Polymer Surfaces with Reversibly Switchable Ordered Morphology

Honeycomb macroporous films fabricated by the “breath figures” method were composed of poly2-vinylpyridine (P2VP) distributed in the holes of poly...
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Polymer Surfaces with Reversibly Switchable Ordered Morphology Liang Cui, Yu Xuan, Xue Li, Yan Ding, Binyao Li, and Yanchun Han* State Key Laboratory of Polymer Physics and Chemistry, Changchun Institute of Applied Chemistry, Chinese Academy of Sciences, Graduate School of the Chinese Academy of Sciences, Changchun 130022, P. R. China Received May 24, 2005. In Final Form: September 5, 2005 Honeycomb macroporous films fabricated by the “breath figures” method were composed of poly2vinylpyridine (P2VP) distributed in the holes of polystyrene (PS). The porous films exhibited reversible behavior responding to water and different solvent vapors. When the porous film was treated with water, the honeycomb pattern would change to the hexagonal islandlike pattern. Once heated to remove the water, the honeycomb pattern emerged again. When the porous film was exposed to different solvent vapors, the same reversible process appeared. Carbon disulfide (CS2), toluene (TOL), and tetrahydrofuran (THF) solvent vapors induced the honeycomb pattern into the ordered islandlike pattern, and ethanol, chloroform, methyl ethyl ketone (MEK), and dimethylformamide (DMF) solvent vapors made the islandlike pattern come back to the honeycomb pattern. The hygroscopic property of P2VP and the polymer-solvent interaction are the driving force for the reversibly switchable morphology. The appropriate control of the hole depth is very crucial in determining the reversible changes.

1. Introduction In recent years, increasing attention has been paid to produce an ordered micropatterned surface that responds to external stimuli such as changes in pH, temperature, humidity, ionic strength, stretch, light, or electrical field due to various uses in electronic,1,2 optical 3, sensor,4 biological technology,5 and mechanical devices.6 etc. The emphasis of the responding micropatterned surfaces is on the switchability and reversibility; that is, the surface properties or topographies can reversibly change with the presence and absence of external effects. Various methodologies have been used to exploit materials responding to external environments.7 Hydrogels with three-dimensional cross-linked polymer network structures show diverse intelligent behaviors due to reversible volume or shape change to different stimuli.8-12 Polyelectrolyte multilayer films with alternating charge demonstrate the reverse change based on swelling and deswelling processes * To whom correspondence should be addressed. Tel.: +86431-5262175. Fax: +86-431-5262126. E-mail: [email protected]. (1) Boltau, M.; Walheim, S.; Schaffer, E.; Mlynek, J.; Steiner, U. Science 1999, 283, 520. (2) Fink, Y.; Winn, J. N.; Fan, S.; Chen, C.; Michel, J.; Joannopoulos, J. D.; Thomas, E. L. Science 1998, 282, 1679. (3) Soukoulis, C. Photonic Band Gap Materials; Kluwer: Dordrecht, The Netherlands, 1996. (4) Nishikawa, T.; Ookura, R.; Nishida, J.; Arai, K.; Hayashi, J.; Kurono, N.; Sawadaishi, T.; Hara, M.; Shimomura, M. Langmuir 2002, 18, 5734. (5) Hirano, Y.; Mooney D. J. Adv. Mater. 2004, 16, 7. (6) Veinot, J. G. C.; Yan, H.; Smith, S. M.; Cui, J.; Huang Q.; Jmarks, T. Nano Lett. 2002, 2, 333. (7) Luzinov, I.; Minko, S.; Tsukruk, V. V. Prog. Polym. Sci. 2004, 29, 635. (8) Motornov, M.; Minko, S.; Eichhorn, K. J.; Nitschke, M.; Simon, F.; Stamm, M. Langmuir 2003, 19, 8077 (9) Draper, J.; Luzinov, I.; Minko, S.; Tokarev, I.; Stamm, M. Langmuir 2004, 20, 4064. (10) Peng, J.; Xuan, Y.; Wang, H. F.; Yang, Y. M.; Li, B. Y.; Han, Y. C. J. Chem. Phys. 2004, 120, 11163. (11) Lahann, J.; Mitragotri, S.; Tran, T. N.; Kaido, H.; Sundaram, J.; Choi, I. S.; Hoffer, S.; Somorjai, G. A.; Langer, R. Science 2003, 299, 371. (12) Crevoisier, G. De.; Favre, P.; Corpart, J.; Leibler, L. Science 1999, 285, 1246.

