Fabrication of Chemically Microstructured Polymer Brushes

Sep 2, 2006 - acid) (PMAA)-poly(hydroxyethyl methacrylate) (PHEMA) is demonstrated. Introduction. The inscription of micropatterns onto solid substrat...
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Langmuir 2006, 22, 8571-8575

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Fabrication of Chemically Microstructured Polymer Brushes Rupert Konradi and Ju¨rgen Ru¨he* UniVersity of Freiburg, Department for Microsystem Engineering (IMTEK), Georges-Ko¨hler-Allee 103, D-79110 Freiburg, Germany ReceiVed May 16, 2006 In this paper, a new and simple pathway to fabricate polymer brush layers with lateral control over the chemical composition is described. The process combines two subsequent free radical grafting from steps: in the first step, a micropatterned polymer brush is grown by photochemical initiation of the polymer growth from the surface through a mask in direct contact. The uncoated areas are then backfilled with a second polymer brush by using the unreacted surface-bound initiator molecules to thermally trigger a second polymerization. As an example for the overall process, the co-assembly of a micropatterned, soft, water-swellable layer consisting of the two-brush system poly(methacrylic acid) (PMAA)-poly(hydroxyethyl methacrylate) (PHEMA) is demonstrated.

Introduction The inscription of micropatterns onto solid substrates to tailor their lateral chemical composition has attracted great attention. Mostly, low molecular weight compounds such as thiols or silanes were assembled on noble metal or oxidic surfaces, and patterns were either generated through conventional photoresist technology, photochemical techniques, microcontact printing, or scanning probe-based lithography.1,2 Frequently, these systems were adopted for the directed immobilization of proteins with potential use for controlled cellular growth or immunoassays.3-5 If initiator molecules, capable of starting a polymerization reactions from the surface, are covalently attached to a surface in a patterned fashion, these patterns can be amplified in a subsequent surface-initiated polymerization to obtain microstructured surface-bound polymer brushes.6-15 Alternatively, patterned polymer brushes can be generated either in situ from an unpatterned monolayer of initiator molecules by photopolymerization through a mask6,7,16 or by post-polymerization modifications.17 These systems have advantages as compared to patterned spincast polymer films as the latter are unstable under many processing conditions due to swelling and dissolution. On the other side, * Corresponding author. E-mail: [email protected]. (1) Ulman, A. Chem. ReV. 1996, 96, 1533-1554. (2) Smith, R. K.; Lewis, P. A.; Weiss, P. S. Prog. Surf. Sci. 2004, 75, 1-68. (3) Blawas, A. S.; Reichert, W. M. Biomaterials 1998, 19, 595-609. (4) Falconnet, D.; Koenig, A.; Assi, F.; Textor, M. AdV. Funct. Mater. 2004, 14, 749-756. (5) Lussi, J. W.; Michel, R.; Reviakine, I.; Falconnet, D.; Goessl, A.; Csucs, G.; Hubbell, J. A.; Textor, M. Prog. Surf. Sci. 2004, 76, 55-69. (6) Tovar, G.; Paul, S.; Knoll, W.; Prucker, O.; Ru¨he, J. Supramol. Sci. 1995, 2, 89-98. (7) Prucker, O.; Habicht, J.; Park, I. J.; Ru¨he, J. Mater. Sci. Eng.,C 1999, 8-9, 291-297. (8) Hyun, J.; Chilkoti, A. Macromolecules 2001, 34, 5644-5652. (9) Shah, R. R.; Merreceyes, D.; Husemann, M.; Rees, I.; Abbott, N. L.; Hawker, C. J.; Hedrick, J. L. Macromolecules 2000, 33, 597-605. (10) Osborne, V. L.; Jones, D. M.; Huck, W. T. S. Chem. Commun. 2002, 1838-1839. (11) Ma, H. W.; Hyun, J. H.; Stiller, P.; Chilkoti, A. AdV. Mater. 2004, 16, 338-41. (12) Husemann, M.; Mecerreyes, D.; Hawker, C. J.; Hedrick, J. L.; Shah, R.; Abbott, N. L. Angew. Chem., Int. Ed. 1999, 38, 647-649. (13) Jeon, N. L.; Choi, I. S.; Whitesides, G. M.; Kim, N. Y.; Laibinis, P. E.; Harada, Y.; Finnie, K. R.; Girolami, G. S.; Nuzzo, R. G. Appl. Phys. Lett. 1999, 75, 4201-4203. (14) Kratzmu¨ller, T.; Appelhans, D.; Braun, H. G. AdV. Mater. 1999, 11, 555-558. (15) Schmelmer, U.; Jordan, R.; Geyer, W.; Eck, W.; Go¨lzha¨user, A.; Grunze, M.; Ulman, A. Angew. Chem., Int. Ed. 2003, 42, 559-563.

