Superhydrophobic Polyolefin Surfaces: Controlled Micro- andNanostructures

Esa Puukilainen, Tiina Rasilainen, Mika Suvanto, and Tapani A. Pakkanen*

Department of Chemistry, UniVersity of Joensuu, P.O. Box 111, FI-80101 Joensuu, Finland

ReceiVed December 12, 2006. In Final Form: April 16, 2007

Superhydrophobic polyolefin surfaces were prepared by simultaneous micro- and nanostructuring. Electropolishedaluminum foil was microstructured with a micro working robot and then anodized in polyprotic acid. The surfacemicrostructure can be tailored by adjusting the settings of the micro working robot and the nanostructure by adjustingthe parameters of the anodization procedure. Surface structuring was done by injection molding where a microstructuredanodized aluminum oxide mold insert was used to pattern the surfaces. Structuring had a marked effect on the contactangle between the injection-molded polyolefins and water. When the optimized microstructure was covered withnanostructure, the static contact angle between polypropylene and water obtained a value of about 165° and the slidingangle decreased to about 2.5°. The superhydrophobic state was achieved.

1. Introduction

Wettability is an important property of a solid surface; it playsa crucial role in nature and in many industrial applications. Thewettability of a solid surface depends on both the surface energyand the surface structure. Lowering of the surface energy enhancesthe hydrophobicity. The lowest recorded value of the surfaceenergy is for regularly aligned closest hexagonally packed-CF3

groups. This surface forms a contact angle of about 120° witha water drop. While clearly hydrophobic, it is not yet super-hydrophobic.1,2 The effect of the surface structure on thewettability can be evaluated with either the Wenzel or the Cassie-Baxter theory. According to Wenzel,3 the liquid fully penetratesthe surface grooves, and the relationship between a flat surfacecontact angle and a rough surface contact angle is as shown ineq 1, whereθ* is the apparent contact angle,r is the roughness

factor (defined as the actual surface divided by the geometricsurface), andθ is Young’s angle. Sincer > 1, both hydrophobicityand hydrophilicity are intensified by roughness. According toCassie-Baxter,4 the drop sits on a composite surface made ofsolid and air, and it only interacts with the top of the roughnesspeaks. The Cassie-Baxter relation is as shown in eq 2, where

θ* is the apparent contact angle,φs is the area fraction of thesolid surface, andθ is Young’s angle. Comparative studies onthe Wenzel and Cassie-Baxter theories have shown that theWenzel model works with surfaces of moderate surface roughnessand when the contact angle hysteresis is large, while the Cassie-Baxter model works with rougher surfaces and when contactangle hysteresis is low. In general, if cosθ* < -1/r, the Cassie-Baxter model gives a lower surface energy and is more favorablethan the Wenzel model. However, metastable states have been

reported as well, and the drop can move from one equilibriumstate to another provided it can overcome the energy barrierbetween the two states.1,5-12

Amazing surfaces are found in nature, where color, UVprotection, superhydrophobicity, and self-cleaning ability areproduced through a combination of specific surface chemistryand highly organized surface structure. Butterfly wings,13cicadawings, water strider’s legs,14and leaves of the lotus plant15,16aregood examples.

The preparation of artificial surfaces with surface propertiessuch as superhydrophobicity is of increasing interest. Severalnew methods have recently been introduced.17 Silica andpolystyrene spheres, direct polymerization of ethylene in solventon silica surfaces, chemical bath deposition, chemical etchingon aluminum, copper, and zinc substrates, and the sol-gel methodhave all been used to prepare superhydrophobic surfaces.18-24

Moreover, solution casting of different polymers, on either a flator a textured substrate, has been employed widely, with goodsuccess.14,25-30 The limitation of most of these methods is the

* To whom correspondence should be addressed. Phone:+358-13-2513345. Fax:+358-13-2513344. E-mail: Tapani.Pakkanen@joensuu.fi.

(1) Nakajima, A.; Hashimoto, K.; Watanabe, T.Monatsh. Chem.2001, 132,31.

(2) Nishino, T.; Meguro, M.; Nakamae, K.; Matsushita, M.; Ueda, Y.Langmuir1999, 15, 4321.

