Polymer Brushes: On the Way to Tailor-Made Surfaces

Transcription

1 1 Polymer Brushes: On the Way to Tailor-Made Surfaces Jürgen Rühe Abstract In recent years, the synthesis of polymer brushes through surface-initiated polymerization reactions has received significant attention. In this overview, several different synthetic strategies for the generation of polymer brushes are reviewed. The unique physical properties of polymer brushes that arise from the covalent anchoring of the polymer chains to the solid substrate are discussed and compared to the properties of polymer layers deposited by other techniques of thin film generation. Finally, examples are provided that highlight some recent developments aimed at strategies for the functionalization of surfaces with polymer brushes, at ways of realizing smart surfaces with switchable properties, and at the generation of micro- and nanostructured polymer monolayers. 1 Growth of Polymer Molecules at Surfaces: Introductory Remarks Thin coatings applied to the surface of materials can improve the properties of objects dramatically as they allow control of the interaction of a material with its environment. This has been known more or less empirically to man for several thousand years. Lacquer generated from tree sap was used in China some 7000 years ago as a protective coating for wooden objects. Cold process coatings were also used around 3000 bc, where Egyptian ship builders used beeswax, gelatin and clay to produce varnishes and enamels and (later) coatings from pitch and balsam to waterproof their ships. The early Greeks and Romans, as well as the ancient Asian cultures in China, Japan and Korea, used lacquers and varnishes applied to homes and ships for decoration and as protective measures against adverse environmental conditions. In modern times, the coatings industry is a multi-billion dollar business and especially if the value of the protected objects is considered a very important contribution to the world economy. Today, however, the application range of coatings extends much beyond the simple decoration and protection aspects, and functional coatings have become an enabling technology in a vast variety of different high-tech Polymer Brushes. Rigoberto C. Advincula (Ed.) Copyright 2004 WILEY-VCH Verlag GmbH & Co. KGaA, Weinheim ISBN:

2 2 Polymer Brushes: On the Way to Tailor-Made Surfaces Figure 1 Schematic depiction of the growth of polymer molecules at a surface of a solid substrate through surface-initiated polymerization. areas. Fields in which such high-tech coatings are applied range from computer chips [1] and hard diskmanufacturing [2] to the use of special coatings in biomedical and aviation applications [3,4]. Accordingly, many different techniques have been developed for the generation of protective coatings, and these will be discussed further below. Surface-initiated polymerization reactions as a new pathway for the preparation of functional, high-tech coatings have recently received much attention [5,6]. This technique is based on the growth of polymer molecules at the surface of a substrate in situ from surface-bound initiators, which results in the attachment of polymer molecules through covalent bonds to this substrate (Figure 1). Polymer layers in which the polymer chains are irreversibly attached to the substrate are especially attractive for a variety of applications, as such layers can have a good long-term stability, even in rather adverse environments. For example, it poses no problem to expose surfaces with such surface-attached coatings to good solvents for the polymers without being concerned that the polymer will be either dissolved or displaced, and that the coating is more or less rapidly removed from the surface. In addition to the issue of stability, the number of functional groups present at a surface can also be greatly enhanced by connecting large polymer molecules with functional groups to the surface instead of binding the functional groups directly to that surface. Such a skyscraper approach allows high densities of functional groups to be obtained at the surface of the substrate through moving from the strictly two-dimensional arrangement of these groups present in typical surfaces to a more three-dimensional situation. An example, which illustrates such a behavior is the attachment of DNA probe molecules to surface-attached polymer chains, which can significantly enhance the sensitivity of a DNA-chip (Figure 2). Systems in which the polymer chains are attached with one end to a solid substrate are very interesting, not only from a chemical but also from a physical point of view. If the grafting density of the polymer molecules is very high, the polymer chains adopt a rather unusual conformation wherein the individual coils overlap [7 9]. Under these conditions, the polymer molecules are strongly stretched away

