- Email: [email protected]
Facile preparation of superhydrophobic polymer surfaces
Polymer 53 (2012) 1180e1188
Contents lists available at SciVerse ScienceDirect
Polymer journal homepage: www.elsevier.com/locate/polymer
Facile preparation of superhydrophobic polymer surfaces Iskender Yilgor*, Sevilay Bilgin, Mehmet Isik, Emel Yilgor Surface Science and Technology Center (KUYTAM), Chemistry Department, Koc University, Istanbul 34450, Turkey
a r t i c l e i n f o
a b s t r a c t
Article history: Received 6 January 2012 Received in revised form 24 January 2012 Accepted 27 January 2012 Available online 1 February 2012
A simple and general method has been developed for the preparation of polymeric materials with superhydrophobic surfaces. The process is applicable to a large number of polymers, thermoplastic or thermoset. In this manuscript preparation and characteristics of superhydrophobic surfaces prepared from a segmented polydimethylsiloxaneeurea copolymer (TPSU), a polyether based polyurethaneurea (TPU), poly(methyl methacrylate) (PMMA), polystyrene (PS), polycarbonate (PC) and a crosslinked epoxy resin (EPOXY) are discussed. All samples were prepared onto glass surfaces by using a simple, multi-step spin-coating procedure. In the first step a thin film of the desired polymer was coated onto the glass slide. This was followed by spin-coating of two layers of hydrophobic fumed silica using a dispersion in tetrahydrofuran. Finally to obtain a durable surface, a very thin film of the parent polymer was spincoated from a very dilute solution containing 2.5% by weight hydrophobic silica and 0.25% by weight matrix polymer in tetrahydrofuran. Surfaces were characterized by scanning electron microscopy (SEM), which clearly showed the formation of rough surfaces with homogeneously distributed silica particles in 1e10 mm range. Static water contact angle and contact angle hysteresis measurements proved the formation of superhydrophobic surfaces. Samples displayed static water contact angles larger than 170 and very low contact angle hysteresis of less than 3 . Ó 2012 Elsevier Ltd. All rights reserved.
Keywords: Superhydrophobic Spin-coating Silica
1. Introduction Preparation and characterization of materials possessing superhydrophobic surfaces with static water contact angles greater than 150 and contact angle hysteresis smaller than 10 , has received dramatic attention during the past 10 years [1e6]. Materials with superhydrophobic surfaces are of interest both in terms of basic scientific research, but also due to their potential technological applications, which include, self-cleaning surfaces, antifouling or foul-release coatings, improved biocompatibility, stain resistant textiles, non-icing or ice repellant surfaces, microfluidics, etc. [1]. It is well documented that hydrophobic surfaces (e. g. highly fluorinated or dimethylsiloxane containing systems) that display roughness in micrometer and/or nanometer scale display superhydrophobicity, which is also called the Lotus Effect, since the surface structure of these materials mimics that of the lotus leaf [7,8]. Therefore, in addition to their surface chemical compositions, topography or the roughness of the polymer surfaces also have a significant effect on their hydrophobicity. A smooth, hydrophobic polymer surface, such as crosslinked silicone rubber, displays a static water contact angle (q) above 90 degrees as schematically shown in Fig. 1a. On the other hand if the surface * Corresponding author. Tel.: þ90 2123381418; fax: þ90 2123381559. E-mail address: [email protected] (I. Yilgor). 0032-3861/$ e see front matter Ó 2012 Elsevier Ltd. All rights reserved. doi:10.1016/j.polymer.2012.01.053
topography of the same sample displays micro-roughness, it usually displays a much higher water contact angle (Fig. 1b). This is due to the substantial reduction in the contact area between the water droplet and the rough hydrophobic surface as a result of the air entrapment within the rough surface microstructure of the polymer. For an ideal smooth surface, the contact angle is determined by the Young’s equation;
gSG gSL ¼ gLG cos q
(1)
where gSG, gSL and gLG are the interfacial free energies (or surface tensions) of solidegas, solideliquid and liquidegas interfaces respectively and (q) is the contact angle. For rough surfaces, Wenzel modified Young’s equation by incorporating a roughness factor (r) to predict the contact angle of a rough surface (qW) [9]. In Wenzel’s approach, the interaction between the liquid droplet and the solid surface is strong and leads to complete wetting, that is; the liquid droplet completely fills the grooves on a rough surface. cos qW ¼ r cos q
(2)
Roughness factor (r) is defined as the ratio of the actual area of a rough surface to the projected geometric area and therefore its value is always greater than 1. It is pointed out that after a critical value of (r) contact angle continues to increase while hysteresis starts to decrease
I. Yilgor et al. / Polymer 53 (2012) 1180e1188
1181
Fig. 1. Schematic representation of the contact angle (q) behavior of a water droplet on the surface of a hydrophobic polymer film (a) smooth surface and (b) surface that has micro-roughness.
leading to a slippery surface [10]. After this value the non-wetted fraction of the solid interface increases, leading to superhydrophobic behavior, which is described by the CassieeBaxter relationship [1,11]. The apparent contact angle (qCB) is given by a weighted average of the cosines of the contact angles on the solid and air surfaces (3), where f is the fraction of the surface on top of the protrusions, (1 f) is the fraction of air pockets and (qg) is the contact angle on the air in the valleys, which is taken to be 180 , leading to Eq. (4): cos qCB ¼ f cos q þ (1 f) cos qg
(3)
cos qCB ¼ f cos q þ f 1
(4)
Combining CassieeBaxter equation (Eq. (4)), with the Wenzel relationship (Eq. (2)), a more general equation is obtained for the apparent contact angles measured on a rough surface (qR). cos qR ¼ r f cos q þ f 1
(5)
Increased surface roughness will therefore affect the liquid contact angle, leading to much higher contact angles for systems which give contact angles >90 on flat surfaces. Several publications, which have used these equations or slightly modified versions to explain the contact angle behavior of rough surfaces have also appeared [12e14]. During the last decade there have been very intense efforts on developing new methods and processes for the preparation of superhydrophobic surfaces that display micro/nanoroughness by employing various techniques, which include; electrospinning [15e19], microphase separation [20e22], etching [23e26], spincoating or dip-coating [1,27e30], sol-gel techniques [31e33], templating [34e37], spraying [38] and others [1,39]. Most of these approaches usually involve many steps and are generally fairly tedious processes and/or may only be applied to specific polymers [40]. In this study, we developed a simple spin-coating method which can be applied to a wide range of polymers, thermoplastic or thermoset. Using this approach we spin-coated commercially available hydrophobic fumed silica on various polymeric substrates, including a polydimethylsiloxaneeurea copolymer (PSU), a polyether based polyurethaneurea (TPU), poly(methyl methacrylate) (PMMA), polystyrene (PS), polycarbonate (PC) and a crosslinked epoxy resin (EPOXY) and superhydrophobic surfaces with static water contact angles greater than 170 and very low contact angle hysteresis of less than 3 were produced.
reported to be 5e30 nm, which increases to 100e250 nm after aggregation [41]. The specific surface area is reported to be 150 m2/g [41]. Poly(methyl methacrylate) (PMMA) (Mn ¼ 46,000, Mw ¼ 93,000 g/mol) and polystyrene (PS) (Mn ¼ 140,000, Mw ¼ 230,000 g/mol) were obtained from Aldrich. Poly(tetramethylene oxide)glycol (PTMO-2000) based polyurethaneurea (TPU) with a hard segment content of 20% by weight was also synthesized in our laboratories [42]. Polycarbonate (PC) (Makrolon 2858) was a product of BayerMaterials. Segmented, thermoplastic polydimethylsiloxaneeurea copolymer (Geniomer 140) (PSU), with a PDMS content of about 92% by weight, was kindly provided by Wacker Chemie [43]. Bisphenol-A based epoxy resin (DER 331) with an epoxy equivalent of 190 g was provided by Dow Chemical. Amine terminated poly(propylene oxide) curing agent (Jeffamine D400) with an amine equivalent weight of 224 g was kindly supplied by Huntsman Chemicals [44]. Reagent grade solvents, toluene (TOL), isopropyl alcohol (IPA), methylene chloride (MC) and tetrahydrofuran (THF) were purchased from Merck and were used as received. 2.2. General procedure used for sample preparation All samples were prepared by spin-coating onto glass slides (20 20 0.15 mm) following a multi-step procedure as
2. Experimental 2.1. Materials Hydrophobic silica HDK H2000 (H2K) was kindly provided by Wacker Chemie, Munich, Germany. Primary particle size for silica is
Fig. 2. Schematic description of the method used for the preparation of polymers with superhydrophobic surfaces. (a) Glass slide, (b) base polymer film, (c) two layers of silica coating and (d) three layers of silica coating with top polymer film.
