pubs.acs.org/Langmuir © 2010 American Chemical Society
Environmentally Responsive Particles: From Superhydrophobic Particle Films to Water-Dispersible Microspheres Juan Rodriguez-Hernandez,*,†,‡ Alexandra Mu~noz-Bonilla,‡ Antoine Bousquet,† Emmanuel Ibarboure,† and Eric Papon† †
Laboratoire de Chimie des Polym eres Organiques (LCPO), CNRS, Universit e Bordeaux I, ENSCPB. 16, Avenue Pey Berland, 33607 Pessac ,Cedex, France, and ‡Instituto de Ciencia y Tecnologı´a de Polı´meros (ICTP-CSIC) C/Juan de la Cierva no. 3, 28006 Madrid, Spain Received October 4, 2010. Revised Manuscript Received November 2, 2010
We describe the preparation, by precipitation copolymerization, of multifunctional divinylbenzene-co-pentafluorostyrene microspheres able to produce superhydrophobic surfaces or disperse in aqueous media upon annealing either in air or water, respectively. For that purpose, an amphiphilic block copolymer, polystyrene-b-poly(acrylic acid), was introduced in the initial feed composed of divinylbenzene and 2,3,4,5,6-pentafluorostyrene. As a result, fluorinated particles were obtained in which the diblock copolymer was encapsulated during the polymerization step. Upon annealing in dry air, the particles are completely hydrophobic and form superhydrophobic surfaces. On the contrary, annealing in water induces the reorientation of the PAA groups toward the particle interface, thus the particles can be dispersed in aqueous media. In addition, the presence of carboxylic acid groups at the particle interface permits us to switch the surface charge between negative and neutral depending on the environmental pH.
The interest in the elaboration of polymer colloids with controlled dimensions and functionality relies on the extensive range of applications of these materials. Microspheres largely used in coatings, adhesives, and inks have recently been employed for other purposes such as medical and biological applications (e.g., bioseparation,1-3 immunoassay, and affinity diagnosis4,5 or as carrier for drug delivery purposes6-9). Polymeric microspheres have also been employed in optical and optoelectrical devices,10 catalysis,11,12 and micropatterning.13 However, the final use of particles for a targeted purpose is conditioned by different parameters, such as suitable, uniform sizes, colloidal stability, and also control over the nature and density of the surface functional groups.14 The preparation of systems that are able to respond and have adequate properties depending on the environmental *Corresponding author. Fax: (34) 91 564 48 53. Tel: (34) 91 258 75 05. E-mail:
[email protected]. (1) Sumi, Y.; Shiroya, T.; Fujimoto, K.; Wada, T.; Handa, H.; Kawaguchi, H. Colloids Surf., B 1994, 2, 419–427. (2) Darkow, R.; Groth, Th.; Albrecht, W.; Luitzow, K.; Paul, D. Biomaterials 1999, 20, 1277–1283. (3) Durrer, C.; Irache, J. L.; Duchene, D.; Ponchel, G. J. Colloid Interface Sci. 1995, 170, 555–564. (4) Dolitzy, Y.; Sturchak, S.; Nizan, B.; Sela, B. A.; Margel, S. Anal. Biochem. 1994, 220, 257–267. (5) Kondo, A.; Uchimura, S.; Higashitani, K. J. Ferment. Bioeng. 1994, 78, 164– 169. (6) Iannotti, J. P.; Baradet, T. C.; Tobin, M.; Alabi, A.; Staum, M. J. Orthop. Res. 1991, 9, 432–444. (7) Kawaguchi, H. Biomedical Applications of Polymeric Materials; CRC Press: Boca Raton, FL, 1993. (8) Heya, T.; Okada, H.; Owada, Y.; Toguchi, H. Int. J. Pharm. 1991, 72, 199– 205. (9) Kiepotin, D. B.; Kinne, R.; Milton, A.; Palombookinne, E.; Emmrich, F. J. Magnet. Magnet. Mater. 1993, 122, 354–359. (10) Hayashi, S.; Seo, T.; Hata, H.; Hirai, T. J. Colloid Interface Sci. 1991, 144, 538–547. (11) Lu, Y.; Mei, Y.; Drechsler, M.; Ballauff, M. Angew. Chem., Int. Ed. 2006, 45 (5), 813–816. (12) Jana, S.; Ghosh, S. K.; Nath, S.; Pande, S.; Praharaj, S.; Panigrahi, S.; Basu, S.; Endo, T. Appl. Catal., A 2006, 313, 41–48. (13) Lenzmann, F.; Li, K.; Kitai, A. H.; St€over, H. D. H. Chem. Mater. 1994, 6, 156–159. (14) Kawaguchi, H. Prog. Polym. Sci. 2000, 25, 1171–1210.
