Evaporative organization of hierarchically structured polymer blend rings

We report the first study of the controlled, evaporative self-organization of a polymer blend from a restricted geometry comprised of a spherical lens upon a Si substrate (i.e., a sphere-on-flat geometry). This geometry facilitated the control over the evaporation rate of solvent, thereby eliminating the temperature gradient and the possible convective instabilities. In this study, a drop of polystyrene (PS) and poly(methyl methacrylate) (PMMA) toluene solution evaporated in the sphere-on-flat geometry. The combination of controlled, consecutive pinning-depinning cycles (i.e., "stick - slip") of the contact line at the edge of the geometry, spontaneous phase separation of incompatible polymers at the microscopic scale, and a dewetting process in the late stage of phase segregation led to the formation of gradient, hierarchically structured polymer blend rings composed of phaseseparated PS and PMMA. The topographic distribution of PS and PMMA phases on the ring surface were revealed after removal of the PS phase with a selective solvent. Namely, the trench-pit structures were formed in the PS-112K/PMMA-133K blend, while for the PS-112K/PMMA-534K blend, pit morphologies were observed. This facile approach offers a new way of simultaneously processing two or more nonvolatile components via controlled evaporation to produce new kinds of structures with hierarchical order in a simple, robust, and one-step manner.