiley Periodicals, Inc. J Polym Sci Part A: Polym Chem 50: 4879-4887, 2012