Perspective pubs.acs.org/Macromolecules
50th Anniversary Perspective: Polymer Brushes: Novel Surfaces for Future Materials Wei-Liang Chen,†,‡ Roselynn Cordero,†,§ Hai Tran,†,‡ and Christopher K. Ober*,† Department of Materials Science & Engineering, ‡Smith School of Chemical and Biomolecular Engineering, and §Department of Chemistry and Chemical Biology, Cornell University, Ithaca, New York 14853, United States
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ABSTRACT: Polymer brushes have become a significant focus of polymer research with the need for straightforward and versatile surface modification. With the development of controlled radical polymerization from surfaces, new theoretical models, and sophisticated characterization tools, the resulting ability to control brush density and brush thickness gives unparalleled control over surface properties and functionality. By increasing brush density, a stretched brush conformation is formed as a result of constraining the cross-sectional area of that brush strand which thereby influences the interactions of molecules with the brush surface. The associated residual stress also gives polymer brushes properties distinct from an equivalent layer of coated polymer chains. Examples of uncharged and charged “grown from” polymer brushes, the effect of architecture on physical behavior, and the influence of nanoscale patterning will be described. The use of brush surfaces in biology relevant applications will be discussed and include resistance to nonspecific binding, cell bioadhesion, their use as platforms for biosensors, thermoresponsive surfaces, and targeted protein binding.
1. INTRODUCTION Polymer brushes are thin films of polymer chains covalently anchored to surfaces. Recent development of new theory to describe polymer brushes combined with new synthetic and characterization tools has led to a better understanding of their unique features, and this, in turn, has triggered a significant growth of interest in the application of these important polymeric materials. Polymerization methods now permit the formation of complex polymer brush architectures with uniform molecular sizes, following growth from initiator sites bound to surfaces. The appropriate surface initiator coupled to controlled radical polymerization methods such as atom transfer radical polymerization (ATRP), nitroxide-mediated polymerization (NMP), and reversible addition−fragmentation chain transfer (RAFT)1−3 also enables control of brush density and, thus, brush chain conformation from the substrate surface. In a typical dense polymer brush, the number of attachment or tethering points is sufficiently high that the crowded polymer chains are forced to stretch normal to a planar surface. This leads to residual tension in polymer brushes and may produce mechanochemical effects, influences location of brush chain ends in planar regions in the brush layer, and affects access by solvent and other molecules to the brush.4 Covalently attached brushes may also be prepared by other methods in addition to “grafting from” surface initiation. “Grafting to” techniques have been extensively studied and may involve attachment of polymer chains to a substrate by exposing photoradical surface-bound initiators in the presence of a polymer film.5 The polymers become chemically linked to the surface and form the brush, enabling brush formation from polymers of extraordinarily high molecular weight. Alternatively, end functional chains have been tethered to surfaces using a © 2017 American Chemical Society
simple thermal process to end-link polymer chains to form a brush layer and create a neutral surface for the directed selfassembly of block copolymers for high resolution patterning.6 These methods cannot readily form the dense, extended brush possible when using initiation from uniform arrays of initiator groups. Stretched conformations, which require neither a confining geometry nor an external field, produce both extension of the chains and restricted mobility. The behavior of polymer brushes can, therefore, be quite different from the typical behavior of flexible polymer chains in solution or the melt where chains adopt a random walk.7 Assuming that in a dense brush the polymer chain is confined to a cylinder centered around its surface attachment point, the increase in brush thickness (L) scales with the degree of polymerization (N). In contrast, a sparsely packed polymer chain behaves more like a random coil whose radius of gyration (Rg) ∼ N1/2. The stretching or lack of it provides polymer brushes with properties that can be tuned and exploited in a variety of surface applications. Brush stretching can lead to interesting mechanical properties in the solid state, including the ability to span channels and gaps over surprising distances as a result of the residual stress. And if brush layers are removed from the substrate, this stiffness enables them to form robust thin films even if not cross-linked.8 De Gennes among others was responsible for recognizing these differences,9,10 and the seminal work by Milner11 has led to a greater understanding of these materials and the resulting interest in brushes as unique surface modifiers. Received: February 28, 2017 Revised: April 15, 2017 Published: May 5, 2017 4089
DOI: 10.1021/acs.macromol.7b00450 Macromolecules 2017, 50, 4089−4113
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Figure 1. Synthesis of polymer brushes on silicon substrate using (a) SI-ATRP,1 (b) SI-RAFT,3 (c) SI-NMP,47 (d) SI-PIMP,53 and (e) light-mediated CRP via photoredox catalyst.59
must be rather high (a radius of a few nanometers) to observe this effect, and it is often associated with brushes on nanoparticles. A new family of materials, hairy nanoparticles, has been developed based on this effect in which the tight curvature permits entanglement of the pendent brushes. While these materials are relatively new, significantly improved thermomechanical, optical, and electrical properties22 have already been observed. More information can be found in recent reviews.23−26 The remainder of this Perspective will focus on polymer brushes on planar substrates and provide a summary of recent studies of “grown from” brush synthesis, theoretical concepts of neutral and charged polymer brushes, new brush architectures, and selected examples of applications involving polymer brushes. In particular, examples where brush density or brush stretching plays an important role are highlighted.
