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Evolution of Surface Morphologies in Multivariant Assemblies of Surface-Tethered Diblock Copolymers after Selective Solvent Treatment Michael R. Tomlinson and Jan Genzer* Department of Chemical & Biomolecular Engineering, North Carolina State University, Raleigh, North Carolina 27695-7905 Received June 8, 2005. In Final Form: October 17, 2005 We study systematically the topography behavior of PHEMA-b-PMMA block as a function of the PHEMA and PMMA block lengths after selectively collapsing the top (PMMA) block by using surface-anchored assemblies of poly(2-hydroxyethyl methacrylate-b-methyl methacrylate), PHEMA-b-PMMA, block copolymer with orthogonally varying lengths of each block. Our experimental results are in excellent qualitative agreement with topology diagrams predicted by self-consistent field calculations of Zhulina and co-workers.
During the past few years, several researchers have reported on tailoring the topology of soft material surfaces by utilizing surface-confined copolymers in conjunction with selectively swelling one of the blocks, while collapsing the other block. For instance, Zhao and co-workers demonstrated that selective swelling and collapse of poly(styrene-b-methyl methacrylate) brushes produced variable and switchable surface topologies.1,2 This simple and yet very powerful method of tailoring substrate topologies has led to exciting developments in utilizing surfacegrafted polymers as potential “soft vehicles” capable of moving nanosized objects.3 In this paper, we present the first systematic study of the dependence of surface topology on the length of the two blocks of surface-tethered block copolymers, after selectively collapsing the top block. Our findings are in very good qualitative agreement with theoretical predictions of Zhulina et al.4,5 and Balazs et al.6 Polymer brush gradients on material substrates provide unique means of programmed material assembly and systematic exploration of a broad parameter space.7-13 The continuous variation of the physicochemical structure and property of the surface-grafted chains, not confounded by intervening gaps, eliminates the requirement of unconfirmed interpolation to determine the brush proper* To whom correspondence should be addressed. E-mail:
[email protected]. (1) Zhao, B.; Brittain, W. J.; Zhou, W.; Cheng, S. Z. D. Macromolecules 2000, 33, 8821. (2) Zhao, B.; Brittain, W. J.; Zhou, W.; Cheng, S. Z. D. J. Am. Chem. Soc. 2000, 122, 2407. (3) Santer, S.; Ru¨he, J. Polymer 2004, 45, 8279. (4) Zhulina, E.; Singh, C.; Balasz, A. C. Macromolecules 1996, 29, 6338. (5) Zhulina, E.; Singh, C.; Balasz, A. C. Macromolecules 1996, 29, 8254. (6) Balazs, A. C.; Singh, C.; Zhulina, E.; Chern, S.-S.; Lyatskaya, Y.; Pickett, G. Prog. Surf. Sci. 1997, 55, 181. (7) Wu, T.; Efimenko, K.; Genzer, J. J. Am. Chem. Soc. 2002, 124, 9394. (8) Wu, T.; Efimenko, K.; Vle`ek, P.; Sˇ ubr, V.; Genzer, J. Macromolecules 2003, 36, 2448. (9) Wu, T.; Genzer, J.; Gong, P.; Szleifer, I.; Vlcˇek, P.; Sˇ ubr, V. in Polymer Brushes; Brittain, B., Advincula, R., Ru¨he, J., Caster, K., Eds.; Wiley & Sons: New York, 2004. (10) Tomlinson, M. R.; Genzer, J. Macromolecules 2003, 36, 3449. (11) Tomlinson, M. R.; Genzer, J. Chem. Commun. 2003, No. 12, 1350. (12) Bhat, R. R.; Tomlinson, M. R.; Wu, T.; Genzer, J. Adv. Polym. Sci. 2005 in press. (13) Bhat, R. R.; Tomlinson, M. R.; Genzer, J. Macromol. Rapid Commun. 2004, 25, 270.
