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Porous Polymer Films and Honeycomb Structures Based on Amphiphilic Dendronized Block Copolymers C. Xia Cheng,† Y. Tian,‡ Y. Qiao Shi,† R. Pei Tang,† and F. Xi*,† State Key Laboratory of Polymer Physics and Chemistry, Joint Laboratory of Polymer Science and Materials, Institute of Chemistry, The Chinese Academy of Science, Beijing 100080, People’s Republic of China, and School of Chemical Engineering and Environment Science, Beijing Institute of Technology, Beijing 100081, People’s Republic of China Received January 23, 2005. In Final Form: May 9, 2005 Fabrication of honeycomb patterned films from our synthesized amphiphilic dendronized block copolymer by “on-solid surface spreading” method and “on-water spreading” method was reported for the first time in this paper. The comparison of the two methods indicated honeycomb-patterned films with smaller size, and larger surface density of micropores can be fabricated by spreading on water but with lower regular arrangement. Furthermore, several influencing factors on the formation of the honeycomb structure and the different morphologies, such as the concentration of the copolymer solution and the relative humidity in the atmosphere and the substrates, were investigated. The results showed that comparably high relative humidity from 80% to 95% was needed, and the mica plate as a spreading substrate was suitable to form orderly porous films for such a copolymer. The best ordered pattern could be formed from the copolymer with concentration of 1.00 mg/mL at the relative humidity of 85% using a mica plate. Besides, strong periodicity, regularity, and a large, defect-free area were notable, which made this structure extremely interesting for applications for templated molecular objects formed via intramolecular metal or metal oxide synthesis.
1. Introduction Macroporous polymers have attracted high interest in recent years for their potential applications in diverse areas such as templates for the synthesis of nanoobjects1 and confined crystallization,2 membranes for selective transport,3 and high surface area supports for catalysis application.4 Furthermore, nanoporous structures with hydrophilic pore environments are useful for biological applications such as antibody or enzyme immobilization and selective transport and separation of biomolecules.5 A large number of methods for preparing micropatterned surfaces have been developed, for example, photolithography,6 photochemical reaction,7 photodecomposition of self-assembled monolayers,8 selective etching,9 microcontact printing,10 and microfluidic networks.11 At the same time, a variety of polymers, such as star and comb * To whom correspondence should be addressed. Fax: +86-106255-9373. Tel.: +86-10-6255-7907. E-mail:
[email protected]. † The Chinese Academy of Science. ‡ Beijing Institute of Technology. (1) (a) Martin, C. R. Science 1994, 266, 1961. (b) Gasparac, R.; Kohli, P.; Paulino, M. O. M.; Trofin, L.; Martin, C. R. Nano Lett. 2004, 4, 513. (2) Ha, J.-M.; Wolf, J. H.; Zalusky, A. S.; Hillmyer, M. A.; Ward, M. D. J. Am. Chem. Soc. 2004, 126, 3382. (3) Hulteen, J. C.; Jirage, K. B.; Martin, C. R. J. Am. Chem. Soc. 1998, 120, 6603. (4) Mitchell, D. T.; Lee, S. B.; Trofin, L.; Li, N.; Nevanen, T. K.; Soederlund, H.; Martin, C. R. J. Am. Chem. Soc. 2002, 124, 11864. (5) Nevanen, T. K.; Soederlund, H.; Martin, C. R. Science 2002, 296, 2198. (6) Nicolau, D. V.; Yaguchi, T.; Taniguchi, H.; Yoshikawa, S. Langmuir 1999, 15, 3845. (7) Pritchard, D. J.; Morgan, H.; Cooper, J. M. Angew. Chem., Int. Ed. 1995, 34, 91. (8) Bhatia, S. K.; Hickman, J. J.; Ligler, F. S. J. Am. Chem. Soc. 1992, 114, 4432. (9) Yoshida, M.; Asano, M.; Suwa, T.; Reber, N.; Spohr, R.; Katakai, R. Adv. Mater. 1997, 9, 757. (10) Lahiri, J.; Ostuni, E.; Whitesides, G. M. Langmuir 1999, 15, 2055. (11) Delamarche, E.; Bernard, A.; Schmid, H.; Michel, B.; Biebuyck, H. Science 1997, 276, 779.