induced by the variation of environment changes.13-15 Liquid crystalline polymers present many switchable properties by taking advantage of a bulk transition between a highly ordered phase and an isotropic phase.16,17 Homopolymers with functional end groups lead to diverse controls of polymer surface and interface properties due to the mutual actions between the functional end groups and external environments.18,19 Polymer brushes are widely developed on the basis of conformational transitions or surface energy change responding to alternative external stimulus.20-25 Moreover, block copolymers present reversible surface morphologies and properties when treated by a selective solvent due to the swelling of one polymer and the collapse of the other.10,26 In our previous work,27 the polymer surface with an ordered array of holes was produced based on the “breath figures” process28 and phase separation of polymer blends. The P2VP located in the holes around which was PS phase. (13) Houbenov, N.; Minko, S.; Stamm, M. Macromolecules 2003, 36, 5897. (14) Lemieux, M.; Usov, D.; Minko, S.; Stamm, M.; Shulha, H.; Tsukruk, V. V. Macromolecules 2003, 36, 7244. (15) Julthongpiput, D.; Lin, Y. H.; Teng, J.; Zubarev, E. R.; Tsukruk, V. V. Langmuir 2003, 19, 7832. (16) Hu, Z. B.; Zhang, X.; Li, Y. Science 1995, 269, 525. (17) Holtz, J. H.; Asher, S. A. Nature 1997, 389, 829. (18) Julthongpiput, D.; Lin, Y. H.; Teng, J.; Zubarev, E. R.; Tsukruk, V. V. J. Am. Chem. Soc. 2003, 125, 15912. (19) Anastasiadis, S. H.; Retsos, H.; Pispas, S.; Hadjichristidis, N.; Neophytides, S. Macromolecules 2003, 36, 1994. (20) Koberstein, J. T.; Duch, D. E.; Hu, W.; Lenk, T. J.; Bhatia, R.; Brown, H. R.; Lingelser, J. P.; Gallot, Y. J. Adhesion 1998, 66, 229. (21) Zhao B.; Brittain, W. J. J. Am. Chem. Soc. 1999, 121, 3558. (22) Sedjo, R. A.; Mirous, B. K.; Brittain, W. J. Macromolecules 2000, 33, 1492. (23) Zhao, B.; Brittain, W. J.; Zhou, W.; Cheng, S. Z. D. J. Am. Chem. Soc. 2000, 122, 2407. (24) Minko, S.; Usov, D.; Goreshnik, E.; Stamm, M. Macromol. Rapid. Commun. 2001, 22, 206. (25) Minko, S.; Mu¨ller, M.; Motornov, M.; Nitschke M.; Grundke, K.; Stamm, M. J. Am. Chem. Soc. 2003, 125, 3896. (26) (a) Minko, S.; Patil, S.; Datsyuk, V.; Simon, F.; Eichhorn, K.-J.; Motornov, M.; Usov, D.; Tokarev, I.; Stamm, M. Langmuir 2002, 18, 289. (b) Minko, S.; Usov, D.; Goreshnik, E.; Stamm, M. Macromol. Rapid. Commun. 2001, 22, 206. (27) Cui, L.; Peng, J.; Ding, Y.; Li, X.; Han, Y. C. Polymer 2005, 46, 5334.

