Responsive Polymer Brushes - Clarkson University

Journal of Macromolecular Sciencew, Part C: Polymer Reviews, 46:397–420, 2006 Copyright # Taylor & Francis Group, LLC ISSN 1558-3724 print/1558-3716 online DOI: 10.1080/15583720600945402

Responsive Polymer Brushes SERGIY MINKO Department of Chemistry and Biomolecular Science, Clarkson University, Potsdam, NY, USA The review focuses on responsive/switching behavior of polymer brushes. The structure of the polymer brushes is analyzed in terms of the constitution and conformation of polymer chains. Responsive properties are widely discussed based on phase transition mechanisms in a controlled environment when changes are induced by solvent quality, temperature, concentration of ions, and interactions with liquids and solids. Potential applications of polymer brushes are also discussed. Keywords

response, grafted layers, phase transition, interactions, applications

Introduction A polymer brush1 – 4 consists of end-tethered (grafted, anchored) polymer chains stretched away from the substrate so that in the given solvent the brush height (h) is larger as compared to the end-to-end distance (kr2l1/2) of the same non-grafted chains dissolved in the same solvent. In polymer brushes the distance between grafting points (d ) is smaller than the chain end-to-end distance. Polymer brushes can be introduced as thin films of end-grafted polymer molecules when the following conditions are satisfied: h . kr2l1/2, d , kr2l1/2. Outside these conditions the grafted layers are in the “mushroom regime”. The chains can be grafted to different substrates that may result in planar brushes (Fig. 1) for planar or “reasonably flat” substrates (root mean square roughness should be much smaller than end-to-end distance of grafted polymer chains), cylindrical brushes (Fig. 2) for fiber- or rod-like substrates (the substrate in this cases can be as small in radius R as a polymer chain backbone. In this case the brushes are called “molecular brushes”), and spherical brushes for substrates of spherical shape (Fig. 3). For the cylindrical and spherical brushes two different regimes can be selected: (1) h . R and (2) h , R (R is the radius of curvature of the grafting surface). The substrates can be in any physical state: solid, liquid, or gas (for example foams stabilized by an amphiphilic blockcopolymer). The grafting points may diffuse laterally on the substrate surface. The grafting can be realized through covalent bonds, electrostatic interactions, hydrogen bonds, or strong van der Waals interactions. However, weak interactions are less interesting. The elastic energy component accumulated in the polymer brush due to the stretching may destroy weak bonds. Soft substrates may be deformed by the brush.5 Received 14 March 2006; Accepted 31 May 2006. Address correspondence to Sergiy Minko, Department of Chemistry and Biomolecular Science, Clarkson University, 8 Clarkson Ave, Potsdam, NY 13699, USA. E-mail: [email protected]

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Figure 1. Planar homopolymer brush: a homogeneous smooth layer of stretched chains in good solvent (a), pinned micelles (b) and a layer of collapsed chains (c) in poor solvent.

Responsiveness of polymer brushes to external stimuli refer to changes of polymer molecule conformations.6 Most research reports on responses affected by changes of macroconformations in solutions and melts when the segmental mobility is reasonably high. The conformational changes in specially tailored polymer brush systems are used in materials and devices to approach demanded effects of switching material properties upon external signals. Theory predicts very large potential to regulate and tune complex interactions between brush-coated surfaces.7 It is noteworthy that polymer brushes represent well-defined grafted polymer chain architecture when the structure (end-grafted chains) is not very complex as compared to other grafted polymer layers (randomly grafted chains, LBL thin polymer films, grafted composite/hybrid films, etc). Polymer brushes can be analyzed theoretically more easily. Interpretations of the experimental study of polymer brushes are based on the welldeveloped theoretical background that facilitates intensive investigations of these responsive systems. The exponential increase of the number of publications on polymer brushes (around 400 papers in 2005 vs. 150 in 2000 and 70 in 1995) is good evidence for that.

Responsiveness in Polymer Systems As in any polymer system, polymer brushes respond to their environment by a change of the polymer chain conformations. The size of polymer chains is sensitive to its

Figure 2. Cylindrical brushes and their responsive conformational changes: bending, contraction, compacting, and coiling. (Reproduced from Ref. 50, Copyright 2006, with permission from Blackwell Publishing).

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Figure 3. Spherical brushes: collapsed and stretched chains of a homopolymer brush in poor and good solvents, respectively (a); polyelectrolyte (PEL) brush swells when ionizable groups dissociate in a polar solvent (b); depending on solvent selectivity a mixed two-component brush switches between the state when chains of the polymer A are stretched and chains of the polymer B are collapsed (left) and the state when the chains of the polymer B are stretched and the chains of the polymer A are collapsed (right) (c).

environment. In Q solvents (attraction and repulsion are compensated) isolated polymer chains of the degree of polymerization N possess ideal coil conformation when kr2l1/2 N1/2. The size of the isolated chain is a function of solvent quality (that may be expressed in terms of the x-Flory-Huggins interaction parameter or Qtemperature). In a good solvent the extended chains have self-avoiding coil statistics: kr2l1/2 N0.588. The isolated chain in a poor solvent collapses into a globule: kr2l1/2 N1/3. In good solvents and at high polymer concentrations the excluded volume effect modifies chain conformations substantially. In semi-diluted polymer solutions the chain size decreases with the 1/8th power of the polymer volume fraction in a good solvent. In theta solvents the polymer chain size is concentration independent. In poor solvents bulk polymer solutions undergo a phase separation into two phases: almost pure solvent and concentrated polymer solution of overlapping Gaussian coils. Both the scaling exponent and the prefactor are sensitive to solvent quality. Constraints due to the end grafting of the polymer chains introduce a specific character of the response which is somewhat different from the response of isolated chains in solution or melt.8 In the crowded grafted layers (polymer brushes) the chains stretch out of the grafting surface until the excluded volume effect is compensated by elastic energy (stretching entropy) of polymer coils. Polymer brushes expand in good solvents and collapse in poor

