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Formation of surface-attached microstructured polyelectrolyte brushes
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Chinese Chemical Letters 19 (2008) 988–991 www.elsevier.com/locate/cclet
Formation of surface-attached microstructured polyelectrolyte brushes Hai Ning Zhang State Key Laboratory of Advanced Technology for Materials Synthesis and Processing, Wuhan University of Technology, Wuhan 430070, China Received 25 January 2008
Abstract Surface-attached micropatterned polyelectrolyte brushes on planar solid surfaces are generated using free radical polymerization photo-initiated by self-assembled initiator monolayers. It is shown that the formed patterns can be either negative or positive with different patterning processes. # 2008 Hai Ning Zhang. Published by Elsevier B.V. on behalf of Chinese Chemical Society. All rights reserved. Keywords: Microstructure; Self-assembly; Surface-initiated polymerization; Polyelectrolyte brush
Polyelectrolyte layers have received a lot of attentions in both theoretical and experimental studies [1,2]. They are also interesting from the point of view of applications in material science [3] and biology [4] as well. On the other hand, micropatterned polyelectrolyte layers are also useful for the fabrication of sensors [5] and the spatial arrangement of bio-macromolecules such as protein and DNA [6,7]. Furthermore, using polyelectrolytes to form patterned surfaces, the formed structures have the potential to form the microchannels to which an electric field can be applied. If a mask is placed over the immobilized photo-active initiator layer and the substrate is exposed to 254 nm-UV light in air, areas shaded by the mask are inactivated since the initiator molecules are decomposed by irradiation. This leaves a latent image for a barrier of further polymer growth. After the initiator monolayer is structured, the radical polymerization is initiated using the remaining intact initiator molecules. This approach yields direct growth of polymer chains in the chosen areas, defined as process A, shown in Fig. 1A. The so-formed patterned layer is a ‘‘positive tone’’ image. When the mask is placed over the immobilized initiator layer and the sample is irradiated by 365 nm-UV light in the presence of monomer, reactive radicals will be generated only at the light-exposed areas, and accordingly, the polymerization will take place only within these areas, defined as process B. Fig. 1B shows the process B for the formation of patterned polymer films. This process leads to the formation of patterned films having a ‘‘negative tone’’ image. Microscopic images of the patterned films using process ‘‘A’’ are shown in Fig. 2. The initiator used in this study is an azo-type molecule, chlorodimethylsilyl- 3-ethylphenylazomethylmalonodonitrile (CSAM) as shown in Fig. 2b, which was first immobilized on the surfaces of silicon oxide substrates. Synthesis and properties of initiator and the
E-mail address: [email protected]. 1001-8417/$ – see front matter # 2008 Hai Ning Zhang. Published by Elsevier B.V. on behalf of Chinese Chemical Society. All rights reserved. doi:10.1016/j.cclet.2008.04.024
H.N. Zhang / Chinese Chemical Letters 19 (2008) 988–991
989
Fig. 1. Schematic depiction of the formation of patterned polymer films. Process A: polymer layer grown from the patterned initiator layer; process B: polymer layer grown directly from the initiator layer.
detailed surface-initiated radical polymerization process have been described elsewhere [8–10]. The sample was patterned by UV irradiation with wavelength of 254 nm for 5 h using structured copper as mask, rinsed extensively and dried in vacuum. Then it was transferred to a Schlenk-tube and surface-initiated polymerization of methacrylic acid (MAA) was carried out in aqueous solution (MAA:H2O 1:1, v/v, 3 h) using 365 nm UV light. After polymerization the sample was extracted with ethanol in order to remove the non-attached polymers and was dried under a nitrogen flow and later under vacuum. The thickness of the poly(acrylic acid) (PMAA) films was measured on areas of substrate which carry no patterns as 210 nm by ellipsometry. Because in those areas the same initiator was used and the same polymerization conditions were applied, it can be assumed that the thickness of the patterned film is the same as that of without patterns. The width of a square shown in Fig. 2 is 200 mm and the line-width is 35 mm. Fig. 3 shows the atomic force microscopy (AFM) image of the lines in the pattern. From the section analysis of the AFM image, the average thickness of the patterned film was calculated as 223 nm, which agrees very well with thickness measured by ellipsometry. The formed pattern has rather sharp boundaries between areas coated with polymer and those without polymer. Interestingly, the thickness of the PMAA layers at the edge of the formed film is
Fig. 2. (a) The microimages of patterned PMAA films formed by the process A, schematically shown in Fig. 1 using the patterned CSAM initiator layers. The polymerization time was 3 h under UV irradiation (365 nm). The thickness of formed films is around 210 nm based upon ellipsometry measurements. (b) Chemical structure of the initiator CSAM.
