Synthesis, Characterization and Amphiphilicity-Driven Self-Assembly of Quantum Dots with Mixed Polymer Brush Layers

The synthesis, characterization and self-assembly behavior of semiconductor quantum dots (QDs) with mixed polystyrene (PS) / poly (methyl methacrylate) (PMMA) polymer brush layers (PS/PMMA-CdS) are described. The environmentally-responsive PS/PMMA-CdS nanoparticles are investigated in various solvents with different polarities. Static and dynamic light scattering results suggest conformational changes in the mixed brush structure in response to different solvent polarities. UV-vis and photoluminescence spectra show that QD sizes and optical properties are independent of the solvent medium due to protection by the block copolymer. Long-term stability of QD size distributions in the studied solvents is demonstrated for period of up to six months. 2D 1H NOESY experiments indicate that PS and PMMA coronal chains are statistically distributed around the QDs within the mixed brush layer. PS/PMMA-CdS nanoparticles are also shown to self-assemble at the polymer/polymer interface of a phase-separating blend of the corresponding homopolymers, forming an encapsulating shell surrounding PMMA islands in a PS matrix. The segregated QDs regulate phase separation during spin-coating and dramatically stabilize the spin-coated blend morphologies during subsequent annealing. Free-standing arrays of QD/polymer rings are developed by selective solvent washing and removal of homopolymers from the spin-coated films. After converting the PMMA coronal chains to poly (methacrylic acid) (PMAA) via a hydrolysis reaction, the resulting amphiphilic PS/PMAA-CdS nanoparticles are found to show rich and tunable self-assembly behavior in mixtures of organic solvents and water. The block copolymer-like self-assembly behavior of PS/PMAA-CdS suggests phase separation of randomly-distributed PS and PMAA chains within the mixed brush structure, leading to anisotropic interactions between nanoparticles mediated by energetic contributions from interfacial tension and chain stretching. As a result, PS/PMAA-CdS forms a wide range of interesting colloidal superstructures, including spherical supermicelles, worms, and vesicles, all with well-defined internal organization of QDs. Based on annealing experiments at a relative low water content above cwc , a mechanism of the formation of worm-like and continent aggregates is proposed. Thermodynamic and kinetic aspects of formation of the various QD/polymer colloids are also described.