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solvents. The change of characteristic size between good and poor solvents is much larger for polymer brushes as compared to the polymer chain in solution. For some applications it is important that the grafting points cannot diffuse laterally. The specific goal of this review is to analyze responsive brushes. There is not an obvious definition which changes of polymer brush properties should be recognized as a responsive behavior. Any material is responsive since changes of its environment always cause some changes of material properties. The term “responsive behavior” is rather a term which reflects applications, and consequently, there is no universal definition of responsiveness. For many applications we suppose to obtain a steep and well noticeable change (switching) of the given property, thus, transitions from the state which can be characterized by some property to the state with the contra property. For example, we may mention the transitions from wetting to non wetting, from swollen to collapsed brush, from adhesive to non adhesive, from adsorbing to non adsorbing, from attractive to repulsive, etc. Typically (but not necessarily), such changes of properties are accompanied by phase transitions in polymeric systems. These transitions are assigned as responsive properties of polymer brushes. For some applications even a small change of brush properties may have a dramatic effect. This change may be also assigned as a responsive property of the brush. Consequently, we may agree that the response of polymer brushes is a noticeable effect for the given application of the brush if this effect appears as a change of the brush properties introduced by a change of the brush environment.

Mechanisms of Responses Homopolymer Brushes In scaling theory and self-consistent field theory8,9 the thickness of the polymer brush is linearly proportional to N: h N (but not obviously in a bad solvent). Experiments proved that this relationship retains unchanged for solvents of different quality.10 However, the prefactor depends on solvent quality and grafting density of polymer chains. Besides the chain constitution, the grafting density is the parameter which affects conformational responsiveness of polymer brushes. At small grafting densities (so called mushroom regime) the response of grafted chains is very similar to that of the bulk polymer solution.8,9 At high grafting densities the collapse is weak and the brush forms a homogeneous layer which is slightly thinner in a poor solvent than in a good solvent or in the Q regime. That is because of the very crowded layer of strongly stretched polymer chains when there is no free space for conformational changes. In the grafting density regime which lays between very low and high surface coverage regimes the brushes are unstable in poor solvents and form clusters (pinned micelles) on the surface (Fig. 1b) to avoid unfavorable interactions with the poor solvents.11,12 In a good solvent the brush is swollen and it forms a homogeneous layer of stretched tethered chains (Fig. 1a). In this grafting density regime (cross-over regime) the polymer brush demonstrates the most pronounced response to solvent quality as it is obvious from the graph in Fig. 4.13 Thus, we may conclude that the largest conformational response in a polymer brush is in the very beginning of the brush regime when chains start to overlap. Switching a range of the thin polymer film properties relevant to density and thickness of the film can be easily approached with the brushes of moderate grafting densities. It is noteworthy, that the optimal grafting density may be affected by a concentration dependency of x-parameter for some polymer-solvent systems. For example, the high responsiveness (collapse in water above a lower critical solution temperature


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Figure 4. Height of the brush layer, scaled by the bulk radius of gyration of the chain in good solvent, as a function of the scaled surface coverage for a variety of qualities of solvents. The thin line marks the onset of instability, that is, it corresponds, for each T, to the surface coverage at which the isothermal compressibility of the grafted chains is zero. (Reproduced from Ref. 12, Copyright 1996, with permission from John Wiley & Sons, Inc.).

308C) was reported for 0.25 nm22 dense poly(N-isopropylacrylamide) (PNIPAM) brushes,14 while for one order less dense brushes no collapse was detected.15 Many properties of brushes can be referred not only to the brush thickness but also to the density profile. Careful considerations of the problem using self-consistent field theory (SCFT) revealed a non-monotonous dependence of the polymer density as a function of the distance from the grafting surface. In the limit of strong stretching (h kr2l1/2) a density profile is parabolic according to the so called classical ISL SCFT of brushes16,17 (refers to the infinite stretching limit (ISL) approximation when the fluctuations around the most probable classical pass can be ignored).18 At a moderate stretching density the profiles obtained with SCFT19 and simulations20 deviate from the classical parabolic profile substantially. Figure 5 presents the density profiles (SCFT results) vs. distance from the grafting surface at different b-stretching parameter values (1/b ¼ d ¼ kr2l1/2/h).21 As the grafting density increases from the mushroom regime (b , 1) to the strong stretching limit (b ¼ 100) the profile changes dramatically.

Figure 5. Self-consistent-field results for the rescaled density profiles as a function of the rescaled distance z from the grafting surface. Shown are results for b ¼ 0.1, 1, 10, and 100 (dotted, dashdotted, dashed, and solid lines, respectively), progressively approaching the asymptotic result (valid for b ! 1), shown as a thick dashed line. (Reproduced from Ref. 21, Copyright 1998, with permission from American Chemical Society).