they possess advantages compared to self-assembled low molecular weight compounds as they allow for the generation of soft and swollen interfaces, smoothen out chemical and physical defects, and allow for the immobilization of a higher amount of functional groups. Such patterned polymer brushes have been prepared on field-effect transistors as substrates and used for the micromanipulation of neuronal cells that were cultured on the patterned polymer layers.18-20 Whereas self-assembled monolayers of low molecular weight compounds have been frequently structured with respect to their lateral chemical composition (i.e., chemically microstructured), the aforementioned investigations to generate patterned polymer brushes usually result in a single patterned polymer layer with no control over the chemical composition of the background region, which is either composed of the initiator monolayer or the bare substrate. A patterning process for the build-up of a continuous polymer brush with chemically distinct areas including independent control over the chemical composition and the thickness of each area, however, is highly attractive since it combines the advantages of polymer brushes with the possibility for chemical patterning as it is known for low molecular weight compounds. Only few approaches to generate chemically microstructured polymer brushes have been described in the literature. Husemann et al. followed a post-polymerization approach in which they covered a homogeneous poly(t-butyl acrylate) brush with a photoacid.17 Irradiation through a mask generated protons in the illuminated areas, which lead to a hydrolysis of the t-butyl acrylate groups to form a patterned poly(acrylic acid)-poly(t-butyl acrylate) brush. Although this process successfully lead to a hydrophilic-hydrophobic micropattern, it encompasses several limitations: first, the patterning relies on a polymer-analogous conversion that is often difficult to perform quantitatively, then the thicknesses of the two brushes are related to each other and cannot be chosen independently, and most importantly, the process (16) Prucker, O.; Schimmel, M.; Tovar, G.; Knoll, W.; Ru¨he, J. AdV. Mater. 1998, 10, 1073-1077. (17) Husemann, M.; Morrison, M.; Benoit, D.; Frommer, J.; Mate, C. M.; Hinsberg, W. D.; Hedrick, J. L.; Hawker, C. J. J. Am. Chem. Soc. 2000, 122, 1844-1845. (18) Offenha¨usser, A.; Ru¨he, J.; Knoll, W. J. Vac. Sci. Technol., A 1995, 13, 2606-2612. (19) Knoll, W.; Matsuzawa, M.; Offenha¨usser, A.; Ru¨he, J. Isr. J. Chem. 1996, 36, 357-369. (20) Ru¨he, J.; Yano, R.; Lee, J. S.; Koberle, P.; Knoll, W.; Offenha¨usser, A. J. Biomater. Sci., Polym. Ed.1999, 10, 859-874.

10.1021/la061379r CCC: $33.50 © 2006 American Chemical Society Published on Web 09/02/2006

