Langmuir 2009, 25, 17-20
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Water Repellency on a Fluorine-Containing Polyurethane Surface: Toward Understanding the Surface Self-Cleaning Effect Wanling Wu,† Qingzeng Zhu,*,‡ Fengling Qing,§ and Charles C. Han*,† Beijing National Laboratory for Molecular Sciences, Joint Laboratory of Polymer Science and Materials, State Key Laboratory of Polymer Physics and Chemistry, Institute of Chemistry, CAS, Beijing, 100190, China, School of Chemistry and Chemical Engineering, Shandong UniVersity, Jinan, 250100, China, Shanghai Institute of Organic Chemistry, CAS, Shanghai, 200032, China ReceiVed September 19, 2008. ReVised Manuscript ReceiVed NoVember 9, 2008 Surface geometrical microstructure and low surface free energy are the two most important factors for a selfcleaning surface. In this study, multiform geometrical microstructured surfaces were fabricated by casting and electrospinning polyurethanes with and without low surface energy segments. The effect of low surface energy on water repellency was evaluated. Low surface energy seems to make a more significant contribution to the static wetting behavior than do dynamic properties such as the improvement of sliding behavior. Sucking disk behavior was brought forward to explain the pinning state of a water droplet on hydrophobic surfaces with high water contact angles (>150°). A better understanding of the relationship between the static contact angle and the dynamic sliding property was provided.
Introduction Naturally water-repellent plant leaves, such as Brassica oleracea, Eryngium ebracteatum, Nelumbo nucifera (lotus), and Colocasia esculenta, are covered with a layer of epicuticular waxes.1,2 This low surface free energy chemical composition that displays nanoscale crystallites and ranges in length from 0.5 to ∼5.0 µm is believed to be closely related to their self-cleaning effect. Generally, surface geometrical microstructure and low surface free energy are the two most important factors for a self-cleaning surface, which is also called a superhydrophobic surface. It has been recognized since 1936 that the roughness plays a very important role in determining the wettability of a surface. Wenzel and Cassie-Baxter theories have pointed out that surface roughness is essential for enhancing hydrophobicity.3,4 Therefore, micro- and nanoscale hierarchical structures that are typically shown on the surface of a lotus leaf can induce a superhydrophobic surface. Researchers have made impressive efforts to prepare artificial surperhydrophobic surfaces during * Corresponding authors. E-mail:
[email protected]. Tel: +86-1082618089. Fax: +86-10-62521519. E-mail:
[email protected]. Tel: +86531-88362866. Fax: +86-531-88564464. † Institute of Chemistry. ‡ Shandong University. § Shanghai Institute of Organic Chemistry.
(1) Neinhuis, C.; Barthlott, W. Ann. Bot. 1997, 79, 667. (2) Barthlott, W.; Neinhuis, C. Planta 1997, 202, 1. (3) Wenzel, R. N. Ind. Eng. Chem. 1936, 28, 988. (4) Cassie, A. B. D.; Baxter, S. Trans. Faraday Soc. 1944, 40, 546. (5) Xie, Q. D.; Fan, G. Q.; Zhao, N.; Guo, X. L.; Xu, J; Dong, J. Y.; Zhang, L. Y.; Han, C. C. AdV. Mater. 2004, 16, 1830. (6) Sun, S. T.; Feng, L.; Jiang, L. AdV. Mater. 2006, 18, 767. (7) Zhang, L.; Zhou, Z. L.; Cheng, B.; DeSimone, J. M.; Samulski, E. T. Langmuir 2006, 22, 8576. (8) Fu¨rstner, R.; Barthlott, W. Langmuir 2005, 21, 956. (9) Shi, F.; Wang, Z. Q.; Zhang, X. AdV. Mater. 2005, 17, 1005. (10) Gao, L. C.; McCarthy, T. J. J. Am. Chem. Soc. 2006, 128, 9052. ; Gao, L.; McCarthy, T. J. Langmuir 2006, 22, 5998. Gao, L.; McCarthy, T. J. Langmuir 2006, 22, 2966. (11) Youngblood, J. P.; McCarthy, T. J. Macromolecules 1999, 32, 6800. (12) Ji, J.; Fu, J. H.; Shen, J. C. AdV. Mater. 2006, 18, 1441. (13) Lu, X. Y.; Zhang, C. C.; Han, Y. C. Macromol. Rapid Commun. 2004, 25, 1606. (14) Hikita, M.; Tanaka, K.; Nakamura, T.; Kajiyama, T.; Takahara, A. Langmuir 2005, 21, 7299.