3 2 Coatings: From First Principles to High-Tech Applications 3 Figure 2 Fluorescence image obtained from a DNA chip based on a oligonucleotide functionalized polymer brush. The pattern and the intensity of the spots allows for the determination of the sequence of the unknown analyte-dna. from the surface and achieve a molecular shape which is far from the typical random coil conformation that polymer molecules assume in solution. Such surfaceattached films with strongly stretched chains are usually referred to as polymer brushes [10]. Polymer brushes are very interesting systems, as the strong stretching of the polymer chains leads to concurrent drastic changes in the physical properties of the systems. For unstretched polymer chains, a slight molecular deformation leads to a moderate increase of the energy stored in the system (entropy elasticity). However, when the molecules are already strongly stretched as is in the case of a polymer brush the energy penalty for the same small deformation is large. Accordingly, in all situations where the stretching of the polymer chains is of concern for example, during the shearing of such surfaces or when the film is penetrated by other polymer chains from solution very strong differences can be observed to the behavior of free coils [11 13]. Whilst systems in which polymer chains have one end tethered to a substrate appeared some years ago to be quite exotic, and significant doubts persisted that such brushes with high grafting densities could be obtained in practice, the development of methods where polymers are grown directly on the surface of a substrate by using surface-initiated polymerization has led to a large number of such systems becoming available. However, before describing more detailed aspects of surface-initiated polymerization, more general aspects of coatings will be briefly discussed. 2 Coatings: From First Principles to High-Tech Applications For a large number of chemical and physical processes both in daily life and in technical applications the bulkproperties of a material as well as the structure and composition of its surfaces determine the performance of the entire system. In order to control the interaction of a material with its environment, coatings consisting of thin organic films are frequently applied to the surfaces of these solids (Figure 3). In many cases, the coating serves simply as a barrier against a hostile envi-

4 4 Polymer Brushes: On the Way to Tailor-Made Surfaces Figure 3 Schematic depiction of use of thin polymer coatings to control the interaction of a material with the surrounding environment. ronment and allows for protection against corrosion or other chemical or photochemical degradation. Although corrosion protection is certainly the most prominent aspect of surface coatings as far as market and materials volumes are concerned, thin organic coatings are also applied in a large number of high-tech applications, ranging form microelectronics [1] to biomedical devices [3,4]. When considering such applications, thin organic coatings are applied to control the interactions between the material and its environment. Examples of interface properties which can be controlled by deposition of a thin organic film onto a surface include friction [11,13 17], adhesion, adsorption of molecules from the surrounding environment, or wetting with water or other liquids. In medical applications, coatings allow control of the interaction of biological cells and biomolecules with artificial materials in order to enhance the biocompatibility of an implant, or to avoid the nonspecific adsorption of proteins onto the active surfaces of an analytical device [18]. It is well known that coatings, even when only a few Angstroms thick, can influence the surface properties of a material so strongly that the chemical nature of the underlying material becomes completely hidden and the interaction of the whole system with the surrounding environment is governed by these extremely thin coatings ( stealth effect ). This is an advantageous situation for materials engineering as it allows optimization of the bulkand surface properties of a material separately from each other. In addition, the application of functional coatings allows the coverage of a surface with groups which interact with other molecules in their environment through specific molecular recognition processes. Such a strategy is, for example, very important for the control of the adhesion of biological cells to artificial substrates. In such a case, thin layers containing cell recognition peptide sequences can induce strong adhesion of the cells to the substrate surfaces, to which they otherwise would show only a very unfavorable adhesion behavior [19]. One example of a system where the covering of a surface with an ultrathin coating is a prerequisite for that system to function is a computer hard disk[2] (Figure 4). If the uncoated surface of a thin film magnetic diskis subjected to strong shear, such as the sliding of a read/write head on the disksurface, then almost instantaneous damage can be observed. The diskshows, even upon the first contact with the head, a strong stick-slip behavior and a high friction coefficient, while the debris

