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From Smart Polymer Molecules to Responsive Nanostructured Surfaces Robert Lupitskyy,† Yuri Roiter,† Constantinos Tsitsilianis,‡ and Sergiy Minko*,† Department of Chemistry, Clarkson University, Potsdam, New York 13699-5810, and Department of Chemical Engineering, University of Patras, 26504 Patras, Greece Received February 14, 2005. In Final Form: July 13, 2005 A heteroarm star block copolymer made from seven polystyrene and seven poly(2-vinylpyridine) arms was grafted onto a solid substrate to fabricate a responsive polymer surface consisting of a densely packed monolayer of copolymer molecules. The grafted layer demonstrates a two-level hierarchical response upon external stimuli combining core-shell transitions of single stars with cooperative transitions of the interacting arms between “dimple” and “ripple” morphologies of the monolayer. The response allows for the switching of the surface properties upon changing solvent selectivity or pH of the aqueous environment.
Introduction We have investigated the switching behavior of the (13 ( 1)-nm-thick polymer layer formed by the poly(2vinylpyridine)-star-poly(styrene) (P2VP7-PS7) heteroarm star-copolymer molecules chemically grafted to the solid surface via P2VP arms. In this study, we present experimental evidence for the combination of two hierarchical levels of environmentally directed self-assembly: (1) intramolecular segregation of the unimolecular spherical mixed brush (where spherical geometry is obtained by grafting different polymer chains to the same core) and (2) the cooperative phase transition in the dense grafted layer of the unimers. We observe reversible morphological transitions between chemically different phase domains 25-50 nm in size that affect the surface composition and properties of the grafted layer. This mechanism is employed to fabricate responsive polymer surfaces. Smart responsive polymer coatings adapt or change surface properties (wetting, reactivity, biocompatibility, adhesion, roughness, optical characteristics, etc.) via external stimuli.1 This behavior has been explored for the fabrication of biomaterials,2 drug delivery systems,3 reversible surface patterning,4 and sensors.5 Rapid, reversible, and reproducible responses are reported for the grafted monomolecular polymer layers.1 These layers are usually made by chemically grafting polymer molecules to the surface with one end (polymer brushes).6 Polymer brushes made of two incompatible polymers (mixed polymer brushes) were shown to be sensitive to the outside medium because of the specific phase-segregation mechanisms,7 where, depending on outside conditions, either one polymer or both polymers are expressed to the top of * Corresponding author. E-mail:
[email protected]. Phone: (315) 268-3807. Fax: (315) 268-6610. † Clarkson University. ‡ University of Patras. (1) Luzinov, I.; Minko, S.; Tsukruk, V. V. Prog. Polym. Sci. 2004, 29, 635-698. (2) Kwon, O. H.; Kikuchi, A.; Yamato, M.; Sakurai, Y.; Okano, T. J. Biomed. Mater. Res. 2000, 50, 82. (3) Kaneko, Y.; Sakai, K.; Okano, T. In Biorelated Polymers and Gels: Controlled Release and Application in Biomedical Engineering; Okano, T., Ed.; Academic Press: New York, 1998; pp 1-69. (4) Ionov, L.; Minko, S.; Stamm, M.; Gohy, J. F.; Jerome, R.; Scholl, A. J. Am. Chem. Soc. 2003, 125, 8302-8306. (5) Tokareva, I.; Minko, S.; Fendler, J. H.; Hutter, E. J. Am. Chem. Soc. 2004, 126, 15950-15951. (6) Polymer Brushes; Advincula, R. C., Brittain, W. J., Caster, K. C., Ruehe, J., Eds.; Wiley-VCH: Weinheim, Germany, 2004.