(3) Wenzel, R. N.Ind. Eng. Chem.1936, 28, 988.(4) Cassie, A. B. D.; Baxter, S.Trans. Faraday Soc.1944, 40, 546.

(5) Johnson, R. E.; Dettre, R. H.AdV. Chem. Ser.1964, 43, 112.(6) Quere, D. Physica A2002, 313, 32.(7) Callies, M.; Chen, Y.; Marty, F.; Pe´pin, A.; Quere, D. Microelectron. Eng.

2005, 78-79, 100.(8) Patankar, N. A.Langmuir2003, 19, 1249.(9) He, B.; Patankar, N. A.; Lee, J.Langmuir2003, 19, 4999.(10) Patankar, N. A.Langmuir2004, 20, 7097.(11) Patankar, N. A.Langmuir2004, 20, 8209.(12) He, B.; Lee, J.; Patankar, N. A.Colloids Surf., A: Physicochem. Eng.

Aspects2004, 248, 101.(13) Kertesz, K.; Balint, Zs.; Vertesy, Z.; Mark, G. I.; Lousse, V.; Vigneron,

J.-P.; Biro, L. P. Curr. Appl. Phys.2006, 6, 252.(14) Sun, T.; Feng, L.; Gao, X.; Jiang, L.Acc. Chem. Res.2005, 38, 644.(15) Barthlott, W.; Neinhuis, C.Planta 1997, 202, 1.(16) Schulz, B.; Frommer, W. B.Science2004, 306, 622.(17) Puukilainen, E.; Koponen, H.-K; Xiao, Z.; Suvanto, S.; Pakkanen. T. A.

Colloids Surf., A: Physicochem. Eng. Aspects2006, 287, 175.(18) Zhang, G.; Wang, D.; Gu, Z.-Z.; Mo¨hwald, H.Langmuir2005, 21, 9143.(19) Ming, W.; Wu, D.; van Benthem, R.; de With, G.Nano Lett.2005, 5,

2298.(20) Yan, L.; Wang, K.; Wu, J.; Ye, L.Colloids Surf., A: Physicochem. Eng.

Aspects2007, 296, 123.(21) Han, W.; Wu, D.; Ming, W.; Niemansverdriet, H. J. W.; Thu¨ne, P. C.

Langmuir2006, 22, 7956.(22) Hosono, E.; Fujihara, S.; Honma, I.; Zhou, H.J. Am. Chem. Soc.2005,

127, 13458.(23) Qian, B.; Shen, Z.Langmuir2005, 21, 9007.(24) Hikita, M.; Tanaka, K.; Nakamura, T.; Kajiyama, T.; Takaraha, A.

Langmuir2005, 21, 7299.(25) Lu, X.; Zhang, J.; Zhang, C.; Han, Y.Macromol. Rapid Commun.2005,

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cosθ* ) r cosθ (1)

cosθ* ) φs(1 + cosθ) - 1 (2)

7263Langmuir2007,23, 7263-7268

10.1021/la063588h CCC: $37.00 © 2007 American Chemical SocietyPublished on Web 05/23/2007

difficulty in controlling the formation of the surface structure,especially on the micrometer scale; additionally, they are suitableonly for planar surfaces and surfaces of limited size. In studieswhere lotuslike dual structures have been prepared, control ofthe formation of the surface structure has been difficult.19,20,27,29,30

Zhang et al.29prepared a microstructured aluminum foil templateby pressing the aluminum with glass spheres. The aluminumwas then anodized to produce nanopores on the template, whichwas used as a mask to produce a lotus-leaf-like topography onthe polymer. The aspect ratio of the microstructure was 0.5. Thepitch of the grid was fixed by the packing geometry of the spheres.

Polyolefins (POs) are widely used in applications wherehydrophobicity and clean surfaces are required.31 We recentlydemonstrated that thehydrophobicityofhigh-densitypolyethylenecan be improved by melt blending with perfluoropolyethers(PFPEs).32We also demonstrated that polyolefin surfaces can bemodified, simultaneously, chemically, and structurally. Modi-fications were done by injection molding; liquid PFPE was addedto the polyolefin melt, and an anodized aluminum oxide (AAO)mold insert was applied to nanopattern the surfaces. A well-ordered nanostructure was obtained over the entire surface.Nanostructuring had a marked effect on the contact angle betweenthe polyolefins and water.17

In the present work we prepared polyolefin surfaces that weremicro- or nanostructured or simultaneously micro- and nano-structured. The formation of both the micro- and nanostructuresneeds to be rigorously controlled in three dimensions to optimizethe surface properties of the injection-molded polyolefin. Witha series of controlled surface structures, the Wenzel and Cassie-Baxter theories can be tested, and a proposal for the optimumsuperhydrophobic surface structure can be made.