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The size and depth of the holes could be regulated through humidity and weight ratio of PS/P2VP. In this paper, we extended the work to develop a patterning surface with reversible change from an ordered honeycomblike structure to hexagonal islands when treated by water or different solvents. The switching mechanisms were discussed in details. 2. Experimental Section Polystyrene (PS) (average weight molecular mass (Mw) is 220 000 and the polydispersity (Mw/Mn) is 1.04) and poly2vinylpyridine (P2VP) (average weight molecular mass (Mw) is 11 000 and the polydispersity (Mw/Mn) is 1.01) were obtained from Aldrich Chemical Co. All solvents (analysis grade), such as carbon disulfide (CS2), toluene (TOL), tetrahydrofuran (THF), ethanol, chloroform, methyl ethyl ketone (MEK), and dimethylformamide (DMF), were from Beijing Chemical Company, China. The THF solutions of PS and P2VP (different weight ratios) with different concentrations were filtered with a 0.22 µm Millipore membrane before casting onto the freshly cleaved mica substrates (1 × 1 cm) by a micro-injector. The volume of each drop of solution was about 50 µL. The liquid film thickness was ca. 1.0 mm. The sample was then put into a vessel plugged with a cork. There were two exhaust pipes on the cork. One opened into the atmosphere, and the other one connected with a vacuum water pump. The rate of solvent evaporation was controlled by a rotameter. The humidity degree inside the vessel was controlled through adjusting the humidity degree of the ambient environment by a humidifier. The humidity degree was controlled to be above 30%. The temperature kept at 25 ( 1 °C. The humidity degree and temperature were measured by means of the hygrothermograph. Solvent evaporation was monitored by weighing the sample at regular time intervals until a constant weight was reached. All of the prepared films were dried at 50 °C in vacuo for 2 days to remove the remaining solvent completely. The dried film thickness was ca. 2.0∼2.2 µm. The topographies of the films were scanned by optical microscopy (OM) and atomic force microscopy (AFM). Then two groups of experiments were performed to observe the reversible switching process of ordered patterns. In the first group, the resulting porous film was kept in water for 20 min and then withdrawn into the atmosphere and promptly dried by nitrogen gas. The topographies of the films were scanned by AFM immediately. Subsequently, the same sample was again kept in water for 1 and 3 h, respectively, to repeat the above process. Then, the film was heated at 60 °C for 1 and 3 h, respectively, to remove water. After each treatment period, the topographies of the films were observed. In the second group, the same procedure as the first group experiment was taken. The difference is that water was replaced by different solvents. The porous film was exposed to a saturated CS2, TOL, or THF solvent vapors for different periods and then put into the atmosphere and promptly dried by nitrogen gas. Next, the film was exposed to saturated chloroform, MEK, ethanol, or DMF solvent vapors for different periods and then put into the atmosphere and promptly dried by nitrogen gas again. After each treatment period, the topographies of the films were scanned by AFM immediately. The two group experiments were repeated for several cycles. Optical microscopy (OM) micrograph was imaged by the use of Leica optical microscopy in reflection mode with a CCD camera attachment. AFM measurements were performed on SPA300HV with an SPI 3800 controller, Seiko instruments industry, Co. Ltd. The images were taken with contact mode and tapping mode at room temperature. The scan rate was in the range of 0.8-1.2 Hz.

3. Results and Discussion In our previous work,27 the ordered porous films were prepared by the “breath figures” method. During a stream (28) (a) Widawski, G.; Rawiso, B.; Franc¸ ois, B. Nature 1994, 369, 387. (b) Srinivasarao, M.; Collings, D.; Philips, A.; Patel, S. Science 2001, 292, 79.

Figure 1. (a) Optical micrograph of a honeycomb macroporous film when the PS/P2VP (5/1)(w/w) solution in THF at a concentration of 4 wt % cast onto the mica substrate in 40% humidity. (b) AFM topographic and (c) phase images in part of (a). Inset at the right and upper side is the 2D fast Fourier transforms (FFT) analysis.

of moist air across a layer of PS and P2VP solution spread on a flat substrate, water droplets would form on the surface of solution films due to the latent heat of solvent evaporation. When humidity reached a critical value (humidity > 30%), the water droplets would assemble into hexagonal arrays which corresponded to the lowest free energy.29 Meanwhile, phase separation occurred when the polymer concentration reached a critical value during the solvent evaporate because PS and P2VP were strongly immiscible. Minor P2VP phase would protrude from the system due to its lower solubility in THF.30 In the case of low ambient humidity (below 30%), islandlike P2VP-rich phase would disorderly distributed on the continuous PS phase surface. When the humidity reached a critical value (humidity > 30%), the P2VP domains were reassembled (29) Thomas E. L.; Kinning D. J.; Alward D. B.; Henkee, C. S. Macromolecules 1987, 20, 2934. (30) Walheim, S.; Bo¨ltau, M.; Mlynek, J.; Krausch, G.; Steiner, U. Macromolecules 1997, 30, 4995.

