Controlled Organization of Colloidal Nanomaterials with Block Copolymer Micelles

Abstract

Colloidal nanoparticles based on organic and inorganic materials have been the core elements of nanotechnology because they exhibit unique photonic, electronic and stimuli-sensitive properties depending on their sizes, shapes and chemical characteristics. Beyond the synthesis and utilization of single colloids, the organization of colloidal nanoparticles while controlling their combination and spatial distribution in a system is attracting much interest to achieve unusual nano- and micro-structures and practical applications of the functionalities of colloidal nanoparticles. The organization of colloidal nanoparticles includes the assembly of colloids with the attachment of building units and the arrangement of nanoparticles into periodic arrays. By employing colloidal nanoparticles as nanoscale building blocks, organized colloidal superstructures can be assembled depending on the interaction between colloidal nanoparticles. The controlled assembly of colloidal nanoparticles enables the production of nature-mimic hierarchical structures and periodic structures with photonic properties. The arrangement of colloids on a solid substrate offer the control of the interparticle distance which is essential to utilize the functionalities of colloidal nanoparticles in device applications as the photonic and electronic properties of colloidal nanoparticles are modified by the interparticle coupling, which is affected by the distance between the colloids. Nanostructures of arranged colloidal nanoparticles on the nanoscale can also be applied as nanomasks and nanotemplates for the fabrication of the nanostructures of other materials. Various colloidal nanoparticles, such as metal nanoparticles and polymeric colloids, serve as building blocks for the construction of organized colloidal structures. Given that block copolymers self-assemble into nanometer-sized micelles with soluble corona blocks and insoluble core blocks in a solvent which is selective for one of the blocks, block copolymer micelles can be utilized as a type of polymeric colloidal nanoparticles. In addition, block copolymer micelles can be applied as templates for the creation of organized metal and inorganic colloidal nanomaterials. In this thesis, we focus on the development of organized structures of colloidal nanoparticles through the use of diblock copolymer micelles. Diblock copolymer micelles, as colloidal building blocks, assemble into colloidal superstructures with controlled morphologies in a solution. We also demonstrate the utilization of diblock copolymer micelles as colloidal templates for the synthesis and arrangement of metal nanoparticles on solid substrates. Metal nanoparticle arrays created from a thin film of diblock copolymer micelles were further combined with fluorophore-encapsulating diblock copolymer micelles to control the interaction between the metal colloids and light emitters through micellar nanostructures, suggesting the potential application for the effective control of the fluorescence. In chapter 1, we offer an 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. Since diblock copolymer micelles can include functional substances, e.g. organic dyes and inorganic precursors, in their core blocks, they are beneficial as colloidal templates for arranging and organizing other colloidal nanomaterials in ordered arrays. In chapter 2, we demonstrate supracolloidal polymer chains of diblock copolymer micelles. With a diblock copolymer composed of a polar block and a non-polar block, typical spherical micelles were initially obtained in a selective solvent for a non-polar block. We cross-linked the polar core of diblock copolymer micelles and then made the solvent preferable to the core block but still compatible with the corona block. The cross-linked core was not dissolved by the favorable solvent but was exposed to the solvent when the corona was rearranged into two separate patches. In other words, typical spherical micelles were converted to colloidal micelles with the corona reorganized into two non-polar patches and the central core directly exposed to the solvent. With the reorganized micelles as colloidal monomers, we were able to polymerize their linear supracolloidal chains by increasing the polarity of the solvent. Furthermore, we applied the same protocol to diblock copolymers of a lower molecular weight and produced small colloidal monomers which were then combined with large colloidal monomers for the synthesis of supracolloidal random and block copolymers. In chapter 3, we synthesize a diblock copolymer, consisting of one block which allows the synthesis of nanoparticles and another block which is selectively removable, by means of the reversible addition-fragmentation chain transfer (RAFT) polymerization. Using a selective solvent for each block, we produced two types of spherical micelles with an inverse position of two blocks, that is, the core of the nanoparticle-synthesizable block and the corona of the removable block and vice versa. We then coated single layers of these two micelles onto substrates, which were successfully employed as templates for the arrangement of spherical gold nanoparticles and their ring-like configuration over a large area. In chapter 4, we utilize diblock copolymer micelles to organize metal nanoparticle arrays on a solid substrate which can be applied for the plasmon-coupled fluorescence. A single layer of diblock copolymer micelles containing metal precursors in the core blocks was coated onto a solid substrate. The nanostructures of the micellar arrays of diblock copolymers were transferred to metal nanoparticle arrays by the reduction of metal precursors and the elimination of the polymers. The distance between the metal nanoparticles was controlled by the intermicellar distance in a single layer of diblock copolymer micelles. These metal nanoparticle arrays were applied as a seed substrate for the growth of larger metal nanoparticles with controlled size and interparticle distances. Diblock copolymer micelles were also used for the isolation of organic fluorophores in the micellar cores. A thin film of diblock copolymer micelles with organic dyes was combined with metal nanoparticle arrays to verify the strategy of metal-coupled fluorescence enhancement.