2. Experimental Section

2.1. Materials and Methods. High-density poly(ethylene-1-butene) copolymer (bPE), MB 7541, density 954 kg/m3, andpolypropylene homopolymer (PP), HD 120 MO, density 908 kg/m3,were from Borealis.

Polyolefin samples were made by injection molding with a DSMMidi 2000 extrudersa microinjection-molding machine. The fol-lowing processing parameters were selected for bPE and PP on thebasis of previous studies:17,32 screw temperature 225°C (bPE)/255°C (PP), mold temperature 40°C (bPE)/50°C (PP), and screwrotation speed 100 rpm for both polymers.

Surface structures of the samples were studied with a HitachiS4800 FE-SEM instrument. The samples were mounted onto a stubwith copper adhesive tape and coated with 5 nm of Pt/Pd. Acceleratingvoltages of 2-3 kV and a general working distance of 8 mm wereapplied.

Contact angle measurements were carried out with a KSV Cam200 contact angle meter. Static contact angle measurements weremade at room temperature with ion-exchanged water. A drop ofwater (5 or 6µL) was placed on the sample, and the sample wasphotographed once a second for 30 s. The contact angle wasdetermined by fitting a Young-Laplace curve around the drop.Values obtained between 6 and 30 s were averaged to obtain thecontact angle for each measurement. Nine parallel measurements

were made of each sample, and average contact angle values werecalculated for each surface. The sliding angle was determined bydropping a 6µL drop of water onto the tilted surface from about1 mm height. The reported value is the value of the tilting anglewhen the drop of water rolls on the surface.

2.2. Fabrication of Microstructured Aluminum Mold Inserts.A piece of an aluminum foil (Alfa Aesar, 0.25 mm thick, Puratronic,99.997%), 3 cm× 3.5 cm, was degreased in acetone andelectropolished in a mixture of perchloric acid and ethanol (1:8)using platinum foil as a counter electrode. The electropolishing currentwas 2.7 A, and the time was 135 s. The foil was then rinsed withdeionized water and allowed to dry at room temperature. Elec-tropolished aluminum foils were microstructured with a microworking robot (RP-1AH) made by Mitsubishi Electric, having aCR1 control unit, and a feedback unit from Delta Enterprise Ltd.Tungsten carbide based needles were made by Gritech Ltd.Microstructured aluminum mold inserts used in polymer patterningwere fabricated by cutting the structured aluminum foil into circularshapes 25 mm in diameter.

2.3. Fabrication of AAO Mold Inserts. Microstructured and flataluminum foils were anodized by the anodization procedure describedearlier,17 except that the anodization time was 48 h for themicrostructured foils, the anodized side was not covered with nailpolish for removal of the unreacted aluminum and barrier layer, andthe barrier layer was removed with 5% H3PO4 solution for 40 min.

The AAO mold inserts used in polymer patterning were fabricatedby gluing the AAO membrane onto a 0.5 mm thick steel plate withheat-stable epoxy glue (Loctite Hysol 9492 A&B). The side onwhich the anodization was done was facing upward.

3. Results and Discussion

3.1. Microstructured Aluminum Mold Inserts. Table 1shows the settings for the micro working robot (the cavity volumeand the step length) and the type of mold insert made with thedifferent settings. Eight different microstructures were fabricated.Six were used for the microstructuring of polyolefins (indicatedas “micro”), and four were anodized after microstructuring toproduce dual-scale structures on polyolefins (indicated as “dual”).