Polymer brushes are well-known for transforming the nature of a surface with a layer just a few nanometers thick. Control of wetting properties, prevention of nonspecific binding of biomolecules, colloidal stabilization, and resistance to fouling are all examples of successful application of polymer brushes. Other uses include polymer compatibilizers,12 new adhesive materials,13,14 chromatographic devices,15 and etch barriers.16 Responsive polymer brushes can be used to change wettability and binding properties, act as valves in microfluidics, control transport of ions, and transduce chemical and biochemical signals.17 Synthetic polymer brushes are increasingly important as interfaces between materials and biological environments, as stimuli-responsive surfaces in drug delivery,18 and as surfaces for cell growth and for bioseparation.19 For example, charged brushes are used in lubrication and wetting and as antimicrobial coatings. Charge controls adsorption of molecules, enables attachment of specific molecules and living cells to surfaces, and greatly influences such factors as nonspecific binding on these surfaces. Nature uses brushes in interfaces to control surface wetting in the cartilage in joints20 or the surfaces of lung tissue, for lubrication,21 or to limit deposition of macromolecules (polysaccharides, proteins) onto surfaces. Interestingly, as curvature increases in a substrate, the brush confinement and resulting chain stretching of planar substrates fade and a more relaxed brush is formed. Typically, curvature
2. BRUSH FORMATION USING CONTROLLED RADICAL POLYMERIZATION FROM A SURFACE 2.1. ATRP. Atom transfer radical polymerization (ATRP) is based on a reversible activation−deactivation process.27 An important advantage of this method is that it is well suited to the polymerization of (meth)acrylates with all the many functionalities that it provides. Ejaz et al. reported one of the earliest examples of the synthesis of polymer brushes via surface initiated (SI)-ATRP.27 The Langmuir−Blodgett (LB) technique was used 4090
DOI: 10.1021/acs.macromol.7b00450 Macromolecules 2017, 50, 4089−4113
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The recently developed techniques from the conventional SI-ATRP have overcome many of these issues and made this technique into an excellent choice for people at different synthetic levels and demonstrated industrial scale manufacture for real-life applications. The fabrication of gradient polymer brushes is also more straightforward with SI-ATRP than with SI-NMP or SI-RAFT (see below). 2.2. RAFT. Reversible addition−fragmentation chain transfer (RAFT) polymerization is a versatile technique that involves a reversible regenerative chain transfer mechanism.35 Baum et al. reported the synthesis of polymer brushes from a silicon substrate via SI-RAFT. A surface-immobilized azo-initiator was used in the presence of a chain transfer agent (CTA) to polymerize styrene, MMA, and N,N-dimethylacrylamide (DMAEMA) with well-controlled thickness (Figure 1b).3 Another strategy is to use a surface immobilized RAFT agent which can be attached to the surface through the leaving and reinitiating group (R group) or through a radical stabilizing group (Z group) (Scheme 1).
to lay down a well-organized layer of initiator on a silicon substrate followed by SI-ATRP of poly(methyl methacrylate) (PMMA) brushes. The free initiator p-toluenesulfonyl chloride (TsCl) was added to control the polymerization due to the extremely low concentration of the initiator on the surface.27 Matyjaszewski et al. reported the synthesis of homopolymer and diblock copolymer brushes in the absence of free initiator (Figure 1a).1 Instead, CuBr2 was added at the beginning of the reaction to ensure a sufficient concentration of deactivating Cu(II) species. Jones et al. demonstrated the accelerating effect of water on SI-ATRP through a much faster formation of thick PMMA brushes compared to prior reports.28 The ability to grow poly(methyl methacrylate)-b-poly(2-hydroxyethyl methacrylate) (|-PMMAb-PHEMA) block copolymer brushes proved the controlled nature of this method.a Huang et al. also confirmed the accelerating effect of water on SI-ATRP as a 700 nm thick PHEMA polymer brush layer was observed in aqueous media while it was only 6 nm in the absence of water.29 The conventional ATRP method discussed above involves an immobilized ATRP initiator on a substrate from which polymer chains will grow. Sedjo et al. reported a modified method called reverse ATRP when a conventional radical initiator was used during the initial addition of Cu(II)/ligand complex.30 Specifically, an immobilized azo-initiator decomposed upon heating and reacted with Cu(II)/ligand complex to generate in situ ATRP initiator and Cu(I)/ligand complex. This simple method was successful in preparing poly(styrene)-b-poly(methyl methacrylate) (|-PS-b-PMMA) block copolymer brushes.30 In addition, conventional ATRP requires stringent deoxygenated conditions as oxygen, if present in the system, can quickly trap propagating radicals. Matyjaszewski and co-workers developed a simple technique called activator regenerated by electron transfer (ARGET) ATRP which involved addition of a minute amount of active copper catalyst in the presence of excess reducing agent while no deoxygenation step is needed.31 As proof, CuCl2 was used together with tin(II) 2-ethylhexanoate as a reducing agent to prepare poly(n-butyl acrylate) (PBA) homopolymer and |-PBA-b-PS block copolymer brushes from silicon wafers.31 The major disadvantages of conventional SI-ATRP are its airfree conditions, the use of metal catalysts which may be harmful for some applications, and the inability to reuse the polymerization solution. As a result, Li et al. introduced electrochemically mediated ATRP (eATRP) to overcome some of these problems.32 A constant electrical potential was applied to generate Cu(I) catalyst through the reduction of Cu(II) in the vicinity of the ATRP-initiator-modified electrode so that the polymerization can be initiated.32 This method was demonstrated by the growth of poly(3-sulfopropyl methacrylate potassium salt) (PSPMA) brushes on a gold electrode in the presence of air. The same group then extended the eATRP technique to nonconducting substrates.33 In this case, an initiator-modified silicon substrate was placed opposite a working electrode, and a potential was applied to reduce Cu(II) to Cu(I) which then diffused to the silicon substrate to initiate the polymerization. More interestingly, the smaller the gap between the electrode and the substrate was, the thicker the polymer brushes formed, so by tilting the electrode with respect to the substrate, a gradient of polymer brushes along the substrate can be produced.33 When a zinc plate was used as a sacrificial anode, it served to reduce Cu(II) to Cu(I) to initiate the polymerization.34 Only a microliter volume of polymerization solution was needed, and tilting the Zn plate also allowed the formation of gradient polymer brushes.
Scheme 1. SI-RAFT via (a) R-Group Approach and (b) Z-Group Approach
The Z-group approach was used to prepare homopolymer and diblock copolymer brushes on a silicon substrate.36,37 However, as this approach resembles a grafting-to method, polymer brushes with lower grafting density and reduced uniformity compared to other “grown from” brushes are more likely to form at high molecular weight.38 The R group approach was reported for polymer brush synthesis of methyl acrylate (MA), MMA, DMAEMA, and their diblock copolymer brushes.39 This approach is similar to the grafting-from technique in ATRP, thus allowing a high density and uniform film, though the molecular weight distribution may be broadened by the bimolecular termination.40,41 Advincula and co-workers reported the synthesis of polymer brushes via SI-RAFT on a conducting Au surface and on polythiophene films on electrode surfaces.42,43 SI-RAFT is a facile technique and is compatible with a wide range of monomers. Another advantage is that no metal catalyst is needed. However, this method generally does not produce polymer brushes as thick as other SI-controlled radical polymerization (CRP) techniques. In fact, polymer brushes with thickness less than 30 nm are often reported using SI-RAFT.36,37,39,44,45 The RAFT agent is also often expensive or not commercially available; therefore, multiple step syntheses may be needed. 2.3. NMP. Nitroxide-mediated polymerization (NMP) is based on the reversible reaction of growing radical chain ends with a stable nitroxide free radical.46 Using a TEMPO-based initiator containing a chlorosilane anchoring group, Husseman et al. successfully synthesized PS brushes from a silicon substrate, 4091
DOI: 10.1021/acs.macromol.7b00450 Macromolecules 2017, 50, 4089−4113
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Figure 2. Scheme of scaling laws of polymer brushes at different conditions. (a) Neutral brushes with general scaling law h ∼ Nσv with variation of v at different conditions. (b) Scaling laws for charged brushes under different conditions. (c) Symbols for the scheme.