this decade.5-16 In practice, surfaces with only nanoscale structures prepared by nanoimprint lithography or other methods show a water-repellent property.17-19 Some research has indicated that it is necessary to modify the rough surface with low surface energy materials such as fluorinated or silicon compounds to fabricate a superhydrophobic surface.20-22 By now, the relationship between surface roughness and wettability is well known. However, a few reports have carefully probed the role of low surface energy in water repellency. The purpose of this study is to separate the effect of low surface energy on the self-cleaning property by fabricating multiform geometrical microstructured surfaces with the same kind of low surface energy polymer. In other words, we would like to have a better understanding of the relationship between the static contact angle and the dynamic sliding property.
Experimental Section Synthesis of Polyurethanes Containing Perfluoropolyether Segments (FPU). The method for preparing FPU was a two-step condensation reaction.23,24 The typical procedure is described as follow: 4,4′-Methylene-bis-(phenylisocyanate) (MDI, Acros Organics) was dissolved in DMAc (N,N-dimethylacetamide, 10.0 wt %) in a three-necked, round-bottomed flask under a nitrogen atmosphere. Polytetrahydrofuran (Mn ) 2000, Sigma-Aldrich) in DMAc (25.0 wt %) was subsequently added to the MDI solution at room temperature. Polymerization was carried out at 70-80 °C for 2 h. (15) Sun, M. H.; Luo, C. X.; Xu, L. P.; Ji, H.; Ouyang, Q.; Yu, D. P.; Chen, Y. Langmuir 2005, 21, 8978. (16) Han, J. T.; Xu, X. R.; Cho, K. Langmuir 2005, 21, 6662. (17) Feng, L.; Song, Y. L.; Zhai, J.; Liu, B. Q.; Xu, J.; Jiang, L.; Zhu, D. B. Angew. Chem., Int. Ed. 2003, 42, 800. (18) Hosono, E.; Fujihara, S.; Honma, I.; Zhou, H. S. J. Am. Chem. Soc. 2005, 127, 13458. (19) Lee, W.; Jin, M. K.; Yoo, W. C.; Lee, J. K. Langmuir 2004, 20, 7665. (20) Han, J. T.; Zheng, Y. L.; Cho, J. H.; Xu, X. R.; Cho, K. J. Phys. Chem. B 2005, 109, 20773. (21) Sun, T. L.; Wang, G. J.; Liu, H.; Feng, L.; Jiang, L.; Zhu, D. B. J. Am. Chem. Soc. 2003, 125, 14996. (22) Zhai, L.; Cebeci, F. C.; Cohen, R. E.; Rubner, M. F. Nano Lett. 2004, 4, 1349. (23) Yoon, S. C.; Ratner, B. D. Macromolecules 1986, 19, 1068. (24) Tonelli, C.; Trombetta, T; Scicchitano, M.; Castiglioni, G. J. Appl. Polym. Sci. 1995, 57, 1031.