the brush. Morphology and surface composition can be reversibly switched via directed self-assembly upon external stimuli.8-11 Recently, we reported on a unimolecular spherical mixed brush represented by the P2VP7-PS7 copolymer, consisting of seven poly(2-vinylpyridine) and seven polystyrene arms emanating from the same core.12 The unimolecular spherical mixed brush demonstrates similar principles to that of directed self-assembly upon external stimuli. In an acidic environment at low concentrations, the copolymer forms unimolecular micelles with the PS segregated to the core and the protonated P2VP shell. Upon exposure of toluene, P2VP7-PS7 undergoes the inverse intramolecular segregation: the P2VP arms form a dense core surrounded by the swollen PS shell. In this work, we apply the mixed spherical brush for grafting onto flat substrates. The fabricated surfaces demonstrate an example of hierarchical transitions of the thin film morphology. However, this complex mechanism is approached using a very simple synthetic route to the responsive material via a one-step grafting procedure in contrast to much more complex procedures reported for the synthesis of mixed brushes. Results and Discussion P2VP7-PS7 was synthesized by anionic polymerization.13 The core of the unimer is formed by 106 cross-linked divinylbenzene molecules. The PS and P2VP arms (Mw(PS arm) ) 20 000 g/mol and Mw(P2VP-arm) ) 57 000 g/mol, where Mw is the weight-average molecular weight) are randomly grafted to the core. The core is highly cross linked and can be considered to consist of dense particles 3-6 nm in diameter embedded into the shell formed by PS and P2VP arms. We did not observe this core in AFM experiments, and we did not obtain any signs of the effect of this core on the visualized morphologies of the structures (7) Minko, S.; Muller, M.; Usov, D.; Scholl, A.; Froeck, C.; Stamm, M. Phys. Rev. Lett. 2002, 88, 035502. (8) Minko, S.; Muller, M.; Motornov, M.; Nitschke, M.; Grundke, K.; Stamm, M. J. Am. Chem. Soc. 2003, 125, 3896-3900. (9) Julthongpiput, D.; Lin, Y. H.; Teng, J.; Zubarev, E. R.; Tsukruk, V. V. Langmuir 2003, 19, 7832-7836. (10) Zhao, B.; He, T. Macromolecules 2003, 36, 8599-8602. (11) Zhao, B. Polymer 2003, 44, 4079-4083. (12) Gorodyska, G.; Kiriy, A.; Minko, S.; Tsitsilianis, C.; Stamm, M. Nano Lett. 2003, 3, 365-368. Kiriy, A.; Gorodyska, G.; Minko, S.; Stamm, M.; Tsitsilianis, C. Macromolecules 2003, 36, 8704-8711. (13) Tsitsilianis, C.; Voulgaris, D. Macromol. Chem. Phys. 1997, 198, 997.
10.1021/la050404a CCC: $30.25 © 2005 American Chemical Society Published on Web 08/12/2005
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Table 1. Advancing Contact Angles and XPS Data for the P2VP7-PS7 Star-Copolymer Layer Surface after Exposure to Different Solvents
solvent
contact angle, deg
toluene chloroform ethanol H2O, pH 2
91 ( 1 78 ( 2 65 ( 1 15 ( 5
nitrogen, at. % (XPS, 30° angle of incidence) 5.47 7.61 9.20
formed by single unimers or thin films of the unimers. The surface of silica substrate was chemically modified with Br-alkyl silane. Then the grafting was performed by a quaternization reaction of P2VP arms with surface Br-alkyl groups (Supporting Information). To investigate the responsive behavior of the starcopolymer grafted layers, the samples were exposed to selective (toluene, ethanol, pH 2 water) and less selective (chloroform) solvents for 10 min. After the exposure to each solvent, the samples were dried and investigated with AFM, XPS, and the contact angle method (Table 1). In Figure 1, we compare the morphology of the grafted monolayer and the morphology of single unimers in different solvents (Supporting Information). A comparison can be made between the conformational transitions of single molecules in very dilute solutions (where the transitions occurs as a single-molecule event without effects of any kind of intermolecular interactions and the solid substrate) with cooperative transition in the dense layer confined by two interfaces and other molecules grafted in close proximity to each other. In all solvents, the single unimers possess a core-shell morphology caused by the phase separation of the PS and P2VP arms to avoid unfavorable interactions in the given solvent. The size of the shell increases as solvent selectivity increases. The largest shell was observed upon treatment with pH 2 water where the P2VP arms are strongly stretched because of the electrostatic repulsion. Similar behavior was observed for the grafted monolayers in selective solvents toluene (for PS) and ethanol or pH 2 water (for P2VP). The PS and P2VP arms in toluene and ethanol, respectively, form nanosize clusters segregated to the unimer cores. That was proven by the XPS and water contact angle data. Upon exposure to toluene, the contact angle value corresponds to the wetting of PS (90°), which completely occupies the top layer (the lowest fraction of N atoms in the XPS spectra). Upon exposure to ethanol or water, the contact angle value corresponds to the wetting of P2VP (63°) (in the case of acidic water, the contact angle value corresponds to the wetting of protonated P2VP), and the XPS spectra reveal the largest fraction of N atoms. Thus, in the monolayer each grafted single smart molecule responds to outside changes via intramolecular phase segregation of PS, and P2VP arms form the first hierarchical level of response. The cooperative character of the conformational transitions in the densely packed monolayer of smart molecules makes up the second level of response. The combination of the smart molecules in one layer introduces new kinds of morphology. The core-shell transitions of unimers are transformed into transitions between different dimple morphologies in the grafted monolayer in selective solvents (Figure 1a, b, and d, and Figure 2a and c). Clusters of the unfavored polymer are embedded in the layer of extended chains (matrix) of the favored polymer. In other words, the core-shell transitions of unimers are transformed into the interplay between lateral and layered phase segrega-
Figure 1. AFM topography images of single unimers (right images, for Z range see cross sections in Supporting Information) and the grafted P2VP7-PS7 star-copolymer layer (left images, 1 × 1 µm2) upon exposure to (a) pH 2 water (pH regulator HCl) (rms ) 3.5 nm, Z scale ) 15 nm); (b) ethanol (rms ) 8.2 nm, Z scale ) 29 nm); (c) chloroform (rms ) 2.2 nm, Z scale ) 12 nm); and (d) toluene (rms ) 4.6 nm, Z scale ) 22 nm).