showing the first example of SI-nitroxide-mediated polymerization (SI-NMP) (Figure 1c).47 The addition of free alkoxyamine initiator is crucial for the success of SI-NMP as it resulted in a high concentration of nitroxide in the polymerization solution. It overcomes the issue of having extremely low concentration of surface bound initiator relative to the monomer concentration, leading to controlled polymer brush growth.47 The linear relationship between brush thickness and the molecular weight of a bulk polymer as well as conversion indicated that the growth of bulk polymer chains and polymer brushes were well controlled. The preparation of block and random copolymer brushes were also demonstrated. SI-NMP can also be done in a vapor phase to prepare homo and block copolymer of various monomers in more efficient way than in solution phase.48 The procedure of SI-NMP discussed so far is a unimolecular system.47 The second system for SI-NMP is bimolecular when an azo-based initiator is anchored on a surface following polymerization in the presence of a nitroxide to form in situ alkoxyamine.49,50 The bimolecular system resulted in a higher grafting density of the initiator due to the smaller size of the azo-initiator compared to the alkoxyamine-based initiator which is thought to be more stable than the azo-initiator.49 In addition, the TEMPO-mediated system is mostly limited to styrene and its derivatives but is not compatible with other types of monomers such as acrylates.2 Thus, other nitroxides have been developed. Parvole et al. reported the synthesis of PBA brushes from silicon substrate using an immobilized azo-initiator and the nitroxide, N-tert-butyl-N-1-diethylphosphono-2,2-dimenthylpropylnitroxyl (DEPN or SG1).50 Hawker and co-workers reported the formation of poly(tert-butyl acrylate) (PtBA) brushes on silicon substrate using an immobilized TEMPO-based alkoxyamine initiator in the presence of
free 2,2,5-trimethyl-4-phenyl-3-azahexane-3-oxyl (TIPNO)based alkoxyamine.51 A similar approach was used to synthesize copolymer brushes of 2-(dimethylaminoethyl) acrylate and styrene on a stainless steel surface.52 The advantage of SI-NMP is that no catalyst or metal is required, so it is an ideal technique to prepare polymer brushes for electronic or biological applications which are sensitive to impurities. Moreover, polymer brushes with thickness from a few nm up to 200 nm for various monomer were reported.48,51 However, as the reaction temperature for SI-NMP is very high, it may not be suitable for a number of thermally sensitive monomers or substrates. Moreover, as TEMPO-based initiators do not work well for many monomers, a thoughtful selection of alkoxyamine is needed when using new types of monomers. The synthesis of some alkoxyamines may be difficult. 2.4. Light-Mediated CRP. Hadziioannou and co-workers reported an early example of using SI-photoiniferter-mediated polymerization (SI-PIMP) to grow polymer brushes from an iniferter monolayer immobilized on a surface (Figure 1d).53 Under UV radiation, the photoiniferter is dissociated into a reactive radical and a persistent dithiocarbamyl radical. Monomers are added to the reactive radical during the propagation step while the dithiocarbamyl radical acts as a transfer agent and reversibly terminates the propagating chain, giving a living characteristic of the polymerization. Homopolymer and diblock copolymer brushes of PS and PMMA were prepared using this method.53 Luo et al. used SI-PIMP to synthesize poly(ethylene glycol) monomethyl ether monomethacrylate (m-PEGMA) on a polymer substrate functionalized with diethyldithiocarbamate photoiniferter.54 Heeb et al. also used SI-PIMP to prepare PMMA brushes using an UV-LED light source in aqueous media at room temperature for the control of polymer thickness with 4092
DOI: 10.1021/acs.macromol.7b00450 Macromolecules 2017, 50, 4089−4113
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Macromolecules irradiation time.55 Rahane et al. carried out a kinetics study of SI-PIMP and found that the initial rate of polymer brush growth had a first-order dependence on monomer concentration, but the reaction rate was nonlinear with the irradiation time due to bimolecular termination arising from insufficient dithiocarbamyl radicals at the surface.56 For this reason, a deactivating species, tetraethylthiuram disulfide (TED), was added, and the maximum thickness of polymer brushes was increased with an increase of TED and a decrease of light intensity, suggesting better control of brush growth.57 Zhang et al. reported a photoinduced SI-ATRP approach using a fluorescent lamp.58 In the study, light reduced a Cu(II) complex to a Cu(I) complex and initiated the polymerization to produce patterned, homopolymer and block copolymer brushes. Poelma et al. developed a novel light-mediated CRP using an Ir-based photoredox catalyst (Figure 1e).59 In this approach, the catalyst fac-[Ir(ppy)3] (where ppy = 2-pyridylphenyl) will absorb visible light and go into an excited state as fac-[Ir(ppy)3]*, which then reacts with an alkyl bromide initiator to generate a highly oxidizing Ir(IV) complex and a reactive radical to which monomers will be added. The propagating radical will then react with the Ir(IV) complex to form a bromo-end-capped polymer chain and the original fac-[Ir(ppy)3].60 The significant development is that this mechanism allows turning on or off light to activate/ deactivate the polymerization. In combination with the use of a photomask, this method can temporally and spatially control polymer brush formation. Brush thickness can be manipulated by the exposure time of a sample to light. Under a photomask, only regions exposed to light will have polymer brushes, allowing the fabrication of 3D nanostructures of polymer brushes. In addition, a neutral density filter allows the formation of gradient polymer brushes.59 No sacrificial initiator or deactivator is needed for this method. The same group then extended the work and developed a light-mediated metal-free living polymerization.61 Phenylphenothiazine (PTH), an organic photoredox catalyst, was used in this case. More importantly, this method does not require airfree conditions, allowing the fabrication of polymer brushes in large samples. Temporal and spatial control of polymerization have also been applied to this method to control the brush thickness and patterned area, respectively. Light-mediated polymerization is a versatile technique for polymer brush synthesis. Thick polymer brushes up to 800 nm were reported using SI-PIMP,62,63 while using light-mediated CRP with a photoredox catalyst, polymer brushes with thickness over 100 nm were prepared.59 Moreover, SI-PIMP and recently developed metal-free redox catalysts open opportunities for this technique to be used in biomedical and electronic applications. Compared with the other CRP methods, light-mediated polymerization has a distinguished advantage on the fabrication of complex 3D microstructure brushes. However, as the reaction requires access to light exposure, this method may be not suitable for a substrate containing microchannels such as a microfluidic platform.
