pubs.acs.org/Langmuir © 2011 American Chemical Society
Solvent and Substrate Contributions to the Formation of Breath Figure Patterns in Polystyrene Films Elisa Ferrari,* Paola Fabbri, and Francesco Pilati Department of Materials and Environmental Engineering, University of Modena and Reggio Emilia, Strada Vignolese 905/a, 41125 Modena, Italy Received November 11, 2010. Revised Manuscript Received December 23, 2010 The generation of ordered porous polymer structures by the breath figures (BFs) method has long been described as a complex phenomenon, in which several parameters combine in a fairly unknown way. The type of polymer and solvent, degree of humidity, and additives are just a few examples of the several parameters that have been described as playing a role in the generation of BFs. This work reports a detailed investigation over the role played by the solvent in the process of BFs generation from polystyrene (PS) solutions spread over different substrates, and discusses the geometrical aspects of the pores via a quantitative point of view by using a purposely developed software for image analysis. Results show that thermodynamic affinity between polymer and solvent is the key parameter for BFs formation, along with other solvent characteristics such as water miscibility, boiling point, and enthalpy. According to our findings, the role played by the substrate is strictly related to the type of solvent used in the generation of BFs.
1. Introduction Breath figures (BFs) formation has long been a fascinating phenomenon in nature and was first investigated by Aitken1 and by Lord Rayleigh2 almost 100 years ago. Only in 1994 did Widawski et al.3 demonstrate how BFs can be exploited to produce porous polymer films that in principle can be used as templates in many fields (tissue engineering,4 photonics,5,6 etc.). When moist air is blown over a polymer solution, evaporative cooling can lead to the formation of water droplets on the liquid surface. These droplets can be arranged into a highly ordered hexagonal array because of different thermofluidodynamic mechanisms such as, for instance, Marangoni convection.7-15 Complete evaporation of both solvent and water results in formation of 2D or even 3D arrays of holes, often ordered hexagonally.7,15 This technique appears extremely attractive, as it allows production of porous films with tunable pore size in the range from 0.2 to 20 μm7 through the preparation conditions. In fact, several parameters can affect BFs formation,16 such as type of *To whom correspondence should be addressed. E-mail: elisa.ferrari@ unimore.it. (1) Aitken, J. Nature 1911, 86, 516. (2) Rayleigh, L. Nature 1911, 86, 416. (3) Widawski, G.; Rawiso, M.; Franc- ois, B. Nature 1994, 369, 387. (4) Beattie, D.; Wong, K. H.; Williams, C.; Poole-Warren, L. A.; Davis, T. P.; Barner-Kowollik, C.; Stenzel, M. H. Biomacromolecules 2006, 7(4), 1072. (5) Yabu, H.; Takebayashi, M.; Tanaka, M.; Shimomura, M. Langmuir 2005, 21, 3235. (6) Park, M. S.; Kim, J. K. Langmuir 2005, 21, 11404. (7) Srinivasarao, M.; Collings, D.; Philips, A.; Patel, S. Science 2001, 292, 79. (8) B€oker, A.; Lin, Y.; Chiapperini, K.; Horowitz, R.; Thompson, M.; Carreon, V.; Xu, T.; Abetz, C.; Skaff, H.; Dinsmore, A. D.; Emrick, T.; Russel, T. P. Nat. Mater. 2004, 3, 302. (9) Karthaus, O.; Maruyama, N.; Cieren, X.; Shimomura, M.; Hasegawa, H.; Hashimoto, T. Langmuir 2000, 16, 6071. (10) Peng, J.; Han, Y.; Yang, Y.; Li, B. Polymer 2004, 45, 447. (11) De Boer, B.; Stalmach, U.; Nijland, H.; Hadziioannou, G. Adv. Mater. 2000, 12, 1581. (12) Maruyama, N.; Koito, T.; Nishida, J.; Sawadaishi, T.; Cieren, X.; Ijiro, K.; Karthaus, O.; Shimomura, M. Thin Solid Films 1998, 327, 854. (13) Yabu, H.; Tanaka, M.; Ijiro, K.; Shimomura, M. Langmuir 2003, 19, 6297. (14) Nishikawa, T.; Ookura, R.; Nishida, J.; Sawadaishi, T.; Shimomura, M. RIKEN Rev. 2001, 37, 43. (15) Pitois, O.; Franc-ois, B. Eur. Phys. J. B 1999, 8, 225. (16) Bunz, U. H. F. Adv. Mater. 2006, 18, 973.