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Figure 2. 3D restructuring images in water over time: (a) air; (b) 20 min; (c) 1 h; (d) 3 h; heated at 60 °C for (e)1 h; (f) 3 h. The image size is 40 µm × 40 µm for (a-e); 50 µm × 50 µm for (f), respectively.

by the water droplets template due to the strong interaction between the P2VP and water. Water droplet would sink into the solution under the action of weight. Thus, the porous films with the hexagonal arrays of holes with P2VP locating inside formed after solvent and water evaporated completely. Figure 1a presents the optical micrograph of a honeycomb macroporous film after the PS/P2VP (5/1)(w/w) solution in THF at a concentration of 4 wt % is cast onto the mica substrate in 40% humidity. AFM images of a part of Figure 1a are shown in Figure 1, panels b and c, respectively. It can be seen that a perfect hexagonal arrangement of the holes is indicated by a fast Fourier transform (FFT) pattern from the inset of Figure 1b. The typical depth of holes is 265 nm, the mean diameter is 3.1 µm, and the interval between adjacent holes is 5.8 µm. The contrast in the phase image (right) shows an ordered porous film with heterogeneous characters. The brighter domains and darker background represent the P2VP-rich phase and PS-rich phase, respectively. The P2VP-rich phase locates in the holes around which the PS phase is. It is very interesting that the micropatterned surface may exhibit a reversibly responding behavior to water. The 3D images in Figure 2a-f showed the reversibly switchable morphology as a function of time after the

porous film was treated by water. After the sample of Figure 1 (its 3D image was shown in Figure 2a) was kept in water for 20 min, islandlike structure appeared (Figure 2b). Larger islands formed in the holes after 1 h (Figure 2c). After 3 h, the islands occupied the holes completely (Figure 2d). Thereafter, the sample was put into a drier and heated at 60 °C in order to remove the water. After 1 h, the volume of islands decreased (Figure 2e). It was completely recovered to the original topography after 3 h (Figure 2f). The cycle could be reproduced again and again. In addition, the porous film may show reversible responding behavior to different solvent vapor environments as well. Figure 3 showed the 3D images of the continuous development process from initial pattern to islandlike pattern emerging and coming back to porous film. Figure 3a was the initial film fabricated by the “breath figures” method after the PS/P2VP (4/1)(w/w) solution in THF at a concentration of 5 wt % was cast onto the mica substrate in 30% humidity. The size and depth of the holes were 2.5 µm and 150 nm, respectively. Then the sample was put into a container full of carbon disulfide (CS2) vapor at 25 °C and kept for 5 min. Subsequently, the film was removed from the container and dried immediately by nitrogen gas. The surface morphology was observed by AFM. The procedure was repeated every 5 min. After the

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Figure 3. 3D topographies of porous film with reversible responding behavior to different solvent vapors. (a) the as-cast film; (b)-(e) the topographies after exposure to CS2 vapor for (b) 5 min; (c) 10 min; (d) 15 min; (e) 20 min; (f)-(g) the topographies after exposure to chloroform vapor for (f) 5 min; (g) 10 min. The image size is 30 µm × 30 µm.

process was repeated 4 times, the sample was put into chloroform vapor for 5 min and removed from the container and dried by nitrogen gas. The change of the surface morphology was observed by AFM. Then, the procedure was repeated again. Some rims emerged along the walls in the holes (Figure 3b) after the sample of Figure 3a was treated by carbon disulfide vapor for 5 min. Meanwhile, there were some little ridges on the PS-rich phase domains. After the sample was again put into CS2 vapor for 5 min and removed from the container, the rims continued to grow up. At the same time, the P2VP in the holes elevated (Figure 3c).