smaller than the polymer stretching distance and reflects the start of the interaction between grafted chains. Following the “Flory argument”, Alexander derived the brush conformation as the balance between the elastic energy of an entropic spring, Fel, and an excluded volume repulsion between segments, Fint. Polymer chains are considered to be ideal and their elastic energy increases quadratically with their end-to-end distance, which is represented by the brush film thickness. In contrast, the excluded volume originates from uniformly distributed monomer segments and is proportional to their volume fraction. The total free energy of a single polymer chain in the brush F = Fel + Fint then can be written as follows: ⎡ 3h2 wN 2σ ⎤ + F = kBT ⎢ ⎥ 2 h ⎦ ⎣ 2Na
(1)
Here a is the monomer diameter, w is the excluded volume, N is the degree of polymerization, and σ is the graft density (number of chains per area). By minimization, the thickness of the polymer brush (h) may be obtained as
h ∼ N (σw)1/3
(2)
A similar relationship was also obtained by de Gennes through simple scaling analysis where polymers in the brush were considered as a series of tension blobs with their steric interaction determining the conformation of the brushes.66 This led to correlation length of polymer chains in the brush to be the distance between grafting points, which is equal to σ−1/2 and is the same in the work of Alexander.64 This description is then referred to as an Alexander−de Gennes brush, and it is the most basic model for a brush. One important difference between the polymer brushes is that the brush dimension is proportional to N instead of N1/2 in its free form. For the chains in the brush, their length could be much larger than Rg due to steric deformation leading to properties that are distinct from free polymers. Similarly, the thickness scaling laws for polymer brushes in a theta solvent and poor solvent were also derived.67−69 The power law dependence of the thickness on grafting density increases from 1/3 to 1/2 to 1 as the solvent quality decreases from good to θ-solvent to poor solvent. In contrast, the dependence on the molecular weight remains constant at N. A similar increase on the dependence could be obtained when the grafting density is further increased from moderate grafting density to high grafting density. At high surface coverage, the excluded-volume effect between segments becomes fully screened like concentrated polymer solutions, and higher order interactions become important. In some cases, the brush height scaling relationship could exhibit a higher power law exponent dependence than 1/3, and it may even be as high as 1.67,70,71 Such brushes are defined as a “concentrated brush” in contrast to the “semidilute brush” where excluded volume repulsion dominates. Alexander−de Gennes brush theory also depicts the brush segment density along the vertical distance from the surface φ(z). The segment density equals σ2/3 above its correlation length. Above a brush of thickness h, the brush segment density drops rapidly to zero. The segment density reaches a plateau when z > σ−1/2 and equals the density of the tension blob. However, this result was later found to be inaccurate due to the oversimplification of the brush structures. In the model, polymer chains are assumed to uniformly stretch normal to the surface and their free ends are located at the outermost edge. In fact, the optimum state of the brush allows polymer chain ends to be buried below the surface without penalty in free energy.
3. FUNDAMENTAL INVESTIGATIONS 3.1. Studies of Neutral Brushes. 3.1.1. Theoretical Studies of Neutral Brushes. A fundamental theory of brush structure was first derived in the work of Alexander.64 Compared to uniformly absorbed polymers, which have a film thickness h smaller than free polymer radius of gyration Rg,65 Alexander noticed that the thickness of end-absorbed polymers could have the opposite behavior at increased grafting density where the chain−chain interaction dominates. The distance between tethering points is 4093
DOI: 10.1021/acs.macromol.7b00450 Macromolecules 2017, 50, 4089−4113
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Macromolecules Skvortsov et al.72 showed that the polymer brush segment density is a parabola, and the chain ends distribute over the entire height of the polymer brush rather than focus on the top. The parabolic profile of segment density distribution is later explained independently by Milner et al.,73−75 Skvortsov et al.,76 and Zhulina et al.77 in different studies. Instead of eq 1, the excluded volume is considered to be the sum of the effect of segment density. In this system, each grafted chain i is represented as a set of N monomer coordinates ri⃗ (n), with n = 0, 1, ..., N and K are the total number of chains. The condition ri⃗ (N) = 0 is set for the end grafted chains which applies no limitation on the positions of free ends. K
F=
∑∫ i=1
0
N
⎡ 1 ⎛ d r ⃗ ⎞2 ⎤ 1 dn⎢ ⎜ i ⎟ ⎥ + w ⎢⎣ 2 ⎝ dn ⎠ ⎥⎦ 2
∫ dV φ 2 ( r ⃗ )
which used a PS block as the brush and the crystalline block as a solid base.81 The brushes showed a transition starting from Σ ∼ 3.7−14.3. It was then concluded that Σ > 5 is a general standard to define the brush state.82 However, this standard is not accurate for semiflexible brushes with rigid chains where a lower threshold for transition at Σ < 1 has been observed.83,58 The swelling properties of a polymer brush, h ∼ N, was first reported by Hadziioannou and Tirrell et al.84,85 while thickness dependence on the grafting density for a good solvent, h ∼ σ1/3, was later observed through different techniques including ellipsometry,86,87 neutron reflectivity (NR),88 surface force apparatus (SFA)89 studies, and atomic force microscopy (AFM).79,90 Early studies generally used brushes made by the “grafting to” technique which places limitations on brush grafting density. With advances in polymerization techniques, polymer brushes with high grafting density could be synthesized and were proven to be different from their low grafting density analogs. Yamamoto et al. found that the power law exponent of thickness scaling h ∼ σv of PMMA brushes gently changed from 1/3 to 2/5 in going from low to high grafting density (σ > 0.4 nm−2).90,91 The increase in power law exponent υ to 2/5 was also observed by Wu et al.79 and then studied systematically by Moh et al.87 PMMA brushes with maximum σ = 0.8 nm−2 were made and demonstrated an increase in υ starting at σ = 0.4 nm−2 (Figure 3b). It was concluded that 0.8 < v < 1.3 for σ > 0.7 nm−2 at the highest grafting density which is consistent with theoretical scaling for v = 1 at high grafting density for concentrated brushes.70,71 PMMA brushes were also tested under different solvent conditions and changed from good to Θ to poor solvents by changing the composition of an acetone/methanol mixture. At medium grafting density (σ < 0.4 nm−2), the exponential dependence increased as the solvent quality decreased but collapsed to a single value of v ∼ 0.8 at high grafting density, which is the same dependence as that in pure methanol. The reason that v < 1 instead of the predicted value of 1 in a poor solvent was due to the “self-solvent” effect since a polymer is a θ-solvent for itself. This becomes obvious at high polymer concentrations. 3.1.3. Experimental Studies of Chain Distribution. Stretching of the polymer chains fundamentally changes the properties of polymer brushes and is a key to engineering target properties.78 The distinct property at the brush region was not revealed until the study of Levicky et al. where Σmax = 28.92 Profiles of polymer segment density became a flat parabola as Σ increased in the strongly stretched region. It could be explained by the fact that at larger Σ the increased entropy penalty caused chains to be redistributed to the bulk. Such a penalty was not observed in the transition region where the higher Σ could only increase segmental density rather than flattening although the parabolic-like segmental density profile has already been shown.93−96 Nevertheless, it was found that the segment density more closely resembled a step-like profile at higher Σ, which was predicted for the Alexander−de Gennes brush with an assumption of strong chain stretching. On the other hand, upon change of solvent quality the brush shrank and the average segmental density increased due to the magnified chain overlap. Surprisingly, the profile was the flattest at good solvent quality and the roundest at medium solvent quality showing the ability of controlled solvent quality as a means to control brush behavior. Another “lever” that provides unique segmental density for brushes is dispersity. Biesalski and Rühe showed that polymer brushes with high dispersity synthesized by free-radical polymerization (FRP) display an exponential-decay segmental density profile.86 Since such a profile was not observed in the PS brushes