990
H.N. Zhang / Chinese Chemical Letters 19 (2008) 988–991
Fig. 3. AFM images of one of the lines in the formed pattern in Fig. 2. The average thickness of the polymer layer is 223 nm.
slightly higher than that in the middle of formed films. This might result from the humid air, in which the water molecules can wet the PMAA layer and minimize the surface energy of the PMAA layer. This phenomenon is somehow like water behaving in a hydrophilic capillary. In the second approach (process B), a mask is placed over the immobilized initiator monolayer and the substrate is placed in monomer solution and irradiated with 365 nm UV light. Hence, the polymerization takes place only in the areas exposed to the UV light, as shown in Fig. 1B. The so-formed films have a negative image. A microimage of a patterned PMAA film formed by this process is shown in Fig. 4. The mask used here was a poly(tetra fluoro ethylene) (PTFE) pellet with a hole (radius = 3 mm). The polymerization was carried out with MAA in aqueous solution (MAA:H2O 1:1, v/v) for 6 h (continuous exposure to UV light with wavelength of 365 nm). After extraction of the non-adsorbed polymer molecules with ethanol from the substrate, the sample was dried under a nitrogen flow and later under vacuum. The thickness of the formed film in the areas, which were exposed to the UV light (through the hole)
Fig. 4. The microimage of a patterned PMAA film formed by the process B as shown in Fig. 1 using the self-assembled CSAM initiator monolayer.
H.N. Zhang / Chinese Chemical Letters 19 (2008) 988–991
991
was measured with ellipsometry as 360 nm. However, also some polymers were attached to those areas, which were not directly exposed to the UV light. This is due to the fact that the mask was placed rather far from the substrate (3– 4 mm) and the scattered UV light, after passing through the mask may decompose the initiator on the latter area as well. This problem can be solved if the mask is placed close enough to the substrate. In summary, surface-attached micropatterned polymer brushes can be easily fabricated using free radical polymerization initiated by an immobilized initiator layer. With different processes, either positive or negative images can be formed. While using process B described above, surfaces are patterned by areas of polymer brushes and areas of unreacted initiator monolayer, which makes the formation of patterned polymer brushes with different components at the surface possible. References [1] [2] [3] [4] [5] [6] [7] [8] [9] [10]
B. Zhao, W.J. Brittain, Prog. Polym. Sci. 25 (2000) 677. J. Ruhe, M. Ballauff, M. Biesalski, et al. Adv. Poly. Sci. 165 (2004) 79. H. Li, W.T.S. Huck, Curr. Opin. Sol. State Mater. Sci. 6 (2002) 3. B.S. Gallardo, V.K. Gupta, F.D. Eagerton, et al. Science 283 (1999) 57. E. Delamarche, A. Benard, H. Schmid, et al. J. Am. Chem. Soc. 120 (1998) 500. F. Caruso, K. Niikura, D.N. Furlong, et al. Langmuir 13 (1997) 3427. G. Ladam, P. Schaaf, F.J.G. Cuisinier, et al. Langmuir 17 (2001) 878. H. Zhang, Polyelectrolyte–Polyelectrolyte Complexes Based on Polymer Brushes, Goettingen Verlag, Goettingen, 2003, 171 pp.. O. Prucker, J. Ruhe, Macromolecules 31 (1998) 592. H. Zhang, J. Ruhe, Macromolecules 38 (2005) 4855.