Figure 1 shows SEM images of the microstructured aluminummold insertsµ2 andµ6. Images A and C, taken with moderatemagnification, show the high controllability and accuracy of themicro working robot. Depressions made with the hard metalneedle appear uniformly over the entire surface. Images B andD, taken with greater magnification, show the shape and size ofthe depressions. The bottom of the circular depression is about15 µm in diameter, and with the deeper depression (D) thediameter at the plane is about 22µm. The sides of the depressionsshow some irregularity due to the abrasion of the hard metalneedle, whereas the polished plane is relatively smooth.

3.2. Anodized Aluminum Oxide Mold Inserts.Anodizationwas carried out in a single step, since some of the aluminum foilshad a microstructure, and two-step anodization would haveflattened the microstructure excessively. The pore walls werecorroded slightly during barrier layer removal, and the porediameters were tuned by adjustment of the removal time. Figure

(26) Han, J. T.; Xu, X.; Cho, K.Langmuir2005, 21, 6662.(27) Sun, M.; Luo, C.; Xu, L.; Ji, H.; Ouyang, Q.; Yu, D.; Chen, Y.Langmuir

2005, 21, 8978.(28) Jin, M.; Feng, X.; Feng, L.; Sun, T.; Zhai, J.; Li, T.; Jiang, L.AdV. Mater.

2005, 17, 1977.(29) Zhang, L.; Zhou, Z.; Cheng, B.; DeSimone, J. M.; Samulski, E. T.Langmuir

2006, 22, 8576.(30) Vogelaar, L.; Lammertink, R. G. H.; Wessling, M.Langmuir2006, 22,

3125.(31) Harper, C. A.Modern Plastics Handbook; The McGraw-Hill Companies:

New York, 2000; Chapter 1.(32) Puukilainen, E.; Pakkanen, T. A.J. Polym. Sci., Part B: Polym. Phys.

2005, 43, 2252.

Table 1. Settings for the Micro Working Robot and the Typesof Mold Inserts

moldinsert

cavityvol (µm3)

step(µm) type

µ1 1100 22 dualµ2 1100 25 micro+ dualµ3 1100 40 dualµ4 2700 25 microµ5 3600 30 microµ6 3600 40 micro+ dualµ7 5700 45 microµ8 >6100 45 micro

7264 Langmuir, Vol. 23, No. 13, 2007 Puukilainen et al.

2 shows the SEM image of an AAO mold insert prepared froma flat aluminum foil and having only nanostructure. On a largescale (A and B), the membrane is homogeneous and pores areperpendicular to the surface. The pore diameter and the wallthickness are similar, about 50 nm (C and D).

Figure 3 shows the SEM image of microstructured AAO moldinsertµ6. Comparison of Figure 3A-D with Figure 1C,D showshow the anodization improves the microstructure; the irregularstructures on the sides of microdepressions are smoothed duringthe anodization. The dimensions of the microstructures remainabout the same; the diameter is about 15µm at the bottom andabout 22µm on the plane, and the depth is about 16µm. Thenanopore structure of the microstructured aluminum foil is similarto that of the flat aluminum surfaces. However, some irregularitiesare visible at the bottom of the microdepressions (Figure 3C).Foils were anodized throughout, and the pores go perpendicularlyeven in the walls of the depressions (see cross-section imagesD-F).

3.3. Microstructured Polyolefins.Figure 4 shows examplesof surface structures of microstructured polyolefins. Micro-structured aluminum mold inserts were filled with PP until theaspect ratio exceeded the value of 1. When molding was donewith molds with shallow depressions (µ2 andµ6), the microbumpsthat were produced had the same irregular shapes as the molds(see images B and D). When the depressions in the mold weredeep (µ8), the tops of the microbumps were smooth, indicating

incomplete filling of the mold insert (see image F). Microstruc-tures were duplicated in polypropylene with high accuracy.

Figure 1. SEM images of microstructured aluminum mold insertsµ2 (A and B) andµ6 (C and D).

Figure 2. SEM images of an AAO mold insert with nanostructureprepared from a flat aluminum foil. (B) and (D) are cross-sectionimages.

Figure 3. SEM images of anodized mold insertµ6. (D)-(F) arecross-section images.

Figure 4. SEM images of PP microstructured withµ2 (A and B),PP microstructured withµ6 (C and D), PP microstructured withµ8(E and F), and bPE microstructured withµ2 (G and H).