Thereafter, these rims changed to columns after 15 min (Figure 3d). Islandlike patterns formed on the surface of the films after 20 min (Figure 3e). The typical size and height of the islands were 4.0 µm and 200 nm, respectively. After the sample was put into chloroform vapor for 5 min and removed from the container and dried by nitrogen gas, the height of the column decreased (Figure 3f). When the sample was again kept in chloroform vapor for 5 min and removed and dried, the islandlike patterns reverted to porous film with time gradually (Figure 3g). We note that the ridges on the PS-rich phase domains changed to small dents. The typical size and depth of the holes are

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4.4 µm and 275 nm, respectively. The size and depth of the holes are larger than the original ones. The reversible change of topographies can be realized through two different group solvents. Islandlike patterns can form on the surface of the porous film after being treated by CS2, TOL, or THF vapor. Furthermore, ethanol, chloroform, MEK, or DMF treatment can make the islandlike patterns come back to the ordered porous films. What is the reason that the micropatterned surface may exhibit a reversibly responding behavior to water and different solvents? We propose that there are two different mechanisms. First, water could not dissolve PS and P2VP but could swell P2VP. The formation of islands originated from the volume increase of P2VP after P2VP absorbed water due to its hygroscopic property. In contrast to P2VP, the ability of PS adsorbing water was very small due to its hydrophobic property. There was nearly no volume change after the PS was treated by water. Therefore, the islandlike structures emerged in the holes when the porous film was kept in water for some minutes due to swelling of P2VPrich domains. Thereafter, the volume would increase gradually over time. Once the sample was withdrawn from the water and dried, the volume would decrease gradually due to the evaporation of water. The islandlike topography would come back to the original state. The main reason for the reversible change to solvents is that different strength of the polymer/solvent interaction which is typically expressed by Flory-Huggins interaction parameters χ between polymer and the solvent. Generally, χ is estimated from solubility parameters by using the following expression:31

χ)

Vs [(δ -δ )2 +(δp1-δp2)2 +(δh1-δh2)2] RT d1 d2

(1)

where R is the gas constant, T is the temperature, Vs is the molar volume of the solvent. δd, δp, and δh are the Hansen’s parameters. δd is the dispersive term, and δp is the polar term. δh accounts for a variety of association bonds, including hydrogen bonds and permanent dipoleinduced dipole. For P2VP, diverse terms of the Hansen’s parameters are not available in the literature. The FloryHuggins interaction parameters χ between P2VP and the solvents cannot be calculated. We can determine the solubility difference between PS and P2VP qualitatively through comparing the solubility difference between PS and poly(methyl methacrylate) (PMMA) in diverse solvents since the solubility difference between PS and PMMA can be found in the literature.30,32-34 If the solubility of PS is higher than that of PMMA in the same solvent, then the solubility of PS must be higher than that of P2VP. Reversely, if the solubility of PMMA is higher than that of PS in the same solvent, then the solubility of P2VP must be higher than that of PS. From the literature,30,32-34 we can know that PS has a higher solubility in TOL, THF than PMMA. CS2 can dissolve PS but not PMMA. PMMA has a higher solubility in chloroform, MEK, DMF than PS. Both ethanol and water cannot dissolve PS and PMMA. From the above results, we can conclude that TOL and THF have higher solubilities for PS than that for P2VP (31) Brandrup, J.; Immergut, E. H.; Grulke, E. A. Polymer Handbook, 4th ed.; John Wiley & Sons: New York, 1999. (32) Xuan, Y.; Peng, J.; Cui, L.; Wang, H. F.; Li, B. Y.; Han, Y. C. Macromolecules 2004, 37, 7301. (33) Ton-That, C.; Shard, A. G.; Bradley, R. H. Polymer 2002, 43, 4973. (34) Dekeyser C. M.; Biltresse, S.; Marchand-Brynaert, J.; Rouxhet, P. G.; Dupont-Gillain, Ch. C. Polymer 2004, 45, 2211.

Figure 4. Column plots of exposure time of different solvents resulting in the same morphology change from (a) holes to islands and (b) islands to holes.