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conditions is a current topic of interest. Tuning the interface for specific applications requires behaviors that are a priori contrary in many cases, such as superhydrophobic or hydrophilic, charged or uncharged, and adhesive or repellent.15 Systems that exhibit such behavior need to incorporate into their structures either responsive polymers16 or different functional groups17 (multifunctional interfaces). In particular, the development of multifunctional materials that may adapt their surface properties by the orientation of one functionality or another depending on the environmental conditions appears to be an interesting approach.18 By using this concept, Luzinov et al.19 elaborated polymeric surfaces in which either a hydrophilic or a hydrophobic segment can be exposed to the interface. Whereas multifunctional systems have been reported on planar surfaces,15 to the best of our knowledge there are only few examples of multifunctional particles. In these cases, the authors generally prepared microspheres enveloped with a second polymer shell,20 using sophisticated preparation methods (i.e., supercritical CO2),21 or modified them chemically to introduce the multifunctional groups: single functional groups22 or polymer chains.23 Several major drawbacks of these previously reported approaches (15) Luzinov, I.; Minkob, S.; Tsukruk, V. V. Prog. Polym. Sci. 2004, 29, 635– 698. (16) Xia, F.; Feng, L.; Wang, S.; Sun, T.; Song, W.; Jiang, W.; Jiang, L. Adv. Mater. 2006, 18, 432–436. (17) Kurkuri, M. D.; Driever, C.; Johnson, G.; McFarland, G.; Thissen, H.; Voelcker, N. H. Biomacromolecules 2009, 10, 1163–1172. (18) Lee, H.; Dellatore, S. M.; Miller, W. M.; Messersmith, P. B. Science 2007, 318, 426–430. (19) Hoy, O.; Zdyrko, B.; Lupitskyy, R.; Sheparovych, R.; Aulich, D.; Wang, J.; Bittrich, E.; Eichhorn, K.-J.; Uhlmann, P.; Hinrichs, K.; M€uller, M.; Stamm, M.; Minko, S.; Luzinov, I. Adv. Funct. Mater. 2010, 20, 2240–2247. (20) See for instance: Zhang, J.; Sunkara, B.; Le, L.; John, V. T.; He, J.; Mcpherson, G. L.; Piringer, G.; Lu, Y. Environ. Sci. Technol. 2009, 43, 8616–8621. (21) (a) Shiho, H.; Desimone, J. M. J. Polym. Sci., Part A: Polym. Chem. 1999, 37, 2429–2437. (b) Zong, M.; Thurecht, K. J.; Howdle, S. M. Chem. Commun. 2008, 5942–5944. (22) As an example: Viswanathan, N.; Sundaram, C. S.; Meenakshi, S. J. Hazard. Mater. 2009, 167, 325–331. (23) Zhao, B.; Zhu, L. Macromolecules 2009, 42, 9369–9383.
Published on Web 11/16/2010
DOI: 10.1021/la103963z
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Figure 1. Approach to the preparation of environmentally responsive particles by combining hydrophobic moieties (fluorinated monomer) and hydrophilic functional groups (carboxylic acid groups present in the poly(acrylic acid) block).
include poor control of the chemical composition of the surface and the requirement of additional postmodification steps. In this letter, we describe an approach to preparing, in a single step, environmentally responsive multifunctional particles with hydrophobic (fluorinated moieties) and hydrophilic functional groups (carboxylic acid) that are able to modify their surface functionality depending on the environment of exposure. A large number of applications can be envisaged for this system, ranging from the formation of superhydrophobic surfaces to its dispersion in aqueous solution. Moreover, when dispersed in water, the surface charge depends on the environmental pH. The particles employed throughout this study were prepared by the precipitation copolymerization of divinylbenzene and 2,3,4,5,6-pentafluorostyrene (DVB/5FS 55/45 v/v) with AIBN as an initiator (Figure 1) and using an acetonitrile/toluene (95/5 v/v) mixture as the solvent. To this mixture and prior to the initiation step, an amphiphilic block copolymer (i.e., polystyrene-b-poly(acrylic acid) (PS31-b-PAA21)) was added. The designed copolymer contains a polystyrene segment similar in nature to the monomers employed in the polymerization in order to favor incorporation within the particles during precipitation. The second block (PAA), which is hydrophilic in nature, will upon segregation toward the particle interface modify the chemical composition thus enabling the particle dispersion. The reaction was carried out for 4 h at 85 °C. The shape and size distribution of the particles were characterized by SEM. As depicted in Figure 2a, the microspheres have an average diameter of between 3 and 4 μm and exhibit narrow size distributions of between 1.07 and 1.1. The presence of the appropriated surface functionality was verified by XPS analysis of the microspheres. Figure 2 depicts the XPS (O 1s and F 1s) curves of the same particles annealed either in air (left) or in water (right). XPS analysis revealed that whereas the O 1s signal (left) was missing upon annealing in air at 90 °C for 3 days, further annealing in water (80 °C, 2 days) increases the oxygen content at the surface as evidenced by the presence of a signal at ∼532 eV. Because the oxygen comes exclusively from the PAA segment of the diblock copolymer, annealing in water enhances the migration of the diblock copolymer toward the interface. On the contrary, the F 1s signal (∼687 eV) is larger when the particles are annealed in air. When in contact with air, 18618 DOI: 10.1021/la103963z
