Nanopore-extrusion Induced Sphere-to-cylinder Transition of Block Copolymer Micelles

Amphiphilic block copolymers can self-assemble into colloidal particles with various structures (phases), including spheres, cylinders and vesicles, in a selective solvent. These phases can be transformed from one to another, conventionally, by an alternation of the polymer-solvent interaction that is affected by the block copolymer composition and the solvent quality. In this thesis study, we found that the extrusion of a block copolymer solution in its spherical micelle phase through a commercial 20-nm nanopore under a proper condition can also induce the sphere-to-cylinder transition to form long wormlike micelles. Not as expected, such formed wormlike micelles do not quickly disassemble back to their original spherical but undergo a slow dissociation process over a long time period (days or even weeks) before returning to their original thermodynamically stable spherical morphology.;To understand such a nanopore-extrusion induced sphere-to-cylinder transition, we further investigated effects of the copolymer composition, chain length, solvent quality and extrusion flow rate. Using a combination of laser light scattering and transmission electron microscopy, we characterized the micelle morphology in the copolymer solution before and after the nanopore-extrusion. In addition, we also monitored the hydraulic pressure drop across the nanopore during the nanopore-extrusion. On the basis of these studies, we have qualitatively learned how those spherical micelles are transformed into long cylindrical micelles in the nanopore extrusion.;As the extrusion started, those spherical micelles with a hydrodynamic radius of 30 nm were naturally blocked at the entrance of smaller pores (20 nm). Under the hydrodynamic force of the elongational flow field, the blocked spherical micelles were squeezed into the nanopore and undergo a inter-micelle fusion inside the nanopore while more spherical micelles are accumulated at the pore entrance, resulting in a rearrangement of individual spherical micelles into a long cylindrical micelle inside each nanopore. Our results showed that it is critically important to control the softness of micelles, especially their core; namely, the interchain iv interaction inside the core should not be too strong so that the inter-micelle fusion can occur under the external compression, but also not too weak so that the resultant cylindrical micelles after the extrusion will not quickly disintegrated back to small spherical micelles. Such interchain interaction (the softness of the core) can be adjusted by the solvent quality.;Further, we quantitatively studied the slow cylinder-to-sphere dissociation kinetics by a combination of laser light scattering and electron microscopy. The results show the weight-average length of the fibers formed after the extrusion gradually decreases. The dissociation mechanism and driving force are discussed on the basis of the TEM analysis of the structures of the fiber at different dissociation times. The simulation of the dissociation using various scission models reveals that the cylinder fragmentation follows a combination of the Gaussian and end scission model and the scission rate constant is essentially a linear function of the fiber length and slightly increases with time.;This thesis study also leads to a novel way to prepare various functional polymeric cylindrical core-shell micelles (nanowires). By incorporating metal nanoparticles inside the initial spherical micelles, long hybrid cylindrical micelles can be prepared by extruding them through nanopores. Such prepared "nanowires" have some unique photo-properties and are potentially useful in nano-devices. More importantly, in comparison with long and disorientated cylindrical micelles normally formed in a selective solvent, the production of billions nanowires in one extrusion using a membrane with parallel nanopores looks promising. These nanowires might be aligned under the elongation flow so that they could be spun into a microscopic fiber bundle (microfiber). Therefore, the extrusion studied in this thesis provides a possible way to manufacture high-quality and functional polymeric microfibers (nanowires) in the future.