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 phase- separated 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.