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is limited to a given combination of two polymers. A more flexible route has been followed by Maeng and Park:21 they deactivated the active (living) ends of a homogeneous brush grown by atom transfer radical polymerization (ATRP) by electron beam irradiation through a mask. In the unexposed areas, a second block was grown on top of this brush from a different monomer. The authors report on the co-assembly of a polystyrene and polystyrene-block-poly(methyl methacrylate) brush. In principle, this strategy should also be applicable to other monomers; however, the first brush must be insensitive to electron beam etching. Overall, the procedure results in the co-assembly of a diblock and a homogeneous brush, where the first block and the grafting density of the two brushes are identical. We have previously reported on a chemically microstructured polymer brush that was obtained through a step-and-repeat procedure.7 This process involved a photoablation step, either of the initiator monolayer before brush growth or of the homogeneous polymer layer after growth of an unpatterned brush. The ablated areas were then reactivated, initiator molecules were deposited, and finally, a second brush of different chemical composition was grown in the initially ablated areas. A similar procedure was established by Zhou et al. using ATRP grown brushes.22 In this case, an additional passivation of the first brush (50 h treatment with NaN3) was needed to allow for the second initiator deposition. These step-and-repeat processes in principle allow for independent control over both thickness and chemical composition of the two brushes and could possibly be used to fabricate even more complicated structures in additional cycles. In this paper, we describe a new strategy to fabricate chemically microstructured polymer brush layers of almost arbitrary composition through the combination of a photopolymerization and a thermal polymerization step. Experimental Procedures Silicon wafers with a native oxide layer of approximately 2 nm were used as substrates and cleaned with a stream of high-velocity dry ice particles (Tectra, Germany) prior to the immobilization to remove dust particles. The monomers and methanol were purified by distillation over a vigreux column, and water was deionized with a Millipore system (resistivityg 18.2 MΩ cm-1). The synthesis of the initiator and its immobilization on hydroxylterminated surfaces have been investigated previously.23 The mechanisms of both thermally23-25 and photochemically7,16 initiated polymerizations using monolayers of this initiator have been described in detail. In the present paper, we have combined a photopolymerization of methacrylic acid (MAA) in water (10 vol %; 1 h) through a mask in direct contact with the substrate and a subsequent thermal polymerization of hydroxyethyl methacrylate (HEMA) in methanol (50 vol %; 6 h) to obtain chemically microstructured brushes. For the photopolymerization, a 500 W high-pressure mercury lamp (LOT-Oriel) with a 1.5 in. water filter and a 90° dichroic beam turner to eliminate infrared light was used. The reaction mixture was degassed in a flat Schlenk flask (≈8-10 mm thick), and the initiatormodified substrate with the mask in direct contact was carefully added under nitrogen. The sample was then irradiated at 20 °C at a distance of ≈4 cm from the light source through the glass wall of the Schlenk flask. The glass functions as a filter for wavelengths below ≈300 nm. The half-life time of the surface-bound initiator molecules for this setup was determined as 59 min. After polymerization, the samples were extracted in methanol and water for 15 h and then placed in a saturated Ca(NO3)2 solution for 1 h to render (21) Maeng, I. S.; Park, J. W. Langmuir 2003, 19, 9973-9976. (22) Zhou, F.; Jiang, L.; Liu, W.; Xue, Q. Macromol. Rapid Commun. 2004, 25, 1979-1983. (23) Prucker, O.; Ru¨he, J. Macromolecules 1998, 31, 592-601. (24) Prucker, O.; Ru¨he, J. Macromolecules 1998, 31, 602-613. (25) Prucker, O.; Ru¨he, J. Langmuir 1998, 14, 6893-6898.

Konradi and Ru¨he

Figure 1. General scheme of the process describing the build-up of chemically microstructured polymer brush systems. It is comprised of surface-initiated photopolymerization through a mask in direct contact and a subsequent thermal polymerization of a different monomer. the PMAA brush hydrophobic by forming the calcium salt. After careful rinsing with deionized water and drying, the samples were transferred in a second Schlenk flask containing the degassed reaction mixture for the thermal polymerization that was carried out in a thermostat at 60.0 ( 0.1 °C. The samples were again extracted for 15 h in methanol and finally placed in diluted hydrochloric acid of pH 3 for 30 min to replace calcium ions by protons. Imaging ellipsometric measurements were performed on a nanofilm I-Elli2000 instrument. A He-Ne laser beam with a wavelength of 633 nm and an intensity of 20 mW was coupled in through a fiberoptic, and the angle of incidence was fixed at 50°. For XPS measurements, the samples were first treated with 2 mol L-1 aqueous AgNO3 solution for 1 h to form the silver salt of the PMAA brush and then with gaseous (CF3CO)2O for 15 h to acetylize the hydroxyl groups of the PHEMA brush. XPS measurements were carried out on a Perkin-Elmer PHI 5600 spectrometer using Mg KR radiation. First, a Ag3d and then a F1s scan with 128 detail spectra were measured along a scan line of 2000 µm. Spectra at each point were recorded at a pass energy of 58.70 eV with a step width of 0.5 eV, and the overall measurement time for each scan was about 2 h. The analyzer was positioned at 45° relative to the substrate surface. The AFM image was recorded in tapping mode on a DI nanoscope multimode 3100 instrument.