ties at a specific surface coordinate. By integrating two individual gradient building blocks, one can produce orthogonal polymer gradient motifs, in which two independent material characteristics vary continuously in orthogonal directions, and where each individual position on such substrates reflects a unique combination of the two orthogonally varying properties. In this paper, we show how to prepare surface-tethered A-B block copolymers of poly(2-hydroxyethyl methacrylate)-b-poly(methyl methacrylate) (PHEMA-b-PMMA) with smoothly varying lengths of both blocks on a single substrate and utilize such structures to systematically map out the influence of the block length on surface morphologies of PHEMAb-PMMA in response to selectively collapsing the top (PMMA) block of the copolymer. Silicon wafers were cut into 5 × 5 cm2 pieces and decorated with a monolayer of (11-(2-bromo-2-methyl)propionyloxy) undecyl-trichlorosilane (BMPUS), the initiator for atom transfer radical polymerization (ATRP).14 The orthogonal diblock gradient was formed by first creating a gradient in molecular weight of PHEMA15 by “grafting from” polymerizing 2-hydroxyethyl methacrylate using the solution draining method,10 followed by rotating the sample by 90°, and repeating the previous step by polymerizing methyl methacrylate (MMA). This procedure resulted in a PHEMA-b-PMMA diblock copolymer brush (cf. Figure 1a) with position-dependent lengths of the two blocks on the substrate. In Figure 1b, we present the dry thickness maps along the PHEMA-b-PMMA specimen. The thickness of each block was determined using ellipsometry after each synthesis step. The data in Figure 1 reveal that PHEMA dry thickness increases linearly along the X (horizontal direction in Figure 1b) direction and the PMMA thickness increases linearly in the Y direction (vertical direction in Figure 1b). Because the grafting density, σ, of all polymers is approximately equal on the entire specimen,10 the dry thickness of each block, h, is directly proportional to its molecular weight, M (h ) σM/FNA, where F and NA are the density and Avogadro’s number, respectively).16 More information about the various copolymer compositions can be obtained by plotting the dry thicknesses of the PHEMA (14) Pyun, J.; Kowalewski, T.; Matyjaszewski, K. Macromol. Rapid Commun. 2003, 24, 1043. (15) To generate linear (un-cross-linked) PHEMA brushes, we follow the procedure suggested by Robinson, K. L.; et al., Macromolecules 2001, 34, 3155.
10.1021/la051523t CCC: $30.25 © 2005 American Chemical Society Published on Web 11/08/2005
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Figure 1. (a) Schematic of the PHEMA-b-PMMA copolymer brush. (b) Dry thickness profile of PHEMA-b-PMMA MW1/MW2 orthogonal brush as a function of the position on the substrate. (c-e) PHEMA (red squares) and total copolymer (blue circles) thicknesses along the directions depicted in the total thickness profile shown in part b. The lines in panels c-e are meant to guide the eye. The thickness error bars are smaller than the size of the symbols.
and PMMA blocks along the various directions indicated by arrows in Figure 1b. The horizontally pointing arrows denote copolymers having a constant PMMA length and a linearly increasing PHEMA length. Two cases are highlighted here, copolymers with a short (1) and long (1’) PMMA block (Figure 1c). The vertical arrows depict block copolymers with a linearly varying length of the PMMA block and a constant length of the bottom PHEMA block. As before, we mark the boundary cases involving a short (2) and long (2’) PHEMA block (Figure 1d). The diagonals in Figure 1b denote copolymers that have: (a) approximately constant fraction of both blocks but an increased total length (3) and (b) those with a constant length but a linearly varying composition (4; Figure 1e). The following solvent treatment was designed to swell the entire copolymer chain then selectively collapse the top PMMA block. The orthogonal gradient specimen was immersed in 40% ethanol/acetone (v/v) solution, a good solvent mixture for both blocks. The solution composition was changed gradually by adding ethanol. When the content of ethanol in the solution reached ≈90%, the sample was immediately transferred to 100% anhydrous (16) The molecular weight of the brush (M) can be estimated from the dry brush thickness (h) from M ≈ 1200h, where M is in Daltons and h is in nanometers. This approximate relation has been obtained by growing chains simultaneously in bulk and on the surface and determining M via size exclusion chromatography and h via ellipsometry. Although it has been found to be valid for a range of methacrylates and acrylates grown from BMPUS initiator layers deposited under identical conditions,12 this relationship should be considered as an estimate only because it assumes that chains grown under confinement possess the same rate of polymerization as those polymerized in solution. Nevertheless, this relationship provides a very reasonable estimate for the chain grafting density (σ): σ ≈ 0.45 chains/nm2.