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the hydrophilic diblock copolymer tends to migrate beneath the surface so that the surface is now enriched in the fluorinated groups provided by the 2,3,4,5,6-pentafluorostyrene monomer employed for the polymerization. More interestingly, consecutive annealing in water and air modifies the chemical composition accordingly between the fluorinated and hydrophilic carboxylic acid groups. As a consequence, the particles’ chemical composition can be easily and reversibly modified depending on the environment of exposure. Changes in the surface chemical composition provide these particles with different properties. As an example of the potential applications herein, we employed the particles annealed in air to obtain superhydrophobic surfaces and the particles annealed in water to both disperse the fluorinated microspheres in aqueous solution and analyze their response to pH. The elaboration of superhydrophobic surfaces (i.e., surfaces with a very large static contact angle and a very small sliding angle) usually requires the combination of two parameters: a hydrophobic surface composition and an appropriate roughness.24 Superhydrophobic surfaces are produced mainly by two different approaches: by creating a rough structure on a hydrophobic surface25 or by modifying a rough surface with low-surface-energy materials.26 An interesting alternative to preparing superhydrophobic surfaces consists of the preparation of films of hydrophobic particles.27 As depicted in Figure 3, the elaboration of films from particles annealed to air by solvent casting leads to the formation of superhydrophobic surfaces with a contact angle of 165 ( 1.6°. Both the chemical composition and the roughness due to the spherical shape of the particles appear to be the main causes of the high contact angles. A completely different wettability is observed when the particles are annealed in water under stirring at 80 °C for 2 days. Under these conditions, the contact angle of the particle films decreases to below 30 ( 1.9°. In spite of the large number of fluorinated moieties contained in its internal structure, the water-annealed particles are able to disperse in neutral aqueous media (Figure 3c). The poly(acrylic acid) block covers the surface of the particle and stabilizes its dispersion. Moreover, as observed by zeta potential measurements, at neutral pH values the particles are negatively charged. This result clearly indicates the presence of deprotonated carboxylic acid groups from the PAA segment of the diblock copolymer at the particle surface. By decreasing the pH of the solution to below 4.5 (the pKa of the COOH functional groups), the charge of the particles disappeared. Thus, the particles functionalized with PAA are not only dispersible but also exhibit a pH-responsive character.
Conclusions We have presented a new and versatile methodology for the fabrication of switchable (super)hydrophobic/(super)hydrophilic smart surfaces prepared from multifunctional particles containing both hydrophobic (fluorinated) and hydrophilic (carboxylic acid functional groups) moieties. The chemical composition on the particle interface was reversibly modified by varying the annealing (24) (a) Blossey, R. Nat. Mater. 2003, 2, 301–306. (b) Sanchez, C.; Arribart, H.; Guille, M. M. G. Nat. Mater. 2005, 4, 277–288. (c) Ma, M.; Hill, R. M. Curr. Opin. Colloid Interface Sci. 2006, 11, 193–202. (25) (a) Minko, S.; Muller, M.; Motornov, M.; Nitschke, M.; Grundke, K.; Stamm, M. J. Am. Chem. Soc. 2003, 125, 3896. (b) Jiang, L.; Zhao, Y.; Zhai, J. Angew. Chem., Int. Ed. 2004, 43, 4338. (26) (a) Zhai, L.; Cebeci, F. C.; Cohen, R. E.; Rubner, M. F. Nano Lett. 2004, 4, 1349. (b) Woodward, I.; Schofield, W. C. E.; Roucoules, V.; Badyal, J. P. S. Langmuir 2003, 19, 3432. (27) (a) Misra, A.; Jarret, W. L.; Urban, M. W. Macromolecules 2007, 40, 6190– 6198. (b) Xue, L.; Li, J.; Fu, J.; Han, Y. Colloids Surf., A 2009, 338, 15–19.
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Figure 2. (a) Scanning electron micrographs of the microspheres. XPS spectra of microspheres annealed either in air or in water. (b) O 1s signal. (c) F 1s signal. Whereas the oxygen content at the surfaces increases upon annealing in water, the exposure to air increases the F 1s signal.