polymers, and block copolymers were used to obtain honeycomb-structure films utilizing a simple method.12 However, various influencing factors on the porous formation process are not studied systematically. Recently, we have reported the first synthesis of dendronized polymers and their block copolymers via atom transfer radical polymerization.13,14 The steric hindrance imposed by the dendrons causes the polymer coil to unwind and behave as a rod.15 Amphiphilic hybrid linear-dendritic rod diblock copolymers have been found to assemble into complex micelles in solution and form Langmuir and Langmuir-Blodgett films on air-water interfaces and monolayers at surfaces.14,16 In this article, this simple method that utilizes the condensation of monodisperse water droplets on the polymer solution, as pioneered by Franc¸ ois et al.,17 was used to prepared honeycomb macroporous films based on our synthesized amphiphilic (12) (a) Stenzel, H. M. Aust. J. Chem. 2002, 55, 239. (b) Karthaus, O.; Maruyama, N.; Cieren, X.; Shimomoura, M.; Hasegawa, H.; Hashimoto, T. Langmuir 2000, 16, 6071. (c) Widawski, G.; Rawiso, M.; Francois, B. Nature 1994, 369, 387. (d) Nishikawa, T.; Nishida, J.; Ookura, R.; Nishimura, S.-I.; Wada, S.; Karino, T.; Shimomura, M. Mater. Sci. Eng. C 1999, 8-9, 495. (e) Karthaus, O.; Cieren, X.; Maruyama, N.; Shimomoura, M. Mater. Sci. Eng. C 1999, 10, 103. (13) Cheng, C. X.; Tang, R. P.; Zhao, Y. L.; Xi, F. J. Appl. Polym. Sci. 2004, 91, 2733. (14) Cheng, C. X.; Tang, R. P.; Xi, F. J. Polym. Sci., Part A: Polym. Chem. 2005, 43, 2291. (15) (a) Prokhorova, S. A.; Sheiko, S. S.; Ahn, C. H.; Percec, V.; Mo¨ller, M. Macromolecules 1999, 32, 2653. (b) Percec, V.; Ahn, C.-H.; Cho, W.-D.; Jamieson, A. M.; Kim, J.; Leman, T.; Schmidt, M.; Gerle, M.; Mo¨ller, M.; Prokhorova, S. A.; Sheiko, S. S.; Cheng, S. Z. D.; Zhang, A.; Ungar, G.; Yeardley, D. J. P. J. Am. Chem. Soc. 1998, 120, 8619. (c) Karakaya, B.; Claussen, W.; Gessler, K.; Saenger, W.; Schlu¨ter, A. D. J. Am. Chem. Soc. 1997, 119, 3296. (d) Ecker, C.; Severin, N.; Shu, L. Schlu¨ter, A. D.; Rabe, J. P. Macromolecules 2004, 37, 2484. (16) (a) Van Hest, J. C. M.; Delnoye, D. A. P.; Baars, M.; van Gendern, M. H. P.; Meijer, E. W. Science 1995, 268, 1592. (b) Aoi, K.; Motoda, A.; Ohno, M.; Tsutsumiuchi, K.; Okada, M.; Imae, T. Polym. J. 1999, 31, 1071. (c) Santini, C. M. B.; Johnson, M. A.; Boedicker, J. Q.; Hatton, T. A.; Hammond, P. T. J. Polym. Sci., Part A: Polym. Chem. 2004, 42, 2784. (17) Pitois, O.; Francois, B. Colloid Polym. Sci. 1999, 277, 574.