(3)
The free energy for addition of a single chain in the system, ΔF, is then similar to the path integral for single particle motion in quantum mechanics by treating ri⃗ (n) as the position of a particle at time n. In the particle system, these conditions together mean that no matter where the particles start their motion, in the end they have to reach the surface at the same time N. As a result, the segment density profile is a parabola and the chain free-end would not have its concentration maximum at the edge of the brushes. The brushes would have stronger excluded volume repulsion in the deeper layers compared to the Alexander−de Gennes brush. The equilibrium chain conformation is determined by the trade-off between excluded repulsion (maximum at low z) and chain entropic stretching (maximum at high z). More detailed comparison between these two models has been discussed elsewhere78 and will not be addressed here. 3.1.2. Experimental Studies on Brush Stretching. In theory, the transition from the low grafting density “mushroom” state to the “brush” state occurs when the reduced grafting density, Σ = σπRg2, is greater than one. Wu et al. studied the transition of the brush thickness with poly(acrylamide) (PAAm) brushes grown on the grafting density gradient prepared with mixed vapor of 1-trichlorosilyl-2-(m/p-chloromethylphenyl)ethane (CMPE) and paraffin oil (PO).79 PAAm brushes showed a sharp transition at Σ ∼ 6 where the thickness scaling changed from σ0 to σ1/3 (Figure 3a). Interestingly, the water contact angle was measured on the same brushes, and it showed a broader crossover region between the two states which fits better to theoretical prediction.80 Another study by Zheng et al. conducted experiments with crystalline−amorphous diblock copolymers
Figure 3. Change of the thickness scaling at different grafting densities and solvent qualities. (a) Mushroom to brush transition of PAAm brushes. Reproduced with permission from ref 79. (b) Transition of PMMA brushes from semidilute brush region to concentrated brush state as σ > 0.4 nm−2 when good solvent, acetone was applied. Such transition was not observed in methanol immersion where scaling remained constant as σ4/5. Reproduced with permission from ref 87. 4094
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its surroundings may control the charge ratio of the brushes and determine their behavior. Such PEBs are referred to as “weak” polyelectrolyte brushes, and they have more tunable properties compared to “strong” polyelectrolyte brushes whose charges are associated with strong acid/base groups. PEB theory has developed since the pioneering work of Pincus,105 but many theories have shortcomings compared to experimental results. For that, this Perspective will focus on the experimental observations on PEBs, and only those matching theories will be addressed and discussed. Readers may find more detailed information on PEB theory in other recently published reviews.106−108 3.2.1. Swelling Properties. Swelling properties of strong PEBs are quite distinct among all the other type of brushes. Results from different groups have confirmed that the wet brush thickness of strong PEBs is independent of the grafting density and only proportional to the molecular weight and the square root of the fraction of charged monomers, f.86,86,109,110 PEBs with such behavior are called an “osmotic brush” since such thickness scaling is caused by the balance between osmotic pressure and the chain stretching.105 The equation describing these brushes is expressed as
synthesized by the same method but with lower molecular weight,97 high molecular weight is another criterion for the appearance of an exponential-decay segmental density profile. A more detailed description of polymer brushes was not revealed until recent years. Spiliopoulos et al.98 conducted NR studies on diblock copolymers of deuterated PS and unlabeled PS (dPS-b-PS) on the last 16% of the free end. The free, nondeuterated chain ends could then be distinguished in a sea of deuterated substrates. The result fit qualitatively to theoretical predications where the chain ends lie within the brush layer rather than aggregating on the surface. In another study by Abbott et al.,99 the segmental density profile was studied with mechanical pressure present. The pressure needed to change the segmental density profile predicted by the self-consistent field (SCF) theory was more than 1 order of magnitude less than experimental results. Experimentally, only ∼1 bar was needed to change the shape from parabolic to step-like, and this magnitude was also obtained in other studies.100,101 The result suggested that the osmotic pressure does not act the same as mechanical pressure in the brush structure, but it is still unclear how those two factors interact. As a polymeric material, polymer brushes are expected to have entanglements just like any other.102 Ruths et al. conducted SFA studies of PS brushes grafted on the both sides of a gap.103 Surprisingly, brush stretching became shorter when the brushes were left at the largest compression for an hour. Since the disentanglement time estimated based on the brush parameters was up to hours, it was concluded that stretching was hindered by an entangled conformation. In addition, even with molecular weight ∼106, the number of entanglements in the brushes was approximately only 3−5 per polymer chain. This could be the reason why such effects were not observed in other SFA studies with shorter polymer chains. Kenji et al. also showed intramolecular entanglements within the PMMA brushes. It was shown that the compressibility of the PMMA brushes was smaller than that of a spin-coated PMMA film. By comparing the brushes to a polymeric network with similar stretching behavior, the number of entanglements was calculated to be 2/3 of that in a PMMA melt as a result of low chain overlapping in brush conformation. Although both experiments showed the resulting effects of brush entanglement, the relationship between grating density, molecular weight, and the number of entanglements is still unclear to people in the field. Using molecular simulation, Hoy and Grest later showed the scaling law for the number of entanglements (Ω) per unit area in brushes versus grafting density to be σ1.78 for a good solvent and σ1.50 for a θ-solvent.104 Although such scaling fits well with the thickness scaling law (σ1/3, σ1/2) when φ2h was used to evaluate Ω, further experimental work is still needed to prove the dependence on σ. On the other hand, since entanglements may also be responsible for the structural integrity of a brush carpet, there still exists the possibility to fabricate highly stable brush carpets from properly entangled brushes.77 3.2. Studies of Charged Brushes. Charged brushes (also known as a polyelectrolyte brush, PEB) have distinct properties compared to neutral brushes. Briefly, additional interactions due to charges must to be considered and increase the complexity of the PEB structure. Additionally, interactions may be dramatically different when the surrounding environment of the PEB is changed. One of the most studied conditions is the addition of electrolyte since it may alter not only the osmotic pressure but also interaction between ions. If the charges on the brush are not permanently associated with certain chemical groups, the pH of
