Robust Hairy Microspheres and Derived Hairy Surfaces by an “Inside

pubs.acs.org/Langmuir © 2009 American Chemical Society

Robust Hairy Microspheres and Derived Hairy Surfaces by an “Inside-Out” Wet Approach Bin Fei,*,† Chao Zhou,‡ Zongyue Yang,† Baitai Qian,† Yeeyee Kong,† and John H. Xin*,† †

Institute of Textiles & Clothing, The Hong Kong Polytechnic University, Hong Kong, P. R. China and ‡ School of Chemical Engineering, Changchun University of Technology, Changchun, 130022, China Received September 8, 2009. Revised Manuscript Received November 24, 2009

Robust hairy microspheres were conveniently synthesized through an “inside-out” protrusion approach consisting of a sol-gel reaction and a radical polymerization, from conventional organosilanes and vinyl monomers. Their hierarchical structures of hard cores and plastic hairs can be adjusted in terms of their sizes and compositions to impart various chemical and physical properties on their surfaces. These hairy microspheres can also be built into hairy surfaces with many useful functions, such as superhydrophobicity. This work opens up a new route for the synthesis of hierarchical particles and enriches the tool library of material and surface engineering.

Introduction Hairy surfaces are ubiquitous in the natural biosphere and play important roles such as enhancing sensing, defending intrusion of foreign substances, preventing surface wetting, and facilitating attachment. Among them, the dry adhesive hairy surface of gecko feet and the superhydrophobic surface of the lotus leaf are particularly attractive to material scientists and engineers.1 Inspired by their hierarchical hairy structures, scientists have endeavored to mimic the intrinsically superhydrophobic and dry adhesive surfaces. However, none of them are close enough to those existing in nature. Jeong et al. have prepared regular hair arrays on planar surfaces by rigiflex lithography, where the hairs were on the submicrometer scale.2 However, according to the Johnson-Kendall-Roberts equation for contact mechanics and the Cassie model for surface wettability,3 thinner hairs and multiscaled hierarchical structures are preferred for the better performance of hairy surfaces. If suitable hairy particles are available as primary assembly units, hairy surfaces with much smaller structures can be obtained through a bottom-up approach even on complex-shaped substrates, which represents an enhancement to the previous lithography approach. Hairy particles4 together with raspberry-like particles5 and Janus particles6 have attracted considerable research interest due to their potential usage in material and surface engineering.7 *Corresponding author. E-mail: [email protected], [email protected]. edu.hk. Tel: 852-2766 6454. Fax: 852-2773 1432. (1) (a) Neinhuis, C.; Barthlott, W. Ann. Botany 1997, 79, 667. (b) Arzt, E.; Gorb, S.; Spolenak, R. Proc. Natl. Acad. Sci. U.S.A. 2003, 100, 10603. (2) (a) Jeong, H. E.; Lee, S. H.; Kim, P.; Suh, K. Y. Nano Lett. 2006, 6, 1508. (b) Jeong, H. E.; Lee, J.-K.; Kim, H. N.; Moon, S. H.; Suh, K. Y. Proc. Natl. Acad. Sci. U.S.A. 2009, 106, 5639. (3) (a) Johnson, K. L.; Kendall, K.; Roberts, A. D. Proc. R. Soc. London, Ser. A 1971, 324, 301–20. (b) Cassie, A. B. D.; Baxter, S. Trans. Faraday Soc. 1944, 40, 546. (4) (a) Li, D.; Sheng, X.; Zhao, B. J. Am. Chem. Soc. 2005, 127, 6248. (b) Pakula, T.; Minkin, P.; Matyjaszewski, K. ACS Symp. Ser. 2003, 854, 366. (c) Min, K.; Gao, H.; Yoon, J. A.; Wu, W.; Kowalewski, T.; Matyjaszewski, K. Macromolecules 2009, 42, 1597. (d) Noble, P. F.; Cayre, O. J.; Alargova, R. G.; Velev, O. D.; Paunov, V. N. J. Am. Chem. Soc. 2004, 126, 8092. (5) (a) Ming, W.; Wu, D.; van Benthem, R.; de With, G. Nano Lett. 2005, 5, 2298. (b) Choi, W. S.; Koo, H. Y.; Huck, W. T. S. J. Mater. Chem. 2007, 17, 4943. (c) Schmid, A.; Tonnar, J.; Armes, S. P. Adv. Mater. 2008, 20, 3331. (6) (a) Gu, H.; Zheng, R.; Zhang, X.; Xu, B. J. Am. Chem. Soc. 2004, 126, 5664. (b) Yu, H. K.; Mao, Z.; Wang, D. J. Am. Chem. Soc. 2009, 131, 6366. (c) Wang, B.; Li, B.; Zhao, B.; Li, C. Y. J. Am. Chem. Soc. 2008, 130, 11594. (d) Shah, R. K.; Kim, J.-W.; Weitz, D. A. Adv. Mater. 2009, 21, 1949.

