A structural definition of polymer brushes - Semantic Scholar

HIGHLIGHT A Structural Definition of Polymer Brushes WILLIAM J. BRITTAIN,1 SERGIY MINKO2 The Department of Polymer Science, The University of Akron, Akron, Ohio 44325-3909 2 Department of Chemistry and Biomolecular Science, Clarkson University, 8 Clarkson Ave., Potsdam, New York 13699

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Received 30 March 2007; accepted 23 April 2007 DOI: 10.1002/pola.22180 Published online in Wiley InterScience (www.interscience.wiley.com).

ABSTRACT: In this article, we consolidate several literature reports in an attempt to provide a structural definition of a polymer brush as a tethered polymer layer with a high grafting density of polymer chains. We will try to distinguish a brush from grafted polymer

WILLIAM BRITTAIN

layers and coated layers and provide a reasonable approach to the description of the brush regime with a single parameter S—reduced grafting density. Furthermore, we will attempt to describe the relevance of this description to applications. This article is timely given the continuing

interest relating to brushes on flat substrates, nanoparticles, and the walls of micro-channels. VC 2007 Wiley Periodicals, Inc. J Polym Sci Part A: Polym Chem 45: 3505–3512, 2007

Keywords: films; nanoparticles

monolayers;

William Brittain is Vice President for Polymer and Surface Sciences at Bausch and Lomb in Rochester, NY, USA. Dr. Brittain received his Ph.D. in chemistry from the California Institute of Technology (1982). Following postdoctoral work at Duke University, he worked in the Central Research and Development Department of E. I. du Pont. In 1990, Dr. Brittain joined the faculty in the Department of Polymer Science at the University of Akron. He was Chair of the department from 1998–2001. He received the Outstanding Researcher of the Year Award from the University of Akron in 2001. In 2001, Dr. Brittain served as Chair of the Division of Polymer Chemistry of the American Chemical Society. In late 2006, he joined Bausch and Lomb. His research focuses on polymer brushes, living radical polymerization, and nanoparticles. Dr. Brittain has published over 100 articles, including 5 book chapters. He holds 2 U.S. patents. [Color figure can be viewed in the online issue, which is available at www.interscience.wiley.com.]

Correspondence to: W. J. Brittain (E-mail: William_j_ [email protected]) Journal of Polymer Science: Part A: Polymer Chemistry, Vol. 45, 3505–3512 (2007) C 2007 Wiley Periodicals, Inc. V

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J. POLYM. SCI. PART A: POLYM. CHEM.: VOL. 45 (2007)

SERGIY MINKO

Sergiy Minko is the Egon Matijevic Chaired Professor of Chemistry at Clarkson University in Potsdam, NY, USA. Dr. Minko received his Ph.D. in chemistry from the Lviv Polytechnic University (1983). In 1986 he became senior staff scientist at the Institute of Physical Chemistry (Ukrainian National Academy of Science). In 1993 he received a Doctor of Science Degree and in 1995 he was granted an honored professor degree by the Ministry of Education of Ukraine. After a stay as an Alexander von Humboldt Fellow at the University of Ulm (1996– 1997) and a visiting professorship in the Institut Charles Sadron (Strasbourg) and the Max Plank Institute for Polymer Research in Maiz, in 1999 he became a senior staff scientist and group leader at the Leibnitz Institute for Polymer Research in Dresden. In 2003 he joined the Department of Chemistry and Bimolecular Science, Clarkson University. His research is focused on the synthesis and study of thin polymer films, polymer brushes, polymer gels and membranes, colloidal particles, and micelles. His present research includes single molecule AFM experiments and the study of lipid bilayers. He combines basic research with applications in material science for stimuli responsive polymer films, smart coatings, biomaterials, microfluidic devices, sensors, and textiles. Dr. Minko has published over 130 articles, including 15 book chapters. He holds 16 patents.