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polymer and solvent, casting and environmental conditions, and also the presence of additives.17 A variety of polymers have been reported to be able to form ordered microporous structures by this “moist” casting method with appropriate solvents, including linear homopolymers,7-10 rod-coil or coil-coil block copolymers,3,9,11,18 starlike homopolymers or copolymers,3,11,19 and amphiphilic polyion complexes.9,12,13 Interestingly, most of the polymers have polystyrene as a component. Generally, the solvents used in making the porous films are highly volatile and water-immiscible. The most reported solvents include carbon disulfide, benzene, toluene, chloroform, dichloromethane, 1,2dichloroethane, and 1,1,2-trichlorotrifluoroethane.16 Although many studies on the hole pattern formation have been reported, correlation between polymer properties and experimental conditions and the formation of BFs surface patterning is far from being completely understood. For instance, polystyrene (PS), with or without polar end groups, has been investigated in several studies using MWs in the range 10 000-700 000 and either static or dynamic humid environments as evaporative conditions.8,10,11,20-26 Some authors obtained ordered BFs using linear PS,8,10,20-22 while others claimed the opposite,11,23-27 even for similar experimental conditions. A recent paper by Peng and co-workers10 indicates that it is possible to obtain ordered BFs on linear PS using toluene as solvent, and it suggests that the viscosity (17) Pilati, F.; Montecchi, M.; Fabbri, P.; Synytska, A.; Messori, M.; Toselli, M.; Grundke, K.; Pospiech, D. J. Colloid Interface Sci. 2007, 315, 210. (18) Franc-ois, B.; Pitois, O.; Franc-ois, J. Adv. Mater. 1995, 7, 1041. (19) Franc-ois, B.; Ederle, Y.; Mathis, C. Synth. Met. 1999, 103, 2362. (20) Limaye, A. V.; Narhe, R. D.; Dhote, A. M.; Ogale, S. B. Phys. Rev. Lett. 1996, 76(20), 3762. (21) Zander, N. E.; Orlicki, J. A.; Karikari, A. S.; Long, T. E.; Rawlett, A. M. Chem. Mater. 2007, 19(25), 6145. (22) Wu, C. Y.; Chiang, T. H.; Hsu, C. C. Opt. Express 2008, 16(24), 19978. (23) Bolognesi, A.; Mercogliano, C.; Yunus, S.; Civardi, M.; Comoretto, D.; Turturro, A. Langmuir 2005, 21, 3480. (24) Wong, K. H.; Hernandez-Guerrero, M.; Granville, A. M.; Davis, T. P.; Barner-Kowollik, C.; Stenzel, M. H. J. Porous Mater. 2006, 13, 213. (25) Madej, W.; Budkowski, A.; Raczkowska, J.; Rysz, J. Langmuir 2008, 24(7), 3517. (26) Sun, W.; Ji, J.; Shen, J. Langmuir 2008, 24(20), 11338. (27) Ghannam, L.; Manguian, M.; Francois, J.; L. Billon, L. Soft Matter 2007, 3, 1492.