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Several methods were previously reported for the fabrication of these surface-engineered particles, which include surface chemical modification,4a hierarchical self-assembly,5a and phase separation.6a Most of these methods employ templates to control the structure or need laborious multistep treatments.4a,,6b There are considerable demands for novel templates and facile methods of preparing hierarchical particles with controllable physical and chemical properties. It is also noticed that previously reported hairy particles are generally fabricated by introducing only macromolecular chains through “grafting-from” or “anchoringto” wet chemistry. These hairy structures, however, would easily lose their hairiness and become smooth after drying.4a-c Only a few examples physically assembled could retain their hairy surfaces after drying.4d Those examples introduced preformed nanorods to the particle surface without any control over the orientation of the nanorods. A facile method is greatly desired for fabrication of robust hairy particles, from which a hierarchical hairy surface can be built. In order to establish such a method, porous silica particles were employed in this study. Porous silica has been widely employed in fabrication of colloidal crystals,8 drug release containers,9 and catalyst carriers,10 because of its high chemical and thermal stability, ease of synthesis at low temperature, low toxicity, large surface area, and surface permeability.11 Using porous silica particles as microreactors, some metal nanoparticles were synthesized (7) (a) Caruso, F. Adv. Mater. 2001, 13, 11. (b) IM, S. H.; Jeong, U.; Xia, Y. Nat. Mater. 2005, 4, 671. (c) Akcora, P.; Liu, H.; Kumar, S. K.; Moll, J.; Li, Y.; Benicewicz, B. C.; Schadler, L. S.; Acehan, D.; Panagiotopoulos, A. Z.; Pryamitsyn, V.; Ganesan, V.; Ilavsky, J.; Thiyagarajan, P.; Colby, R. H.; Douglas, J. F. Nat. Mater. 2009, 8, 354. (d) Lou, X. W.; Archer, L. A.; Yang, Z. Adv. Mater. 2008, 20, 3987. (e) Wang, Y.; , Price, A. D.; , Caruso, F., J. Mater. Chem. 2009, 19, DOI:10.1039/b901742a. (8) (a) Yin, Y.; Xia, Y. Adv. Mater. 2002, 14, 605. (b) Ge, J.; He, L.; Goebl, J.; Yin, Y. J. Am. Chem. Soc. 2009, 131, 3484. (c) Nakamura, T.; Yamada, Y.; Yano, K. J. Mater. Chem. 2007, 17, 3726–32. (9) (a) Chen, J.-F.; Ding, H.-M.; Wang, J.-X.; Shao, L. Biomaterials 2004, 25, 723. (b) Li, Z.-Z.; Xu, S.-A.; Wen, L.-X.; Liu, F.; Liu, A.-Q.; Wang, Q.; Sun, H.-Y.; Yu, W.; Chen, J.-F. J. Controlled Release 2006, 111, 81. (10) (a) Ge, J.; Zhang, Q.; Zhang, T.; Yin, Y. Angew. Chem., Int. Ed. 2008, 47, 8924. (b) Zhang, Q.; Zhang, T.; Ge, J.; Yin, Y. Nano Lett. 2008, 8, 2867. (c) Park, J.-N.; Forman, A. J.; Tang, W.; Cheng, J.; Hu, Y.-S.; Lin, H.; McFarland, E. W. Small 2008, 4, 1694. (11) (a) Kobler, J.; Bein, T. ACS Nano 2008, 2, 2324–30. (b) Blas, H.; Save, M.; Pasetto, P.; Boissiere, C.; Sanchez, C.; Charleux, B. Langmuir 2008, 24, 13132–13137. (c) Davis, R. W.; Flores, A.; Barrick, T. A.; Cox, J. M.; Brozik, S. M.; Lopez, G. P.; Brozik, J. A. Langmuir 2007, 23, 3864–3872.