INTRODUCTION This article was stimulated by the continuing number of scientific publications that use the term ‘‘polymer brush.’’ In many cases, this term is used for polymer chains endgrafted to a supporting surface (interface), or in other words, it is broadly used as a synonym of the term ‘‘tethered polymer chain.’’ Nevertheless, some authors associate the term polymer brush with a layer of tethered polymer chains under specific conditions; specifically, when the behavior of the tethered layer is dictated by strong interactions between densely grafted polymer chains. Here, we attempt to simplify a suggested use of this terminology. We do not suggest that this article is a comprehensive review, but rather a discussion about one of the key fields in polymer nanotechnology. The connection between polymer brushes and nanotechnology stems from the typical dimensions of brush systems. Polymer brushes (or tethered polymers) first gained attention in the 1950s when it was discovered that flocculation could be prevented by grafting polymer molecules to colloidal particles.1–6 Some of the first quantitative treatments of polymer brushes were reported by Alexander,7 de Gennes,8 and Semenov.9 All these reports introduced theoretical approaches to study polymer brushes. Although numerous experimental results on polymer grafting on solid substrates were published even earlier, lack of well-developed characterization methods for thin polymer films did not allow for the accurate study and identification of the grafted layers as polymer brushes. In this article, we will concentrate on

polymers grafted to solid surfaces, although it is widely recognized that other examples of end-immobilized polymer systems exist in other heterogeneous systems. Several groups have performed detailed studies on polymer brushes in the early 1990s and helped to create an increased awareness of these type of structures.10–12 Polymer brushes have become increasingly studied and most macromolecular and/or surface journals now routinely contain articles on polymer brushes. It is generally recognized that a polymer brush corresponds to an array of macromolecular chains attached to a surface and in sufficient proximity so that the unperturbed solution dimensions (in a good solvent) of the chains are altered. Furthermore, this close proximity causes overlap of adjacent chains and thus significantly alters the conformational dimensions of individual polymer chains such that they extend or alter their normal radius of gyration to avoid unfavorable interactions. Films composed of polymer chains that extend along a direction normal to the grafting surface can exhibit properties distinctly different from chains in solution, and can exhibit many novel and different properties relative to chains in solution and thus make polymer brushes an interesting field for novel properties. Examples of distinctly different properties include (1) interfacial localization of terminal groups,13 (2) diffusion control,14 (3) regulation of steric repulsive forces,15 (4) control of phase-segregation in response to external stimuli,16,17 (5) wetting control,18–24 (6) control of protein and cell adsorption,25 (7) adsorption of molecules,26 (8) lubrication,27 and (9) flocculation control.28

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Figure 1. Methods of polymer formation.

SYNTHESIS OF GRAFTED POLYMER CHAINS Much of the early work on polymer brushes focused on systems formed by physisorption; that is, the selective adsorption of one block of a diblock copolymer to a surface.29–31 The surface and solvent were chosen to maximize preferential adsorption of one block to a solid surface while the solvent was chosen to preferentially interact with the other block. One example is the physisorption of polystyrene-b-poly(ethylene oxide) (PS-bPEO) from a toluene solution. The PEO segment was attracted to a mica surface while the PS block was preferentially solvated. Although this is a simple method of grafted polymer formation, these physisorbed blocks can be unstable under certain conditions of solvent and temperature, or can be displaced by other adsorbents. The covalent binding of polymer chains at the interface has attracted interest because of the enhanced stability of the tethered polymer layers. Covalently attached polymer chains (chemisorption) can be synthesized by either the ‘‘grafting-to’’ or ‘‘grafting-from’’ method. The scheme in Figure 1 depicts representations for physisorption and chemisorption. ‘‘Grafting-to’’ involves the chemical reaction of preformed, functionalized polymers with surfaces containing complementary functional groups.32 The primary advantage of this method is a technically simple synthesis and more accurate characterization of the preformed polymers. The grafting can be performed from polymer solutions or melt and, in general, higher concentrations of polymer lead to higher grafting densities by the ‘‘grafting-to’’ method. Often it has been argued that the ‘‘grafting-to’’ technique will not produce dense polymer brushes because the attachment of chains may be impeded by the existence of previously attached chains. Grafting density can be regulated by grafting time. As soon as the grafting process is terminated (by separation of the support with grafted polymer from the polymer solution/melt or by rapid cooling) the residual ungrafted polymer can be removed by solvent extraction. ‘‘Grafting-from’’ involves an in situ polymerization of an initiator functionalized surface with monomer. Many living and conventional vinyl polymerization techniques have been reported and applied using this method.33,34 In general, the ‘‘grafting-from’’ technique