10.1021/la050187d CCC: $30.25 © 2005 American Chemical Society Published on Web 06/08/2005
Porous Polymer Films and Honeycomb Structures Scheme 1. Chemical Structure and Schematic Representation of the Block Copolymer
dendronized block copolymer. We noticed that, in recent years, several research groups in the literature18 have reported such a regular hexagonal microporous structure from the cast film of rod-coil block copolymers, of which rod blocks were mainly aromatic, conjugated rigid chains with small rod dimensions. However, due to the complexity of the phenomenon, there are still many questions remaining concerning the formation of these regular structures, such as the role of the polymer during the casting process. In particular, no definitive correlation between polymer properties and working conditions on the formation of microstructure was found. Herein, dendronized polymers are rodlike in shape, and the polymer backbone may form supramolecular helical structures containing a mixture of left- and right-handed helices in a condensed state due to the steric hindrance imposed by the bulky dendritic side groups attached to each repeating unit (Scheme 1). Furthermore, dendronized polymers have larger rod dimensions and were semirigid. As far as we know, it is the first time that honeycomb-like films are fabricated from such a rod-coil block copolymer containing a dendronized block. Tuning the surface tension by means of dendritic side groups allowed the film morphology to be modified compared with conventional block copolymers. Moreover, several influencing factors on the formation of the honeycomb structure and the different morphologies, such as the concentration of the copolymer solution and the relative humidity (RH) in the atmosphere and the substrates, were investigated schematically. This work raised the possibility that such structures could be formed in amphiphilic dendronized block copolymers and extended the family of source materials. 2. Experimental Section 2.1. Materials. A copolymer containing a dendronized polymethacrylate block and a linear poly(ethylene oxide) block (Scheme 1, copolymer 1) was applied to the fabrication of microporous films. Copolymer 1 was synthesized according to a (18) (a) Jenekhe, S. A.; Chen, X. L. Science 1999, 283, 372. (b) Duan, H. W.; Kuang, M.; Wang, J.; Chen, D. Y.; Jiang, M. J. Phys. Chem. B 2004, 108, 550. (c) de Boer, B.; Stalmach, U.; Nijland, H.; Hadziioannou, G. Adv. Mater. 2000, 12, 1581. (d) Hayakawa, T.; Horiuchi, S. Angew. Chem., Int. Ed. 2003, 42, 2285. (e) Stenzel M. H. Aust. J. Chem. 2002, 55, 239. (f) Bolognesi, A.; Mercogliano, C.; Yunus, S. Langmuir 2005, 21, 3480.
Langmuir, Vol. 21, No. 14, 2005 6577 previous publication.14 The monomer unit ratio of the hydrophobic part to the hydrophilic part of the copolymer is about 1:3, and its total molecular weight is 3.76 × 104, which was determined by the proton NMR analysis. The molecular weights of the copolymer is Mw ) 2.79 × 104 and Mn ) 2.49 × 104 (Mw/Mn ) 1.12), which were measured by gel permeation chromatography. Water was purified by a Millipore system (Milli-Q, Millipore). Chloroform was spectroscopy grade. 2.2. Film Preparation. “On-solid surface spreading” method: The patterned films were prepared directly by casting of the chloroform solution of the copolymer (40 µL) on hydrophilic substrates (mica, glass, or silicon surface) at 18 °C. The concentration ranged from 0.25 to 2.0 mg/mL, and the RH ranged from 70% to 95%. “On-water spreading” method: As described elsewhere,19 a Petri dish was filled with Milli-Q grade water, and 40 µL of the copolymer solution was spread onto the water surface for film fabrication (18 °C and 85% RH). After solvent evaporation, a thin opaque film remained on the water surface. The film was transferred onto a solid substrate and left in an air-conditioned room to dry. Transmission electron microscopy was carried out on a JEM-2010 electron microscope operated at 105 kV; the photo of the self-assembly morphologies were taken with a charge-coupled device camera and photographic plate. Scanning electron microscopy (SEM) images were obtained using a Hitachi S-4300 instrument operated at 15 kV and 10 µA. 2.3. Observation of Surface Morphology. The wettability of the copolymer with different substrates and the hydrophilicity of the substrates were characterized by the contact angle on a contact angle meter (FACE CA-D, Kyowa, Kaimenkagaku Co.). A honeycomb film was characterized by atomic force microscopy (AFM) and SEM. AFM images were recorded by a Digital Instrument Nanoscope III Multimode system with a silicon cantilever (resonance frequency 300 kHz, spring constant 35 mN/ m) using tapping mode. AFM images were recorded in height mode without any image process except flattening. SEM was carried out on a Hitachi S-4300 at 15 kV and 10 µA.