Chinese Chemical Letters 19 (2008) 988–991 www.elsevier.com/locate/cclet
Formation of surface-attached microstructured polyelectrolyte brushes Hai Ning Zhang State Key Laboratory of Advanced Technology for Materials Synthesis and Processing, Wuhan University of Technology, Wuhan 430070, China Received 25 January 2008
Abstract Surface-attached micropatterned polyelectrolyte brushes on planar solid surfaces are generated using free radical polymerization photo-initiated by self-assembled initiator monolayers. It is shown that the formed patterns can be either negative or positive with different patterning processes. # 2008 Hai Ning Zhang. Published by Elsevier B.V. on behalf of Chinese Chemical Society. All rights reserved. Keywords: Microstructure; Self-assembly; Surface-initiated polymerization; Polyelectrolyte brush
Polyelectrolyte layers have received a lot of attentions in both theoretical and experimental studies [1,2]. They are also interesting from the point of view of applications in material science [3] and biology [4] as well. On the other hand, micropatterned polyelectrolyte layers are also useful for the fabrication of sensors [5] and the spatial arrangement of bio-macromolecules such as protein and DNA [6,7]. Furthermore, using polyelectrolytes to form patterned surfaces, the formed structures have the potential to form the microchannels to which an electric field can be applied. If a mask is placed over the immobilized photo-active initiator layer and the substrate is exposed to 254 nm-UV light in air, areas shaded by the mask are inactivated since the initiator molecules are decomposed by irradiation. This leaves a latent image for a barrier of further polymer growth. After the initiator monolayer is structured, the radical polymerization is initiated using the remaining intact initiator molecules. This approach yields direct growth of polymer chains in the chosen areas, defined as process A, shown in Fig. 1A. The so-formed patterned layer is a ‘‘positive tone’’ image. When the mask is placed over the immobilized initiator layer and the sample is irradiated by 365 nm-UV light in the presence of monomer, reactive radicals will be generated only at the light-exposed areas, and accordingly, the polymerization will take place only within these areas, defined as process B. Fig. 1B shows the process B for the formation of patterned polymer films. This process leads to the formation of patterned films having a ‘‘negative tone’’ image. Microscopic images of the patterned films using process ‘‘A’’ are shown in Fig. 2. The initiator used in this study is an azo-type molecule, chlorodimethylsilyl- 3-ethylphenylazomethylmalonodonitrile (CSAM) as shown in Fig. 2b, which was first immobilized on the surfaces of silicon oxide substrates. Synthesis and properties of initiator and the
E-mail address: [email protected]. 1001-8417/$ – see front matter # 2008 Hai Ning Zhang. Published by Elsevier B.V. on behalf of Chinese Chemical Society. All rights reserved. doi:10.1016/j.cclet.2008.04.024
H.N. Zhang / Chinese Chemical Letters 19 (2008) 988–991
989
Fig. 1. Schematic depiction of the formation of patterned polymer films. Process A: polymer layer grown from the patterned initiator layer; process B: polymer layer grown directly from the initiator layer.
detailed surface-initiated radical polymerization process have been described elsewhere [8–10]. The sample was patterned by UV irradiation with wavelength of 254 nm for 5 h using structured copper as mask, rinsed extensively and dried in vacuum. Then it was transferred to a Schlenk-tube and surface-initiated polymerization of methacrylic acid (MAA) was carried out in aqueous solution (MAA:H2O 1:1, v/v, 3 h) using 365 nm UV light. After polymerization the sample was extracted with ethanol in order to remove the non-attached polymers and was dried under a nitrogen flow and later under vacuum. The thickness of the poly(acrylic acid) (PMAA) films was measured on areas of substrate which carry no patterns as 210 nm by ellipsometry. Because in those areas the same initiator was used and the same polymerization conditions were applied, it can be assumed that the thickness of the patterned film is the same as that of without patterns. The width of a square shown in Fig. 2 is 200 mm and the line-width is 35 mm. Fig. 3 shows the atomic force microscopy (AFM) image of the lines in the pattern. From the section analysis of the AFM image, the average thickness of the patterned film was calculated as 223 nm, which agrees very well with thickness measured by ellipsometry. The formed pattern has rather sharp boundaries between areas coated with polymer and those without polymer. Interestingly, the thickness of the PMAA layers at the edge of the formed film is
Fig. 2. (a) The microimages of patterned PMAA films formed by the process A, schematically shown in Fig. 1 using the patterned CSAM initiator layers. The polymerization time was 3 h under UV irradiation (365 nm). The thickness of formed films is around 210 nm based upon ellipsometry measurements. (b) Chemical structure of the initiator CSAM.
