Owing to conformational flexibility of Si–O–Si backbone and low surface energy, (poly)siloxanes are often employed to prepare materials for oil-water separation. However, the effect of molecular structure and chain mobility of siloxanes on surface wettability was neglected. In this work, sponge surfaces were functionalized with three (poly)siloxanes (i.e., mercaptopropyltris(trimethylsiloxy)silane (Si-4), polymercaptopropyl-methylsiloxane (P-1), and mercaptopropylmethylsiloxane-dimethysiloxane copolymer (P-2)) to construct small molecule layer, polymer layer with low chain mobility and polymer layer with high chain mobility, respectively. The surface morphology, elemental composition, wettability, and surface forces were systemically investigated, and the oil absorption and oil-water separation using the as-prepared polyurethane sponges were also investigated. The chain mobility greatly restricted surface coverage of polysiloxane coatings, resulting in less hydrophobicity of P-1 coated sponge, compared with P-2 coated sponge. The surface coverage and hydrophobicity of small molecule (i.e., Si-4) coated surfaces falls between immobilized P-1 and free-rotating P-2 coatings. The atomic force microscopy force measurement also demonstrates the strongest attraction force between P-2 coating and an oil droplet, compared to P-1 and Si-4 coatings. Despite the overall performance of Si-4/P-1/P-2 coated polyurethane sponges are excellent in oil adsorption and oil-water separation, slightly difference was observed following the trend of P-1 < Si-4 < P-2, which agrees well with the trend of hydrophobicity. This work demonstrated a non-fluorinated (super)hydrophobic surface driven by siloxane chain mobility, enabling scalable fabrication of hydrophobic sponges for high-efficiency oil-water separation.
Tan et al. (Wed,) studied this question.