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can produce polymer brushes of high grafting density because the grafted layer is swollen by the monomer solution that feeds the growing chains. In this case, the growth of chains is not limited by the diffusion of monomer unless a very high grafting density is approached. The ‘‘grafting-from’’ technique may suffer complications because of the limitation of initiator surface coverage, initiator efficiency, and the rate of diffusion of monomer to active polymerization sites. The effect of side reactions may be more important as compared to the bulk polymerization because of the high local concentration of polymer chains in the grafted layers; e.g., bimolecular termination in radical polymerization. Hence, the ‘‘grafting-from’’ polymerization may lead to a broader molecular mass distribution.

THE DEFINITION OF A POLYMER BRUSH BY CHARACTERISTIC PARAMETERS OF GRAFTED LAYERS The structure of a surface-immobilized polymer can be evaluated by the inverse value of the distance between grafting points (D) (Fig. 2). As the size of grafted polymer chains approaches the distance between grafting points, the grafted chains overlap. This point is a transition point between a single grafted chain (mushroom) regime and brush regime. A commonly used literature parameter for quantitative characterization of this transition is the reduced tethered density (S); S ¼ r p Rg2, where Rg is radius of gyration of a tethered chain at specific experimental conditions of solvent and temperature. The definition of grafting density (r) is determined by r ¼ (h q NA)/Mn (h, brush thickness; q, bulk density of the brush composition; and NA, Avogadro’s number). Grafting density (r) is sometimes determined by r ¼ 1/ D2.11 The physical interpretation of S is the number of chains that occupy an area that a free nonoverlapping polymer chain would normally fill under the same experimental conditions. Thus, S as defined here is a reliable parameter for judging the brush-like character of a

Figure 2. Characteristic parameters of the polymer brush: h-height and D-distance between grafting points.

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grafted film. It is generally recognized that three regimes occur in brush formation: (1) the ‘‘mushroom’’ or weakly interacting regime (S < 1), (2) the crossover regime (S & 1), and (3) the highly stretched regime (S [ 1). However, in real systems, the transition between single grafted chains and a polymer brush is less sharp because of the statistical characteristic of grafting and polydispersity of the tethered chains. Resulting fluctuations of the average distance between grafting points can cause an inhomogeneous distribution of S across the grafting surface.35 The ‘‘true brush’’ regime can be approached at a significantly high stretching of polymer chains that is typically characterized by S [ 5. However, this value is strongly affected by excluded volume and can vary from system to system. For example, Kent29 studied Langmuir-Blodgett layers of poly(dimethylsiloxane)-b-PS block copolymers and reported that the highly stretched regime occurs for S [ 12. A study was reported by Cheng and coworkers36,37 in which they studied PS brushes formed by the block-selective crystallization of PEO-b-PS and poly(L-lactic acid)-b-PS copolymers. This crystal surface engineering approach generated PS brushes covering the top and bottom basal surfaces of the crystallized poly(L-lactic acid) (PLLA) or PEO crystals. By controlling the crystallization temperature, solvent, and molecular weights of the blocks, the grafting density of the PS brushes could be systematically controlled. Although this study does not involve the covalent attachment of polymers to a surface, it is relevant as the PS chains are constrained to a specific 2-dimensional geometry by the structure of the underlying crystal surface. They reported the onset of PS chain overcrowding at S ¼ 3.7–3.8 and the formation of highly stretched PS chains at S & 14. One key experimental study relating grafting density to the highly stretched regime is that reported by Genzer and coworkers,38 where they prepared polymer brushes by the more common technique of covalent grafting. In this study, a gradient of initiator coverage was created by differential vapor diffusion of a trichlorosilane initiator onto a silicon substrate. The authors studied the dependence of film thickness versus initiator density for a polyacrylamide brush and observed a crossover regime where the relationship of brush thickness to r changed from zero order to 1/3 order, consistent with self-consistent theory predictions.29 The Mw of the polyacrylamide was 17,000 g/mol with PDI ¼ 1.7. For this system, Genzer and coworkers observed a mushroom-to-brush transition around r & 0.065 chains/nm2 (S & 6) (Fig. 3). These reports suggest that the transition to the true brush regime takes place at S [ 5. It is noteworthy that S depends on the thermodynamic quality of the solvent. Consequently, in some cases, the same brush can be

