PhD Thesis
Design, Synthesis and Application of Cylindrical Polymer Brushes: From Nanostructures to Advanced Materials
Zhicheng Zheng (11/2013)
Support: Axel H. E. Müller
Summary
This thesis focuses on the design, synthesis and application of cylindrical polymer brushes (CPBs). Herein, we investigated the scission behavior of polyelectrolyte CPBs on different surfaces, developed novel synthetic pathways for well-defined CPBs via reversible addition-fragmentation chain transfer (RAFT) polymerization, designed and prepared complex functional CPBs for light-harvesting and energy transfer, and utilized CPBs as templates for the synthesis of novel one-dimensional (1D) organic/inorganic hybrid nanostructures. The ‘grafting-from’ approach was chosen as the general method to synthesize well-defined CPBs with various chemical and structural compositions. The linear polymer backbones (polyinitiators) were obtained by anionic polymerization or RAFT polymerization, whereas the side chains were grafted by atom transfer radical polymerization (ATRP) or RAFT polymerization. The obtained CPBs possess a narrow molecular weight distribution in both the backbone and the side chains. The polymer backbone of core-shell CPBs consisting of a poly(oligo(ethylene glycol) methyl ether methacrylate) (POEGMA) core block and a poly(2-(dimethylamino)ethyl methacrylate) (PDMAEMA) shell block was ruptured upon drying on solid surfaces, when suf-ficient Coulombic interactions between the shell block and the surface were formed. We controlled this scission behavior by tuning the surface interactions through switching the surface nature, shell quaternization, varying pH, or adding multivalent counterions. This study demonstrates that core-shell CPBs serve as a tool to directly compare the weak intermolecular forces with the strong carbon-carbon covalent bonds. A novel ‘grafting-from’ approach was developed to overcome the challenges of synthesizing well-defined CPBs from a linear polymer backbone with a high density of RAFT functionalities. In this so-called “CTA-shuttled” R-group approach, a certain amount of low-molecular-weight chain transfer agent (CTA) was added to the polymerization system, serving as shuttles to transfer active radicals among the individual growing CPBs. Well-defined CPBs with polystyrene or poly(tert-butyl acrylate) branches and core-shell CPBs with polystyrene-block-poly(N-isopropylacrylamide) branches were synthesized, with the molecular weight distribution much narrower than that from the conventional R-group approach. Monte Carlo simulations confirmed that the advantage of the “CTA-shuttled” R-group approach consists in the release of the active radicals from the trapping CPB systems. Imitating the natural “energy cascade” architecture, we developed single-molecular, rod-like nano-light harvesters (NLHs) on the basis of CPBs. Herein, a number of block copolymer side chains carrying light absorbing antennae groups (9,9-diethylfluorene, energy donors) were tethered to a linear polymer backbone containing emitting groups (anthracene, energy acceptors). These NLHs provide very efficient energy absorption and energy transfer from antennae to energy acceptors. Furthermore, we were able to manipulate the efficiency of energy transfer by tuning the distance between energy donors and energy acceptors in physical and/or chemical ways. This CPB-based NLH architecture presents a novel concept to design light harvesting materials and can readily be transplanted to any other applications in photoelectronic devices. Core-shell CPBs with a poly(acrylic acid) (PAA) core block and a PDMAEMA shell block were employed as templates for the preparation of various rare-earth metal cations (Ln3+) incorporated silica hybrid nanoparticles (NPs). A tight chelation of Ln3+ ions in the PAA core and a crosslinked silica layer deposited on the shell provide a very stable encapsulation of Ln3+ ions within the hybrid NPs and thus a high biocompatibility. The silica hybrid NPs obtain unique and diverse properties from the incorporated Ln3+ ions, such as visible photoluminescence, paramagnetic behavior, and a longitudinal relaxation time (T1) shortening effect. This novel template-directed approach succeeds in combining different functional centers via loading in-situ mixed Ln3+ ions into individual CPBs resulting in multicomponent hybrid NPs, which possess both visible photoluminescence and T1 contrast enhancement and can thus be applied as multimodal bioimaging probes.