1182
I. Yilgor et al. / Polymer 53 (2012) 1180e1188
schematically depicted in Fig. 2. It is also possible to use unsupported polymer films. Glass surfaces were cleaned by wiping with THF and IPA successively several times before the coating is applied. Specific coating procedure was as follows: Parent polymer was dissolved in a solvent (or solvent mixture) to make a solution with a concentration of 15e25% solids by weight. Silica dispersions used in the coating process were prepared in THF with a concentration of 2.5% by weight and were sonicated for 3 h at room temperature prior to use. Dynamic light scattering (DLS) measurements on hydrophobic silica suspensions in THF indicated fairly homogeneous distribution of the nanoparticles, with a number average size distribution of 40.0 9.0 nm, which is in good agreement with the supplier’s specifications. 3 drops of polymer solution was placed onto the glass slide and spin-coated at 1000 rpm for about 70 s. This resulted in a polymer film thickness of about 20e30 mm depending on the concentration of the polymer solution used (Table 1). Immediately afterwards 3 drops of silica dispersion was placed onto the film and spin-coated after waiting for 1 min (to allow penetration of silica into the polymer matrix). This step was repeated in order to form a denser silica layer. To obtain a durable coating, a final top coating was also applied, which contained 2.5% by weight silica and 0.25% by weight of the parent polymer in THF solution. All samples were dried at 50 C under vacuum and kept in a desiccator until further testing. For crosslinked epoxy samples, stoichiometric amounts of epoxy resin and curing agent were thoroughly mixed and a thin film was applied onto the glass substrate using a doctor blade. Sample was partially cured at 60 C in air oven for 16 h. Two successive layers of silica (from 2.5% by weight dispersion in THF) were applied onto the partially cured sample by spin-coating, followed by a final layer of coating, which contained 2.5% by weight silica and 0.25% by weight of the (epoxy þ curing agent) mixture in THF. Coated sample was cured in 100 C air oven for 16 h. Table 1 provides detailed description of the samples prepared, solvents used and concentrations of the coating solutions.
2.3. Characterization methods Static, advancing and receding water contact angle measurements were performed on a Krüss G-10 goniometer, fitted with a high resolution digital camera (Spot Insight Color, by Diagnostic Instruments, Inc.) at room temperature (23 1 C). For static contact angle studies 10 mL deionized, triple distilled water was used. Average of 10 readings from different locations on the surface was reported as the contact angle for each sample. Contact angle hysteresis measurements were conducted by dynamic sessile drop method. A total of 10 mL water drop was dispensed from the syringe tip to touch the surface and the volume of the sessile drop was increased at a rate of 0.2 mL/s up to 50 mL. The largest possible angle was taken as the advancing water contact angle value. Then, volume of water drop was decreased at a rate of 0.1 mL/s and receding angle was taken as the lowest contact angle value at when the contact line between water and surface started to decrease with a satisfactory drop shape. Table 1 Polymers, solvents and concentrations of polymer solutions used for sample preparation. Code
Polymer description
Solvent
Conc. (wt %)
TPSU TPU PMMA PS PC EPOXY
Thermoplastic siliconeeurea copolymer Polyether based polyurethaneurea Poly(methyl methacrylate) Polystyrene Bisphenol-A polycarbonate Bisphenol-A epoxy þ polyamine
IPA THF/IPA (1/2) THF/TOL (5/2) THF/TOL (5/1) MC/THF (6/1) None
25 20 15 20 15 Neat
Surface structures of the samples were examined using a scanning electron microscope (SEM) (Zeiss EVO LS 10) and field-emission scanning electron microscope (FESEM) (Zeiss Ultra Plus Scanning Electron Microscope) operated at 2e10 kV. The spin-coated films were coated with a thin layer of gold (approx. 2.2 nm) prior to SEM examinations to prevent/minimize charging. Dynamic Light Scattering (DLS) measurements on silica suspensions were performed on Malvern ZetaSizer Nano-S Instrument with the software Nano-S. Glass cuvettes with square aperture were used as sample holders. 3. Results and discussion Preparation and characterization of polymeric materials with superhydrophobic surfaces has received widespread attention during the past 10 years [1e6]. A search with the keyword “superhydrophobic” for the decade covering 1990e1999 in the Web of Science database produced only 9 references. Same search for the years 2000e2009 resulted in about 1500 references, indicating an incredible 167-fold increase. Even more impressive was the same search for 2010e2011 period (only 17 months), which produced 900 references. As already discussed in the introduction section, numerous processes and methods have been developed for the preparation of superhydrophobic polymer surfaces [1,15e40]. Most of these methods included several, fairly tedious steps and were generally applicable to specific polymers. In this study we developed a simple spin-coating process for the preparation of superhydrophobic polymeric surfaces. The method is applicable to both thermoplastic and thermosetting polymers and leads to the formation of durable surfaces. To prepare materials with a superhydrophobic surface, a thermoplastic polymer is dissolved in a suitable solvent and spin-coated on a substrate, such as glass. Hydrophobic silica is separately suspended in THF by ultrasonication and spin-coated onto the polymer film. Since THF is a good solvent for all base polymers, it dissolves/swells the surface layer of the polymers and helps silica particles to penetrate and get firmly embedded into the polymer matrix, to form a robust surface, as clearly shown by SEM studies, discussed below. Finally, to obtain a permanent surface topography, a top layer was spin-coated, which contained silica in a very dilute polymer solution in THF. To demonstrate the applicability of the process to a wide range of polymers, thermoplastic or thermoset, superhydrophobic surfaces were prepared by using a polydimethylsiloxaneeurea copolymer (TPSU), a polyether based polyurethaneurea (TPU), poly(methyl methacrylate) (PMMA), polystyrene (PS), bisphenol-A polycarbonate (PC) and an amine cured epoxy resin (EPOXY) . 3.1. SEM studies on surface structure and morphology As already discussed, hydrophobic surfaces (such as a highly fluorinated or PDMS containing surface) that display roughness in micrometer scale are superhydrophobic [1e13]. In order to obtain superhydrophobic behavior it is reported that: (i) surface roughness is more important than low surface energy, and (ii) a hierarchical structure with microscale and nanoscale roughness is beneficial but not necessary [45]. To investigate the surface morphology and roughness of the samples prepared by our process and their effect on the superhydrophobic behavior, we performed extensive SEM studies at various stages of sample preparation. Fig. 3 shows the SEM images of polystyrene (PS) surface spin-coated on a glass substrate with a thickness of approximately 30 mm and surfaces obtained after coating this thin PS film with hydrophobic silica. As can be seen in Fig. 3a, the surface of the spin-coated PS film is very smooth and featureless,
I. Yilgor et al. / Polymer 53 (2012) 1180e1188
1183
Fig. 3. SEM images of PS at various stages of sample preparation. (a) Uncoated PS, (b) one layer of silica coated sample, (c) two layers of silica coated sample, (d) three layers of silica and PS coated sample (1000).
which is expected. After one layer of silica coating the surface displays micro-roughness with a fairly homogeneous distribution of silica particles in 1e5 mm range with some particles as large as 10 mm in size (Fig. 3b). After two layers of silica coating (Fig. 3c)
surface concentration of silica particles increases without any noticeable change in the particle size. After the final coating with PS containing silica solution, a much denser silica layer with a fairly homogeneous distribution of particles is observed
Fig. 4. SEM images of silica coated PS. (a) Two layers of silica coated sample, (b) three layers of silica and PS coated sample, (c) two layers of silica coated sample at 45 tilt angle, (d) three layers of silica and PS coated sample at 45 tilt angle (all at 3000X).