Published on Web 12/03/2009

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in situ.12 In 1999, Kageyama et al. reported the use of porous silica as reactors for polymer nanofiber synthesis.13 Thereafter, no report on the similar technique was seen. After successfully preparing a variety of core-shell particles,14 we started using them as assembly units and microreactors to create hierarchical structures with potential functions.15 In this study, we present a facile “inside-out” wet chemistry for synthesizing hairy microspheres from conventional organosilanes and vinyl monomers, with porous silica spheres as microreactors. This is a quite novel method, as the synthesis involved an “inside-out” protrusion of polymer nanofibers perpendicular to the colloidal particle surfaces. These nanofibers or “hairs” possess the features of tunable stiffness and robustness that can endure drying and heating. In addition, these novel hairy particles are readily used to build hierarchical hairy surfaces on substrates of complex shapes.

Experimental Section Methyl trimethoxysilane (MTMS), 3-aminopropyl trimethoxysilane (ATMS), azobisisobutyronitrile (AIBN), benzoyl peroxide (BPO), 2,2-dimethoxy-2-phenylacetophenone (DMPA), potassium bromide (KBr), tetrahydrofuran (THF), and hydrogen fluoride (HF) were all purchased from Sigma-Aldrich Co. and used as received. Styrene (St), divinyl benzene (DVB), and benzylmethacrylate (BMA) were purchased from Sigma-Aldrich Co. and distilled in vacuum before use. HF was diluted into 4 wt % aqueous solution for etching organosilica. Deionized water was used in all experiments. A typical synthesis procedure was described as follows (recipe 1): a mixture of 1.00 g MTMS, 0.50 g ATMS, 1.0 g St and 10.0 mg AIBN was quickly added into 50 g water under vigorous stirring. After sol-gel reaction for 1 h at room temperature, a white dispersion was obtained. Its solid fraction was collected by vacuum filtration and quickly washed with water to remove non-encapsulated free monomers. This solid was redispersed into an aqueous medium and stirred in a 70 °C bath for 5 h. The final colloidal particles were collected by filtration. The samples were cast on silica substrates and observed by a scanning electron microscope (SEM) (JEOL, JSM-6335F) at 3 kV. They were also dropped on copper grids with holey carbon films for transmission electron microscope (TEM, JEOL 2010, 200 kV) measurement. Fourier transform infrared spectrometry (FTIR, Perkin-Elmer System 2000) was employed to investigate the compositions of hybrid spheres that were pressed into KBr tablets. The dispersion of hairy particles was processed for 10 min by ultrasonication (Bronsonic 2510-MT at 100 w and 42 kHz) to harvest polymer nanofibers from particles. The harvested polymer nanofibers were scanned by an atomic force microscope (AFM, Digital Instruments NanoScope IV from Veeco Co., with a probe BS-Tap300Al from Budget Sensors Co.) on a glass substrate. A high-pressure mercury lamp (Mejiro Precision Co., SHG-200, 500 w) was employed in UV-irradiation of samples. Contact angles (CA) of sample films were measured using a contact angle system OCA 15 plus (Dataphysics Instrument Co., Germany).