Published on Web 01/12/2011
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Article Table 1. Solvents Used to Produce PS BFs, and Their Properties
solvent
density34 @ 20 °C (g/cm3)
viscosity34 @ 25 °C (mPa 3 s)
solubility in water35 @ 20 °C (g/100 mL)
0.7899 1.2632 1.4832 0.9003 0.8054
0.306 0.352 0.537 0.423 0.405
miscible 0.22 0.8 7.9 29
23.46 31.58 26.5 23.39 23.97
1.3266 0.8892 0.8669
0.413 0.456 0.560
2 miscible 0.05
27.2 26.4 27.93
acetone carbon disulfide chloroform ethyl acetate methyl ethyl ketone dichloromethane tetrahydrofuran toluene
interfacial enthalpy of surface tension34 tension with @ 25 °C water36,37 @ 25 °C boiling point34 vapor pressure34 vaporization34 (°C) @ 25 °C (kPa) (kJ/mol) (mN/m) (mN/m) 48.1 33.5 6.8 1.0 28.3 36.1
56 46 61 77 80
30.8 48.2 26.2 12.6 12.6
30.99 27.51 31.28 35.60 34.79
40 65 111
58.2 21.6 3.79
28.82 31.99 38.01
Table 2. Experimental Details on Substrates Preparation and Calculated Surface Energies substrate glass, G glass, GW silicon wafer, WSi silanized glass, GWG silanized glass, GWO fluorinated glass, GWF
treatment treated with piranha solution (H2SO4/H2O = 5:1) for 15 min washed with RCA1 solution (NH4OH/H2O2/H2O = 1:1:6) @ 70 °C for 30 min treated with piranha solution (H2SO4/H2O = 5:1) for 15 min and then silanized with 3-glycidoxypropyltrimethoxysilane treated with piranha solution (H2SO4/H2O = 5:1) for 15 min and then silanized with octyltriethoxysilane treated with piranha solution (H2SO4/H2O = 5:1) for 15 min and then treated with a fluorinate silane
polyethylene, PE polyvinylchloride, PVC polyethylene terephthalate, PET
of the system is the key factor in the formation of ordered structures, rather than the presence of polar groups. Zander et al.21 agree with results obtained by Peng,10 and also B€oker et al.8 and Wu et al.22 were able to produce porous PS films under various experimental conditions. For Franc-ois et al.,18,19 attempts to prepare honeycomb membranes from linear polystyrenes were unsuccessful, while they obtained BFs using star shaped PS, PS block copolymers, and linear PS with a polar terminal groups. Also, Bolognesi et al.23 and Srinivasarao et al.7 were able to prepare BFs using PS with polar end groups. Similar results have been obtained by Billon et al.,27 Wong et al.,24 Sun et al.,26 and Madej.25 Each of them claimed that the presence of polar end groups is mandatory to get ordered BFs. Although it is well accepted that the presence of polar groups or additives can promote the formation of ordered BFs,12,17,27-30 the above-reported controversial results confirm that, at the moment, the phenomenon of BFs formation is simple to be exploited but difficult to be completely understood. In the complex framework of scientific discussion over this topic, this work brings a contribution of detailed investigation over the role played by the solvent in the process of BFs formation from linear PS solutions. Discussion has also been enriched with evaluations on the importance of the substrate used to generate the porous film, which has been scarcely commented in the literature until now,27,31-33 while we demonstrate that it is a central piece to draw a coherent puzzle. Finally, since few papers (28) Fukuhira, Y.; Kitazono, E.; Hayashi, T.; Kaneko, H.; Tanaka, M.; Shimomura, M.; Sumi, Y. Biomaterials 2006, 27(9), 1797. (29) Sun, H.; Li, H.; Wu, L. Polymer 2009, 50, 2113. (30) Jiang, X.; Zhou, X.; Zhang, Y.; Zhang, T.; Guo, Z.; Gu, N. Langmuir 2010, 26(4), 2477. (31) Cheng, C.; Tian, Y.; Shi, Y.; Tang, R; Xi, F. Langmuir 2005, 21, 6576. (32) Wang, C. Y.; Mao, Y. D.; Wang, D. Y.; Qu, Q. S.; Yang, G. J.; Hu, X. Y. J. Mater. Chem. 2008, 18, 683. (33) Li, X.; Wang, Y.; Zhang, L.; Tan, S.; Yu, X.; Zhao, N.; Chen, G.; Xu, J. J. Colloid Interface Sci. 2010, 350, 253.
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water CA (deg)
surface energy (mN/m)
59.6 ( 3.4 28.0 ( 2.6 55.8 ( 2.3 52.5 ( 3.6
43.1 62.8 48.1 49.0
32.6 ( 2.6
61.5
107.1 ( 1.4
13.8
87.0 ( 2.4 79.5 ( 2.2 71.7 ( 2.2
34.5 38.0 41.0
commented on the quantitative aspects of BFs formation and pore distributions, we report the results obtained by using purposely developed software for studying the regularity of pore shape and order.