1184
I. Yilgor et al. / Polymer 53 (2012) 1180e1188
(Fig. 3d). Roughness of the surface also increases without any remarkable change in the particle size after the final coating process. Application of the top PS layer acts as a protective coating for the silica and leads to the formation of a durable surface. This can be better seen in the SEM images obtained at higher magnifications and at 45 tilt angle, as shown in Fig. 4. After two layers of coating, silica particles are firmly imbedded in the PS matrix, as shown in Fig. 4a and c. On the other hand as shown in Fig. 4b and d, after the application of a third layer of silica in a dilute PS solution, complete surface coverage of the silica particles by a fine layer of PS film can clearly be seen.
Polymeric surfaces prepared by spin-coating of silica on TPSU, PMMA, PC and TPU displayed similar surface topography and roughness, as observed in the surface SEM images of silica coated PS films discussed above. Comparative SEM images of the surfaces of these materials (TPSU, PMMA, PC and TPU) after two layers of silica and three layers of silica plus polymer coatings are provided in Fig. 5. After the final coating with polymer, the surface topography does not change significantly, however, silica particles present on the surface of the polymer film are covered by a thin polymer film to generate durable surfaces, as can be seen in Fig. 5b. SEM images of the silica coated, crosslinked epoxy resin samples which display superhydrophobic surfaces are reproduced in Fig. 6.
Fig. 5. SEM images of silica coated TPSU, PMMA, PC and TPU surfaces. (a) Two layers of silica coated sample, (b) three layers of silica and polymer coated sample (1000).
I. Yilgor et al. / Polymer 53 (2012) 1180e1188
1185
Fig. 6. Surface SEM images of coated epoxy resin (EPOXY) surfaces. (a) Two layers of silica coated sample, (b) three layers of silica and polymer coated sample (1000).
In these materials also after coating the silica surfaces with a thin layer of epoxy resin produces durable surfaces. These results are similar to superhydrophobic surfaces obtained by spraying the substrate with a mixture of micro/nano particles and polymer in an organic solvent [37]. 3.2. Static water contact angle measurements SEM images clearly demonstrated the formation of surfaces with micro-roughness through spin-coating of hydrophobic silica on various thermoplastic or thermoset polymers. The ultimate test to prove the formation of superhydrophobic surfaces is the static water contact angle and contact angle hysteresis measurements. Contact angle measurements were performed at room temperature (23 1 C). Using the simple spin-coating process, for all polymeric samples investigated (thermoplastic or thermoset) we were able to obtain superhydrophobic surfaces with very high static water contact angle values (well above 150 ) and in most cases above 170 . In addition, as will be discussed later on in the manuscript, the samples also displayed very low contact angle hysteresis values of less than 3 . Fig. 7 shows the images of the water droplets on the TPSU surfaces at various stages of the coating process. Virgin TPSU, which is composed of about 92% by weight of polydimethylsiloxane (PDMS), shows a fairly hydrophobic surface with a static water contact angle of 107.7 2.1 as shown in Fig. 7a. After coating with two layers of hydrophobic silica, the contact angle dramatically increases to 170.5 1.6 (Fig. 7b), clearly showing the formation of a superhydrophobic surface. After the final coating with silica dispersion containing very small amount of TPSU, contact angle reaches to 172.6 1.2 (Fig. 7c).
Very similar static water contact angle behaviors were observed on the hydrophobic silica coated thermoplastic polymer films based on PS, PMMA, TPU and PC as reproduced in Fig. 8. As can clearly be seen in Fig. 8, depending on their chemical structures all uncoated polymers display static water contact angles below 90 , PMMA displaying the smallest contact angle of 65.9 1.5 or indicating the most hydrophilic surface. On the other hand when they are coated with two layers of hydrophobic silica all samples display superhydrophobic surface behavior with static water contact angles above 160 as shown on Table 2. Upon application of the third silica layer, which also contains a very small amount of the base polymer, it is possible to obtain durable surfaces, which are also superhydrophobic with static water contact angles greater than 170 as shown in Fig. 8b. Fig. 9 provides the images of the water droplets on the silica coated, crosslinked epoxy resin surfaces. Very similar to the thermoplastic polymers, we have demonstrated that by applying our fairly simple coating process it was possible to completely modify the surface of a reasonably hydrophilic crosslinked epoxy resin with a static water contact angle of 69.2 to a superhydrophobic surface with a water contact angle above 175 . Table 2 provides a summary of the results obtained on the static water contact angle measurements on all uncoated polymer surfaces and on surfaces after two layers and three layers of hydrophobic silica and a fairly thin layer of the parent polymer coating, which were denoted as “control”, “two layers H2K” and “three layers H2Kþpolymer” respectively. Each contact angle value reported is an average of at least 10 measurements. It is very interesting to note that an uncoated PMMA surface, which displays a reasonably hydrophilic character with a static water contact angle
Fig. 7. Images of static water contact angles on TPSU. (a) Base TPSU (107.7 2.1 ), (b) two layers of silica coated sample (170.5 1.6 ), (c) three layers of silica plus TPSU coated sample (172.6 1.2 ).
1186
I. Yilgor et al. / Polymer 53 (2012) 1180e1188
Fig. 8. Images of the water droplets on the silica coated PS, PMMA and TPU surfaces at various stages of the coating process. (a) Base polymer, (b) three layers of silica and polymer coated sample.
of 65.9 becomes superhydrophobic after two and three layers of silica coating, which displays static water contact angle values of 168.4 and 172.9 respectively. Very similar behaviors are observed for samples prepared using thermoplastic PS, PC, TPSU and TPU and thermosetting epoxy resin, as can easily be seen in Figs. 8 and 9, and Table 2.
Table 2 Static water contact angles obtained on the virgin polymers and hydrophobic silica coated polymer surfaces at room temperature. Polymer
PS PMMA TPU PC TPSU EPOXY
Static water contact angle ( ) Control
Two layers H2K
Three layers H2K þ polymer
89.3 1.6 65.9 1.5 84.4 1.5 89.3 0.8 107.7 2.1 69.2 1.75
168.9 2.1 168.4 2.4 161.9 1.5 161.7 3.7 170.5 1.6 175.3 0.8
173.7 0.5 172.9 1.2 170.8 1.7 164.6 1.9 172.6 1.2 174.8 0.7
3.3. Studies on the contact angle hysteresis A very important characteristics expected from a superhydrophobic surface is that it should also display very small contact angle hysteresis (CAH), usually <10 . Contact angle hysteresis is defined as the difference between the advancing and the receding contact angles. CAH is mainly due to the presence of numerous metastable states at the solid/liquid/vapor interface (due surface heterogeneity and roughness) while a liquid drop moves on the surface of a solid. CAH also signifies the amount of energy irreversibly dissipated during the flow of a liquid droplet on a surface. Images of the advancing (pictures a) and receding (pictures b) water contact angles for 3 layer silica plus polymer film coated PS, PMMA, TPU, TPSU, PC and EPOXY are provided in Fig. 10. Based on these measurements, contact angle hysteresis calculated for these materials are provided in Table 3. These values, which are measured to be between 0 and 3 are extremely small and are as good or better than the data reported in the literature for superhydrophobic surfaces [44].