Superhydrophobic Polyolefin Surfaces Langmuir, Vol. 23, No. 13, 20077265

Dimensions of the microstructures could be tuned freely in allthree dimensions to produce favorable surface microstructure.Duplication of microstructures to polyethylene did not succeedwith high accuracy. Filling of the mold inserts, even those withshallow depressions, was incomplete. This can be seen in imagesG and H, which show the surface structure of bPE molded withtheµ2 mold insert. Somewhat higher microbumps were producedwhen molding was done with a mold insert having deeperdepressions, but in every case the filling was incomplete. Injectionmolding of polyethylene was tested with higher processingtemperatures, since the replication of submicrometer featuresreportedly improves at elevated melt and mold temperatures.33

However, no marked improvement was achieved in the fillingof the mold insert. Adhesion energy is reported to have a majoreffect on the replication of surface structures by injection

molding.34 Low adhesion energy between the polyethylene andaluminum mold insert could be the reason why the polyethylenedid not completely fill the aluminum mold insert.

3.4. Nanostructured and Dual-Structured Polyolefins.Nanostructured polyolefin surfaces were prepared by injectionmolding with a flat AAO foil described in section 3.2. Figure5 shows SEM images of nanostructured PP (A and B) and bPE(C and D). Surface structures of the two polyolefins are similar.The shape and size of the structures are about the same as onthe AAO foil. Cross-section images (B and D) show that thediameter of the structure is about 50 nm and the aspect ratio isas much as 3, so that the structure mimics well the lotuslikenanostructure.15

Dual-structured polyolefin surfaces were prepared by injectionmolding with the microstructured AAO foils described in section3.2. Figures 6 and 7 show the SEM images of dual-structuredpolyolefin surfaces. Both micro- and nanostructures wereduplicated on the polyolefins with high accuracy. Well-orderedmicrostructure was found over the entire surface on bothpolypropylene (Figure 6A-D) and polyethylene (Figure 7A-D). As can be seen at higher magnification, a high aspect rationanostructure covered the surfaces on the plane as well as on themicrobumps (images E and F). The quality of the surfaces wasimproved: the adhesion between the polymer melt and the ceramicAAO foil was sufficient, and structures were now duplicatedalso well with polyethylene.

3.5. Contact Angle Measurements.Contact angle measure-ments were made for a more quantitative understanding of the

(33) Monkkonen, K.; Hietala, J.; Pa¨akkonen, P.; Pa¨akkonen, E. J.; Kaikuranta,T.; Pakkanen, T. T.; Ja¨askelainen, T.Polym. Eng. Sci.2002, 42, 1600.

(34) Pranov, H.; Rasmussen, H. K.; Larsen, N. B.; Gadegaard, N.Polym. Eng.Sci.2006, 46, 160.

Figure 5. SEM images of nanostructured PP (A and B) and bPE(C and D). (B) and (D) show the cross-section images.

Figure 6. SEM images of dual-structured PP molded with anodizedµ6.

Figure 7. SEM images of dual-structured bPE molded with anodizedµ2.

Figure 8. Side view of the roughness geometry of the microstructure.

7266 Langmuir, Vol. 23, No. 13, 2007 Puukilainen et al.

microstructured, nanostructured, and dual-structured surfaces.Figure 8 shows howa, b, andH were estimated from the SEMimages for each microstructure;a is the diameter at the top ofthe microbump,b is the distance between the top corners, andH is the height of the barrel-shaped structure.

r andφs can be calculated from eqs 3 and 4, which are derivedfrom the geometry of a barrel-shaped structure.

Tables 2 and 3 report the results of the contact anglemeasurements and the calculated theoretical values.a, b, andHare the roughness parameters of the microstructures measuredfrom the SEM images,θE is the experimental contact angle withstandard deviation,r is the roughness factor for microstructuredsurfaces,θW is the theoretical contact angle according to theWenzel equation,φs is the area fraction of the solid surface formicrostructured surfaces,θCB is the theoretical contact angleaccording to the Cassie-Baxter equation, and SA is theexperimental sliding angle. In the case of the microstructuredsurfaces, theoretical values of the contact angles were calculated

so that the experimental contact angles of flat polyolefins weretreated as Young’s contact angles. Similarly, in the case of thedual-structured surfaces the experimental contact angles ofnanostructured polyolefins were treated as Young’s contactangles. Figure 9 shows a comparison of the experimental andtheoretical contact angles. Numbers refer to the mold insertswith which the structures were made, and superscripts indicatecalculations with the Wenzel (W) and the Cassie-Baxter (C)equations. Straight lines show the exact correlation between thetheoretical and the experimental values. Graphs A and B show