2. Experimental Section 2.1. Materials. Linear PS with a molecular weight of 192 000 g/mol was purchased from Sigma-Aldrich (Milan, Italy) and used without further purification. All solvents, listed in Table 1, were purchased from Sigma-Aldrich (Milan, Italy) and used as received. 2.2. Preparation and Characterization of Porous PS Films. PS solutions (1 wt %/vol) were prepared at room temper-
ature and spread (liquid film thickness: 250 μm) on various substrates inside a glovebag previously inflated with moist air (static condition). Alternatively, the porous films were obtained under a vertical flow of humid air (dynamic condition, flow rate from 0.4 to 1.8 L/min). The humid air passed through a glass frit before reaching the solution. Moist air (RH = 75 ( 2%) was produced using a humid air generator (HG-1 Humidity Generator, Michell Instruments). Nine substrates with different surface energies and wettabilities were prepared according to the treatments shown in Table 2. Contact angles were measured using either bidistilled water or polymer solution as drop phase at room temperature for all substrates using a DataPhysics OCA20 instrument in open-air atmosphere. Reported values were averaged over at least five drops (Table 2). Surface energy was calculated from contact angle measurements by applying the approach proposed by Owens and Wendt38 (liquids employed: bidistilled water, n-hexadecane, formamide, (34) Lide, D. R. CRC Handbook of Chemistry and Physics, 80th ed.; CRC Press: Boca Raton, FL, 1999. (35) Various MSDS. (36) Demond, A. H.; Lindner, A. S. Environ. Sci. Technol. 1993, 27(12), 2318. (37) Freitas, A. A.; Quina, F. H.; Carroll, F. A. J. Phys. Chem. B 1997, 101(38), 7488. (38) Owens, D. K.; Wendt, R. C. J. Appl. Polym. Sci. 1969, 13, 1741.
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Figure 1. SEM micrographs of surface patterns formed from 1 wt %/vol PS solutions on GW in static conditions (RH = 75%, 23 °C): (a) in chloroform, (b) in dichloromethane, (c) in carbon disulfide, and (d) in methyl ethyl ketone. diiodomethane). The surface-patterned films were observed by scanning electron microscopy (SEM; ESEM Quanta-200, Fei Company-Oxford Instruments) operating at 1.5-10 kV accelerating voltage. Samples were made conductive by deposition of a gold layer (10 nm) in a vacuum chamber. A quantitative evaluation of the order of BFs patterns was made by using the Voronoi polygon construction.20,39-41 To run this evaluation, a software able to analyze SEM images was developed. SEM images were elaborated to obtain black and white images at high resolution. The software automatically generates the Voronoi tessellation and calculates the entropy of the pattern. In addition, hole dimensions and circularity were evaluated by using the freeware image analysis software called ImageJ.
3. Results and Discussion The aim of this paper is to investigate the possibility to generate BFs and to predict and design the main features of BFs patterns (i.e., pore size and degree of order of the patterns) by deriving correlations with the kind of solvent and substrate used. For this purpose, a polymer soluble in various solvents such as PS was chosen. 3.1. Effect of the Solvent. As the solvent used to dissolve the polymer is expected to be of utmost importance,42 playing a role in almost all the phenomena involved in BFs generation (surface cooling, solidification rate, etc.), various solvents have been used in this study as listed in Table 1. Solvents were selected having either lower or higher density than water, complete or partial miscibility, and interfacial tension with water ranging from 1 to 48 mN/m (at 25 °C). This selection was purposely wide enough to allow comparison with several former studies reported in the literature, frequently reporting inconsistent results. Figure 1 reports examples of extended area with ordered BFs that we obtained under static humid environment. No or poor-ordered BFs or other morphologies were obtained using acetone, ethyl acetate, tetrahydrofuran (THF), and toluene. Confirming what is reported in the literature,8,21 chloroform and dichloromethane allowed the formation of regular patterns (Figure 1a and b), differently from carbon disulfide, that did not allow the formation of an extended hexagonal array of pores on hydrophilic glass GW (Figure 1c) as previously described by Wu et al.;22 it seems that a too fast solidification of the polymer film occurred, favored by the relatively low boiling point and high vapor pressure of carbon disulfide. The other solvents used were unable to generate porous structures, independently from the substrate. Interestingly, after solvent evaporation, the surface of samples prepared from acetone, methyl ethyl ketone, and THF was characterized by the presence of polymer microspheres rather than porous polymer films (see Figure 1d). This last behavior, (39) Steyer, A.; Guenoun, P.; Beysens, D.; Knobler, C. M. Phys. Rev. B 1990, 42 (1), 1086. (40) Knobler, M.; Beysens, D. Europhys. Lett. 1988, 6, 707. (41) Park, M. S.; Kim, J. K. Langmuir 2004, 20, 5347. (42) Servoli, E.; Ruffo, G. A.; Migliaresi, C. Polymer 2010, 51, 2337.