I. Yilgor et al. / Polymer 53 (2012) 1180e1188
Fig. 9. Images of static water contact angles on cured epoxy resin. (a) Cured resin, (b) three layers of silica and epoxy resin coated sample.
Fig. 10. Images of the advancing (a) and receding (b) water contact angles for three-layer silica plus polymer coated PS, PMMA, TPU, TPSU, PC and EPOXY.
1187
1188
I. Yilgor et al. / Polymer 53 (2012) 1180e1188
Table 3 Water contact angle hysteresis obtained on three-layer silica plus polymer film coated PS, PMMA, TPU, TPSU, PC and EPOXY surfaces. Sample
PS
PMMA
TPU
TPSU
PC
EPOXY
CA hysteresis ( )
2
2
3
z0
2
3
4. Conclusions A simple and general process has been developed for the preparation of polymeric materials with superhydrophobic surfaces. Method is based on a fairly simple, multi-step spincoating process, where hydrophobic silica is coated onto a polymer surface from a dispersion in THF. Superhydrophobic polymer surfaces obtained, which display micro-roughness are robust and durable. It has been demonstrated that the process is applicable to a large number of polymers with different nature and chemical structure, thermoplastic or thermoset. To demonstrate the broad applicability of the method, in this study superhydrophobic polymer surfaces were prepared using a segmented polydimethylsiloxaneeurea copolymer, a polyether based polyurethaneurea copolymer, poly(methyl methacrylate), polystyrene, polycarbonate and a crosslinked epoxy resin. In all cases it was possible to obtain superhydrophobic surfaces with static water contact angles higher than 170 , which also displayed extremely low contact angle hysteresis of less than 3 . Although we only discussed a multi-step spin-coating process in this manuscript, we believe it is possible to obtain silica coated superhydrophobic polymer surfaces by using dipping method or employing doctorblade coating techniques. Acknowledgement Authors thank to Turkish Ministry of Development for the financial support provided for the establishment of Koc University Surface Science and Technology Center (KUYTAM). References [1] Roach P, Shirtcliffe NJ, Newton MI. Soft Matter 2008;4:224e40. [2] Blossey R. Nat Mater 2003;2:301e6. [3] Chen W, Fadeev AY, Hsieh MC, Oner D, Youngblood J, McCarthy TJ. Langmuir 1999;15:3395e9. [4] Callies M, Quere D. Soft Matter 2005;1:55e61.
[5] [6] [7] [8] [9] [10] [11] [12] [13] [14] [15] [16] [17] [18] [19] [20] [21] [22] [23] [24] [25] [26] [27] [28] [29] [30] [31] [32] [33] [34] [35] [36] [37] [38] [39] [40] [41] [42] [43] [44] [45]
Feng L, Li SH, Li YS, Li HJ, Zhang LJ, Zhai J, et al. Adv Mater 2002;14:1857e60. Sun TL, Feng L, Gao X, Jiang L. Acc Chem Res 2005;38:644e52. Barthlott W, Neinhuis C. Planta 1997;202(1):1e8. Neinhuis C, Barthlott W. Ann Bot London 1997;79:667e77. Wenzel RN. Ind Eng Chem 1936;28(8):988e94. Dettre RH, Johnson RE. In: Fowkes FM, editor. Contact angle, wettability and adhesion. Adv chem ser, vol. 43. Washington, DC.: ACS; 1964. p. 136e44. Cassie ABD, Baxter S. Trans Faraday Soc 1944;40:546e51. Onda T, Shibuichi S, Satoh N, Tsujii K. Langmuir 1996;12:2125e7. Extrand C. In: Somasundaran P, editor. Encyclopedia of surface and colloid science. 2nd ed. New York: Taylor and Francis; 2006. p. 5854e68. Erbil HY, Cansoy CE. Langmuir 2009;15(24):14,135e14,145. Ma M, Hill RM, Lowery JL, Fridrikh SV, Rutledge GC. Langmuir 2005;21: 5549e54. Agarwal S, Horst S, Bognitzki M. Macromol Mater Eng 2006;291:592e601. Ma M, Gupta M, Li Z, Zhai L, Gleason KK, Cohen RE. Adv Mater 2007;19:255e9. Ogawa T, Ding B, Sone Y, Shiratori S. Nanotechnology 2007;18:165,607. Acatay K, Simsek E, Ow-Yang C, Menceloglu YZ. Angew Chem Int Ed 2004;43: 5210e3. Shirtcliffe NJ, McHale G, Newton MI, Perry CC, Roach P. Mater Chem Phys 2007;103(1):112e7. Li XH, Chen G, Ma Y, Feng L, Zhao H. Polymer 2006;47:506e9. Han JT, Xu XR, Cho KW. Langmuir 2005;21(15):6662e5. Busscher HJ, Stokroos I, Golverdingen JG, Schakenraad JM. Cells Mater 1991; 1(3):243e9. Busscher HJ, Stokroos A, Vandermei HC, Rouxhet PG, Schakenraad JM. J Adhes Sci Technol 1992;6(3):347e56. Teshima K, Sugimura H, Inoue Y, Takai O, Takano A. Langmuir 2003;19(25): 10,624e10,627. Vourdas N, Tserepi A, Gogolides E. Nanotechnology 2007;18:125,304. Tsai PS, Yang YM, Lee YL. Langmuir 2006;22(13):5660e5. Shiu JY, Kuo CW, Chen PL, Mou CY. Chem Mater 2004;16:561e4. Zhu L, Jin Y. Appl Surf Sci 2007;253:3432e9. Wang JX, Wen YQ, Feng XJ, Song YL, Jiang L. Macromol Rapid Commun 2006; 27(3):188e92. Shirtcliffe NJ, McHale G, Newton MI, Perry CC. Langmuir 2003;19:5626e31. Hikita M, Tanaka K, Nakamura T, Kajiyama T, Takahara A. Langmuir 2005;21: 7299e302. Mahltig B, Bottcher HJ. Sol-Gel Sci Technol 2003;27:43e52. Bico J, Marzolin C, Quere D. Europhys. Lett 1999;47(2):220e6. Zhang L, Zhou Z, Cheng B, DeSimone JM, Samulski ET. Langmuir 2006;22: 8576e80. He B, Patankar NA, Lee J. Langmuir 2003;19:4999e5003. Sun H, Luo MCX, Xu LP, Ji H, Qi OY, Yu DP, et al. Langmuir 2005;21:8978e81. Manoudis PN, Karapanagiotis I, Tsakalof A, Zuburtikudis I, Panayiotou C. Langmuir 2008;24:11,225e11,232. Shirtcliffe NJ, McHale G, Atherton S, Newton MI. Adv Coll Interface Sci 2010; 161:124e38. Reick FG. US Patent 4,142,724; March 6, 1979. http://www.wacker.com/cms/media/publications/downloads/6180_EN.pdf. Yilgor I, Mather B, Unal S, Yilgor E, Long TE. Polymer 2004;45(17):5829e36. http://www.wacker.com/cms/en/products-markets/trademarks/geniomer/ geniomer.jsp. http://www.huntsman.com/performance_products/Media/JEFFAMINE_D400_US.pdf. Nosonovsky M, Bhushan B. Adv Func Mater 2008;18:843e55.