ethanol. To preserve the chain “equilibrium” conformations and minimize the reordering of the copolymer structures upon drying, the sample was vitrified by immersing into liquid ethane (freezing point, -211 °C; boiling point, -88 °C). The ethane and the sample were allowed to increase in temperature until the ethanol was absorbed completely by the ethane phase. The sample was then immersed in liquid nitrogen and transferred to a vacuum oven at room temperature. Almost a decade ago, Zhulina and co-workers studied the swelling of surface-tethered copolymers using selective solvents.4-6 Their self-consistent field calculations and scaling arguments revealed that grafted copolymers exposed to a solvent that is a theta solvent for the bottom block and a poor solvent for the top block exhibit several distinct morphologies: flat (I), pure B pinned micelles (PMB), A-legged micelles (MAB), starlike micelles (MA), and a bicontinuous phase (BAB). The type of morphology the copolymer adopts depends on the lengths of the individual blocks. Our sample design is ideally suited for testing the predictions of Zhulina and co-workers. Having copolymers with independently and smoothly varying lengths of each block on a single sample, we can systematically screen the entire parameter space in a reproducible and fast manner. The effect of PHEMA-b-PMMA copolymer composition on the surface morphology after solventcollapsing the top PMMA block was determined from scanning the specimens with tapping mode scanning force microscopy (SFM). SFM images were taken from equally spaced locations on the sample. In Figure 2, we plot the morphology diagram deduced from multiple SFM scans (>30) collected from several
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Figure 2. Top and middle panels: Morphologies of ethanol-quenched PHEMA-b-PMMA brushes collected by scanning force microscopy (SFM) on various positions on the sample corresponding to different combinations of the PHEMA and PMMA block lengths. Specifically, the top row denotes specimens with approximately constant PHEMA block length and increasing PMMA block length (marked as V#: vertical position in phase diagram shown in the bottom panel). The middle panel presents images collected from samples with nearly constant PMMA block length and increasing PHEMA block length (marked as H#: horizontal position in phase diagram shown in the bottom panel). The height scale of the SFM images ranges from 0 (dark brown) to 30 nm (white). Each image is accompanied with a characteristic topography scan (plotted as a height different relative to the flat surface). Bottom: Phase diagram for ethanol-quenched PHEMA-b-PMMA brushes plotted as a function of the PHEMA and PMMA block lengths expressed in terms of dry PHEMA (hPHEMA) and PMMA (hPMMA) thickness. The phase diagram reveals the existence of flat (F), micellar (M), and bicontinuous (BC) morphologies. The F/M and M/BC regions denote samples, whose morphology could not be identified unambiguously.
areas on two discrete PHEMA-b-PMMA orthogonal samples. Although more detailed analysis of the surface morphologies is outside the scope of this paper and will be presented in an upcoming publication,17 we note that our results reveal the presence of three very distinctive, clearly-to-resolve morphologies of the PHEMA-b-PMMA copolymer that range from flat topology (F) to micelles (M) to a bicontinuous phase (BC). We note that these experimentally observed trends are in very good qualitative agreement with the predictions of Zhulina and coworkers.4-6 The SFM results can be broken down into two basic effects: i) the increase in size of the micelles (17) Tomlinson, M. R.; Genzer, J. in preparation.
(dependent primarily on the PMMA block length) and ii) the lateral confinement of the micelles (dependent primarily on the PHEMA block length). In the region of short PHEMA, the PMMA micellar cores are pinned close to the surface. As the PMMA block increases the micellar cores increase in size; they are forced to approach one another, begin to aggregate (V2), and eventually form a bicontinuous honeycomblike morphology (V3 and V4). Increasing the PHEMA block length dramatically enhances the lateral freedom of the micelles. Comparison of copolymer morphologies having the same PMMA thickness and increasing PHEMA reveals that the copolymer micelles become less laterally confined and more freefloating on the PHEMA brush surface (H1 f H4). Solely
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on the basis of the SFM images, we cannot make any direct comparison to the exact types of micellar morphologies predicted by Zhulina and co-workers. Clearly, more work is needed to study these structures in more detail. Static contact angle readings performed using deionized water support these transitions as well.17 Although it is challenging to make conclusions about chain conformations solely on the basis of contact angle measurements, because these can be affected greatly by surface roughness,18 several trends are worth mentioning. The transition from isolated micelles to the bicontinuous structure associated with increasing the PMMA length is accompanied with a sharp decrease of contact angles from ≈68-70° down to ≈50-55°. Concurrently, the M f BC transition is accompanied by a decrease in the root-meansquare roughness from ≈3-5 nm to ≈2-3 nm, indicating (18) Ulman, A. An Introduction to Ultrathin Organic Films from Langmuir-Blodgett to Self-Assembly; Academic Press: New York, 1991.
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that the observed contact angle decrease does not result from increased surface roughness (Wenzel’s law of wetting18). These observations thus support the notion that as the length of PMMA increases, the PMMA block (contact angle ≈75°) is almost completely collapsed thus exposing the PHEMA block (contact angle ≈49°) to the surface. Acknowledgment. We are grateful to the National Science Foundation for providing financial support for this work. We also thank Professor Greg Parsons (NCSU Chemical & Biomolecular Engineering) for the use of his SFM, Professor Richard Spontak (NCSU Chemical & Biomolecular Engineering) for expert help in developing a sample freezing method, and Jan Singhass (NCSU Glass Shop) for her expertise in construction of the polymerization chamber. LA051523T