10.1021/la803089y CCC: $40.75 2009 American Chemical Society Published on Web 11/24/2008
18 Langmuir, Vol. 25, No. 1, 2009
Letters Scheme 1. Chemical Structure of FPU
Stoichiometric perfluoropolyether alcohol (CF3CF2CF2O (CFCF3CF2O)2CFCF3CH2OH, Shanghai Institute of Organic Chemistry) was added drop by drop to the prepolymer solution, and the reaction was allowed to proceed for 1 h. Then 1,4-butanediol as chain extenders dissolved in DMAc were added. The molar ratio of MDI to polytetrahydrofuran to butanediol was 2:1:1.5. The chainextension reaction was continued for 1 to 2 h at 70-80 °C. The resulting polymers were precipitated in excess water and dried in a vacuum oven at 60 °C. They were redissolved in DMAc and precipitated in an excess methanol-water mixture to remove lowmolecular-weight materials. The purified polymers were washed with methanol and then deionized water and dried in a vacuum oven at 60 °C. The prepared polymer is an elastic material and has good mechanical properties. The perfluoropolyether alcohol content in the FPU specimen is 10.0 wt %. Pure polyurethanes (PU) with similar structure were prepared under the same conditions without perfluoropolyether alcohol added. FPU was prepared from polytetrahydrofuran, MDI, and monofunctional fluorinated oligomers. The linear molecular chains based on urethanes were capped by fluorinated oligomers. Its chemical structure is shown in Scheme 1. Preparation of Hydrophobic Surfaces. Surfaces with different geometrical structures were prepared by electrospinning with varying concentrations of FPU or PU solutions in DMF/THF (30:70 v/v) mixed solvent. The concentration of the PU or FPU solution is 10 mg/mL for the bead-thread membrane, whereas for the nanofibrous membrane the concentration of PU or FPU is 30 or 25 mg/mL respectively. A DC high-voltage generator was applied to produce voltages ranging from 0 to 50 kV. The electrospinning solutions were placed into a 5.0 mL syringe with a no. 5 gauge needle having an inner diameter of 0.3 mm. A syringe pump was used to feed polymer solution into the needle tip. A sheet of aluminum foil, connected to the ground, was placed under the syringe as a collector. The environmental temperature was kept at 50 °C. The DC highvoltage applied was 16 kV. The feeding rate was 50 µL/min, and the tip-to-collector distance was 15.0 cm. Characterization. Gel permeation chromatography was performed to estimate the molecular weights of FPU using a 1515 system (Waters) equipped with 2414 refractive index and Styragel gel columns calibrated with narrow-molecular-weight polystyrene standards. Thermal transitions were measured by DSC using a Mettler Toledo DSC822e. The contact angle was measured using a sessile drop method. Before contact angles were measured by a sessile drop method, all samples were kept in air for a week after being prepared. Each water CA value was an average of at least five measurements on different locations on the surface. A 5.0 µL water drop was used. X-ray photoelectron spectroscopy data were obtained with an ESCALab220i-XL electron spectrometer from VG Scientific using 300 W Al KR radiation. The binding energies were referenced to the C 1s line at 284.8 eV from adventitious carbon. Morphologies of the sample surfaces were observed with scanning electron microscopy (JEOL JSM-6700F, Japan) and AFM (Digital Instruments, Nanoscope III A, tapping mode).
Results and Discussion In this study, polyurethanes containing low surface energy perfluoropolyether segments were designed and prepared. It is a light-yellow elastomer with a molecular weight of 220 000 g/mol and a polydispersity index of 1.94 characterized by GPC. DSC results showed that the glass transition temperature, cold crystallization, and melting temperature were -60, -17, and 18 °C, respectively. Membranes with different surface geometrical
Figure 1. SEM images of membranes prepared by casting FPU (A) or PU (B) solution. Insets are the profiles of water droplets on the corresponding surfaces.
microstructures (e.g.,. flat, bead-thread, and nanofibrous membranes) were fabricated using this material by casting and electrospinning. Membranes with a smooth surface were prepared by casting a DMF solution of FPU on a silicon wafer. Angle-dependent XPS was employed to quantify the surface composition of the FPU membrane. The fluorine atomic percentage among C, N, O, and F atoms in the bulk was 8.2% according to XPS measurements. According to the reactants, the theoretical fluorine atomic percentage among C, N, O, and F atoms in the bulk is 5.1%. XPS analysis showed that the fluorine atomic percentage was 27.1% in an ∼10 nm surface layer and 39.8% in an ∼5 nm surface layer. The fluorine atomic concentration on the outermost surface was much higher than that in the bulk for the casted membranes. The surface was enriched with fluorine-containing segments. Low surface energy components usually migrate to substrate-air interfaces.25,26 These perfluoropolyether segments, which are linked only on one side in the polyurethane chains, can move more freely than do the backbone segments in polymer chains, so they could migrate easily to the outermost polyurethane surface. Membranes with a smooth surface were also prepared from a DMF solution of polyurethane (PU), which had a similar chemical structure to that of FPU without perfluoropolyether end segments. Scanning electron microscopy (SEM) images showed the flat surfaces of FPU- and PU-cast membranes with a slightly sunken, raised terrain (Figure 1). The static water contact angles (WCAs) on FPU and PU membranes are 113 ( 1.7 and 85 ( 0.9°, respectively. The FPU surface is hydrophobic, and its wettability is similar to that of plant epicuticular wax, which exhibits a WCA of 110° on its flat surface.27 However, the PU surface was slightly hydrophilic (WCA