Contents lists available at SciVerse ScienceDirect
Polymer journal homepage: www.elsevier.com/locate/polymer
Facile preparation of superhydrophobic polymer surfaces Iskender Yilgor*, Sevilay Bilgin, Mehmet Isik, Emel Yilgor Surface Science and Technology Center (KUYTAM), Chemistry Department, Koc University, Istanbul 34450, Turkey
a r t i c l e i n f o
a b s t r a c t
Article history: Received 6 January 2012 Received in revised form 24 January 2012 Accepted 27 January 2012 Available online 1 February 2012
A simple and general method has been developed for the preparation of polymeric materials with superhydrophobic surfaces. The process is applicable to a large number of polymers, thermoplastic or thermoset. In this manuscript preparation and characteristics of superhydrophobic surfaces prepared from a segmented polydimethylsiloxaneeurea copolymer (TPSU), a polyether based polyurethaneurea (TPU), poly(methyl methacrylate) (PMMA), polystyrene (PS), polycarbonate (PC) and a crosslinked epoxy resin (EPOXY) are discussed. All samples were prepared onto glass surfaces by using a simple, multi-step spin-coating procedure. In the first step a thin film of the desired polymer was coated onto the glass slide. This was followed by spin-coating of two layers of hydrophobic fumed silica using a dispersion in tetrahydrofuran. Finally to obtain a durable surface, a very thin film of the parent polymer was spincoated from a very dilute solution containing 2.5% by weight hydrophobic silica and 0.25% by weight matrix polymer in tetrahydrofuran. Surfaces were characterized by scanning electron microscopy (SEM), which clearly showed the formation of rough surfaces with homogeneously distributed silica particles in 1e10 mm range. Static water contact angle and contact angle hysteresis measurements proved the formation of superhydrophobic surfaces. Samples displayed static water contact angles larger than 170 and very low contact angle hysteresis of less than 3 . Ó 2012 Elsevier Ltd. All rights reserved.
Keywords: Superhydrophobic Spin-coating Silica
1. Introduction Preparation and characterization of materials possessing superhydrophobic surfaces with static water contact angles greater than 150 and contact angle hysteresis smaller than 10 , has received dramatic attention during the past 10 years [1e6]. Materials with superhydrophobic surfaces are of interest both in terms of basic scientific research, but also due to their potential technological applications, which include, self-cleaning surfaces, antifouling or foul-release coatings, improved biocompatibility, stain resistant textiles, non-icing or ice repellant surfaces, microfluidics, etc. [1]. It is well documented that hydrophobic surfaces (e. g. highly fluorinated or dimethylsiloxane containing systems) that display roughness in micrometer and/or nanometer scale display superhydrophobicity, which is also called the Lotus Effect, since the surface structure of these materials mimics that of the lotus leaf [7,8]. Therefore, in addition to their surface chemical compositions, topography or the roughness of the polymer surfaces also have a significant effect on their hydrophobicity. A smooth, hydrophobic polymer surface, such as crosslinked silicone rubber, displays a static water contact angle (q) above 90 degrees as schematically shown in Fig. 1a. On the other hand if the surface * Corresponding author. Tel.: þ90 2123381418; fax: þ90 2123381559. E-mail address: [email protected] (I. Yilgor). 0032-3861/$ e see front matter Ó 2012 Elsevier Ltd. All rights reserved. doi:10.1016/j.polymer.2012.01.053
topography of the same sample displays micro-roughness, it usually displays a much higher water contact angle (Fig. 1b). This is due to the substantial reduction in the contact area between the water droplet and the rough hydrophobic surface as a result of the air entrapment within the rough surface microstructure of the polymer. For an ideal smooth surface, the contact angle is determined by the Young’s equation;
gSG gSL ¼ gLG cos q
(1)
where gSG, gSL and gLG are the interfacial free energies (or surface tensions) of solidegas, solideliquid and liquidegas interfaces respectively and (q) is the contact angle. For rough surfaces, Wenzel modified Young’s equation by incorporating a roughness factor (r) to predict the contact angle of a rough surface (qW) [9]. In Wenzel’s approach, the interaction between the liquid droplet and the solid surface is strong and leads to complete wetting, that is; the liquid droplet completely fills the grooves on a rough surface. cos qW ¼ r cos q
(2)
Roughness factor (r) is defined as the ratio of the actual area of a rough surface to the projected geometric area and therefore its value is always greater than 1. It is pointed out that after a critical value of (r) contact angle continues to increase while hysteresis starts to decrease
I. Yilgor et al. / Polymer 53 (2012) 1180e1188
1181
Fig. 1. Schematic representation of the contact angle (q) behavior of a water droplet on the surface of a hydrophobic polymer film (a) smooth surface and (b) surface that has micro-roughness.
leading to a slippery surface [10]. After this value the non-wetted fraction of the solid interface increases, leading to superhydrophobic behavior, which is described by the CassieeBaxter relationship [1,11]. The apparent contact angle (qCB) is given by a weighted average of the cosines of the contact angles on the solid and air surfaces (3), where f is the fraction of the surface on top of the protrusions, (1 f) is the fraction of air pockets and (qg) is the contact angle on the air in the valleys, which is taken to be 180 , leading to Eq. (4): cos qCB ¼ f cos q þ (1 f) cos qg
(3)
cos qCB ¼ f cos q þ f 1
(4)
Combining CassieeBaxter equation (Eq. (4)), with the Wenzel relationship (Eq. (2)), a more general equation is obtained for the apparent contact angles measured on a rough surface (qR). cos qR ¼ r f cos q þ f 1
(5)
Increased surface roughness will therefore affect the liquid contact angle, leading to much higher contact angles for systems which give contact angles >90 on flat surfaces. Several publications, which have used these equations or slightly modified versions to explain the contact angle behavior of rough surfaces have also appeared [12e14]. During the last decade there have been very intense efforts on developing new methods and processes for the preparation of superhydrophobic surfaces that display micro/nanoroughness by employing various techniques, which include; electrospinning [15e19], microphase separation [20e22], etching [23e26], spincoating or dip-coating [1,27e30], sol-gel techniques [31e33], templating [34e37], spraying [38] and others [1,39]. Most of these approaches usually involve many steps and are generally fairly tedious processes and/or may only be applied to specific polymers [40]. In this study, we developed a simple spin-coating method which can be applied to a wide range of polymers, thermoplastic or thermoset. Using this approach we spin-coated commercially available hydrophobic fumed silica on various polymeric substrates, including a polydimethylsiloxaneeurea copolymer (PSU), a polyether based polyurethaneurea (TPU), poly(methyl methacrylate) (PMMA), polystyrene (PS), polycarbonate (PC) and a crosslinked epoxy resin (EPOXY) and superhydrophobic surfaces with static water contact angles greater than 170 and very low contact angle hysteresis of less than 3 were produced.
reported to be 5e30 nm, which increases to 100e250 nm after aggregation [41]. The specific surface area is reported to be 150 m2/g [41]. Poly(methyl methacrylate) (PMMA) (Mn ¼ 46,000, Mw ¼ 93,000 g/mol) and polystyrene (PS) (Mn ¼ 140,000, Mw ¼ 230,000 g/mol) were obtained from Aldrich. Poly(tetramethylene oxide)glycol (PTMO-2000) based polyurethaneurea (TPU) with a hard segment content of 20% by weight was also synthesized in our laboratories [42]. Polycarbonate (PC) (Makrolon 2858) was a product of BayerMaterials. Segmented, thermoplastic polydimethylsiloxaneeurea copolymer (Geniomer 140) (PSU), with a PDMS content of about 92% by weight, was kindly provided by Wacker Chemie [43]. Bisphenol-A based epoxy resin (DER 331) with an epoxy equivalent of 190 g was provided by Dow Chemical. Amine terminated poly(propylene oxide) curing agent (Jeffamine D400) with an amine equivalent weight of 224 g was kindly supplied by Huntsman Chemicals [44]. Reagent grade solvents, toluene (TOL), isopropyl alcohol (IPA), methylene chloride (MC) and tetrahydrofuran (THF) were purchased from Merck and were used as received. 2.2. General procedure used for sample preparation All samples were prepared by spin-coating onto glass slides (20 20 0.15 mm) following a multi-step procedure as
2. Experimental 2.1. Materials Hydrophobic silica HDK H2000 (H2K) was kindly provided by Wacker Chemie, Munich, Germany. Primary particle size for silica is
Fig. 2. Schematic description of the method used for the preparation of polymers with superhydrophobic surfaces. (a) Glass slide, (b) base polymer film, (c) two layers of silica coating and (d) three layers of silica coating with top polymer film.