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which leads to morphologies quite similar to those recently described and named reverse breath figures,43 will be discussed in more detail in an upcoming work. One main question arising from these results is about which solvent characteristics are essential to generate ordered BFs. Table 1 reports the main characteristics of the solvents used. Of course, at room temperature and under static conditions, low boiling point (high vapor pressure) and low boiling enthalpy are prerequisites to induce a surface cooling able to start water condensation from the environment.27,44 However, this is not enough to explain the observed results. Acetone has a boiling point lower than that of chloroform and similar boiling enthalpy, but it does not allow the formation of BFs. In several studies,7,10,20 BFs were generated with solvents having either lower or higher density than that of water, but, under our experimental conditions, ordered BFs patterns were obtained only with solvents having density higher than that of water. In addition, it has to be noticed that BFs were obtained only for solvents with a low miscibility with water. Such a requirement has already been confirmed, and for this reason it was recently claimed that carbon disulfide is the best solvent for generating honeycomb films from polystyrene solutions.27 As shown by Figure 1 and Table 1, solvents such as dichloromethane and chloroform, having higher water solubility than carbon disulfide, give more regular BFs patterns on hydrophilic glass substrate. Low water solubility in the solvent is a requirement, but the lowest solubility is not necessarily the best one. A further related possible explanation can be based on the interfacial tension between solvent and water. All the solvents with a high interfacial tension with water, except toluene, were able to generate BFs. This can be explained by considering that toluene has low volatility and evaporates too slowly at room temperature to allow the required surface cooling to condense water on the surface of the solution. This is in agreement with Billon et al.27 and Tian et al.,44 who concluded that under static conditions only solvents with a low boiling point can form BFs. Indeed, the formation of BFs from toluene solutions of PS has been reported7 under a flow of humid air, which promotes faster toluene evaporation. In a recent paper, Tian and co-workers44 suggested that the formation of honeycomb structures depends on the thermodynamic affinity between polymer and solvent. They investigated the behavior of PPO in various solvents and concluded that a thin polymer film can be formed on the surface of water droplets only for good solvents. This film decreases the surface tension between the solvent and the water droplets, thus hindering their coalescence. When a poor solvent is used, migration of polymer chains to the water/solution interface is restricted, resulting in coalescence of water droplets and poor regularity of pores or no BFs formation. Inspired by the work of Tian et al., we used Hansen (43) Xiong, X.; Zou, W.; Yu, Z.; Duan, J.; Liu, X.; Fan, S.; Zhou, H. Macromolecules 2009, 42, 9351. (44) Tian, Y.; Jiao, Q.; Ding, H.; Shi, Y.; Liu, B. Polymer 2006, 47, 3866.