found in different regimes in different solvents. Thus, tethered polymer chains can be characterized by three major regimes in terms of grafting density: the mushroom regime at S < 1, mushroom-to-brush transition regime at 1 < S < 5, and brush regime at S [ 5. (The condition S [ 5 is selected because this value is a rough average of several studies). In terms of a recommendation to the field, one question is whether we should combine all three of these regimes of tethered polymer layers under the general term ‘‘polymer brushes’’ or we should use the term ‘‘polymer brush’’ only for the third regime at S [ 5. In other words, is it correct to use the terms ‘‘polymer brush’’ and ‘‘tethered polymer layer’’ as synonyms, or better to say ‘‘polymer brush in mushroom regime’’? Common practice in the literature is to broadly use the term ‘‘polymer brush’’ as a synonym for ‘‘tethered polymer layers’’ and ‘‘end-grafted polymers.’’ This practice sometimes does not distinguish polymer brush structures from adsorbed polymer layers, different kinds of deposited layers (casting, layer-by-layer deposition) thin polymer films and polymer layers formed by polymer chains grafted to the substrate via many monomer units per one chain. Our recommendation is that the term ‘‘polymer brush’’ only be used whenever the regime of the system is indicated (if known) or a value of S is provided. Many brush studies today involve stimuli–responsive systems that are composed of more complicated architectures that include multiblock polymers, gradient polymers, and Y-shaped polymers. With these systems, characterization of the tethered layer with the single S parameter becomes difficult, usually, because measurement of Rg is more difficult. It is worthwhile to consider the relative merits of the ‘‘grafting-to’’ versus ‘‘grafting-from’’ techniques in terms of a reduced grafting density. Many of the complicated architectures being reported recently have often relied on the ‘‘grafting-to’’ technique. For the ‘‘graftingto’’ technique, it is common to see relatively lower film

Figure 3. Transition between the ‘‘mushroom’’ regime and ‘‘true brush’’ regime observed in the experiment with a grafting density gradient of polyacrylamide brushes reported by Genzer et al.38

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thicknesses (typically 5–8 nm). We recommend that S be determined to evaluate whether a ‘‘grafting-to’’ method produces a film in the brush regime. Penn and coworkers39,40 have arguably performed some of the most systematic research on the ‘‘grafting-to’’ technique. They have reported results that suggest the ‘‘grafting-to’’ technique can produce sufficiently high grafting densities to qualify as conformationally stretched chains. The study focused on functionalized PS chains and reported that high values for r occur only at lower molecular weights (specifically in their study, Mn ¼ 14,500 g/mol) and concentrated solutions of PS. The highest value of r reported by Penn and coworkers was 0.14 chains/nm2 that corresponds to S & 4.0. Therefore this work indicates that for lower molecular weights of PS and sufficiently high polymer concentrations, it is possible to obtain chains that, at least, exist in the ‘‘mushroom-to-brush’’ transition regime. Luzinov and coworkers41 have reported ‘‘grafting-to’’ synthesis of poly(ethylene glycol) (PEG) brushes (Mn ¼ 5000 g/mol), which approached r ¼ 1.6 chains/nm2 or S & 24. Long and coworkers42 reported a ‘‘graftingto’’ approach where they measured the Rg of the precursor polymer. Thus, they were able to authoritatively establish in which regime the tethered polymer chains existed. They concluded that the end-grafted chains they prepared existed in a ‘‘moderately stretched’’ state by comparing film thickness to Rg. Using their data, we calculate that S ¼ 10 for linear polystyrene (Mn ¼ 3800 g/ mol) where this film exhibited a thickness of 3.7 nm (Rg ¼ 2.3 nm). These examples demonstrate that systems prepared by the ‘‘grafting-to’’ method can vary significantly in terms of approaching high S values; in some cases, the ‘‘true brush’’ regime can be approached using the ‘‘grafting-to’’ method. In most cases, the ‘‘grafting-from’’ technique will produce polymer brushes providing the density of initiating sites is sufficiently high and initiation efficiency is good. For many cases in the literature that have reported initiation efficiency, 10% is an average value.43 To verify whether the ‘‘grafting-from’’ method has produced a polymer brush, the molecular weight of the surface-immobilized polymer should be measured. Ideally, degrafting the polymer brush and subsequent molecular weight analysis is ideal. However, this is often impractical given the low surface area of many substrates, unless the ‘‘grafting-from’’ technique is applied to particles. When living radical polymerization is used for a ‘‘graftingfrom’’ process, a sacrificial initiator is often used, which leads to the formation of free polymer in solution. It has been reported that there is a reasonable correlation between molecular weight and polydispersity for free polymer and the polymer brush for brush growth via atom transfer radical polymerization.43 Therefore, in