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The impenetrable grafting surface causes a decrease of the polymer density close to the grafting surface when the grafting density maximum is located in some distance from the grafting surface. This distance increases as the grafting density decreases. The grafting density profile is much more sensitive to the brush characteristic at moderate grafting densities as compared to a very high stretching regime. Concluding this paragraph, both brush thickness and density profile are sensitive to grafting density and solvent quality. The responsive behavior of the homopolymer brush is referred to changes of free energy of the brush in its environment due to the change of solvent quality. Any application of the responsive homopolymer brushes will be grounded on this principle. Polyelectrolyte (PEL) Brushes (Fig. 6) Macromolecules with ionizable groups carry electrical charges in polar solvents. PEL in aqueous solutions attract great interest because of their relevance to many biological systems. Interactions which involve charged macromolecules are strongly modified by Coulomb forces. The charge density on a polymer chain in a polar solvent depends on the chain constitution and degree of dissociation ( f ) of ionizable groups. If ionizable groups are strong acids or bases (strong PE) f is equal to 1 and is not affected by the environment. If ionizable groups are weak acids or bases (weak PEL) f depends on local pH. For the latter case charges are mobile within the polymer chain. For the dense strong PEL brush (high f and grafting density) all counterions are trapped inside the brush (Fig. 6b). The brush height is determined by the balance between osmotic

Figure 6. Planar polyelectrolyte (PEL) brush with ionizable groups (a) swells when the groups dissociate a polar solvent (b); the dissociation is pH dependent for weak polyelectrolytes (c); addition of salt leads to the shrinkage of the PEL brush (d).


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pressure of the trapped counter-ions and the stretching entropy of the chains (so called osmotic brush regime). The contribution of the excluded volume effect depends on the grafting density. At very high densities the excluded volume effect may dominate while at moderate densities the electrostatic nature will have a major contribution. The latter will be reflected in the prefactor in the scaling relationship h N. These brushes are insensitive to local pH. Added salt does not affect the brush unless the ionic strength of the solution approaches the level of the ionic strength inside the brush (Fig. 6d). In that case the prefactor is an inverse cubic root function of the external salt concentration and the grafting density (so called salted brush regime).22,23 Thus, in terms of responsive applications strong PEL are interesting for design of responsiveness to humid and aqueous environments when the high swelling of the brush in water or a humid atmosphere is resulted from strong osmotic pressure of trapped counterions (Fig. 6 a,b). Weak PEL brushes represent one of the most interesting responsive behaviors. They demonstrate responsiveness to changes in external pH and ionic strength. Weak PEL brushes carrying basic functionalities expand upon a decrease of pH, while acidic PEL brushes expand upon an increase of pH (Fig. 6 c,d). At a high salt concentration weak PEL brushes shrink due to the same mechanism as strong PEL brushes. However, it is noteworthy, that in some range of pH values they shrink also at no salt added or at very small salt concentrations, thus, expressing non-monotonous dependence of the brush height vs. salt concentration. This behavior originates from the sensitivity of f for weak PEL(s) to the local electric field.24 At a low ionic strength electrostatic interactions result in the condensation of counterions in the brush (the neutral polymer brush regime). If ionic strength is increased the screening of the electrostatic interactions affects an increase of f and brings the system in the osmotic regime. The graph of the brush thickness vs. salt concentration has a maximum.25 In mixed brushes (Fig. 7) two or more different polymers grafted to the same substrate constitutes the brush. The mechanism of responsiveness of the mixed brushes is very different from homopolymer brushes. Unlike polymers in the mixed brush segregate into nanoscopic phases. The size of the nanophases scales with kr2l1/2 value. The phase

Figure 7. Mixed two-component brush: lateral phase segregation of chains in non-selective solvent (a), and combined lateral and layered segregation in a selective solvent (b, c).

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segregation is a lateral segregation in a nonselective solvent when unlike polymers form spherical or elongated clusters. Both the polymers are exposed on the top of the brush (Fig. 7a). In selective solvents the mixed brush structure may be seen as a combination of lateral and layered segregation mechanisms.26 In the latter case one polymer preferentially segregates to the top of the brush, while another polymer forms clusters segregated to the grafting surface (Fig. 7b, c). The most important difference of the mixed brush as compared to the homopolymer brush is that not only the height and density profile but also the composition profile depends on solvent quality. In other words, the surface composition of the brush is switched by a change in its environment. If two unlike polymers are of the same chemical constitution but they differ by molecular weight the brush stratifies.27 – 29 The polymer of the largest molecular weight will occupy the top of the brush. If two unlike polymers are of different chemical compositions the lateral and layered segregation takes place. A combination of both kinds of heterogeneity (by chemical composition and molecular weight) results in the more complex phase diagram when the whole range of parameters of the system can be used to design the brush responsiveness.30 The further modification of the responsiveness can be obtained for mixed PEL brushes (Fig. 8). In this case electrostatic interactions are used to regulate the mechanism of the phase segregation. Weak mixed PEL brushes are of special interest. At isoelectric points they form a neutral PEL complex (Fig. 8a), while outside the isolectric point one PEL dominates in the topmost layer and affects the electrical charge of the brush (Fig. 8 b, c). Thus, the surface composition of the mixed PEL brush can be switched just by a change of external pH.31 – 33 Block-Copolymer Brushes (Fig. 9) In this case two or more chemically different polymers constitute a polymer brush with block-copolymer architecture. The responsiveness of these brushes is determined

Figure 8. Mixed weak polyelectrolyte brush forms a neutral complex in the isoelectric point (a); in acidic or basic solutions positively or negatively charged polymers, respectively, are stretched while a counterpart chains are collapsed and segregated to the grafting surface (b, c).