Results and Discussion The “inside-out” approach explored here is a two-step procedure: vinyl monomers are encapsulated in monodisperse parent spheres of organosilica (SiOx) by a sol-gel reaction in an aqueous medium; then, the isolated parent spheres are heated or (12) Cavaliere-Jaricot, S.; Darbandia, M.; Nann, T. Chem. Commun. 2007, 2031. (13) Kageyama, K.; Tamazawa, J.; Aida, T. Science 1999, 285, 2113. (14) (a) Fei, B.; Lu, H.; Wang, R. H.; Xin, J. H. Chem. Lett. 2006, 35, 622. (b) Fei, B.; Lu, H.; Xin, J. H. Polymer 2006, 47, 947. (c) Fei, B.; Lu, H.; Qi, K.; Shi, H.; Liu, T.; Li, X.; Xin, J. H. J. Aerosol Sci. 2008, 39, 1089. (15) (a) Fei, B.; Hu, Z.; Lu, H.; Xin, J. H. Small 2007, 3, 1921. (b) Fei, B.; Yang, Z.; Xin, J. H. Macromol. Rapid Commun. 2008, 29, 1886.

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Scheme 1. Schematic Illustration of the “Inside-Out” Approach for Synthesis of Hairy Spheres and Surfaces

UV-irradiated for in situ polymerization and protrusion of polymer hairs (see Scheme 1). A typical synthesis of parent capsule spheres is by adding a mixture of methyl trimethoxysilane (MTMS), 3-aminopropyl trimethoxysilane (ATMS), styrene (St), and azobisisobutyronitrile (AIBN) into water under vigorous stirring at 25 °C (MTMS/ATMS/St/AIBN/H2O=1.0:0.5:1.0:0.01: 50 by wt for recipe 1). The sphere product (St@SiOx) has a high level of monodispersity with a diameter of ∼700 nm. The particles under SEM (Figure 1a) have clean surfaces without any cavity, as confirmed by the TEM image (Figure 1b). These solid spheres can be matrix capsules, in which the enclosed St distributes homogeneously. When they are filtered, washed, and heated in a fresh aqueous medium, these microspheres increase in size (to ∼750 nm) with their surfaces covered by many fluffy embroideries (Figure 1c). Under TEM, these embroideries are identified as interesting “hairs” or nanofibers and the corresponding bundles protruded from the parent spheres, resembling the funny “grass doll” (Figure 1d, Supporting Information Figure S1; the hairs are marked by arrows.). A typical sphere produces copious nanofibers that densely cover its whole surface and present an evident roughness. The sphere components are verified as organosilica and polystyrene (PSt) by FTIR spectra. In comparison to the pure organosilica sphere, this composite sphere shows increasing absorbance at 1637 cm-1 and 1410 cm-1, which are the diagnostic peaks of PSt, as shown in Supporting Information Figure S2.16 To confirm the protrudent nanostructures as PSt rather than organosilica, the composite spheres are thoroughly extracted with tetrahydrofuran, and naked spheres (∼700 nm) are observed under both SEM and TEM (Supporting Information Figure S3a,b). Notably, an interior cavity with diameter of 20) (as shown in Figure 2b, two typical nanofibers are colored in red). Atomic force microscope (AFM) images (Supporting Information (19) Zhu, Z. Y.; Wang, S. Q. J. Rheol. 2004, 48, 571.