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Article Table 3. Hansen’s Parameters of PS and Solvents and Their RED
solubility parameters45 (HSP) to evaluate the thermodynamic affinity between PS and the solvents. The approach followed in the present paper is based on the solubility parameter distance Ra (MPa1/2), Ra2 ¼ 4ðδD2 - δD1 Þ2 þ ðδP2 - δP1 Þ2 þ ðδH2 - δH1 Þ2 where δDi, δPi, and δHi are the HSP components for polymer and solvent, respectively. Solubility, or high thermodynamic affinity, requires Ra to be lower than Ro, where Ro is the radius of interaction of a HSP sphere. The ratio Ra/Ro has been called the relative energy difference (RED) number. If the RED number equals zero, there is no energy difference. RED numbers lower than 1 indicate high thermodynamic affinity; RED numbers equal to or close to 1 are a boundary condition, and progressively higher RED numbers indicate progressively lower affinities. RED numbers calculated for our PS/solvent couples are shown in Table 3. According to our experimental results, only solvents with RED lower than 1 were able to generate ordered BFs (cf. Figure 1), while solvents with RED higher than 1 did not allow the formation of the pattern, independently from the substrates on which the solutions were spread. Hence, our findings are in full agreement with the results of Tian et al., supporting the idea that the polymer/solvent thermodynamic affinity is a key parameter for BFs formation. Again, toluene is an exception; with a RED lower than 1, it does not allow generation of BFs patterns on the PS film surface. However, as said before, this can be due to its low volatility, which under our experimental conditions does not provide the necessary cooling. According to the above-reported results, either low water miscibility, interfacial tension with water, or thermodynamic affinity can be used as a criterion to discriminate between solvents able or unable to generate BFs but, at present, it is difficult to state which criterion is the most important. The use of humid airflow instead of static conditions should make solvent evaporation easier and allow BFs generation even for solvents with a relatively high boiling point. However, attempts to generate porous films under dynamic conditions using the apparatus described in the Experimental Section lead to poorly ordered surfaces. 3.2. Geometrical Features of BFs Patterns. The control over pore size and the quantification of the degree of order of the patterns represent important aspects related to BFs formation. An analysis of the pore size (diameter, D) and degree of order (circularity, circ = 4π(area)/(perimeter2) and conformational (45) Hansen, C. M. Hansen Solubility Parameters: A User’s Handbook; CRC Press: Boca Raton, FL, 1999.
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entropy, S) has been performed, using a purposely developed software or the commercially available ImageJ. Results are summarized in Tables 4 and 5. Pores with diameters ranging from about 1 to 6 μm were obtained, depending on the selected solvent and substrate. In general terms, pores showed a high circularity, except chloroform solutions cast on silicon wafers. As far as pore size distribution is concerned, most of the BFs patterns obtained using dichloromethane, carbon disulfide, and chloroform presented a monomodal distribution. On the contrary, PS/chloroform solutions showed a bimodal distribution when spread on hydrophilic glass GW and silanized glass GWO. This agrees with the results recorded by Limaye et al.20 Ordering of pores can be quantitatively evaluated by the construction of Voronoi polygons, defined as the smallest convex polygons surrounding points whose sides are perpendicular bisectors of lines connecting a given point (at the center of a hole) with those of its neighbor holes. The analysis of Voronoi constructions has been performed for the most regular BFs patterns obtained in our experiments, and made in terms of the coordination number n of a polygon (i.e., the number of sides of a Voronoi polygon) and of Pn, the fraction of the number of polygons having the coordination number n. According to the literature,39 a conformational entropy of the patterns can be calculated as S = -ΣPn ln Pn and compared with the value 1.71 obtained for the case of a random distribution and with the zero packing entropy of a perfectly ordered array.20 In our calculations, n equal to 5, 6, or 7 has been considered. The lowest S of conformation, that is, maximum order of BFs array, was obtained using PS/dichloromethane solutions spread on either silanized glass GWO (S = 0.51) or piranha treated glass GW (S = 0.76). Higher S values, that is, less ordered arrays, were obtained for carbon disulfide on glass (S = 0.87) and chloroform on silanized glass GWO, hydrophilic glass GW, and silicon wafer (S close to 1). Results obtained for chloroform are comparable with those reported by Limaye et al.20 As the highest pore order was obtained for dichloromethane (see Figure 2c), the solvent with the lowest boiling point and the highest vapor pressure, we have to conclude that a fast evaporation (and the related fast change in the solution viscosity) is not a limiting factor to achieve highly ordered BFs. Attempts to find a quantitative correlation between solvent properties and the order features of the BFs patterns were unsuccessful. 3.3. Effect of the Substrate. Several authors reported different BFs surface patterns using the same polymer solution over different substrates.27,31-33 An obvious question arises: how and why does the substrate affect the BFs pattern formation? In order to investigate the effect of substrate in the formation of BFs on PS DOI: 10.1021/la104500j