Figure 3. Water contact angle of particle arrays (a) upon annealing in dry air and (b) after annealing in water. (c) Dispersion of the particles obtained after polymerization (left) and upon annealing in water. (d) Zeta potential measurements of the water-annealed particles. The surface charge can be modified when the pH is varied from acidic to neutral values.
environment between water and air. Films formed from airand water-annealed particles produced superhydrophobic and hydrophilic surfaces, respectively. Moreover, the PAA present at the particle interface in the water-annealed particles allowed us to disperse them in water in spite of the elevated amount of 5FS employed for the copolymerization. Finally, the carboxylic acid groups at the particle surface confer pH-responsive character to the particles. Langmuir 2010, 26(24), 18617–18620
Experimental Section Chemicals. 2,3,4,5,6-Pentafluorostyrene (5FS) and divinylbenzene (DVB) were purchased from Sigma-Aldrich. Azobisisobutyronitrile (AIBN, 98%) as the initiator was recrystallized from methanol before use. All other solvents were used as received unless otherwise specified. Characterization. 1H NMR spectra of the diblock copolymers were recorded at room temperature on a Bruker Avance DOI: 10.1021/la103963z
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Letter 400 MHz spectrometer using the residual proton resonance of the deuterated solvent as an internal standard. Average molar masses and molar mass distributions were determined by size exclusion chromatography (SEC) using a Varian 9001 pump with both a refractive index detector (Varian RI-4) and a UV detector (Spectrum Studies UV 150). Calibration was obtained using narrowly distributed PS standards and THF as the mobile phase at a flow rate of 0.5 mL min-1.
Preparation and Characterization. Synthesis of the Diblock Copolymer (PS-b-PAA). The synthesis of the diblock copolymer was accomplished following previously reported procedures.28 The diblock copolymer employed was PS31-b-PAA21 (Mn =4600 g/mol, PD=1.17) Preparation of Particles. The preparation of the particles has been carried out by precipitation polymerization based on the following procedure: 0.79 mL of 5FS, 0.79 mL of DVB, and 72 mg of AIBN were dissolved in 30 mL of an acetonitrile/toluene (v/v 95%/5%) mixture. In a separate step, the diblock copolymer, PS-b-PAA (10% weight related to the total amount of 5S/DVB), was dissolved in the same solvent mixture and added to the previous solution. The polymerization was carried out at a constant temperature of 85 °C for 4 h. After the polymerization was complete, the soluble polymer was separated from the insoluble fraction by vacuum filtration. The insoluble microspheres were extensively washed with tetrahydrofuran. Scanning Electron Microscopy (SEM). The morphological characterization of the functionalized microspheres was carried out with a scanning electron microscope (SEM, JEOL JSM-5200 scanning electron microscope). The particles were dropped onto a sample holder, placed under vacuum at room temperature, and (28) Bousquet, A.; Ibarboure, E.; Labrugere, C.; Papon, E.; RodriguezHernandez, J. Langmuir 2007, 23, 6879–6882.
18620 DOI: 10.1021/la103963z
Rodriguez-Hernandez et al. coated with gold prior to examination. The particle size distributions were obtained from the statistical treatment of representative SEM images using ImageJ software (http://rsb.info.nih.gov/ij/). The PDI was calculated according to PDI w/Dn where P = DP Dw is the weight-average diameter, Dw P = niDi4/ niDi3, and Dn is the number-average diameter Dn = niDi/ni (where ni is the number of particles with diameter Di). X-ray Photoelectron Spectroscopy (XPS). XPS spectra were recorded with a 220i-XL ESCALAB from VG. The particles, supported on indium, were put under UHV to reach the 10-8 Pa range. A nonmonochromatized Mg X-ray source was used at 100 W, and a flood gun was used to compensate for the nonconductive samples. The spectra were calibrated in relation to the C 1s binding energy (284.6 eV), which was applied as an internal standard. Fitting of the high-resolution spectra was provided through the AVANTAGE program from VG. Contact Angle Measurements. Water contact angles were measured using a KSV theta goniometer. The volume of the droplets was controlled to be about 3 μL. Zeta Potential Measurements. Measurements were conducted using a Malvern Zetasizer NanoZS instrument (Malvern Instruments). The zeta potential of the microspheres was determined as a function of pH in the presence of 1 mM NaCl, using NaOH or HCl to adjust the pH as required.
Acknowledgment. We gratefully acknowledge financial support from the Centre National de la Recherche Scientifique, the Agence National de la Recherche (Jeunes Chercheurs program ANR-07-JCJC-0148). This work was also supported by the Spanish National Research Council (CSIC) through the PI 200860I037, CCG08-CSIC/MAT-3643, and MAT2009-12251 (subprograma MAT) programs.
Langmuir 2010, 26(24), 18617–18620