1182
I. Yilgor et al. / Polymer 53 (2012) 1180e1188
schematically depicted in Fig. 2. It is also possible to use unsupported polymer films. Glass surfaces were cleaned by wiping with THF and IPA successively several times before the coating is applied. Specific coating procedure was as follows: Parent polymer was dissolved in a solvent (or solvent mixture) to make a solution with a concentration of 15e25% solids by weight. Silica dispersions used in the coating process were prepared in THF with a concentration of 2.5% by weight and were sonicated for 3 h at room temperature prior to use. Dynamic light scattering (DLS) measurements on hydrophobic silica suspensions in THF indicated fairly homogeneous distribution of the nanoparticles, with a number average size distribution of 40.0 9.0 nm, which is in good agreement with the supplier’s specifications. 3 drops of polymer solution was placed onto the glass slide and spin-coated at 1000 rpm for about 70 s. This resulted in a polymer film thickness of about 20e30 mm depending on the concentration of the polymer solution used (Table 1). Immediately afterwards 3 drops of silica dispersion was placed onto the film and spin-coated after waiting for 1 min (to allow penetration of silica into the polymer matrix). This step was repeated in order to form a denser silica layer. To obtain a durable coating, a final top coating was also applied, which contained 2.5% by weight silica and 0.25% by weight of the parent polymer in THF solution. All samples were dried at 50 C under vacuum and kept in a desiccator until further testing. For crosslinked epoxy samples, stoichiometric amounts of epoxy resin and curing agent were thoroughly mixed and a thin film was applied onto the glass substrate using a doctor blade. Sample was partially cured at 60 C in air oven for 16 h. Two successive layers of silica (from 2.5% by weight dispersion in THF) were applied onto the partially cured sample by spin-coating, followed by a final layer of coating, which contained 2.5% by weight silica and 0.25% by weight of the (epoxy þ curing agent) mixture in THF. Coated sample was cured in 100 C air oven for 16 h. Table 1 provides detailed description of the samples prepared, solvents used and concentrations of the coating solutions.
2.3. Characterization methods Static, advancing and receding water contact angle measurements were performed on a Krüss G-10 goniometer, fitted with a high resolution digital camera (Spot Insight Color, by Diagnostic Instruments, Inc.) at room temperature (23 1 C). For static contact angle studies 10 mL deionized, triple distilled water was used. Average of 10 readings from different locations on the surface was reported as the contact angle for each sample. Contact angle hysteresis measurements were conducted by dynamic sessile drop method. A total of 10 mL water drop was dispensed from the syringe tip to touch the surface and the volume of the sessile drop was increased at a rate of 0.2 mL/s up to 50 mL. The largest possible angle was taken as the advancing water contact angle value. Then, volume of water drop was decreased at a rate of 0.1 mL/s and receding angle was taken as the lowest contact angle value at when the contact line between water and surface started to decrease with a satisfactory drop shape. Table 1 Polymers, solvents and concentrations of polymer solutions used for sample preparation. Code
Polymer description
Solvent
Conc. (wt %)
TPSU TPU PMMA PS PC EPOXY
Thermoplastic siliconeeurea copolymer Polyether based polyurethaneurea Poly(methyl methacrylate) Polystyrene Bisphenol-A polycarbonate Bisphenol-A epoxy þ polyamine
IPA THF/IPA (1/2) THF/TOL (5/2) THF/TOL (5/1) MC/THF (6/1) None
25 20 15 20 15 Neat
Surface structures of the samples were examined using a scanning electron microscope (SEM) (Zeiss EVO LS 10) and field-emission scanning electron microscope (FESEM) (Zeiss Ultra Plus Scanning Electron Microscope) operated at 2e10 kV. The spin-coated films were coated with a thin layer of gold (approx. 2.2 nm) prior to SEM examinations to prevent/minimize charging. Dynamic Light Scattering (DLS) measurements on silica suspensions were performed on Malvern ZetaSizer Nano-S Instrument with the software Nano-S. Glass cuvettes with square aperture were used as sample holders. 3. Results and discussion Preparation and characterization of polymeric materials with superhydrophobic surfaces has received widespread attention during the past 10 years [1e6]. A search with the keyword “superhydrophobic” for the decade covering 1990e1999 in the Web of Science database produced only 9 references. Same search for the years 2000e2009 resulted in about 1500 references, indicating an incredible 167-fold increase. Even more impressive was the same search for 2010e2011 period (only 17 months), which produced 900 references. As already discussed in the introduction section, numerous processes and methods have been developed for the preparation of superhydrophobic polymer surfaces [1,15e40]. Most of these methods included several, fairly tedious steps and were generally applicable to specific polymers. In this study we developed a simple spin-coating process for the preparation of superhydrophobic polymeric surfaces. The method is applicable to both thermoplastic and thermosetting polymers and leads to the formation of durable surfaces. To prepare materials with a superhydrophobic surface, a thermoplastic polymer is dissolved in a suitable solvent and spin-coated on a substrate, such as glass. Hydrophobic silica is separately suspended in THF by ultrasonication and spin-coated onto the polymer film. Since THF is a good solvent for all base polymers, it dissolves/swells the surface layer of the polymers and helps silica particles to penetrate and get firmly embedded into the polymer matrix, to form a robust surface, as clearly shown by SEM studies, discussed below. Finally, to obtain a permanent surface topography, a top layer was spin-coated, which contained silica in a very dilute polymer solution in THF. To demonstrate the applicability of the process to a wide range of polymers, thermoplastic or thermoset, superhydrophobic surfaces were prepared by using a polydimethylsiloxaneeurea copolymer (TPSU), a polyether based polyurethaneurea (TPU), poly(methyl methacrylate) (PMMA), polystyrene (PS), bisphenol-A polycarbonate (PC) and an amine cured epoxy resin (EPOXY) . 3.1. SEM studies on surface structure and morphology As already discussed, hydrophobic surfaces (such as a highly fluorinated or PDMS containing surface) that display roughness in micrometer scale are superhydrophobic [1e13]. In order to obtain superhydrophobic behavior it is reported that: (i) surface roughness is more important than low surface energy, and (ii) a hierarchical structure with microscale and nanoscale roughness is beneficial but not necessary [45]. To investigate the surface morphology and roughness of the samples prepared by our process and their effect on the superhydrophobic behavior, we performed extensive SEM studies at various stages of sample preparation. Fig. 3 shows the SEM images of polystyrene (PS) surface spin-coated on a glass substrate with a thickness of approximately 30 mm and surfaces obtained after coating this thin PS film with hydrophobic silica. As can be seen in Fig. 3a, the surface of the spin-coated PS film is very smooth and featureless,
I. Yilgor et al. / Polymer 53 (2012) 1180e1188
1183
Fig. 3. SEM images of PS at various stages of sample preparation. (a) Uncoated PS, (b) one layer of silica coated sample, (c) two layers of silica coated sample, (d) three layers of silica and PS coated sample (1000).
which is expected. After one layer of silica coating the surface displays micro-roughness with a fairly homogeneous distribution of silica particles in 1e5 mm range with some particles as large as 10 mm in size (Fig. 3b). After two layers of silica coating (Fig. 3c)
surface concentration of silica particles increases without any noticeable change in the particle size. After the final coating with PS containing silica solution, a much denser silica layer with a fairly homogeneous distribution of particles is observed
Fig. 4. SEM images of silica coated PS. (a) Two layers of silica coated sample, (b) three layers of silica and PS coated sample, (c) two layers of silica coated sample at 45 tilt angle, (d) three layers of silica and PS coated sample at 45 tilt angle (all at 3000X).