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by phase segregation of unlike polymers, however, the mechanism depends on whether the AB block copolymer is tethered by the more (A) or the less (B) soluble block.34,35 In poor solvents the brushes grafted by the B blocks form dumbbell-like micelles with the B-polymer segregated to the grafting surface and the A-polymer micelles on the top (Fig. 9b). The brushes grafted by the A blocks form B-polymer micelles shielded by A-polymers (Fig. 9a). If the solvent quality is improved (increased selectivity of solvent) for the A-polymers the dumbbell-like micelles of the B-grafted brushes are transformed to the structures which are similar to the mixed brushes in selective solvents. The B-polymers form pinned micelles segregated to the grafted surfaces and shielded by the brush of the A-polymers. In general, the block-copolymer brushes have a complex phase diagram affected by the ratio between the molecular weights of the A and B blocks and the grafting density. The major difference between mixed and block-copolymer brushes can be found for the case of the A-grafted brushes in a selective solvent for the A-blocks. The mixed brushes of the A and B polymers form pinned B-micelles segregated to the grafting surface. The AB blockcopolymer brushes form B-micelles located at the top of the brush (Fig. 9a). Consequently, block-copolymer brushes introduce unique mechanisms of the phase segregation. The experimental studies36 – 39 have demonstrated reach responsive behavior of blockcopolymer brushes based on the rearrangement of segregated phases in good agreement with theoretical predictions. PEL block-copolymer brushes are less investigated. The A-B PEL block-copolymer brushes grafted by the neutral blocks A demonstrated no rearrangement upon treatment by different solvents.40 However, it seems to be a specific case of the brush structure. Generally, electrostatic interaction can modify interactions in the block copolymer brushes resulting in additional responsive mechanisms that could be a subject of future investigations.

Figure 9. The block-copolymer brush constituted of more soluble A blocks and less soluble B blocks. The brushes grafted by the A-blocks (a) form B-polymer micelles shielded by the A-polymers (a, right). In good solvent for the A-polymers B-micelles are located at the top of the brush (a, left). The brushes grafted by the B-polymers (b) form dumbbell-like micelles (b, right). In good solvent for the A-polymers the A-blocks are stretched and the B-blocks form micelles segregated to the grafting surface (b, left).

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Non-Planar Brushes In many important applications the grafting surface is curved. Most brush applications are for colloidal objects of spherical and cylindrical shapes. The traditional application of the brushes against flocculation of colloidal particles is well known.41 However, responsive brushes for colloidal systems are considered to be an under-investigated field which promises numerous exciting applications for a new generation of functional materials: smart coatings and detergents, drug delivery systems, smart containers for biomolecules, elements of miniaturized sensors, etc. for textiles, paints, cosmetics, microscopic devices, medicine, and biotechnology. A curved grafting surface may strongly modify properties of the brushes. First of all, the volume available for the grafted chains is a function of distance from the grafting surface (z) and changes with curvature (Fig. 10) In terms of theoretical analysis the boundary conditions imposed by curved grafting surfaces are different than those for a planar grafting surface.13 It seems that these “difficulties” have postponed intensive studies in the area of nonplanar polymer brushes, although industrial needs and published research reports give well- justified prospects for success.25,42 – 45 Fabrication of mixed brushes on the curved surfaces46 – 48 sounds very promising for the design of a range of various interaction mechanisms in colloidal systems.49

Molecular Brushes Molecular brushes are of special interest because they represent a tailored responsiveness on the level of single molecule events.50 Brush-like molecules are formed by grafting polymer arms to a molecular (polymer) backbone. They change conformations in response to their environment. Changes of solvent quality induce switching between rodlike and globule conformations. The basic mechanism of responsiveness is the same as for planar brushes: solvent quality and entropic elasticity balance the conformation. The flexible backbone also contributes to the balance and in turn changes its conformation.

Figure 10. Density profiles for chains tethered at spherical surfaces for a variety of radii R. z is the radial distance from the tethered surface. The full line with filled circles is for a planar film, dotdashed line R ¼ 100l, long dashed line R ¼ 20l, dashed line R ¼ 10l, dotted line R ¼ 5l, and a full line R ¼ 2l, where l is the bond length of a chain. (Reproduced from Ref. 13, Copyright 1996, with permission from John Wiley & Sons, Inc.)


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Thus, the cylindrical brushes change length and stiffness with changes in solvent quality. If the polymer arms shrink in a poor solvent the cylindrical brush undergoes rod-to-globule phase transition.51 Adsorbed brush-like molecules undergo conformational transitions strongly modified by interactions with the solid substrate52 (Fig. 2).