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Table 4. Summary of BFs Patterns Prepared from 1 wt %/vol PS Solutions on Various Inorganic Substrates in Static Conditions (RH = 75%, 23 °C, Marker = 100 μm)
films, several substrates, both inorganic and organic, with different surface energies, were prepared and used under the same test conditions. As evidenced in Tables 4 and 5, the substrate plays an important role. 1878 DOI: 10.1021/la104500j
As far as glass is concerned, carbon disulfide was the only solvent able to generate ordered BFs, with holes of about 3 μm. As glass is the most common substrate used to prepare microporous films, this can explain why carbon disulfide is Langmuir 2011, 27(5), 1874–1881
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Table 5. Summary of BFs Patterns Prepared from 1 wt %/vol PS Solutions on Various Organic Substrates in Static Conditions (RH = 75%, 23 °C, Marker = 100 μm)
considered the best solvent to prepare BFs from PS solutions.27 However, with different inorganic substrates, either poor or no BFs were formed. Focusing on organic substrates, according to the results reported by Ghannam and co-workers,27 we obtained Langmuir 2011, 27(5), 1874–1881
ordered BFs also on PVC support. PE substrate allowed the formation of less ordered BFs patterns, while not hexagonally packed pores were obtained spreading the PS/carbon disulfide solutions on PET films. DOI: 10.1021/la104500j
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Figure 2. SEM micrographs of porous films generated from 1 wt %/vol PS/dichloromethane in static conditions (RH = 75%, 23 °C) on (a) PET, (b) PVC, (c) silanized glass GWO, and (d) piranha treated glass GW.
Dichloromethane led to very ordered patterns using GWO and GW substrates (S equal to 0.51 and 0.76, respectively), but again no BFs were formed with other inorganic substrates. Interesting results were recorded with the PS/dichloromethane solution on organic supports: PET and PVC led to the formation of quite well ordered arrays. Pores formed using the PVC support had a wide variability in terms of diameter, probably because dichloromethane slightly dissolved the PVC surface. However, this is interesting evidence that potentially introduces a new parameter in the process of BFs formation, that is, the interaction between solvent and support. Chloroform is a solvent able to generate BFs on most of the substrates used, including silicon wafer. It is the more robust solvent and less affected by the change of substrate. Of course, an effect of the substrate is still present, as shown in Tables 4 and 5. As already found by several authors, the substrate can affect pore size.27,31-33 Hydrophilicity has been suggested as a parameter that makes the BFs formation easier, and also the wettability of the substrate with the polymer solution has been found to play an important role. Based on wettability results obtained using either water or polymer solutions as the drop phase on mica, glass, or silicon, Xi et al.31 suggested that the generation of BFs patterns is probably dependent on the amount of water adsorbed onto the surface of hydrophilic substrates. Under their experimental conditions, mica was the most suitable substrate being the most hydrophilic and wettable. The authors explained those results by proposing that the formation of water droplets onto the substrate, generating a template, is a key step for the formation of porous films. Similarly, Hu et al.32 investigated the influence 1880 DOI: 10.1021/la104500j
of the substrate on the structure of PS-b-PAA microporous films using mica (hydrophilic and hydrophobic), glass, and silicon wafer as substrates. They found that the pore density increased as hydrophilicity increased, and that it was difficult to form microporous films using hydrophobic mica. Based on the wettability of the substrates with the polymer solution, they concluded that, in agreement with Xi et al.,31 the wetting of solid substrates is beneficial for the periodicity and regularity of pores. More recently, Xu et al.33 studied the effect of the substrate on the formation of BFs patterns on polycyanoacrylate films using glass, silicon, and PP as solid substrate. Contrary to what has been claimed in other papers, the best periodicity and regularity of pores was obtained with the less hydrophilic PP substrate. Wettability measurements led us to conclude that the higher the wettability of the substrate with the polymer solution, the better the BFs patterns, while hydrophilicity was not important. The disagreement