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some cases, the free polymer molecular weight can be used to verify the polymer brush structure.

ARE EXTENDED POLYMER CHAINS REQUIRED FOR USEFUL APPLICATIONS? The significant interest in surface attached macromolecules raises a range of interesting questions; is a polymer brush simply another form of a thin polymer film. Do polymer brushes constitute a unique thin film structure that justifies more complicated synthetic processes? Furthermore, is there a unique aspect of extended polymer chains? To all of these questions, we argue that the answer is ‘‘yes.’’ Stretched polymer chains in the brush generate an entropic penalty that changes the energy of grafted chains. There are also well expressed examples of a very noticeable change of enthalpy of interactions with solvent as a function of grafting density.44 Properties of polymer brushes change with grafting density. There have been recent reviews of the potential applications of polymer brushes, but none of the reviews directly studied the relationship between grafting density and utility of a brush in a particular application.25,45,46 The nonmonotonous character of the structure versus grafting density relationship for polymer brushes implies a concept of an optimum grafting density for a given application and experimental conditions (since the excluded volume effect may modify the brush profile). Here, we have selected several examples to demonstrate that applications of polymer brushes are strongly dependent on the grafting density regime. There have been several studies on the temperaturedependent conformational change for surface-immobilized poly(N-isopropylacrylamide) (PNIPAM). For chains with a low grafting density, the conformational change of PNIPAM is subtle as the temperature increases above the lower critical solution temperature (LCST).47 The conformational change was deduced using neutron reflectivity. At higher grafting densities (0.25 chains/nm2), Kent and coworkers44 observed a strong contraction of the PNIPAM layer as the temperature passed through the LCST. Huck and coworkers48 reported that an 80 nm thick poly(4-diphenylaminobenzyl acrylate) (PTPAA) brush exhibited a current density three orders of magnitude higher than a spin-coated PTPAA film. The much-improved conductivity for the covalently attached film suggested a different structure that permitted clearer pathways for charge transport. Spin-coated films are likely to exhibit a certain degree of chain alignment in the plane of the film. Thus, charges will need to hop between chains more frequently than the covalently attached PTPAA film which will significantly impede charge transport.

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strongly interact with the polymer brush at moderate grafting density disperse freely within the polymer brush, while polymer ‘‘insoluble’’ particles tend to aggregate in the brush [Fig. 5(a)]. The big aggregates are expelled towards the brush-air interface [Fig. 5(b)].57 At a very high grafting density the particles will be expelled from the brush [Fig. 5(c)]. This example introduces nicely the concept of an optimum grafting density. The brush-particle composites can be fabricated at small and moderate grafting densities, while the protection of materials from particle adsorption can be approached at high grafting densities. Gast and Leibler49 have predicted the effect of attractive interactions of colloids stabilized by high density brushes (Fig. 6). Doi and coworkers50 have presented experimental evidence for an optimum grafting density (around S ¼ 8) for dispersing particles in polymer melts. A similar situation was discovered for wetting brushes of different grafting densities (Fig. 4) by Gay,51 Mass et al.,52 Voronov and Shafranska,53 Muller and MacDowell.54

Figure 4. Liquid interacting with polymer brushes: (a) dewetting at the low grafting density due to the interaction of the liquid with the substrate (the substrate is not wettable by the liquid; (b) wetting (h2 & 0) at moderate grafting densities; (c) dewetting at high grafting densities due to a poor miscibility with stretched chains.