Hierarchically Structured Brushes These brushes are a combination of different architectures. Discovery and manufacturing of complex brush architectures appear in response to the demand of the development of hybrid functional nanomaterials with a complex response mimicking interactions in life systems. To assemble nanoscale building blocks into responsive hierarchical nanostructures, new synthetic techniques are developing that are capable of linking a wide range of length and response-time scales. An important challenge is to precisely manipulate and quantify the response (switching) of a given property in the material based on external stimuli. Multiple levels of functionality can be integrated into complex brushes by carefully designing their architecture. Recent research attempts to combine several architectural approaches in polymer brushes. Y-shaped amphiphilic brushes combine two dissimilar polymer chains attached to a single focal point or a leg. These brushes represent the first step to a more complex architecture (Fig. 11).53,54 Although the responsive behavior of mixed and Y-shaped brushes can be explained by very similar mechanisms, the difference between randomly mixed polymer brushes and Y-shaped brushes is in the polymer distribution on the surface. Incompatibility between two unlike polymers may affect the distribution between grafting points for mixed brushes and for Y-shaped brushes differently. The Y-shaped brushes could potentially form more regular structures because the grafting of two different polymers is always coupled due to the Y-shaped architecture. This morphology may be promising for the precise design of complex interactions. Tightly grafted to the planar solid substrate spherical,47 cylindrical brushes (bottlebrush brushes),55 or hyperbranched polymers56 represent new hierarchically structured brush architectures.

Figure 11. Molecular graphics representation of the proposed structural rearrangements in the Y-shaped brushes. (a) Internally segregated pinned micelle composed of seven grafted polystyrene — poly(acrylic acid) (PS-PAA) molecules spaced 3.5-nm apart. Upon treatment with toluene and subsequent drying, the PS arms (gray) form a corona covering the micelle’s core consisting of seven PAA (dark gray) arms. (b) Representation of the same seven molecules swelled in a nonselective solvent (light gray). (c) Top-open craterlike structure containing seven collapsed PS arms partially covered by seven PAA chains. (Reproduced from Ref. 54, Copyright 2003, with permission from American Chemical Society).

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Reversibly Self-Assembled Brushes These brushes are formed (1) if polymer arms are tethered to the grafting surface via non-covalent bonds and (2) if a grafted interface is formed just as a result of self-assembling of polymer chains. Examples of these brushes encounter but are not limited to: end adsorbed polymer chains, adsorbed block-copolymer layers, thin block-copolymer films, block-copolymer micelles and vesicles, lipid-bilayers and vesicles, etc. These brushes demonstrate at least three levels of responsiveness if assembling forces are strong enough to sustain conformational changes of the polymer arms. The first level is the same as for “classical” planar polymer brushes: balance between entropic spring and excluded volume effect. The second level evolves bending of the grafting surface similar to cylindrical brushes. Finally, the third level of response is an entire reorganization of the structure when the brush may completely rearrange the supra-structure: for example the transitions between micelles and vesicles of different shapes,57,58 and the inversion of block-copolymer micelles.59,60 These structures possess a very complex response. Selfassembled structures of amphiphilic molecules share many characteristics with brushes. These systems are the building blocks of synthetic membranes (e.g. polymersomes61) or cell membranes that are found in many living systems resulting in them being one of the major targets of modern synthetic polymer chemistry.

Synthesis Synthesis of polymer brushes of various structures was thoroughly introduced and described in several reviews4,6,23 and in the book on polymer brushes.2 Thus, only the major concepts are listed in this paragraph. Three well known approaches are broadly used for synthesis of polymer brushes: “grafting from,” “grafting to,” and “grafting through.” The major advantage of the “grafting from” approach is that the tightly tethered chains may be grown from the grafting surfaces. The active end-centers of the growing chains are easily accessible for monomer units when small monomers swell the brush. Thus, the highest grafting density can be approached with the “grafting from” approach. Although, almost all possible polymerization mechanisms were employed to grow tethered chains from different substrates, the most popular mechanisms (by number of publications) are classical and controlled radical polymerizations. The controlled radical polymerization seems to be the most technically simple method. However, the “grafting from” approach is a synthesis of the polymer at a high local concentration of polymer chains. In these conditions side reactions (chain transfer to polymer) should be considered as a source of defective molecules. This problem seems to be underinvestigated yet. An important step of the “grafting from” approach is surface immobilization of the initiator of the polymerization. In the “grafting to” approach end-functionalized polymer chains are grafted directly to a solid substrate via a chemical reaction of the end-groups with complimentary groups on the grafting surface. In this case the functionalization of the grafting surface is an important step. The “grafting to” approach is limited by diffusion kinetics of end-functionalized polymer chains through the crowed grafted brush-like layer. The brushes of a moderate grafting density can be obtained by the “grafting to” methods. Nevertheless, the major conformational response of the brushes was indeed observed for the brushes with moderate grafting densities. Thus, the “grafting to” is considered