3. Results and Discussion The formation process of the honeycomb patterned film20 is described as follows: (1) Water droplets are condensed at the surface of the casting solution due to cooling by solvent evaporation. During this process, the surface turns turbid apparently due to the condensation of water vapor of the humidified air. (2) Water droplets are closely packed. Coalescence of water droplets is inhibited by polymers adsorbed at an interface between the water droplet and the polymer solution. (3) After solvent evaporation, the polymer film having honeycomb structure templated by water droplets is formed. It is obvious that the size and regularity of the pores can be controlled by changing any experimental parameters that affect the three steps, such as amphiphiles, solvents, temperature, and mechanical force applied to the dispersing of water droplets. 3.1. Influence of Substrates on the Pattern Formation. Morphology of the film is probably dependent on how water droplets are adsorbed onto the surface of hydrophilic slides. Mica, glass, and metal-coated glass slides and so forth were all used as the substrate for the formation of porous polymer films. At this point, an obvious question arises: Does the substrate affect the pattern formation and how? To answer this question, we first tested contact angles of spreading water or polymer solution onto hydrophilic substrates. Measurement results were listed in Table 1. Table 1 showed that it was easiest for the polymer solution to be spread on mica, while there was no distinct difference between the glass slide and the silicon slide. On the other hand, the spreading ability of (19) Nishikawa, T.; Ookura, R.; Nishida, J.; Arai, K.; Hayashi, J.; Kurono, N.; Sawadaishi, T.; Hara, M.; Shimomura, M. Langmuir 2002, 18, 5734. (20) Nishida, J.; Nishikawa, K.; Nishimura, S.-I.; Wada, S.; Karino, T.; Nishikawa, T.; Ijiro, K.; Shimomura, M. Polym. J. 2002, 34, 166.
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Figure 1. SEM images of the honeycomb structure of films prepared at 18 °C and 90% RH by spreading 40 µL of the copolymer solution (0.75 mg/mL) onto (a) mica, (b) glass, and (c) silicon plates, respectively. The bar is 20 µm. Table 1. Measurement Results of Contact Angles of Spreading Water or Polymer Solution onto Hydrophilic Substrates run
mica
glass (deg)
silicon (deg)
water polymer solution
a a
20.0 8.0
45.5 9.5
a
The spreading occurred too quickly to measure.
Figure 2. SEM images of the honeycomb structure of films prepared in different RHs. (a) 95%; (b) 90%; (c) 85%; (d) 80%. Other conditions: polymer concentration, 0.75 mg/mL; spreading volume, 40 µL; room temperature, 18 °C. The bar is 10 µm.
water on the three solid surfaces was in the following order: silicon, glass, and mica. It is a well-known fact that the wetting and dewetting of solid substrates with liquid polymer films is significantly affected by the interaction between the liquid and the substrates.21 Wetting of solid substrates with polymer solution was beneficial to the periodicity and regularity of holes. Figure 1 shows SEM images of the honeycomb structure of films prepared by spreading 40 µL of the copolymer solution (0.75 mg/mL) onto (a) mica, (b) glass, and (c) silicon plates at 18 °C and 90% RH, respectively. As shown in Figure 1, the average pore size was 2.0 µm for the mica plate and 1.6 µm for the glass plate, while attempts to prepare ordered porous films on the silicon plate under identical conditions were unsuccessful, which was explained as follows: It is the easiest for water droplets to be adsorbed onto the mica slide as a template for the formation of porous polymer films. While the least hydrophilic silicon slide makes its function as a steady mold become weaker, (21) Henn, G.; Bucknall, D. G.; Stamm, M.; Vanhoorne, P.; Je´roˆme, R. Macromolecules 1996, 29, 4305. Wyart, F. B.; Daillant, J. Can. J. Phys. 1990, 68, 1084.
the formation of porous films becomes difficult. Besides, the periodicity and regularity of holes using the mica slide were better than those using the glass slide, although both of them displayed some imperfections (Figure 1a,b). On the basis of the formation mechanisms of imperfections proposed by Han et al.,22 water droplets were carried away by the evaporating solvent vapor before condensation onto the solution or before having time to be trapped into the solution, which resulted in the formation of imperfections. On the whole, mica was the most suitable substrate among them for such a copolymer, which was confirmed by the porous films prepared under other humidities or concentrations (data not shown). 3.2. Influence of RH on the Pattern Formation. The two-dimensional array of water microspheres is a template for the porous structure of the honeycomb film, and the size of a water microsphere is one of the determining parameters of the pore size, so the humidity of the atmosphere influences the water condensation at the air-polymer solution interface and, consequently, influences the pore size and regularity of the micropore arrangement.23 To investigate the influence of the RH on pattern formation, the copolymer chloroform solution (0.75 mg/mL) was used to fabricate porous polymer films keeping other conditions constant. As shown in Figure 2, orderly porous films could be found under a controlled range of RHs between 80 and 95%. It is safe to say that high humidity is needed to form orderly porous films for such a copolymer, but when too many water droplets condensed onto the solution surface under comparably high humidity, coalescence of some water droplets could not be avoided (Figure 2a), which gave both a larger pore size and a thinner film width, and also led to lower regularity of micropore size and their arrangement. Evaporation at low humidity (