A common application of polymer brushes is to regulate interactions that include interactions of the polymer brushes with liquids, solids, particles, proteins, cells, etc.45,49–56 The effect of the grafting density on the interaction can be generalized as follows. At low grafting densities (S < 1) molecules or particles can mix with grafted polymer chains, penetrate the brush, and interact with the underlying substrate (Figs. 4 and 5). In this case, van der Waals interactions may strongly dominate. As the grafting density increases (S [ 1) and if the brush is swollen by a good solvent, repulsive forces may dominate (e.g., colloidal stabilization, resistance to protein adsorption). A further increase of grafting density results in a rise in attractive forces. The mechanism of attractive interactions can vary (depletion forces versus van der Waals interactions), but the origin of these mechanisms is largely due to poor miscibility of solvents, polymers, and particles within the dense polymer brushes. Interaction with the brush also affects the spatial organization of particles in the brush. Small particles that

Figure 5. Intraction of particles with polymer brush: (a) strong interaction between particles and polymer, low grafting density-particles are mixed with polymer chains; (b) poor interaction between particles and polymer chains, low grafting density-particles segregate from the grafted chains; (c) high grafting density-steric repulsion of the particles.

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Figure 6. Interaction between spherical brushes: (a) attraction at low grafting density (bridging effect); (b) steric repulsion at moderate grafting density; (c) attraction (Van der Waals interaction, depletion effect) between brushes at very high grafting density.

Various applications of polymer brushes for responsive materials suggest the possibility to not only control interactions, but also control physical properties of the brush based on an expansion-shrinkage effect when solvent quality changes. In this case, one should also consider the idea of an optimum grafting density. At low grafting densities, a small expansion of the brush in good solvent versus poor solvent can be attributed to a small change in osmotic pressure. At very high grafting densities, highly stretched brushes will not mix with solvents. Genzer and coworkers55 have experimentally shown that the degree of swelling of polyelectrolyte brushes decreases with the grafting density For brushes with a more complex architecture (e.g., block-copolymer brushes, mixed brushes, and spherical mixed brushes) the idea of optimum grafting density can also be applied. The responsive behavior of these brushes is due to the microphase segregation of unlike polymers. Because the grafting density affects the immiscibility of the polymers, an increase of the grafting density will change the sensitivity of the brushes to their environment.56

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The kinetic aspect of grafting density for responsive brushes has not yet been explored. An increase of grafting density will decrease the rate of the brush response. The response of the brush is associated with conformational changes of grafted chains via segmental diffusion. For high grafting densities, the local viscosity of the brush can be very high and will likely result in a slow response. A good example of this effect was recently reported by Horwarter and Youngblood58 where the stimuli-responsive nature of perfluorinated end-capped poly(ethylene glycol) layers was studied. Chain conformation was qualitatively assessed by comparing experimental film thicknesses to the calculated thickness for a fully extended chain. Optimal behavior (fast rearrangement simultaneous with continuous surface coverage) was observed in a mushroom-brush transition regime; denser layers were detrimental to performance. Clearly, the grafting density should be optimized for the given application. We cannot suggest a universal value for a grafting density that could satisfy all possible applications and conditions. However, a literature survey suggests several regimes that could be used for the selection of the appropriate behavior of polymer brushes: (1) substrate dominated regime at S 1, (2) chain-dominated regime with a moderate (5 [S [ 1) and strong (S [ 5) repulsion effect in good solvent, and (3) high stretching regime for poorly miscible brushes S [ 10. These numbers must be regarded as rough estimations. A complex combination of interactions (for example, additional presence of electrostatic interactions in polyelectrolyte brushes and specific donor-ligand interactions) may strongly modify these numbers. The optimum grafting density may or may not be approached using the ‘‘grafting-to’’ method. Thus, the development of more complicated synthetic processes for the ‘‘grafting-from’’ method is justified. Lastly, the authors recommend that the term ‘‘polymer brush’’ only be used whenever the regime of the system is indicated (if known) or the value of S is provided (or compare the value of Rg to D or film thickness). Brittain (DMR-0353746) and Minko (DMR-0602528) would like to acknowledge the support of the National Science Foundation.

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