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by the author of this review as the best method for the fabrication of responsive brushes for many applications because of two major advantages: (1) the grafted polymer can be thoroughly synthesized and characterized by traditional methods in solutions, many commercially available polymers can be used, (2) the “grafting to” procedure is very simple technically. The “grafting through” approach is the polymerization of macromonomers. This method is used for the fabrication of cylindrical brushes. Two new tendencies have appeared in the field of synthesis of responsive brushes: patterned surfaces and combinatorial approach. Surfaces which are patterned on the nano- and microscale with responsive polymer brushes can be used in sensing and actuation devices. These applications have stimulated the development of synthetic approaches to nanopatterning of stimuli-responsive brushes.62 Several methods were suggested for the fabrication of the patterned brushes. The template methods refers to the fabrication of the patterns of an initiator on solid substrates using photolithography63 or soft lithography methods.64 Alternative methods explore scanning probe lithography: nanoshaving, anodization lithography, and dip-pen lithography. In the nanoshaving approach a resist is mechanically removed by the AFM tip and the obtained patterned are backfield with the initiator for the “grafting from” process.65 AFM anodization lithography was used to generate silicon oxide nanopatterns on silicon substrates by electrochemical anodic oxidation with the conductive AFM cantilever and using a resist layer on the surface of the silicon substrate. Local electrochemical processes near the AFM tip results in the decomposition of the resist and generation of silicon oxide patterns. The patterns are treated by the initiator for grafting polymer brushes.62 Dip-pen lithography was used for direct patterning of the thiol initiator.66 Lift-off electron beam lithography was used to initially fabricate gold nanopatterns on Si-wafers which later were surface modified with the initiator for “grafting from” polymerization.67 The combination of the common lithography methods and surfaceinitiated polymerization resulted in the development of the method for the fabrication of 70 nm wide polymer brushes. Responsive properties of brushes strongly depend on polymerization degree, grafting density, and brush composition (for mixed and block-copolymer brushes). The precise design of responsiveness can thus be approached by optimization of the brush structure. This request was realized through the development of combinatorial methods to study and optimize responsive polymer brushes. The combinatorial approaches explored the fabrication of the samples of planar brushes with a 1D or 2D gradual change of the brush characteristics such as grafting density, molecular weight, and composition. Both the “grafting to” and “grafting from” methods were used for the fabrication of the gradient brushes. The brush grafting density gradient was used to study the effect of grafting density on the brush structure in solvents.68 – 70 The mixed brushes71 – 73 and blockcopolymer brushes74 with composition gradients were used for the study of the brush rearrangement in different solvents.

Applications of the Responsive Brushes: So Far The major objective for the application of responsive polymer brushes is to regulate, adjust, and switch interaction forces between the brush and its environment constituted of liquid, vapor, solid, another brush, particles, etc. The simplest formulation of the

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problem is switching between attraction and repulsion. For example, the polymer brushlike layer stabilizes colloidal dispersion, however, upon change of its environment the colloid coagulates because the repulsive forces of the brush have been “switched off.” This simple effect has numerous important applications in various technologies and it is not fully explored and engineered yet. The same simple problem is important if the friction coefficient, adhesion, or wetting could be rapidly changed to switch off and on capillary flow, cell adhesion, protein adsorption, cell growth, membrane permeability, and drug release. The complexity of the problem, however, is substantially increasing if we have in mind the design of complex responsive systems which mimic functions of life systems. Such artificially created systems will be capable of: reporting on toxins, diagnosing cancer cells, monitoring important parameters of organs, and targeting the release of drugs. Currently, research is focused on how the interactions with polymer brushes may be precisely tuned and monitored in a controlled environment. Wetting by Solvents Wetting of responsive brushes by solvents is the well-explored field of research. The most practical case relevant to many applications is wetting by water. Spreading of paints, adhesives, deposition of proteins and microorganisms, cleaning and self-cleaning surfaces—all these processes are frequently used in different technologies and in everyday life. Switching between wetting and nonwetting, thus, may help to improve efficiency of many technological processes and may result in the fabrication of smart materials with switchable wetting behavior. Generally, wetting of polymer brush includes the interaction of the liquid with solid substrate (grafting surface) and the interaction of the liquid with the tethered polymer chains. The interaction with the brush is the most important component to approach complete wetting of the substrate. A good solvent swells the brush and the complete wetting is expected in this case. However, experiments show that complete wetting can not be easily approached. Many studies report on finite contact angle for water on the planar polymer brushes prepared from water soluble grafted polymer layers. Thus, wetting of the brush is strongly affected by the structure of the brush-water-gas interfaces. One of the possible explanations for this effect is the surface activity of water soluble polymers which results in polymer bridges between the substrate and the solvent-vapor interface. This effect may cause partial wetting.75 If the solvent quality is tuned toward a poor solvent the water contact angle will increase due to the increased polymer brush –solvent interfacial tension. This tendency, however, is not obvious. The scenario depends on the constitution of the grafted chains and their microconformations in the topmost layer of the brush. A rearrangement of polymer chains way results in “self-adaptive” behavior of the brush. The microconformation of the polymer chain at interface and the location of end-functional groups will adapt the lowest polymer brush – solvent interfacial energy. The changes of contact angle, therefore, will not be big. For example, a transition between good and poor solvent conditions induced by temperature change for PNIPAM brushes has almost no effect on the wetting behavior of the brush.76 In weak PEL brushes switching of wetting can be approached by a change of pH.23,32 For example, weak PEL brushes prepared from poly(2-vinylpyridine) (hydrophobic backbone) are much better wettable by acidic water than by neutral or basic aqueous solutions. Ionization of PEL brushes results in their strong swelling in aqueous solutions77 and, therefore, in increased wettability. If polyvalent groups are explored in