h ∼ Nf 1/2
(4)
Weak PEBs are rather complicated since now the charges may be altered under different environmental conditions. Rühe et al.111,112 showed that pH could affect the swelling of poly(methacrylic acid) (PMAA) brushes by affecting its dissociation. Later, Wu et al.113 conducted experiments on weak PEBs made of poly(acrylic acid) (PAA), and it was found that the wet brush thickness versus grafting density behaved similar to a neutral brush with a power law exponent dependence of 1/3 which is opposite to the prediction by mean-field theory.114 Similar behavior was observed by Lego et al.,115 and it was suggested that it might originate from low dissociation level for polymers in brush state as observed in other studies.112,113,116 The same mechanism was not only predicted by the theory46 but also contributes to the inhomogeneous ionization along the brush as proven by Ober et al. by comparing pH changes derived from titration and FTIR studies.117 3.2.2. Interaction with Monovalent Electrolytes. The addition of electrolytes to the PEBs turns them into so-called “salted brushes” where the charges are screened. Theory suggests that the transformation happens when the solution electrolyte concentration, Cs, equals that inside the PEB layer where the osmotic pressure is decreased by penetration of electrolytes.105 The scaling of a salted brush is expressed by eq 5: h ∼ Nσ 1/3Cs−1/3
(5)
Experimental demonstrations have been provided by studies regarding scaling of strong PEBs. Poly(styrenesulfonate, sodium salt) (PSS) brushes were tested at different electrolyte concentrations, and their thickness scaling power exponent was determined to be −0.27 for NR measurement109 and −1/3 for SFA measurement.110 Such deviation could originate from the difference in the measurement technique since brushes were in contact with each other at the time of measurement. Furthermore, a SFA measured force profile fit the theory except for the high compression region where ion concentration inside the brushes was not predicted precisely. The −1/3 power exponent was also obtained by Biesalski et al. with cationic poly(N-methyl-4vinylpyridinium iodide) P(MeVP) brushes and halogen ions.118 4095
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Figure 4. PEB reaction with multivalent ions. (a) Structure of two complexes between divalent ions and PMAA brushes. Reproduced with permission from ref 121. (b) Swelling properties of PMAA brushes at different metal ion Me(NO)3 concentrations. Reproduced with permission from ref 124. (c) Neutron scattering density versus distance away from grafting surface (z) of PSS brushes when soaked in different divalent ion D2O solution. It reflects the distribution of monomer and ions along the z direction. Reproduced with permission from ref 129. (d) Scheme of the variation of ion−brush complex structures at different ion concentrations and ion species determined by QCM study with PSS brushes. Reproduced with permission from ref 131.
Interestingly, when the ions were changed from chloride to bromide to iodide, deviation from scaling predictions took place with more shrinkage of the brushes. But at even higher iodide concentration reswelling of the brushes was observed. Weak PEBs behaved the same as strong PEBs in the salted brush regime. Equation 5 has been proven in the experimental studies of PAA113 and PMAA112,116 brushes. However, under low electrolyte concentration, thickness scaling is proportional to electrolyte added since the dissociation level increases with it. Birshtein et al.114 examined the effect of dissociation and derived its corresponding scaling as h ∼ Nσ −1/3Cs1/3
Lego et al. showed that PAA brushes have a unique thickness scaling at pH 9. The power exponent for grafting density increased from 0.87 to 0.96 rather than 1/3 as the salt concentration increased from 0 to 1 × 10−3 M. In addition, in the salted brush regime, it has been shown that h ∼ σ1.09Cs−0.21, and such scaling is different from previous studies.115 This was close to the scaling of the Pincus brush as expressed in eq 7.105 The Pincus brush was developed to model brushes with low grafting density/extent of dissociation whose counterion cloud perturbs the brush region surroundings. Under such conditions it cannot maintain its charge neutrality, and charge repulsion is dominant. Although the Pincus brush has long been developed, very few studies use it due to its narrow regime and complexity. It remains unclear why their brushes behaved like Pincus brushes rather than those reported in other studies.
(6)
Weak polyelectrolyte brushes behaved as predicted at low electrolyte concentration, and the sensitivity of the swelling change decreased as either pH values increased or grafting density decreased.112,113,116 As electrolyte concentration increases, charge screening starts to dominate at Cs,max where weak PEBs reach their maximum thickness. This maximum value, Cs,max, is proportional to the grafting density (Cs,max ∼ σ) as predicted,52 but its value could be lower due to the counterion condensation.110 An interesting aspect of eq 6 is that the thickness is a decreasing function of grafting density, which originates from the fact that looser brushes have higher dissociation level.120,121 Nevertheless, it has not yet been observed experimentally. At low salt concentration, the thickness of PAA brushes is proportional to the grafting density but with completely different scaling exponent of 1/3.113
h ∼ N3σf 2
(7)
3.2.3. Interactions with Multivalent Electrolytes. Multivalent electrolytes induce completely different structural changes in PEBs depending on their own specific ionic properties. Because of their ability to form complex cross-links, nature uses them when additional stability is needed.122,123 Konradi and Rühe, on the other hand, have conducted a series of studies on weak PEBs, PMAA interacting with multivalent ions.121,124 Using FTIR, the relationship between grafting density, complexation, and dissociation was quantitatively studied. As the grafting density increased, the degree of complexation decreased as did the level of dissociation. Two geometries of complexation (chelated bidentate/bridging bidentate, Figure 4a) were observed in 4096
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Figure 5. (a) Thermoresponsive polypeptide brushes with oligo(ethylene glycol) side chains. Reproduced with permission from ref 147. (b) Stimuli responsive diblock copolymer brushes. Reproduced with permission from ref 136. (c) Poly(acrylic acid) brushes with grafting density gradient. Reproduced with permission from ref 113. (d) Molecular weight gradient polymer brushes via sacrificial-anode ATRP (sa ATRP). Reproduced with permission from ref 34. Copyright 2013 Wiley-VCH Verlag GmbH & Co. KGaA. (e) Nanoscale structural rearrangements of the Y-shaped brushes to form (i) a corona with PS chains (blue) covered a core of poly(acrylic acid) chains (red) in toluene, (ii) swelling state of the brushes in nonselective solvent, and (iii) PAA arms partially covered PS core in a good solvent for PAA. Reproduced with permission from ref 174. (f) Reconstruction of mixed brushes in swollen state and dry state for both nonselective (1, 2) and selective (3, 4) solvents, respectively. Reproduced with permission from ref 168.