Results and Discussion Synthesis. We have previously introduced an initiator system23-25 that allows for the use of two complementary patterning pathways.6,7,16 This initiator bears a monochlorosilane functionality for the covalent linkage to oxidic surfaces such as silicon wafers or glass and an azo-moiety to initiate a polymerization from the surface. The fact that this azo-moiety can be cleaved both thermally and photochemically is the key to the process described here. In Figure 1, the overall strategy is schematically depicted. In the first step, a mask (different TEM grids were used as masks) was placed in direct contact with the initiator-modified substrate and covered with the oxygen free reaction mixture, and a polymer brush was grown through photochemical initiation in the uncovered areas. After removal of free polymer by extraction with a good solvent, the polymer brush-modified substrate was covered with a solution of another monomer in the second step, and the still-intact initiator molecules in the formerly shaded areas were then cleaved thermally at 60.0 °C to yield the second polymer brush in the still-uncoated areas. Both initiation pathways have been individually investigated previously16,23-25 and allow for the use of a wide variety of different monomers in both steps as well as for an independent control over the brush thicknesses, molecular weights, and grafting densities. A complementary pattern can be obtained if the sequence of the two monomers is reversed.

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Figure 2. Thickness of thermally grown PMAA brushes after the initiator-modified substrate covered with pure water had been irradiated for a given time using the photopolymerization setup. A half-life time of 59 min for the photochemical cleavage of the surfaceattached initiator was determined from an exponential fit of the data (solid line).

As the spectral profile of the UV lamp and the absolute flux of photons at the surface of the sample are not easily determined and as the orientation and packing of the tethered initiator molecules can strongly influence the molar extinction coefficient and the quantum yield, the half-life time of the initiator molecules upon photochemical activation has been determined for the given setup as shown in Figure 2. Here, an initiator-modified substrate was irradiated for different times in six macroscopic areas. However, in this case, the substrate was placed in contact with pure water instead of a monomer-containing reaction mixture. As expected, no polymer brush was formed. Then, this substrate was used to thermally initiate a polymerization of MAA in water (25 vol %; 3 h at 60.0 °C), and the resulting PMAA brush thicknesses in the six areas were plotted as a function of the initial passivation time. From an exponential fit of the data, it has been determined that half of the initiator molecules were decomposed photochemically after approximately 59 min. The corresponding half-life time for the thermal cleavage of the initiator at 60.0 °C has been determined as approximately 20 h.24 For both activation processes, an increasing polymerization time results in an increased conversion of the initiator and therefore an increased grafting density that in turn yields a higher brush thickness. As an example for the overall process, we have chosen to first polymerize methacrylic acid (MAA) photochemically and then hydroxyethyl methacrylate (HEMA) thermally (see Figure 3). The architecture of a patterned PMAA-PHEMA brush is an attractive and challenging case as both polymers are hydrophilic and swellable in aqueous environments. Only one of them, PMAA, is tunable in its charge density and in its swelling behavior via adjustment of external parameters such as the pH value or through the addition of different kinds of cations.26-29 The resulting chemically microstructured polymer brush is expected to be swellable as a whole ensemble with a pattern inscribed that can be switched on or off or even inversed by external stimuli. In a first attempt, the thermal polymerization of HEMA in the presence of a PMAA brush resulted not only in the build-up of a PHEMA brush but also in an unexpected increase in thickness of the PMAA brush. This observation could be attributed to PHEMA incorporation into the PMAA brush as confirmed by (26) Konradi, R.; Ru¨he, J. Macromolecules 2005, 38, 4345-4354. (27) Konradi, R.; Ru¨he, J. Macromolecules 2004, 37, 6954-6961. (28) Biesalski, M.; Johannsmann, D.; Ru¨he, J. J. Chem. Phys. 2002, 117, 4988-4994. (29) Ru¨he, J.; Ballauff, M.; Biesalski, M.; Dziezok, P.; Gro¨hn, F.; Johannsmann, D.; Houbenov, N.; Hugenberg, N.; Konradi, R.; Minko, S.; Motornov, M.; Netz, R. R.; Schmidt, M.; Seidel, C.; Stamm, M.; Stephan, T.; Usov, D.; Zhang, H. AdV. Polym. Sci. 2004, 165, 79-150.