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Figure 3. TEM images of small hairy spheres (a) and two typical spheres (b) from recipe 2 (MTMS/ATMS/St/AIBN/H2O = 1.0: 1.0:0.5:0.01:100 in wt); SEM images of large hairy spheres from recipe 3 (MTMS/ATMS/St/DVB/AIBN/H2O=1.0:0.1:0.5:0.1:0.01: 50 in wt) after ultrasonic process (c, pores are marked by an arrow) and hairy spheres from recipe 4 (MTMS/ATMS/St/DVB/ AIBN/DMPA/H2O=1.0:0.5:0.5:0.06:0.01:0.01:50 in wt) by UVradiation (d).

Figure S6a,b) of the PSt nanofibers harvested by ultrasonication are consistent with the TEM results, in which typical nanofibers are marked by arrows. Another trial with BPO instead of AIBN as the initiator produces raspberry-like spheres without hairs. It demonstrates the importance of releasing N2 for the hair formation. The abundant silane/monomer library and the general sol-gel/polymer chemistry allow reasonable adjustment of the parent sphere composition, leading to various compositions and parameters of the hairy spheres. The size of the parent sphere increases with the ratio of MTMS/ATMS and their concentrations. By adding cross-linker divinyl benzene (DVB) and reducing ATMS quantity to improve the microphase separation, the number of nanofibers is reduced to only a few on each sphere, with their size increased to ∼100 nm diameter and over 1 μm in length, as shown in Figure 3c (from recipe 3, MTMS/ATMS/St/ DVB/AIBN/H2O = 1.0:0.1:0.5:0.1:0.01:50 in wt). Notably, these cross-linking hairs are robust against organic solvents and above their glass transition point (Tg). After ultrasonic harvest of hairs, corresponding pores are randomly exposed on the sphere surface (marked by arrow in Figure 3c). In addition to the rather stiff PSt hairs with Tg of ∼100 °C, many other hydrophobic monomers and functional reagents are available for synthesis of hairs with variable stiffness and additional functions. However, when the Tg is much lower than the synthesis temperature, the protrusion becomes a continuous film rather than hairs over parent spheres, e.g., poly(1H,1H,7H-dodecafluoroheptyl methacrylate) with Tg of ∼15 °C. Interestingly, the hairy spheres can also be produced in air at room temperature by UV-irradiation. Using a highly efficient UV initiator 2,2-dimethoxy-2-phenylacetophenone (DMPA) together with AIBN to enhance the photopolymerization of St, hairy spheres are quickly obtained within 10 min under 500 W UV light, from the cast parent spheres (see Figure 3d, from recipe 4, MTMS/ATMS/St/DVB/AIBN/DMPA/H2O = 1.0:0.5:0.5:0.06: 0.01:0.01:50 in wt). Without AIBN, however, only raspberry-like spheres are obtained (not shown here), due to the lack of N2 gas 1438 DOI: 10.1021/la903375t

Figure 4. Images of water droplets on the coatings composed of spheres from recipe 4 before (a, CA 140°) and after (b, CA 155°) UV-irradiation, and those from recipe 2 by casting (c); the dynamic CA changes during the droplet advancing and receding between 4 and 8 μL at a constant rate (d); and the SEM image of a typical hairy surface assembled from hairy spheres (inset) by UV-irradiation (e).

from AIBN. This UV process allows the synthesis of low Tg hairs, e.g., poly(benzylmethacrylate) with Tg of ∼50 °C. It also allows convenient application on various substrates, meaning potential usage in coating and patterning by UV-irradiation.20 To summarize all these hairy spheres, their hair diameters are adjustable on a scale of less than 100 nm. The distribution of hairs and their aspect ratio can also be adjusted, supporting a “bottomup” assembly route to a hierarchical hairy surface. Physically, the assembled hairy surface would have robust heterostructure in both micro- and nanoscales, mimicking those of lotus leaf and gecko feet.1,5a To verify the lotus leaf effect, we deposited the parent spheres from the recipe 4 on a glass slide and compare their superhydrophobic performance before and after UV-irradiation. On the surface built by parent spheres, with the surface roughness increasing significantly compared to that of the glass substrate, the contact angle (CA) reached as high as 140° (see Figure 4a). However, it is still at the Wenzel regime suggested by a CA hysteresis (the difference between advancing and receding CA’s) of higher than 10° for a 4 μL droplet change.5a After UVirradiation of the above surface, a massive hairy surface was formed due to the growth of polymer hairs and polymer linkage among hairs, which caused a further increase of the surface roughness into the Cassie regime,5a where air pockets were allowed to form between substrate and water droplet, effectively turning the surface superhydrophobic (CA = 155° and the CA hysteresis is below 2°; see Figure 4b,d and Supporting Information (20) (a) Zhang, H.; Lee, Y. Y.; Leck, K. J.; Kim, N. Y.; Ying, J. Y. Langmuir 2007, 23, 4728–4731. (b) Morigaki, K.; Kiyosue, K.; Taguchi, T. Langmuir 2004, 20, 7729–7735.

