Nano/microstructured polyhedral oligomeric silsesquioxanes-based hybrid copolymers: Morphology evolution and surface characterization

Unique "micro-bean sprouts" with "nano-tails" were electrohydrodynamically prepared from poly[(propylmethacryl-heptaisobutyl-polyhedral oligomeric silsesquioxane)-co-(methylmethacrylate)] (POSS-MMA). The nano/microstructured POSS-MMA substrates reveal superhydrophobic nature, with contact angles >160°. Contact angle versus time plots show that water droplet evaporation from the substrates with "micro-bean sprouts" falls into two stages: the initial stage, marked by steadily decreasing contact angles and the later stage, marked by rapidly decreasing contact angles. The turning point differentiating these two stages occurs at ∼120°-130°. The substrates consisting of "micro-bean sprouts" with larger "heads" experience pinned triple-line phase during the early stage of droplet evaporation. In contrast, the triple line for a droplet placed on "micro-bean sprouts" with smaller "heads" can move easily, leading to a rapid decrease in contact diameter. Meanwhile, the contact angle decreases only slightly during the rapidly moving triple-line phase. The contact angles of the fibrous substrates measured during the initial stage of water droplet evaporation are much lower than those of "micro-bean sprouts", even though the time period of the initial stage is extended. Both the architectures and the sizes of these POSS-MMA nano/microstructures were shown to affect the energy barrier for the triple-line motion during water droplet evaporation and therefore the contact angle hysteresis.