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weak PEL brushes several wetting regimes can be obtained for the brush based on an association-dissociation equilibrium. A three-stage switching of surface wetting was demonstrated using phosphate-bearing polymer brushes.78 Strong PEL brushes are less sensitive to pH, however, they were found to be responsive to the nature of counterions. The brushes prepared from poly(2-(methacryloyloxy)ethyltrimethylammonium chloride) have shown the collapse and transition from 378 to 798 water contact angle if Cl2 counterions were replaced by ClO2 4 less hydrated ions.79 Surface texture may strongly amplify wetting and non-wetting behavior. If the brush is grafted on the textured surface the switching between ultrahydrophobic and hydrophilic states can be approached.80 Mixed and block-copolymer brushes offer an interesting opportunity to freeze wetting behavior in a non-equilibrium state. If the brush was treated by a selective solvent for hydrophobic chains they will occupy the topmost layer of the brush. As soon as the solvent evaporates the hydrophobic state will be “frozen.” Water will not wet the layer and will not switch the brush because the topmost hydrophobic layer is not permeable for water.6 Wetting by Polymers Responsiveness of polymer brushes to polymer liquids of the same chemical structure as the polymer brush or in other words wettability of polymer brushes by thin liquid polymer films was in a focus of theoretical81 – 84 and experimental85 – 87 works because this problem is very important for thin film technologies. The stability of a molten homopolymer film on a polymer brush of identical chemical composition is of particular interest. It was found that the stability of the film depends on the grafting density of the brush. In the case of the brush grafted to the non-wettable solid substrate the polymer film demonstrates two wetting/dewetting transitions. At low grafting densities the grafted polymer chains do not change significantly the interfacial tension between the polymer melt and the solid substrate (gsm). The film tends to dewet. As the grafting density rises, gsm decreases to the level, when the spreading parameter (S ¼ gs 2 gsm 2 gm, where gs and gm are the surface tensions at the vacuum/brush and vacuum/homopolymer melt interfaces, respectively) becomes positive and the film begins to wet the substrate. In this region of the film stability the grafted chains are totally or partially intermixed with the free chains of the melt. The width of the brush/homopolymer interface is determined by a balance of two effects.82 The polymer melt is a solvent. Therefore, its penetration into the brush is entropically favored. However, that implies an extra stretching of the grafted chains with an entropic cost of the elastic free energy. Thus, the interface width decreases with an increase of the grafting density because the densely grafted chains create an entropic barrier for the penetration of free polymer chains of the melt. The decrease in the interface width is accompanied by the growth of the interfacial tension gsm. The same effect on gsm has the increase of temperature.87 As gsm increases the spreading parameter again becomes negative and the polymer film dewets. This phenomenon has been called autodewetting or autophobicity. This behavior was observed experimentally.85 – 87 Interesting wetting behavior has been reported for diblock copolymer films on random copolymer brushes.88,89 The grafting density of the brush was adjusted to obtain a complete wetting of the brush by the block-copolymer film. Block copolymers are known to exhibit a variety of ordered microdomain morphologies produced by the

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phase separated polymer blocks. The important examples are lamellar and cylindrical morphologies, because the orientation of lamellae and cylinders in thin films is affected by the film interfaces. Even a small difference in interfacial tensions between the substrate and each polymer (constituting the block copolymer) leads to the parallel orientation of lamellar and cylindrical microdomains with respect to the substrate plane. In this state the polymer block that has the lowest interfacial tension wets a substrate surface thus allowing for minimization of the interfacial energy. The perpendicular orientation, however, is more preferable in terms of a potential application of block copolymer films as nanolithographic templates. This orientation implies that both phases wet the substrate surface, therefore, the substrate has no preference with respect to any block. Such a neutral surface can be prepared by modification of a substrate surface with a random copolymer brush. A random copolymer brush allows for precise control over the interfacial interactions simply by changing the copolymer composition. That has been demonstrated for films of a polysterene-block-poly(methyl methacrylate) deposited on silicon substrates modified with brushes of a random copolymer of styrene and methyl methacrylate.88,89 The perpendicular orientation of the lamellar and cylindrical microdomains was observed for the certain copolymer compositions. Brush-Brush, Brush-Solid, and Brush-Particle Interactions These interactions represent a classical field of colloid science.90,91 Polymer brushes stabilize colloids because of repulsive forces (originated from the excluded volume effect) which oppose the brush compression. At a moderate grafting density the interpenetration of the two interacting brushes may lead to an attraction if the interaction of grafted chains with the interacting surface dominates. Both repulsive and attractive interactions were well documented in experiments (for example).92 – 94 For PEL brushes electrostatic interactions may strongly modify the interactions resulting in strong repulsion between similarly charged brushes.95 Therefore, the interactions between two brushes or the brush and solid surface is a tiny balance between volume excluded effect, attractive interactions (van der Waals) and Coulomb interactions. The balance can be tuned by a change of environmental conditions (temperature, pH, ionic strength). In the case of mixed polymer brushes or the block-copolymer brushes the balance between attractive and repulsive interactions can be switched depending on which polymer preferentially segregates to the topmost layer of the brush or on how the interaction with the solid surface will change the phase segregation in mixed/block-copolymer brushes.7,49,96 It is difficult to overestimate the importance of the interaction regulation between polymer-coated solids. The responsiveness of a polymer brush to the insertion of a small impenetrable object is the problem of a special and very important interest. These phenomena are critical for the regulation/control of particle, dust, protein, and cell adsorption on polymer brushes. Theoretical analysis shows that the insertion of a small object in the polymer brush creates a repulsive pressure field which decays exponentially away from the region of insertion.97,98 Interaction with the brush also affects the spatial organization of particles in the brush. Small particles that strongly interact with the polymer brush disperse freely within the polymer brush, while polymer “insoluble” particles tend to aggregate in the brush. The big aggregates are expelled towards the brush-air interface.99 Therefore, switching particle-polymer interactions via the change of solvent quality will affect the particle organization in polymer brushes100 and can be used for a controlled release of particles from the surfaces.101