(defined as the first group solvents (I)) and chloroform, MEK, and DMF have higher solubilities for P2VP than that for PS (defined as the second group solvents (II)). For a further demonstration of the difference in solubility between PS and P2VP in diverse solvents, the dissolving time of 30 wt % PS and P2VP in different solvents were observed. We found that less time is needed for CS2, TOL, and THF to dissolve PS than P2VP and more time is needed for chloroform, MEK, and DMF to dissolve PS than P2VP. In addition, CS2 could dissolve P2VP (may be because the molecular weight of P2VP is very low). Ethanol only dissolved P2VP and could not dissolve PS. Then, we discuss the reason of a reversibly responding behavior to different solvents. When both PS and P2VP phase exposed to CS2, TOL, and THF solvent vapors (the first group solvents (I)), they would swell due to the absorption of some solvents. It was observed by Krausch et al.35 through ellipsometry that, when the samples were exposed to solvent vapors continuously, TOL and THF led to only weak swelling of P2VP, whereas PS was swollen the most. Why did the holes change to islands after the solidification of the samples? We propose that there are two key points. First, the minor P2VP domains are distributed in the continuous PS surface. When they are swollen by solvent, the isolated P2VP phase will show more strong swelling behavior than the continuous PS phase. Second, after the film is removed from the solvent vapor, the lower solubility phase (P2VP phase) will solidify first, whereas the higher solubility phase (PS phase) will still be able to shrink. Therefore, islandlike P2VP domains are frozen on the film surface when the samples exposed to the first group solvents (I) are removed to the atmosphere. When the islandlike pattern surface expose to (35) Elbs, H.; Krausch, G. Polymer 2004, 45, 7935

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Figure 5. Porous films with hole depth of 730 nm ((a) topographic and (b) frictional images) after kept into water for 8 h ((c) topographic and (d) frictional images). (e) the schematic illustrations for the change of P2VP swollen by water when the holes depth is deeper.

ethanol, chloroform, MEK, and DMF solvent vapors (the second group solvents (II)), the PS and P2VP will absorb some solvents and further swell. After the film is removed from the solvent vapor quickly, the lower solubility phase (PS phase) will solidify first, whereas the higher solubility phase (P2VP phase) will still be able to shrink. Since the second group solvents have higher solubility for P2VP than for PS, the volume shrinking speed of the minor P2VP phases is quicker than that the continuous PS phase. Thereafter, islandlike patterns change to the holes because

the collapse of P2VP is frozen after the sample is withdrawn from the second group solvents. Furthermore, the different time is needed for the two group solvents to drive the morphologies change. Figure 4 presents the different exposure times of the first group of solvents resulting in a morphology change from holes with 4.0 µm size and 70 nm depth to islands with 3.0 µm size and 55 nm height and those of the second group of solvents resulting in the reversed change, respectively. We propose that the time difference is mainly determined

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Figure 6. (Top) AFM topographic images: (a) the as-cast film and the porous film was treated by THF vapor for (b) 1 h and (c) 2 h. (Down) The profiles along the lines in the corresponding top images.

by the ability to dissolve the minor P2VP phase. We have known that the ability of the first group of solvents to dissolve P2VP from least to greatest is CS2 < TOL < THF. From Figure 4a, we can conclude that the exposure time increases with the increase of the ability of solvents dissolving P2VP when the morphology is induced from the holes to islands. After the film was removed from the solvent vapor, the order of the solidity of P2VP is CS2 > TOL > THF. Therefore, more time is needed for the solvents which have bigger solubility for P2VP to reach the island structure. On the other hand, the abilities of solvents dissolving P2VP are all very strong for the second group of solvents. After the sample with islandlike structures is put into the second group of solvents, the swollen P2VP phase will be shrunk to holes and is frozen after the sample is withdrawn from solvent vapors. It is found that the exposure time increases with the increase of boiling point of the solvents except ethanol (boiling point (°C): ethanol, 78; chloroform, 61; MEK, 80; DMF, 153). The higher the boiling point of the solvent is, the lower the vapor pressure of the solvent is. The amount of solvent adsorbed by P2VP decreases with a decrease of the vapor pressure of a solvent leading to the increase of polymer concentration. The polymer-solvent interaction parameters strongly decrease with increasing polymer concentration.35 Therefore, the actual ability of solvents dissolving P2VP decreases with the increase of the boiling point of the solvents leading to the increase of the exposure time when the morphology changes from the islands to holes. The exposure time of ethanol vapor is shortest among these solvents. It may be because ethanol is the strong selective solvent for the P2VP phase. The depth of hole which can be changed by varying the humidity may influence the restructuring behavior. Figure 5a,b shows the topographic and frictional images of PS/ P2VP(5/1) (w/w) blend films cast from THF solution at a concentration of 4 wt % in a higher humidity of 70% than that of Figure 1a. The hole depth is 730 nm, larger than 265 nm of Figure 1a. After being kept in water for enough time, the hole depth increased to 780 nm and hole size increased to 4.5 µm There was no restructuring process. Only some P2VP droplets were distributed on the film surface (Figure 5c,d). From the frictional image, we cannot see an obvious contrast. The increase of hole depth led to the increase of the areas of P2VP distribution in the holes.

