Rayleigh Instability Induced Cylinder-to-Sphere Transition in Block Copolymer Micelles: Direct Visualization of the Kinetic Pathway.

Direct visualization of morphological evolution remains extremely challenging despite its critical importance to understand the basic fundamentals behind the transition. Here we report on the detailed observation of a spontaneous cylinder-to-sphere morphological transformation of amphiphilic poly(2-vinylpyridine)--poly(ethylene oxide) (P2VP--PEO) diblock copolymer micelles in aqueous solution, which first provides experimental evidence that the fragmentation pathway is driven by Rayleigh instability showing the distinctive signatures during the transition. Owing to the instability of cylindrical micelles and the fluidity of micellar cores, our results show that the cylindrical micelles spontaneously undulate and transform into spherical micelles through distinct intermediate states, including undulated cylinders and pearl-necklace-like micelles with a perfect sinusoidal wave throughout the length. Moreover, the present system with transitional morphology is proved to be able to act as a model to encapsulate hydrophobic guests in the micellar cores, which displays a relatively sustained release behavior. The specific kinetic pathway provides new insight into the mechanism of block copolymer micellar morphological transition; meanwhile, the dynamic system might serve as a promising candidate for unique nanostructure design as well as contribute to the transition-coupled guest delivery and controlled release.