One-Step Synthesis of a Durable and Liquid-Repellent Poly(dimethylsiloxane) Coating

Abstract

Coatings with low sliding angles for liquid drops have a broad range of applications. However, it remains a challenge to have a fast, easy, and universal preparation method for coatings that are long-term stable, robust, and environmentally friendly. Here, a one-step grafting-from approach is reported for poly(dimethylsiloxane) (PDMS) brushes on surfaces through spontaneous polymerization of dichlorodimethylsilane fulfilling all these requirements. Drops of a variety of liquids slide off at tilt angles below 5°. This non-stick coating with autophobicity can reduce the waste of water and solvents in cleaning. The strong covalent attachment of the PDMS brush to the substrate makes them mechanically robust and UV-tolerant. Their resistance to high temperatures and to droplet sliding erosion, combined with the low film thickness (≈8 nm) makes them ideal candidates to solve the long-term degradation issues of coatings for heat-transfer surfaces.

Keywords: adhesion; liquid-repellency; poly(dimethylsiloxane); polymer brushes; wetting.

Figure 1

Ultrafast processing of PDMS brushes. a) Schematic illustration of the formation of PDMS brushes from dichlorodimethylsilane (DCDMS) monomers. b) Si 2p peaks (red line) measured by X‐ray photoelectron spectroscopy (XPS) of the PDMS brushes grafted onto a silicon wafer. The fit indicates the presence of O—Si—O (green line) and —O—Si(CH₃)₂—O— (blue line) bonds. Reaction time: 30 min. The sample was washed with toluene for five times. c) Autophobicity of PDMS brushes on glass: after 30 s grafting time, the reactant can be fully removed by simple tilting. d) Sliding of n‐hexadecane on the PDMS‐brushes‐coated glass slide. Liquid volume: 5 µL. e) Sliding of water on the PDMS‐brushes‐coated glass slide. Liquid volume: 10 µL. Scale bar (c–e): 1 cm. f) Advancing contact angle (Θ _ACA) and contact angle hysteresis (∆Θ) of water as function of grafting time. g) Advancing contact angle and contact angle hysteresis of n‐hexadecane as function of grafting time.

Figure 2

Robust liquid‐repellent PDMS brushes. a) Time‐sequence images of hexane (left), ethanol (middle), and toluene (right) droplets sliding down a tilted silicone brush surface. To increase visibility of the hexane drop, a small air bubble was injected on the top of hexane droplet. Scale bar: 2 mm. b) Contact angle hysteresis of various liquids on PDMS brushes. Grafting time: 30 min. c) Sliding angle of drops of various liquids (from left to right: n‐hexadecane, DMF, DMSO, diiodomethane, water) on PDMS brushes. The black line shows sliding angles of liquid drops (10 µL) versus surface tension plotted according to Equation (2). The mean, advancing, and receding contact angles were measured values of the five kinds of liquids. d) Water (Θ _ACA: and ∆Θ: ) and n‐hexadecane (Θ _ACA: and ∆Θ: ) repellency of the PDMS brushes versus sonication‐washing time. e) Water (Θ _ACA: and ∆Θ: ) and n‐hexadecane (Θ _ACA: and ∆Θ: ) repellency of the PDMS brushes resisting tape‐peeling tests.

Figure 3

Durability of the PDMS brushes under high‐temperature and water vapor treatment. a) Water repellency of the PDMS brushes versus aging time at 100 °C and 250 °C. b) Hexadecane repellency of the PDMS brushes versus aging time at 100 °C and 250 °C. c) Durability of wetting property of water on PDMS brushes and fluorinated surface under water vapor treatment at 70 °C. d) Image sequence shows sliding of condensed water droplets during water vapor treatment. Vapor temperature: 70 °C. e) Image sequence shows sliding of condensed toluene droplets during toluene vapor treatment. Vapor temperature: 70 °C. Scale bar in (d,e): 0.5 cm.

Figure 4

Applications of PDMS brushes. a) A window glass plate (soda‐lime glass, 20 × 10 cm²) was covered with a piece of commercial paper (80 g m⁻²). Then the reactant solution (200 µL) was applied. The paper was used to hold the reaction solution and promote its spreading on the surface. Scale bar: 2 cm. b) UV–vis spectra show the film transmittance as a function of modification time. The inset shows a window glass (30 × 20 cm²) coated for 30 min. c) Wearing tolerance of PDMS brushes on window glass (15 × 7 cm²). Grafting time: 3 min. d) Images show sliding of various liquids on coated glass. Liquids from left to right: red wine, white wine, seed oil, castor oil, polystyrene solution in toluene (1 wt%), poly(ethylene glycol) aqueous solution (50 wt %), and detergent aqueous solution (10 wt%). Scale bar: 1 cm. e) Pouring of red wine from wine glass and no staining on the coated surface (iii). A layer of wine stayed on the glass surface after pouring (iii'). f) Washing of concentrated detergent (0.3 mL) on a coated glass with water flux (25 mL). Grafting time: 3 min. Scale bar: 2 cm. Inset shows a water drop after the detergent was removed. Scale bar: 2 mm. g) Metal‐oxide surfaces with nanostructures. Surfaces from left to right: titanium dioxide (TiO₂) nanotexture, silicone nanofilament (SiO _x ), and zinc oxide nanopillars (ZnO). Scale bars (left to right): 500 nm, 500 nm, 1 µm. h) Impacting and bouncing of a 5 µL water droplet shows superhydrophobic performance of TiO₂ hierarchical surface after grafting with PDMS brushes in 3 min. Dropping height: 2 cm. Scale bar: 1 mm.