ion was seen to decrease with an increase in total ionic strength. Another interesting finding is the appearance of attractive forces between brushes in SFA experiments with brushes on both sides of a gap. CV experiments confirmed ion complexation and brush attraction. Such results were solely observed for multivalent ions (Ru(NH3)63+/La3+/Al3+/Ca2+) but not for monovalent ions. The attractive force induced by La3+ could be so strong that the force−distance curve “jump-in” occurred at a shorter distance without increase of compressive force during a compression experiment. By comparing the attractive forces between trivalent ions, it was concluded that La3+ > Al3+ > Ru(NH3)63+. This trend followed the reverse order of their hydration radius. They pointed out the importance of hydration and charge density in complexation attraction. In addition, such interactions were not observed for brushes at high grafting density with Y3+ due to the reduction of chain interpenetration. The complexation-induced structural change was also characterized by NR.129 A segmental density profile transition was observed to evolve from a parabolic form to a Gaussian-like form when certain multivalent ions were used (Figure 4c), in contrast to the general condition of monovalent ions.130,131 Quartz crystal microbalance (QCM-D) has also been applied as a useful way to study the dynamics of PEB complexation. Liu and co-workers characterized the conformations of PEBs under complexation by monitoring brush changes (Figure 4d).132,133 Together with other studies the big picture of brush complexation may be understood. In the future, a study that combines QCM-D, FTIR, and SFA results will offer a better picture of the complete mechanism and structure of PEB complexation.
PMAA brushes, and the amount of complexation had a strong dependence on the grafting density. Different ions had different coordination preferences and thus swelling properties. Through comparison with FTIR spectra, it was shown that Cu2+ bridging bidentate links could collapse at very low ion concentration of 10−6 M and Al3+ could induce even stronger collapse at the same concentration. Only alkaline-earth metal ions could reswell the network at high ion concentration similar to the case of PMeVP brushes with iodide ion.118 Such trends also followed the order of the ion radius, where the swelling extent of brush under Sr2+ > Ca2+ > Mg2+ (Figure 4b). Unexpectedly, the collapse of brushes with Cu2+ or Al3+ was weaker than the collapse with alkaline-earth metals due to the higher hydrophilicity of the bridging complex and associated steric repulsion, which was different from the result observed by Tirrell et al. with the same metal ions.125 Tirrell and co-workers conducted studies with PSS brushes soaked in metal ions with different charges at fixed ionic strength.125−129 Compared to the situation with only monovalent ions, tri- or divalent ions can induce stronger collapse in the salted brush regime as a result of lower osmotic pressure, and the process happens at lower ion concentration. These multivalent ions may be released only at high monovalent ion concentration. Since the amount of complexation of Ru(NH3)63+ in PEBs could be quantified by cyclic voltametry (CV), it was confirmed that additional collapse of the brushes was induced by the complexation of Ru(NH3)63+, and the mechanism of complexation was different at different total ionic strength. At high total ionic strength (salted brush regime), high concentrations of monovalent ions competed with the trivalent ion and the complexation thus built on the kinetic equilibrium between ions. In contrast, at lower total ionic strength, trivalent ions diffused easily into brushes without much competition and achieved saturation. The maximum uptake of the trivalent
4. ADVANCED BRUSH ARCHITECTURE With the use of living polymerization methods, variations of polymer brush architecture such as block copolymer brushes, 4097
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kept relatively short (10 < DP < 20). In addition, with the proper design of side arms, effects of charge spacing, and charge periodicity can be readily studied. Tethered bottlebrush or molecular brush polymers are rarer and consist of a backbone anchored to a surface with long side chains hanging from that backbone. There are very few examples of synthetic charged tethered polyelectrolyte bottlebrush polymers. Hydrophilic tethered bottlebrush polymers typically consist of a hydrophobic backbone (e.g., PMMA) with side chains such as PEG having a DP ranging from 30 to 40.144 Hawker has shown the formation of tethered bottlebrush polymers using light-mediated growth of the polymer backbone and then growth of hydrophilic side arms. In these studies grayscale light exposure controls brush height and molecular weight.145 In another example, Klok grew PEG brushes with thermoresponsive behavior having either PMMA or peptide backbones (using vapor polymerization146 in the latter case) (Figure 5a).147 No cleavage effects have been reported due to side-chain crowding likely because reported tethered bottlebrushes have relatively short side groups. In additional examples of coated but not tethered bottlebrush polymers, Claesson et al. describe a bottlebrush polymer with a charged backbone and PEO side groups that has good protein absorption resistance.148 The presence of PEG stops the protein binding while the charged backbone binds to a surface. Recently, Müller,149 Lienkamp,150 and others151−153 have reported the synthesis of cylindrical PEBs.154,106 A characteristic feature of PEBs in general is the strong confinement of the counterions within the brush layer.105,155 Müller prepared polymers with long MMA backbone segments with side chains of quaternized PDMAEMA (DP ∼ 80). Interestingly they were found to form helical bottlebrush bundles under certain process conditions.156 4.3. Gradient Brushes. Polymer brushes may come in the form of a gradient when there is a gradual variation in one of its properties (e.g., brush density) in a given direction. Two main types of gradient polymer brushes, based on (i) molecular weight and (ii) grafting density, are discussed below. Wu et al. utilized the diffusion of CMPE in a vapor phase followed by backfilling with an inert silane to create a density gradient of this initiator along a substrate which was then amplified into a grafting density gradient of poly(acrylamide) (PAAm) brushes via SI-ATRP (Figure 3a).79 Similarly, but in reverse order, an inert silane may be vapor deposited on a surface before backfilling with CMPE for the growth of density gradient brushes (Figure 5c).113 Wang et al. applied an electrochemical potential gradient to spatially remove hexadecanethiol and then backfill the bare area with an ATRP initiator which was later used to grow polymer brushes with a chain density gradient.157 Genzer and co-workers developed a draining method employing a polymerization solution in a chamber containing a silicon substrate with immobilized ATRP initiator that was gradually removed with a micropump, resulting in a molecular weight brush gradient.158 When the draining method was used twice, diblock copolymer brushes with molecular weight gradient of both blocks could be prepared.159,137 Xu et al. used microchannel confined SI-ATRP to create a molecular weight gradient of PHEMA brushes.160 Yan et al. introduced an approach called sacrificial-anode ATRP (sa-ATRP) to prepare a molecular weight gradient polymer brush under ambient conditions (Figure 5d).34 In this approach a zinc metal surface and an initiator immobilized surface were sandwiched together while a polymerization solution was introduced into the gap between them. Tilting the Zn surface with respect to an initiator-immobilized substrate caused