tion. Comparing samples a and b (Figure 1) shows that the increase in solvent selectivity results in an increase in upper layer thickness. The PS clusters are poorly recognized under the relatively thick carpet of P2VP arms (Figure 1a). However, the P2VP7-PS7 molecules possess new specific morphology in the grafted monolayer exposed to chloroform, where PS and P2VP arms segregate to different sides of the core (Figures 1c and 2b). The result of the phase-segregation mechanism appears as a ripple morphology of the layer. Both the P2VP and PS arms are exposed to the top and form alternating stripes. This may be concluded from the AFM image, XPS, and contact angle data. Chloroform is slightly selective for P2VP, and this promotes the formation of the ripple structures. The
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Figure 2. Schematic representation of the switching behavior of the P2VP7-PS7 star-copolymer layer chemically grafted to the surface upon exposure to solvents: (a) selective for P2VP (dashed lines); (b) nonselective; and (c) selective for PS (solid lines).
switching between different morphologies is a reversible process. We observed the transitions more than 10 times upon treatment of the sample with different solvents. Here we should mention that the thin-layer morphologies are equilibrium morphologies obtained upon treatment with solvents that dissolve both polymers PS and P2VP (for example, chloroform and toluene). However, treatment with ethanol and water may not show a completely equilibrated morphology because these solvents do not dissolve PS and the equilibration could be slow kinetically. We performed a series of experiments to demonstrate that the observed morphologies were not corrupted by capillary forces acting while the samples were rapidly dried. Two types of AFM measurements were performed in solutions. (1) We tried to scan the surfaces under ethanol and water (pH 2). In both the cases, we were unable to receive a stable image for the layers because the top layer
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was highly swollen by solvent. That is consistent with the model of the strong layered segregation. (2) We introduced the sample to a selective solvent (methylethylketone, MEK) that is miscible with water. Then we rapidly dried the sample and analyzed the dry sample. In a second experiment, MEK was substituted by pH 5.5 water (we have shown in the reference experiments that water at pH >5 did not effect the morphology of the grafted layer) by multiple dilution with water of the solvent bath containing the embedded sample so that we avoid drying while the sample was transferred from the MEK bath to the water bath. Afterward, we scanned the sample under water. We found no changes in the sample morphology as compared to that of the dry sample (Supporting Information). It is noteworthy that the morphologies of the layer formed by the densely packed “spherical brushes” are very similar to the morphologies of “flat” mixed brushes.7 In both cases, the response is expressed in the form of morphological transitions between the dimple and ripple segregated phases. In conclusion, we demonstrate a simple one-step route to the fabrication of responsive coatings from star heteroarm copolymers. The cooperative interactions in the grafted layer of the responsive star polymer molecules allow for switching surface properties of the film and extend the responsive behavior (as compared to that of the single molecules) of the assembly where new morphologies appear (dimples and ripples in the layer vs coreshell morphology of unimers). Acknowledgment. We thank Igor Luzinov and Bogdan Zdyrko for assistance and helpful discussions. Y.R. acknowledges support from the North Atlantic Treaty Organization under the NSF award DGE 0411649 and the NSF award CTS 0456548. Supporting Information Available: Grafting method, ellipsometry, AFM, and XPS data. This material is available free of charge via the Internet at http://pubs.acs.org. LA050404A