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Videos S1 and S2) and allowing a 10 μL water droplet to roll off the surface at a slope as small as 4°. This superhydrophobic behavior is superior to that of hair arrays prepared by lithography,2 which is clearly attributed to the superior hierarchical structure of the hairy sphere assembly. A typical robust coating assembled from hairy spheres by UV-irradiation is presented in Figure 4e, showing the assembly ability of the spheres on surfaces with complex shapes. It is confirmed that the hairy spheres from recipe 2 by heating are also easily assembled into a hairy surface. Due to the smaller spheres and the thinner hairs than those from recipe 4, this assembled surface shows a higher superhydrophobicity, with a CA of 160° and a CA hysteresis near 0° (Figure 4c and Supporting Information Video S3). In addition to imparting superhydrophobicity, these hairy spheres may also be developed into a Velcro-type (hook-and-loop) fastener and gecko-feetlike dry adhesive that provide reversible and strong physical attachment.21 Furthermore, these hairy spheres have tunable chemical properties. The parent spheres have many amino groups on the surface, allowing further functionalization and assembly on the large scale; the hairy spheres can be modified into a variety of functional groups by hydrolysis or oxidation, e.g., sulfonic groups from PSt by H2SO4 oxidation, aldehyde groups from polymethylstyrene by tert-butyl hydroperoxide oxidation, and carboxylic groups from polymethacrylates by hydrolysis.22 These chemical (21) Murphy, M. P.; Aksak, B.; Sitti, M. Small 2009, 5, 170. (22) (a) Yang, Z.; Niu, Z.; Lu, Y.; Hu, Z.; Han, C. C. Angew. Chem., Int. Ed. 2003, 42, 1943. (b) Li, P.; Xu, J.; Wu, C. J. Polym. Sci., Polym. Chem. 1998, 36, 2103.

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groups allow a wide scope of functional applications such as protein immobilization and metal catalyst deposition.23

Conclusions In summary, novel hairy particles have been conveniently produced through an “inside-out” wet approach from conventional organosilanes and vinyl monomers. These hairy particles have been adjusted over a range of components and parameters according to the proposed mechanism and easily assembled into an intrinsically superhydrophobic hierarchical surface that mimics the lotus leaf and gecko feet. The dry adhesive property of the hairy assembly will be our forthcoming research target. This “inside-out” strategy introduces a new route to fabrication of hairy particles and surfaces and enriches the tool library of material and surface engineering. The well-developed sol-gel chemistry and polymer chemistry allow the combination of versatile chemical and physical properties on these hairy particles, which may have great potential in surface applications. Acknowledgment. We gratefully acknowledge the PolyU 5309/07E from RGC and the PolyU Niche area fund J-BB6L. Supporting Information Available: FTIR, SEM, TEM, AFM and video data concerning the colloidal particles and derivative surfaces. This material is available free of charge via the Internet at http://pubs.acs.org. (23) Lee, E. P.; Chen, J.; Yin, Y.; Campbell, C. T.; Xia, Y. Adv. Mater. 2006, 18, 3271.

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