990
H.N. Zhang / Chinese Chemical Letters 19 (2008) 988–991
Fig. 3. AFM images of one of the lines in the formed pattern in Fig. 2. The average thickness of the polymer layer is 223 nm.
slightly higher than that in the middle of formed films. This might result from the humid air, in which the water molecules can wet the PMAA layer and minimize the surface energy of the PMAA layer. This phenomenon is somehow like water behaving in a hydrophilic capillary. In the second approach (process B), a mask is placed over the immobilized initiator monolayer and the substrate is placed in monomer solution and irradiated with 365 nm UV light. Hence, the polymerization takes place only in the areas exposed to the UV light, as shown in Fig. 1B. The so-formed films have a negative image. A microimage of a patterned PMAA film formed by this process is shown in Fig. 4. The mask used here was a poly(tetra fluoro ethylene) (PTFE) pellet with a hole (radius = 3 mm). The polymerization was carried out with MAA in aqueous solution (MAA:H2O 1:1, v/v) for 6 h (continuous exposure to UV light with wavelength of 365 nm). After extraction of the non-adsorbed polymer molecules with ethanol from the substrate, the sample was dried under a nitrogen flow and later under vacuum. The thickness of the formed film in the areas, which were exposed to the UV light (through the hole)
Fig. 4. The microimage of a patterned PMAA film formed by the process B as shown in Fig. 1 using the self-assembled CSAM initiator monolayer.
H.N. Zhang / Chinese Chemical Letters 19 (2008) 988–991
991
was measured with ellipsometry as 360 nm. However, also some polymers were attached to those areas, which were not directly exposed to the UV light. This is due to the fact that the mask was placed rather far from the substrate (3– 4 mm) and the scattered UV light, after passing through the mask may decompose the initiator on the latter area as well. This problem can be solved if the mask is placed close enough to the substrate. In summary, surface-attached micropatterned polymer brushes can be easily fabricated using free radical polymerization initiated by an immobilized initiator layer. With different processes, either positive or negative images can be formed. While using process B described above, surfaces are patterned by areas of polymer brushes and areas of unreacted initiator monolayer, which makes the formation of patterned polymer brushes with different components at the surface possible. References [1] [2] [3] [4] [5] [6] [7] [8] [9] [10]
B. Zhao, W.J. Brittain, Prog. Polym. Sci. 25 (2000) 677. J. Ruhe, M. Ballauff, M. Biesalski, et al. Adv. Poly. Sci. 165 (2004) 79. H. Li, W.T.S. Huck, Curr. Opin. Sol. State Mater. Sci. 6 (2002) 3. B.S. Gallardo, V.K. Gupta, F.D. Eagerton, et al. Science 283 (1999) 57. E. Delamarche, A. Benard, H. Schmid, et al. J. Am. Chem. Soc. 120 (1998) 500. F. Caruso, K. Niikura, D.N. Furlong, et al. Langmuir 13 (1997) 3427. G. Ladam, P. Schaaf, F.J.G. Cuisinier, et al. Langmuir 17 (2001) 878. H. Zhang, Polyelectrolyte–Polyelectrolyte Complexes Based on Polymer Brushes, Goettingen Verlag, Goettingen, 2003, 171 pp.. O. Prucker, J. Ruhe, Macromolecules 31 (1998) 592. H. Zhang, J. Ruhe, Macromolecules 38 (2005) 4855.