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The interaction between particles/proteins and a brush in real systems is much more complex since many different forces are involved in the phenomenon. The amount of protein adsorbed on the surfaces depends on the surface density of the polymer, the protein-surface interaction, the polymer-surface interaction, the size of the polymer segments, and the ability of the proteins to undergo conformational changes upon adsorption.102 – 105 For the example of polyethyleneoxide (PEO) brushes, the adsorption of proteins is governed by the interplay between the attractive interaction of the protein with the surface, the attractive interaction of the adsorbed PEO segments with the grafting surface, and the entropic changes associated with the deformation of the PEO layer due to the adsorbing proteins. In the case of the grafting surface which is not attractive for PEO the amount of protein adsorbed depends on how the adsorbing protein deforms the PEO brush. The mechanism responsible for the prevention of protein adsorption by grafted PEO results from the ability of the grafted polymer to block the adsorption sites on the grafting surface. Therefore, the fact that the PEO is attracted to the surface results in more effective control of nonspecific protein adsorption. It was found experimentally104 that the surface concentration of adsorbed proteins decreased as the surface density of the grafted PEO increased, but the surface concentration of the adsorbed protein never reached zero. An analysis of the protein adsorption dynamics shows that brushes of the same constitution express a different efficiency in the prevention of protein adsorption at different time scales. The polymers that are not attracted to the surface are effective for kinetic control but not good for equilibrium reduction of protein adsorption, while polymers with attractions to the surface show exactly the opposite behavior.106 This result demonstrates the importance of the brush response in dynamics. These problems are still very poorly investigated. If considering PEL brushes, the electrostatic interactions between proteins and the brush should be also taken into account.107 The electrostatic interactions are easily tuned by pH change since proteins are polyampholytic molecules constituted of weak acids and bases. Ionic strength also strongly contributes to protein adsorption on polyelectrolyte brushes that opens really broad possibilities to regulate protein adsorption/desorption processes.108 Interaction with biological molecules is a very complex phenomena. Precisely tailored materials for controlled interactions with proteins and cells will explore a complex architecture when a wide spectrum of forces will be used. To this end the functional polymer brushes bearing ionizable groups and other type of functional groups located at different distances from the grafting surfaces are promising candidates to design and tune the brush-layer response.109 Application in Devices One of the targets is the application of the responsive brushes for smart devices such as drug delivery devices, microfludic analytical devices, and sensors. Smart drug delivery devices are seen as a drug loaded capsule coated with a brush-like shell. The first approach to drug delivery devices comprised the fabrication of stealth particles coated with PEO brushes. The brush masks the particles from macrophages and results in an increase in the blood circulation half-life of the particles by several orders of magnitude.110 – 112 The next step could be seen as a fabrication of the particle shell from a mixed spherical brush constituted of polymer chains (which control nonspecific adsorption) and embedded selective centers for the specific adsorption and targeting the drug release. Intensive investigations in this field for controlled drug release are outside

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of the frames of this review. The precise design of the capsule will need intensive research on the development of the fundamental backgrounds for understanding the interactions of responsive brushes with biological objects. The first experiments show that responsive polymer brushes can switch the protein adsorption upon external signal.113,114 Expansion and shrinking of responsive polymer brushes can be used to fabricate mechanical actuators.115,116 The effect of switching of wetting behavior of the mixed weak PEL brushes upon a change of pH was recently explored for the fabrication of “smart” microfluidic devices. The passage of liquids through the microfluidic channels was regulated by responsiveness of the mixed brushes of different compositions.117 Reversible changes of mixed brush morphologies in solvents of different thermodynamic quality were used for the motion of nanoparticles deposited on the brush surface.100 The simplest device which explores a polymer brush responsiveness is a sensor working on the principle of the brush expansion – collapse transitions upon changes in its environment. For example, the strong PEL brush expands in a humid atmosphere. That can be detected as a change of a bending of the micromechanical cantilever (with a single-sided brush layer)118,119 or as a change of optical characteristics of the brush layer using ellipsometry,32 surface plasmon resonance spectroscopy23 or fluorescence intensity.120 The brush expansion-collapse was also detected as a shift of an absorption band in visual spectra based on the principle of the transmission plasmon spectroscopy when the responsive brush was synthesized on the top of gold nanoislands deposited on the glass substrate. The brush was coated by a monolayer of gold nanoparticles. The change in the distance between gold nanoparticles and nanoislands upon expansion and collapse of the polymer brush resulted in the shift of the absorbance band in visual spectra.121

Conclusions The field of responsive polymer brushes is a continuously expanding area of research. The expansion is not very fast because of the complexity of the systems for the fabrication as well as for investigations. Nevertheless, the continuous and successful development of the field is predetermined by the fact that the polymer brushes are the most effective structures to regulate complex interactions in synthetic colloidal and natural living systems. The potential for the design of the interactions is very high. Mimicking natural systems and designing new structures will accompany the development of the field of polymer brushes. That will stimulate expansion of theoretical and experimental investigations. We may also benefit from the combination of polymer brushes and gels in complex responsive devices. A number of reports on the application of polymer brushes for devices is still very small. Further expansion of the field will appear in the near future.

Acknowledgement Work described in this review was supported in part by the NSF award CTS 0456548 and US ARO grant W911NF-05 – 1– 0339.

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