Figure 3. Overall reaction scheme for the fabrication of the PMAAPHEMA brush assembly.

infrared spectroscopy. The PHEMA turned out to be very difficult to remove from the PMAA brush even by rigorous extraction procedures or changes in the pH value of the system. Different mechanisms may be the origin of this finding: first, after 1 h of irradiation during the polymerization of the PMAA brush, about 50% of the initiator molecules remained active. Since PMAA is swollen in methanol, HEMA may diffuse into the PMAA brush, and the growth of a PHEMA brush may be triggered within the PMAA brush areas. Such a formation of a mixed brush could be avoided if the first brush is grown until the initiator in the exposed areas is fully depleted (e.g., after 5 h of irradiation, 97% of the initiator is depleted). Second, in each polymerization step, not only surface-attached but also free polymer in solution is formed. On one hand, growing polymer chains in solution could transfer a radical function to surface-attached PMAA chains resulting in a covalent binding of PHEMA to the PMAA brush. One the other hand, solution polymerized PHEMA could form a strong interpolymer complex with PMAA through multiple hydrogen bonds. All these pathways can be avoided if the first generated polymer brush is insoluble in the reaction mixture, which is used for the generation of the second polymer brush. Particularly, remaining active initiator molecules will be blocked by the first collapsed brush during the polymerization of the second brush. Therefore, when the pattern combines a hydrophilic and a hydrophobic brush, the contamination of the photopolymerized brush by thermally polymerized monomer is unlikely. In the present case of two hydrophilic polymers, we have chosen to protect the patterned PMAA brush by making it temporarily insoluble through the addition of calcium ions before the polymerization of HEMA was carried out. We have previously shown that this complexation with calcium ions leads to a strong deswelling of the PMAA brush and, furthermore, that it can be quantitatively reversed under mild acidic conditions.26,27 It was observed that the PMAA layer thickness remained almost unchanged during calcium addition, growth of the PHEMA chains, and subsequent calcium replacement with protons. Thus, the use of calcium ions as protecting groups allowed us to coassemble two hydrophilic polymers. Characterization. Figure 4 shows imaging ellipsometric micrographs of the photopolymerized PMAA brush (Figure 4a,b)

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Figure 4. Imaging ellipsometric micrographs of a 27 nm thick photopolymerized PMAA brush (a and b) and of a chemically microstructured PMAA-PHEMA brush (30 and 60 nm) assembly (c and d). Panels a-d show complementary pictures of the same area minimized on the PMAA brush (a and c) and on the initiatormodified substrate (b) or the PHEMA brush (d).

Figure 5. Panels a and b show 3-D thickness representations of the data shown in Figure 4a,b and 4c,d, respectively.

and of the chemically microstructured PMAA-PHEMA brush (Figure 4c,d). In Figure 4a,b, the polarizer and analyzer were rotated with respect to minimize the signals originating from the PMAA brush layer and from the bare silane-modified substrate, respectively. The respective average thicknesses as determined from measurements on different spots were 27.3 ( 1.0 and 0.6 ( 1.0 nm. The error was estimated from maximum deviations and from the accuracy of the instrument. As the mask, a 200 mesh (125 µm pitch size) TEM copper grid with 28 µm wide lines and 97 µm wide squares was used. Figure 4c,d represents the corresponding micrographs for the very same brush after co-assembly of the PHEMA brush by thermal polymerization as described previously. The PMAA brush thickness was determined to be 29.6 ( 1.0 nm; the minimization is shown in Figure 4c. The slight increase in thickness by less than 10% is most probably due to residual PHEMA adsorption. Whereas the PMAA brush thickness remained almost unchanged during the thermal polymerization, the areas that had been shaded during the initial photopolymerization were filled in with a 60.0 ( 1.0 nm PHEMA brush (Figure 4d). A 3-D representation of the data is shown in Figure 5 for the microstructured PMAA brush (Figure 5a) and for the two-brush system (Figure 5b). This graph was obtained by variation of the polarizer angle in 20 steps of 0.2° (Figure 5a) or 1° (Figure 5b) around the minimum to yield a ∆ map from which a thickness map was calculated. The thickness map correspondingly consisted of the superposition of 20 vertical slices of the 3-D structure. It clearly reveals that the areas that were left uncoated during the first polymerization were successfully filled during the second polymerization. As can be seen in the micrographs, the resolution and homogeneity of the brushes were not affected by the second polymerization. Note that the brushes appear in the outer edges of the micrographs somewhat rougher than they actually are due to inhomogeneities in the laser beam profile. This assumption was confirmed by AFM measurements.