1184
I. Yilgor et al. / Polymer 53 (2012) 1180e1188
(Fig. 3d). Roughness of the surface also increases without any remarkable change in the particle size after the final coating process. Application of the top PS layer acts as a protective coating for the silica and leads to the formation of a durable surface. This can be better seen in the SEM images obtained at higher magnifications and at 45 tilt angle, as shown in Fig. 4. After two layers of coating, silica particles are firmly imbedded in the PS matrix, as shown in Fig. 4a and c. On the other hand as shown in Fig. 4b and d, after the application of a third layer of silica in a dilute PS solution, complete surface coverage of the silica particles by a fine layer of PS film can clearly be seen.
Polymeric surfaces prepared by spin-coating of silica on TPSU, PMMA, PC and TPU displayed similar surface topography and roughness, as observed in the surface SEM images of silica coated PS films discussed above. Comparative SEM images of the surfaces of these materials (TPSU, PMMA, PC and TPU) after two layers of silica and three layers of silica plus polymer coatings are provided in Fig. 5. After the final coating with polymer, the surface topography does not change significantly, however, silica particles present on the surface of the polymer film are covered by a thin polymer film to generate durable surfaces, as can be seen in Fig. 5b. SEM images of the silica coated, crosslinked epoxy resin samples which display superhydrophobic surfaces are reproduced in Fig. 6.
Fig. 5. SEM images of silica coated TPSU, PMMA, PC and TPU surfaces. (a) Two layers of silica coated sample, (b) three layers of silica and polymer coated sample (1000).
I. Yilgor et al. / Polymer 53 (2012) 1180e1188
1185
Fig. 6. Surface SEM images of coated epoxy resin (EPOXY) surfaces. (a) Two layers of silica coated sample, (b) three layers of silica and polymer coated sample (1000).
In these materials also after coating the silica surfaces with a thin layer of epoxy resin produces durable surfaces. These results are similar to superhydrophobic surfaces obtained by spraying the substrate with a mixture of micro/nano particles and polymer in an organic solvent [37]. 3.2. Static water contact angle measurements SEM images clearly demonstrated the formation of surfaces with micro-roughness through spin-coating of hydrophobic silica on various thermoplastic or thermoset polymers. The ultimate test to prove the formation of superhydrophobic surfaces is the static water contact angle and contact angle hysteresis measurements. Contact angle measurements were performed at room temperature (23 1 C). Using the simple spin-coating process, for all polymeric samples investigated (thermoplastic or thermoset) we were able to obtain superhydrophobic surfaces with very high static water contact angle values (well above 150 ) and in most cases above 170 . In addition, as will be discussed later on in the manuscript, the samples also displayed very low contact angle hysteresis values of less than 3 . Fig. 7 shows the images of the water droplets on the TPSU surfaces at various stages of the coating process. Virgin TPSU, which is composed of about 92% by weight of polydimethylsiloxane (PDMS), shows a fairly hydrophobic surface with a static water contact angle of 107.7 2.1 as shown in Fig. 7a. After coating with two layers of hydrophobic silica, the contact angle dramatically increases to 170.5 1.6 (Fig. 7b), clearly showing the formation of a superhydrophobic surface. After the final coating with silica dispersion containing very small amount of TPSU, contact angle reaches to 172.6 1.2 (Fig. 7c).
Very similar static water contact angle behaviors were observed on the hydrophobic silica coated thermoplastic polymer films based on PS, PMMA, TPU and PC as reproduced in Fig. 8. As can clearly be seen in Fig. 8, depending on their chemical structures all uncoated polymers display static water contact angles below 90 , PMMA displaying the smallest contact angle of 65.9 1.5 or indicating the most hydrophilic surface. On the other hand when they are coated with two layers of hydrophobic silica all samples display superhydrophobic surface behavior with static water contact angles above 160 as shown on Table 2. Upon application of the third silica layer, which also contains a very small amount of the base polymer, it is possible to obtain durable surfaces, which are also superhydrophobic with static water contact angles greater than 170 as shown in Fig. 8b. Fig. 9 provides the images of the water droplets on the silica coated, crosslinked epoxy resin surfaces. Very similar to the thermoplastic polymers, we have demonstrated that by applying our fairly simple coating process it was possible to completely modify the surface of a reasonably hydrophilic crosslinked epoxy resin with a static water contact angle of 69.2 to a superhydrophobic surface with a water contact angle above 175 . Table 2 provides a summary of the results obtained on the static water contact angle measurements on all uncoated polymer surfaces and on surfaces after two layers and three layers of hydrophobic silica and a fairly thin layer of the parent polymer coating, which were denoted as “control”, “two layers H2K” and “three layers H2Kþpolymer” respectively. Each contact angle value reported is an average of at least 10 measurements. It is very interesting to note that an uncoated PMMA surface, which displays a reasonably hydrophilic character with a static water contact angle
Fig. 7. Images of static water contact angles on TPSU. (a) Base TPSU (107.7 2.1 ), (b) two layers of silica coated sample (170.5 1.6 ), (c) three layers of silica plus TPSU coated sample (172.6 1.2 ).
1186
I. Yilgor et al. / Polymer 53 (2012) 1180e1188
Fig. 8. Images of the water droplets on the silica coated PS, PMMA and TPU surfaces at various stages of the coating process. (a) Base polymer, (b) three layers of silica and polymer coated sample.
of 65.9 becomes superhydrophobic after two and three layers of silica coating, which displays static water contact angle values of 168.4 and 172.9 respectively. Very similar behaviors are observed for samples prepared using thermoplastic PS, PC, TPSU and TPU and thermosetting epoxy resin, as can easily be seen in Figs. 8 and 9, and Table 2.
Table 2 Static water contact angles obtained on the virgin polymers and hydrophobic silica coated polymer surfaces at room temperature. Polymer
PS PMMA TPU PC TPSU EPOXY
Static water contact angle ( ) Control
Two layers H2K
Three layers H2K þ polymer
89.3 1.6 65.9 1.5 84.4 1.5 89.3 0.8 107.7 2.1 69.2 1.75
168.9 2.1 168.4 2.4 161.9 1.5 161.7 3.7 170.5 1.6 175.3 0.8
173.7 0.5 172.9 1.2 170.8 1.7 164.6 1.9 172.6 1.2 174.8 0.7
3.3. Studies on the contact angle hysteresis A very important characteristics expected from a superhydrophobic surface is that it should also display very small contact angle hysteresis (CAH), usually <10 . Contact angle hysteresis is defined as the difference between the advancing and the receding contact angles. CAH is mainly due to the presence of numerous metastable states at the solid/liquid/vapor interface (due surface heterogeneity and roughness) while a liquid drop moves on the surface of a solid. CAH also signifies the amount of energy irreversibly dissipated during the flow of a liquid droplet on a surface. Images of the advancing (pictures a) and receding (pictures b) water contact angles for 3 layer silica plus polymer film coated PS, PMMA, TPU, TPSU, PC and EPOXY are provided in Fig. 10. Based on these measurements, contact angle hysteresis calculated for these materials are provided in Table 3. These values, which are measured to be between 0 and 3 are extremely small and are as good or better than the data reported in the literature for superhydrophobic surfaces [44].