Because the amount of P2VP was fixed, there was little P2VP phase at the bottom of the holes compared to the shallow ones. The P2VP distributed mainly in the interior walls of holes in the deeper holes. Figure 5e showed the schematic illustration for the change of P2VP swollen by water when the hole depth was very deep. After the sample was put into water, the P2VP phase was swollen. The repellence between PS and the P2VP phase absorbing water increased due to the hydrophobicity of PS. Meanwhile, the movement ability of P2VP increased due to the permeation of water. Because they could not be dissolved by water, the P2VP could not enter into water directly. Thus, P2VP would move to the film surface along the walls and spread on the film surface. Hereafter, they distributed isolated as little droplets on the film surface in order to decrease the surface energy after the water evaporation. Therefore, there was no appearance of the structure restructuring. Meanwhile, the restructuring behavior of solvent vapor is influenced by the hole depth as well. Figure 6 shows the surface morphology development as a function of time when the porous film (430 nm in depth) is treated by THF vapor. Figure 6a is the initial topographic image which is obtained after the PS/P2VP (4/1) (w/w) solution in THF at a concentration of 5 wt % cast onto the mica substrate in 50% humidity. The typical size and depth of the hole are 2.8 µm and 430 nm, respectively. Figure 6b exhibits the image after this porous film is kept in THF vapor for 1 h and then removed to the atmosphere. The typical size and depth of the hole change to 2.0 µm and 90 nm, respectively. Meanwhile, we can see that the level plateau domains change to dome from the line profile. When the sample is kept in THF vapor for another 2 h, the typical size and depth of the hole decrease to 1.1 µm and 50 nm (Figure 6(c)), respectively. The sample was destroyed when further exposed to THF vapor for some time. There is no appearance of an islandlike pattern during the THF vapor treatment. From the change of holes size and depth, it can be concluded that both P2VP phase in the holes and PS phase around the holes are swollen by THF vapor. The increase of P2VP phase volume makes the hole bottom elevate. At the same time, the PS phase is also swollen in the horizontal and perpendicular directions. The plateau domains hunch and the holes shrink which results in the decrease of hole size and surface undulation. Owing to

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the fact that there is not enough P2VP to fill in the holes, the islandlike pattern is not on the surface before the film is destroyed. From the change of hole depth and size, we can also see that the change of the continuous PS phase is mainly in the horizontal direction and the change of the minor P2VP phase is mainly in the vertical direction. 4. Conclusions In summary, we present a facile process to fabricate micropatterned polymer surfaces with reversibly switchable morphology from an ordered honeycomblike structure to hexagonal islands by water and different solvent vapors treatment. The reason for the reversibly responding behavior to water originates from the volume increase of P2VP absorbing water due to its hygroscopic property, and the volume decrease is due to the removal of water. The reversible change of topographies to diverse solvents can be realized through two different group solvents. Generally, those solvents that have higher solubility for PS than that for P2VP will lead the porous films to switch into an ordered island-like pattern due to a lesser extent to volume contraction of P2VP than that of PS. The

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exposure time increases with the increase of the ability of solvents dissolving P2VP when the morphology is induced from the holes to islands, whereas those solvents that have a higher solubility for P2VP than that for PS can induce the island-like pattern into a porous film due to a greater extent of volume shrinkage of P2VP than that of PS. The exposure time increases with the increase of boiling point of the solvents except ethanol when the morphology changes from the islands to holes. The appropriate control of the hole depth is important in determining the reversible changes. Acknowledgment. This work is subsidized by the National Natural Science Foundation of China (50125311, 20334010, 20274050, 50390090, 50373041, 20490220, 20474065, and 50403007), the Ministry of Science and Technology of China (2003CB615601), the Chinese Academy of Sciences (Distinguished Talents Program, KJCX2SW-H07), and the Jilin Distinguished Young Scholars Program (20010101). LA051376Z