with the results previously reported by other authors can be explained by the different hydrophobic nature of the polymer considered; more hydrophobic polymers work better with hydrophobic substrates and vice versa. They tentatively attributed the effect of the substrate to its affinity with the polymer solution which leads to a restricted movement of the polymer chains. Analysis of the literature reveals a confusing picture about the effect of the substrate in the formation of BFs. In our case, all polymer solutions showed contact angles with the various substrates lower than 10°. So we cannot explain the results observed only on the basis of a different wettability of the substrates. However, our attempt to prepare BFs on scarcely wettable fluorinated Langmuir 2011, 27(5), 1874–1881
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substrates (cf. Table 2) was unsuccessful; this confirms that wettability certainly plays an important role. As far as the order of BFs is concerned, it seems that a substrate with higher surface energy promotes the formation of long-range ordered BFs for the PS/chloroform solution, but a completely opposite behavior was observed for the PS/carbon disulfide solution; in this case, the best results were obtained for PVC (with a lower surface tension). According to our findings, there is no a quantitative correlation between the substrate surface energy or wettability and pore size and order. From a qualitative point of view, however, the effect of the substrate (nature, hydrophilicity, wettability) is clear. All the above-reported results suggest that the role of the substrate in the overall mechanism of BFs formation is strictly related to the type of solvent used. A convincing explanation for the experimental evidence it is not yet given; however, on the basis of our and literature results, we can suggest the following mechanism. As it is difficult to expect that the chemical characteristics of the substrate (hydrophilicity, wettability, affinity for the polymer) affect water condensation on the surface solution after application of a polymer solution hundreds of micrometers thick, we have to suppose that the role of the substrate lies in a combined effect with the characteristics of the solution. In particular, the combined effect of solvent and substrate could affect the nucleation of water droplets. Water adsorbed on the substrate before the deposition of the polymer can be removed after the application of the polymer solution and transported to the surface by the Marangoni convective movements. More hydrophilic substrates are able to bind higher amounts of water, and this can make the formation of water nuclei faster when the solution is able to remove efficiently this water from the substrate surface. The solvent characteristics with respect to wettability of the substrate and water solubility can explain the different behaviors of chloroform, carbon disulfide, and dichloromethane. Furthermore, the presence of suitable polar groups in the polymer can favor this step and facilitate the formation of water nuclei. A
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Article
faster formation of water nuclei makes shorter the start of the growth step, which in turn can allow an easier water droplet organization (at shorter times, when the viscosity of the solution is lower) and a longer growth step before solidification (which allows increased pore sizes). This mechanism can also explain the role of the polar groups in PS and of amphiphilic additives; the presence of polar groups or additives can favor and/or stabilize the formation of water nuclei by behaving like surfactants. The abovedescribed complex behavior of polymer/solvent/substrate systems in the mechanism of BFs formation can also explain the controversial results reported by different authors for the same polymer/ solvent system.
4. Conclusions In this paper, we showed that is possible to obtain ordered BFs from linear PS using appropriate solvents and substrates. According to our results, the thermodynamic affinity between polymer and solvent is a key parameter for BFs formation; however, other characteristics of the solvent such as water miscibility, boiling point, and boiling enthalpy can be also important. As already recognized in few previous papers, the solid substrate on which the polymer solution is cast can play an important role both for the pore size and for the pore order. To explain the role of the substrate, we suggest a possible mechanism that qualitatively accounts for hydrophilicity, wettability, and polymer characteristics. Acknowledgment. The authors thank Prof. Giovanni Sebastiano Barozzi of the University of Modena and Reggio Emilia (Italy) for the fruitful scientific discussion about thermodynamic effects occurring in BFs generation. Dr. Elena Fabbri and Dr. Costantino Grana from the same University are acknowledged for SEM analysis and for the development of a dedicated software. This work has been funded by the Italian Ministry of University and Research (PRIN 2007 Program).
DOI: 10.1021/la104500j
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