bottlebrush, mixed brushes, gradient brushes, free-standing brushes, and patterned brushes are possible and have been reported. Differences in architecture, for example, may lead to better control of surface formation and coverage, provide better access to specific functionality, and enable structured surface formation. 4.1. Block Copolymer Brushes. Husseman et al. utilized nitroxide-mediated polymerization (NMP) to prepare block copolymer brushes composed of a first block of polystyrene and a second block of a random copolymer of PS and PMMA on silicon substrates.47 Matyjaszewski et al. used SI-ATRP to synthesize |-PS-b-PMA and |-PS-b-PtBA diblock copolymer brushes.1 In addition, ARGET ATRP was reported for the preparation of |-PBA-b-PS copolymer from silicon substrates.31 The formation of |-PS-b-PMMA diblock copolymer brushes can also be done by reverse-ATRP.30 Moreover, SI-RAFT was successfully used to grow |-PDMAEMA-b-PHEMA diblock brushes from silicon substrate with immobilized chain transfer agent.36 Similar approaches using SI-RAFT were employed for the synthesis of |-PMMA-b-PDMAEMA, |-PMMA-b-PS and |-PS-b-PMA.39 Baum et al. reported the synthesis of polystyreneb-poly(N,N-dimethylacrylamide) (|-PS-b-PDMA) and |-PDMAb-PMMA via SI-RAFT from surface-immobilized azo-initiators in the presence of 2-phenylprop-2-yl dithiobenzoate as chain transfer agent.3 Well-defined block copolymer brushes can be produced by a combination of ATRP and RAFT techniques.134 Specifically, the bromine end groups of homopolymer brushes prepared via SI-ATRP were modified with a RAFT agent which was subsequently used to grow a second block in the brushes.134 The phase separation of |-PS-b-PMMA brushes upon solvent treatment was explored to create patterned nanostructures as an array of micelles with an ellipsoidal shape.135 Kumar et al. demonstrated the application of diblock copolymer brushes prepared with SI-ATRP as potential stimuli responsive materials for the control of dye release triggered by changes in temperature, light, and pH (Figure 5b).136 Not only diblock but also triblock copolymer brushes can be prepared from a silicon substrate137 or gold substrate138 via SI-ATRP. 4.2. Bottlebrush Architecture. Bottlebrush polymers differ significantly from more conventional single-strand polymers. The majority of these materials are uncharged with reported side chains having a degree of polymerization (DP) ranging from 20 to 60 or more. They provide a different structural motif than the typical single-strand brush and, as a result of the bulky side chains, may place the backbone under stress. As an example, Sheiko et al. reported an untethered bottlebrush polymer coated on a surface with a PMMA backbone and DP 25 side chains of n-butyl acrylate and studied main chain breakage kinetics of these polymers.139 Persistence length scales as side chain length: DP ∼ 10 is flexible and DP ∼ 100 is a rigid rod. These studies indicate that relatively short side chains will not place undue stress on a backbone chain.140 Nature uses polyelectrolyte bottlebrush polymers to create specific interfaces and solves the backbone stress problem by using spacing between the side chains and also spacing between the charges.141 For example, in contrast to most synthetic PEBs the charge is not placed directly on the backbone but is instead placed in the side chains offering some relief from Coulombic stress.142,143 In addition, charge stress is also minimized by avoiding placement of anionic or cationic groups on each repeat of the side chain. As a result, tethered polyelectrolyte bottlebrush polymers offer a new and interesting architecture for the investigation of polyelectrolyte rushes provided that the side arms are 4098
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of PS−PMMA mixed brushes from this Y-initiator was demonstrated and confirmed by near-edge X-ray absorption fine structure spectroscopy (NEXAFS) and AFM measurements. Another Y-shaped initiator for ATRP/RAFT brushes has also been reported.177 In general, mixed brushes are a developing aspect of brush surfaces. New chemistry and examples of mixed brushed with either a dual function initiator for combinations of initiator types have been used. The promise of the ability to make wellcontrolled phase-separated structures has been realized on a curved surface but not yet on planar surfaces. 4.5. Patterned Brushes. Patterned brushes are useful in many applications when specific dimensions of brush coverage are needed. Patterned brushes due to their surface-attachment properties may have unique response characteristics compared to other patterned polymers. Generally patterned brushes are created by either “bottom-up” or “top-down” processes, and each offers its own distinct advantages. “Bottom-up” patterning first forms patterned regions (e.g., initiator zone) and then the polymer is grafted to them. Since polymer grafting is the final chemical step, contamination is minimized. However, these patterned brushes will adopt an already stretched, lens-like shape due to the free-space presented in the grafting process and the mobility provided during synthesis.178,179 As the pattern size decreases, the interaction between brushes and surfaces become dominant and morphology is changed.180 UV photolithography has been used by Ober and co-workers to create patterned photoresist as the templates for initiator backfilling at sub-micrometer level.181,182 Alternately, e-beam lithography (EBL)180,183 has been used by Jonas et al. to create such patterns as small as to 20 nm. Another study by Ahn et al. deposited a thin gold film to create chemical contrast sites for the growth of patterned brushes.184 Even without photoresist, small molecules may also be patterned as a template for subsequent pattern formation. With the lithography process, those small molecules could be either removed to expose a reaction site185,186 or directly reacted to form the chemical group for the grafting.187−189 Steenackers et al. fabricated patterned templates on a surface with EBL which created thin layers of hydrocarbon (