Figure 9. Comparison of the experimental and theoretical contact angles of microstructured polypropylene (A), microstructured polyethylene(B), dual-structured polypropylene (C), and dual-structured polyethylene (D) surfaces. Numbers refer to the mold inserts defined in Table1, and superscripts indicate calculations with the Wenzel or Cassie-Baxter equations.

r )[(a + b)2 + πaH]

(a + b)2(3)

φs ) πa2

4(a+ b)2(4)

Table 2. Results of the Contact Angle Measurements ofPolypropylene Surfaces

molda

(µm)b

(µm)H

(µm)θE

(deg) rθW

(deg) φs

θCB

(deg)SA

(deg)

flat 103( 1 >45µ2 17 8 5 104( 5 1.4 109 0.36 136 >45µ4 14 11 12 140( 4 1.8 115 0.25 144µ5 14 13 15 148( 4 1.9 115 0.21 147µ6 15 25 16 153( 9 1.5 109 0.11 156 ∼10µ7 15 30 25 156( 3 1.6 111 0.09 159 ∼10µ8 17 28 27 152( 3 1.7 113 0.11 156 ∼10nano 136( 14 >45a_µ1 17 5 5 146( 8 ∞ 150 ∼10a_µ2 17 8 5 153( 9 ∞ 154 ∼10a_µ3 17 23 5 152( 9 147 164a_µ6 15 23 18 164( 5 ∞ 165 ∼2.5

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the results for microstructured surfaces and graphs C and D theresults for dual-structured surfaces.

A relationship between the surface structure and the contactangle is evident, and the experimental values follow well eitherthe Wenzel or the Cassie-Baxter equation. The distance andheight of the microstructures must be adjusted precisely to achievethe Cassie-Baxter state, where the water droplet stays on themicrostructure. If the structure is too low or wide, the dropletfollows it and the Wenzel state is presented, as for the surfacesmade with mold insertsµ2 and anodizedµ3. When the optimizedmicrostructure is covered with nanostructure, the static contactangle between polypropylene and water reaches a value of about165°, the sliding angle decreases to about 2.5°, and thesuperhydrophobic state is achieved. The best recorded value ofthe contact angle was over 170° with virtually zero sliding angle.

4. Conclusions

Polyolefin surfaces were simultaneously micro- and nano-structured. Structuring was done by injection molding, wheremicrostructured aluminum and dual-structured AAO moldinserts were used to pattern surfaces. Structuring had a markedeffect on the contact angle between the polyolefins and water.The surface microstructure can be tailored by adjusting thesettings of a micro working robot and the nanostructure byadjusting the parameters of the anodization procedure. Theheight of the microstructures and distance between them mustbe adjusted precisely to achieve the Cassie-Baxter state, wherethe water droplet stays on the microstructure. When the optimizedmicrostructure was covered with nanostructure, the staticcontact angle between polypropylene and water obtained avalue of about 165° and the sliding angle decreased to about2.5°. The superhydrophobic state was achieved. We concludefrom these findings that our method allows the fabrication ofinjection-molded plastic components with superhydrophobicproperties.

Acknowledgment. We acknowledge Hanna-Kaisa Koponenfor her expertise in AAO fabrication and Janne Hirvi for hisadvice in the theoretical analysis. Borealis is gratefully ac-knowledged for their donation of materials.

LA063588H

Table 3. Results of the Contact Angle Measurements ofPolyethylene Surfaces

molda

(µm)b

(µm)H

(µm)θE

(deg) rθW

(deg) φs

θCB

(deg)

flat 99 ( 1µ2 17 8 2 100( 1 1.2 101 0.36 134µ4 20 5 5 114( 4 1.5 104 0.50 125µ5 20 8 7 128( 5 1.6 104 0.40 132nano 135( 24a_µ2 17 8 5 151( 6 ∞ 153a_µ3 17 23 5 137( 16 146 163

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