To confirm the chemical identity of the two-brush system, we have derivatized the brushes in two polymer-analogous reactions to introduce elements with high X-ray photoelectron spectroscopy (XPS) sensitivity factors as specific labels for each brush. First, the substrate was placed in a 2 mol L-1 aqueous AgNO3 solution for 1 h to replace protons of the PMAA brush carboxylic acid units with silver ions. We have investigated this complexation in detail previously27 and expect that more than 80% of the carboxylic acid units have reacted to form the silver salt. Then, the substrate was treated with trifluoroacetic anhydride in the gas phase for 15 h to acetylize the hydroxyl groups of the PHEMA brush. As a result of the two polymer-analogous reactions, the PMAA brush is labeled with silver, whereas the PHEMA brush is labeled with fluorine. Both reactions are probably incomplete; however, a quantitative conversion is not needed for a qualitative confirmation of the chemical identity by XPS. Note that we did not attempt to quantify the XPS measurements as both polymers gradually decompose under X-ray exposure. Figure 6a depicts a 3-D representation of a PMAA-PHEMA (31.2 ( 1.1 and 56.3 ( 1.6 nm) brush assembly. Since a 120 µm aperture had to be used to obtain sufficient sensitivity in the XPS experiment, a mask with rather broad lines of similar dimensions as the aperture was used for the fabrication of this two-brush system. Figure 6b shows the Ag3d (9) and F1s (O) line scans across this structure after the brushes had been labeled with silver and fluorine. The maxima and minima of each scan coincide with the respective minima and maxima of the complementary scan. The Ag3d scan shows maxima in the areas where the PMAA brush had been attached and minima where the PHEMA brush was grown. Accordingly, the F1s scan shows maxima in the PHEMA brush domains and minima in the PMAA brush domains. These findings clearly confirm the successful process of subsequent deposition of two chemically different brushes with control over the lateral pattern on a micrometer scale. Furthermore, this demonstrates that only the PMAA brush is being complexed in the presence

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Figure 7. AFM image and section of an edge between the two brushes of a chemically microstructured PMAA-PHEMA brush assembly. Figure 6. Ag3d (9) and F1s (O) XPS line scans (panel b) of a chemically microstructured PMAA-PHEMA brush assembly after derivatization with AgNO3 and (CF3CO)2O as shown in panel a.

of silver ions, even though the whole brush system was brought into contact with these ions. We have recently shown that a PMAA brush responds to complexation by concentrated silver nitrate solutions with a strong shrinkage of the swollen brush, indicating that the two-brush systems have the potential to build up micropatterned polymer brush coatings that can be addressed and tuned by external stimuli.26 The boundaries between the chemically different microstructure domains are represented in the XP line scan as broad and smooth transitions rather than as sharp edges due to the rather large aperture size that was of the same order as the domain sizes. Hence, the XP line scan is not suitable to determine the real size of the domain boundaries. We therefore performed an atomic force microscopy (AFM) investigation on a PMAA-PHEMA (33.4 ( 1.2 and 50.3 ( 1.3 nm) brush assembly to obtain topological information about the domain boundaries. Figure 7 shows the corresponding micrograph together with a section analysis. One can see that the boundary between the two brushes is on the order of 1 µm. This critical lateral dimension for the overall process is similar to that of single micropatterned polymer brushes that were obtained by a photopolymerization process7,16 or processes involving the microcontact printing of an initiator monolayer.13,30 Therefore, the deposition of the second brush does not lower the lateral resolution of the micropattern. This

seems reasonable as in the given process, the pattern is exclusively written into the polymer layer during the first step.

Conclusion In this paper, we have introduced a novel process for the preparation of chemically microstructured polymer brush layers. Two individually well-known methods for the initiation of a free radical polymerization from the surface, the photochemical and thermal cleavage of a surface-bound azo-moiety, have been successfully combined. The process is suitable to co-assemble polymers from a great variety of different monomers with independent control over the graft densities, molecular weights, and chemical compositions of the individual pattern areas. Such a procedure is especially interesting if two relatively similar polymers are to be assembled and therefore, it has great potential for the construction of sophisticated surface architectures including switchable polymeric micropatterns. Acknowledgment. Financial support by the Deutsche Forschungsgemeinschaft, DFG (Schwerpunkt: Polyelektrolyte mit Definierter Moleku¨larchitektur) under Grant Ru489/6-3 is gratefully acknowledged. Martin Scho¨nstein is thanked for technical assistance. LA061379R (30) Lackowski, W. M.; Ghosh, P.; Crooks, R. M. J. Am. Chem. Soc. 1999, 121, 1419-1420.