I. Yilgor et al. / Polymer 53 (2012) 1180e1188
Fig. 9. Images of static water contact angles on cured epoxy resin. (a) Cured resin, (b) three layers of silica and epoxy resin coated sample.
Fig. 10. Images of the advancing (a) and receding (b) water contact angles for three-layer silica plus polymer coated PS, PMMA, TPU, TPSU, PC and EPOXY.
1187
1188
I. Yilgor et al. / Polymer 53 (2012) 1180e1188
Table 3 Water contact angle hysteresis obtained on three-layer silica plus polymer film coated PS, PMMA, TPU, TPSU, PC and EPOXY surfaces. Sample
PS
PMMA
TPU
TPSU
PC
EPOXY
CA hysteresis ( )
2
2
3
z0
2
3
4. Conclusions A simple and general process has been developed for the preparation of polymeric materials with superhydrophobic surfaces. Method is based on a fairly simple, multi-step spincoating process, where hydrophobic silica is coated onto a polymer surface from a dispersion in THF. Superhydrophobic polymer surfaces obtained, which display micro-roughness are robust and durable. It has been demonstrated that the process is applicable to a large number of polymers with different nature and chemical structure, thermoplastic or thermoset. To demonstrate the broad applicability of the method, in this study superhydrophobic polymer surfaces were prepared using a segmented polydimethylsiloxaneeurea copolymer, a polyether based polyurethaneurea copolymer, poly(methyl methacrylate), polystyrene, polycarbonate and a crosslinked epoxy resin. In all cases it was possible to obtain superhydrophobic surfaces with static water contact angles higher than 170 , which also displayed extremely low contact angle hysteresis of less than 3 . Although we only discussed a multi-step spin-coating process in this manuscript, we believe it is possible to obtain silica coated superhydrophobic polymer surfaces by using dipping method or employing doctorblade coating techniques. Acknowledgement Authors thank to Turkish Ministry of Development for the financial support provided for the establishment of Koc University Surface Science and Technology Center (KUYTAM). References [1] Roach P, Shirtcliffe NJ, Newton MI. Soft Matter 2008;4:224e40. [2] Blossey R. Nat Mater 2003;2:301e6. [3] Chen W, Fadeev AY, Hsieh MC, Oner D, Youngblood J, McCarthy TJ. Langmuir 1999;15:3395e9. [4] Callies M, Quere D. Soft Matter 2005;1:55e61.
[5] [6] [7] [8] [9] [10] [11] [12] [13] [14] [15] [16] [17] [18] [19] [20] [21] [22] [23] [24] [25] [26] [27] [28] [29] [30] [31] [32] [33] [34] [35] [36] [37] [38] [39] [40] [41] [42] [43] [44] [45]
Feng L, Li SH, Li YS, Li HJ, Zhang LJ, Zhai J, et al. Adv Mater 2002;14:1857e60. Sun TL, Feng L, Gao X, Jiang L. Acc Chem Res 2005;38:644e52. Barthlott W, Neinhuis C. Planta 1997;202(1):1e8. Neinhuis C, Barthlott W. Ann Bot London 1997;79:667e77. Wenzel RN. Ind Eng Chem 1936;28(8):988e94. Dettre RH, Johnson RE. In: Fowkes FM, editor. Contact angle, wettability and adhesion. Adv chem ser, vol. 43. Washington, DC.: ACS; 1964. p. 136e44. Cassie ABD, Baxter S. Trans Faraday Soc 1944;40:546e51. Onda T, Shibuichi S, Satoh N, Tsujii K. Langmuir 1996;12:2125e7. Extrand C. In: Somasundaran P, editor. Encyclopedia of surface and colloid science. 2nd ed. New York: Taylor and Francis; 2006. p. 5854e68. Erbil HY, Cansoy CE. Langmuir 2009;15(24):14,135e14,145. Ma M, Hill RM, Lowery JL, Fridrikh SV, Rutledge GC. Langmuir 2005;21: 5549e54. Agarwal S, Horst S, Bognitzki M. Macromol Mater Eng 2006;291:592e601. Ma M, Gupta M, Li Z, Zhai L, Gleason KK, Cohen RE. Adv Mater 2007;19:255e9. Ogawa T, Ding B, Sone Y, Shiratori S. Nanotechnology 2007;18:165,607. Acatay K, Simsek E, Ow-Yang C, Menceloglu YZ. Angew Chem Int Ed 2004;43: 5210e3. Shirtcliffe NJ, McHale G, Newton MI, Perry CC, Roach P. Mater Chem Phys 2007;103(1):112e7. Li XH, Chen G, Ma Y, Feng L, Zhao H. Polymer 2006;47:506e9. Han JT, Xu XR, Cho KW. Langmuir 2005;21(15):6662e5. Busscher HJ, Stokroos I, Golverdingen JG, Schakenraad JM. Cells Mater 1991; 1(3):243e9. Busscher HJ, Stokroos A, Vandermei HC, Rouxhet PG, Schakenraad JM. J Adhes Sci Technol 1992;6(3):347e56. Teshima K, Sugimura H, Inoue Y, Takai O, Takano A. Langmuir 2003;19(25): 10,624e10,627. Vourdas N, Tserepi A, Gogolides E. Nanotechnology 2007;18:125,304. Tsai PS, Yang YM, Lee YL. Langmuir 2006;22(13):5660e5. Shiu JY, Kuo CW, Chen PL, Mou CY. Chem Mater 2004;16:561e4. Zhu L, Jin Y. Appl Surf Sci 2007;253:3432e9. Wang JX, Wen YQ, Feng XJ, Song YL, Jiang L. Macromol Rapid Commun 2006; 27(3):188e92. Shirtcliffe NJ, McHale G, Newton MI, Perry CC. Langmuir 2003;19:5626e31. Hikita M, Tanaka K, Nakamura T, Kajiyama T, Takahara A. Langmuir 2005;21: 7299e302. Mahltig B, Bottcher HJ. Sol-Gel Sci Technol 2003;27:43e52. Bico J, Marzolin C, Quere D. Europhys. Lett 1999;47(2):220e6. Zhang L, Zhou Z, Cheng B, DeSimone JM, Samulski ET. Langmuir 2006;22: 8576e80. He B, Patankar NA, Lee J. Langmuir 2003;19:4999e5003. Sun H, Luo MCX, Xu LP, Ji H, Qi OY, Yu DP, et al. Langmuir 2005;21:8978e81. Manoudis PN, Karapanagiotis I, Tsakalof A, Zuburtikudis I, Panayiotou C. Langmuir 2008;24:11,225e11,232. Shirtcliffe NJ, McHale G, Atherton S, Newton MI. Adv Coll Interface Sci 2010; 161:124e38. Reick FG. US Patent 4,142,724; March 6, 1979. http://www.wacker.com/cms/media/publications/downloads/6180_EN.pdf. Yilgor I, Mather B, Unal S, Yilgor E, Long TE. Polymer 2004;45(17):5829e36. http://www.wacker.com/cms/en/products-markets/trademarks/geniomer/ geniomer.jsp. http://www.huntsman.com/performance_products/Media/JEFFAMINE_D400_US.pdf. Nosonovsky M, Bhushan B. Adv Func Mater 2008;18:843e55.