Synthesis of Block Copolymer Micelles for Supracolloidal Chains and Surface-Functionalized Nanoparticles

Abstract

Self-assembled nano-building blocks into controlled superstructures is of significant importance in technological applications as well as of great interest in basic science because cooperative electronic, photonic, and magnetic properties of individual nano-objects are determined by the collective interactions in their ensembles. In particular, uniform nanoparticles of metals, semiconductors, oxides, and polymers have been assembled into supracolloidal assemblies upon the controlled attraction between nanoparticles. Especially, patchy nanoparticles have been employed as colloidal building blocks which can be effectively polymerized into linear supracolloidal chains. Colloidal particles with well-ordered patches have been developed mainly for mimicking the valency in an atomic structure to demonstrate an artificial atom in the large scale. However, colloidal particles with multivalent patches have not been utilized for controlling branching or crosslinking. In addition, it would not be trivial to synthesize colloidal patchy particles smaller than 100 nm. In this thesis, we focus on the controlled branching and eventual crosslinking in supracolloidal chains by introducing well-defined trifunctional patchy micelles. Three patches in the micelles worked as the distinct parts for crosslinking as well as branching, analogues to multifunctional groups in classical gelation of small molecular monomers. These branched and crosslinked supracolloidal chains were well compared with long linear chains only with bifunctional micelles. Furthermore, we carried real visual images on branching and crosslinking in chain-like structures which cannot be directly imaged in conventional gelation of small multifunctional monomers. We also demonstrate that diblock copolymer micelles can be used as surface-functionalized particles and they can be coated with Ag or TiO2 nanoparticles without surface modification. We obtained dopamine-functionalized diblock copolymers which were synthesized by the reversible addition fragmentation chain transfer polymerization and followed by the post-polymerization modification. By dissolving this amphiphilic diblock copolymer in water, spherical micelles with the dopamine-functionalized coronas were induced, which are essentially equivalent to polymeric particles with dopamine-functionalized surface. Chapter 1 gives a brief overview of the self-associating characteristics of diblock copolymers, which assemble into micelles with soluble coronas and insoluble cores in a selective solvent for one of the blocks. The structure and dimension of block copolymer micelles can be precisely tuned by the molecular weight of polymers and the weight ratio of the blocks. These diblock copolymer micelles can be potentially employed as nano-sized polymeric colloids. The synthesis and post-polymerization modification of block copolymers for functionalization is also introduced. In Chapter 2, we demonstrate that controlled branching and eventual crosslinking in supracolloidal chains by introducing well-defined trifunctional patchy micelles. Uniform micelles having three patches were induced from core-crosslinked micelles of diblock copolymers. Three patches in the micelles served as functional groups for crosslinking as well as branching in supracolloidal polymerization with bifunctional patchy micelles. Thus, by the addition of trifunctional micelles, supracolloidal chains showed branches originated only from the trifunctional units and were eventually crosslinked into the network structure, in sharp contrast to long linear chains of bifunctional patchy micelles. Formation of crosslinked supracolloidal chains of patchy micelles was understood by the classical gelation theory. We also delivered visual images on branching and crosslinking in chain-like structures which cannot be directly imaged in conventional gelation of small multifunctional monomers. In Chapter 3, we describe that diblock copolymer micelles can be used as surface-functionalized particles and they can be coated with Ag or TiO2 nanoparticles without surface modification. We first obtained dopamine-functionalized diblock copolymers which were synthesized by the reversible addition fragmentation chain transfer polymerization and followed by the post-polymerization modification. By dissolving this amphiphilic diblock copolymer in water, spherical micelles with the dopamine-functionalized coronas were induced, which are essentially equivalent to polymeric particles with dopamine-functionalized surface. Thus, without additional surface functionalization, we were able to directly decorate these particles with Ag and TiO2 nanoparticles due to the dopamine functionality on their surface.