Hydrophobic and Ultrahydrophobic Multilayer Thin Films from Perfluorinated Polyelectrolytes

Abstract

Interest in surfaces exhibiting large water contact angles is stimulated both by important technological implications and by novel aspects of surface physics. Since wetting can be controlled by a single monolayer of material, novel materials or surface treatments can yield a fascinating range of properties. Such treatments, which are capable of generating particularly hydrophobic surfaces, include conventional coating processes such as spraying or dipping, [1a] vacuum deposition techniques, [1b] and such surface-modification technologies as diffusion, [1c] laser and plasma processes, [1d] chemical plating, [1e] grafting [1f] or bonding hydrogel encapsulation, [1g] and bombardment with high-energy particles.[1h]The use of the layer-by-layer sequential adsorption [2] method for the assembly of an ultrathin film has very recently been reported for making ultrahydrophobic surfaces. [3,4] These studies address the critical issue of inducing surface roughness, which is required for true ultrahydrophobic behavior. [5][6][7][8][9][10][11][12][13][14][15] Soeno et al. [3] described a multilayered polyelectrolyte/silica nanoparticle system which was heated to sinter the particles and burn off the polymer, then treated with a fluorosilane to induce the required hydrophobicity. Zhai et al. [4] reported the formation of a pH-sensitive multilayer which undergoes a porosity-inducing phase transition in acidic solutions. Additional treatment steps, including crosslinking, deposition of silica nanoparticles, fluorosilane treatment, and thermal annealing, yielded stable ultrahydrophobic materials. Although the targeted surface properties were obtained, multiple processing steps are not desirable. Herein, we describe a multilayering process that uses both fluorinated polyanions and polycations, with a roughening step, by utilizing a naturally occurring nanorod that can be inserted conveniently into the layering sequence.The polyelectrolytes used were nafion (Scheme 1), which is a commercially available sulfonated fluorinated material, and a polycation synthesized from poly(vinylpyridine) and a fluorinated alkyl iodide. Polyelectrolyte components were deposited on silicon wafers under ambient conditions using dilute solutions/dispersions. Although fluorinated solvents, such as trifluoroethanol, were useful for dispersing the polyelectrolytes, we were interested in employing more common solvents as vehicles for manipulating all the components. Methanol was effective in this respect. Figure 1 shows the layer-by-layer build-up of the fluorinated polyelectrolytes. The regular growth of the multilayer alleviated our concerns that the polyelectrolytes, probably existing as micellar, nanoparticulate dispersions rather than true solutions, might not produce acceptable multilayering behavior. Some curvature of the build-up curve hints at exponential growth (a topic of recent interest [16,17] ), but the fact that such curvature is so slight suggests it might result from increasing surface roughness, which was found to be 2.9, 3.2, 4....

Synthesis of Stable Super Water‐ and Oil‐Repellent Polythiophene Films

“…In particular, the difference between them (hysteresis) is essential for verifying ultrahydrophobic behavior 46. 47 Measurements on chemically formed poly(fluorinated thiophene) surfaces showed a high hysteresis value (>70°, see Table 1). In this case, constant receding values were not obtained, and an initial receding contact angle is given.…”

“…Thus, these surfaces can be defined as ultrahydrophobic, that is, they exhibit both large water contact angle and small hysteresis 46. 47…”

Nanostructured thin films for icephobic applications =

“…[95], phase separation [96], sublimation [97], template methods [98], layer-by-layer methods [99], etc., exist in preparing superhydrophobic surfaces. A list of papers on the processing of superhydrophobic surfaces and films up to the year 2000 has been provided by A. Nakajima et al in Table 1 in their "invited review" article [65].…”

Polymeric ionic liquids for the fast preparation of superhydrophobic coatings by the simultaneous spraying of oppositely charged polyelectrolytes and nanoparticles

“…5 The use of the LbL sequential absorption method for the assembly of ultrathin films has been reported for making ultrahydrophobic surfaces including oppositely charged polyelectrolytes and silica nanoparticles. 6,7 However, the conventional LbL technique is quite time consuming and difficult to implement over large areas. The method of simultaneous spraying of polyelectrolyte solutions onto a substrate allows LbL thin films to be produced over large areas and in short times.…”