Slow morphology evolution of block copolymer-quantum dot hybrid networks in solution

When 2-propanol is added to a chloroform solution containing a mixture of CdSe quantum dots (QDs) plus a polystyrene-block-poly(4-vinylpyridine) diblock copolymer (PS₄₀₄-b-P4VP₇₆, the subscripts refer to the degree of polymerization), the block copolymer assembles into uniform 40 nm diameter spherical micelles with a PS core and a thin P4VP corona to which the QDs are attached. As we reported in Macromolecules, 2010, 43, 5066-5074, vigorous magnetic stirring of this mixture over two days leads to a change in morphology to finite (1 to 2 μm) networks consisting of interconnected cylindrical PS struts surrounded by a P4VP corona in which the QDs are embedded. Here we find that these networks settle but do not lose their structure if allowed to age without stirring. Continued vigorous stirring over one month leads to a complete change in morphology, to form clusters of oval-shaped vesicles similar in size to the networks, in which individual vesicles are a few hundred nm long. Some of the vesicles appear to have popped open to form pairs of bowl-like structures. These bowls are single-walled structures, with walls similar in thickness (35 to 40 nm) to the core of the micelles formed in the initial step. We believe the walls consist of a PS core with P4VP chains protruding from the inner and out surfaces. The QDs are attached to both surfaces of both the vesicles and the bowls, presumably still embedded in the P4VP corona. These colloidal nanocomposites provide a striking example of kinetically controlled self-assembly in which the system evolves from small spheres of high curvature to cylinders of lower curvature, to larger vesicles, and each step takes place on its own characteristic time scale.