5 2 Coatings: From First Principles to High-Tech Applications 5 a) b) Figure 4 (a) Computer hard disks are protected against mechanical wear by ultrathin layers of perfluorinated polymers. (b) Hard disk in an accelerated wear test: (i) unlubricated and (ii) after application of 1.5 nm chemisorbed and 1 nm physisorbed lubricant; the high friction coefficient and the strong noise indicate a strong stick-slip behavior, which is the beginning of a catastrophic failure of the system. (Reprinted with kind permission from Ref. [2]; American Society of Mechanical Engineering, 1996.) generated by this damage leads to rapid failure of the disk. However, if a film of a perfluoropolyether of typically only 2 4 nm thickness is attached to the disk, the tribological properties are greatly improved, the wear is reduced, and the mean time to failure of the diskis greatly prolonged (Figure 4). A second example where ultrathin organic coatings control the performance of the whole system is the control of interface properties of materials in contact with blood. If artificial materials are brought into contact with blood, then blood proteins such as fibrinogen adsorb very rapidly to the surfaces of the implant or sensor surface, followed by the adhesion of blood cells to these protein layers. This reaction cascade leads almost immediately to strong changes of the surface composition of the active surfaces of the sensor or implant. After a short period of time, blood clots Figure 5 SEM image of a fibrin network and thrombocytes on the surface of an artifical heart value (picture courtesy of Dr. A. Schlitt, University Hospital Mainz, Germany).

6 6 Polymer Brushes: On the Way to Tailor-Made Surfaces become attached to the surfaces of the blood-exposed materials (Figure 5). Eventually, the blood clots can breakoff from the surface into the blood stream, where they pose a life-threatening situation for the patient [18]. It has been shown, that the application of just a polymer layer which is just a couple of nanometers thickcan dramatically reduce the adhesion of the blood proteins and thereby greatly improve the blood compatibility of the material [20 22]. 3 Surface-Coating Techniques Depending on the type of interaction between the molecules which are constituents of the coating and the substrate which is to be modified, two classes of strategies for the deposition of thin organic coatings can be distinguished. In one of these, the molecules interact with the substrate by physical forces [7 9], whilst the other class consists of molecules which are attached to the surfaces through chemical bonds. In the latter case, a monomolecular layer or a surface-attached networkis very strongly ( irreversibly ) attached to the surface. This classification is not simply a formality, but the type and accordingly the strength of interaction also has a very strong influence on the physical properties of the coating, the film thicknesses which can be obtained through such a method, and the long-term stability of the coating in problematic environments. A number of technologically important coating techniques rely on physical interactions between the deposited molecules and the substrate, including:. painting/droplet evaporation. spray coating. spin coating. dip coating. doctor blading Although being quite different in detail, a common feature of all of these processes is that the molecules are deposited from solution and the solvent evaporates during the coating process (Figure 6). The techniques described above are somewhat empirical in nature, as certain parameters such as the rate of evaporation of the solvent depend on specific details of the individual process and are accordingly difficult to predict a priori, but in many cases are simple to reproduce. Accordingly if the deposition conditions are properly controlled, layers with well-defined thickness and good homogeneity can be generated without major effort. Several of these processes, such as dip- and spin-coating, allow the deposition of extremely thin film coatings (starting from just a few nanometers thickness), but essentially no upper limit to film thickness exists, if appropriate conditions are applied. In contrast to these rather empirical processes, more sophisticated coating techniques have been developed, including the Langmuir-Blodgett technique [23], the adsorption of monomolecular layers of homo- and blockcopolymers [7] from solution, and the Layer-by-Layer (LbL) [24] technique in which multilayer stacks of oppo-

7 3 Surface-Coating Techniques 7 a) b) c) Figure 6 Schematic illustration of different processes used for the deposition of organic molecules and/or polymers on surfaces: (a) spin-coating. (b) Langmuir-Blodgett-Kuhn (LBK) technique; (c) adsorption from solution. sitely charged polyelectrolytes are deposited onto a (charged) substrate. These techniques allow for much better control of the internal structure of the deposited layers, and also for extremely high precision with regard to the thickness of the coatings. All of the coating techniques, except perhaps for the Langmuir-Blodgett technique, are from a technological viewpoint rather simple, and the generation of layers typically requires no complicated set-ups to generate the coatings. The molecules are attached to their substrates by physical interactions, and consequently the forces holding them at the surface are rather weak. In some cases this situation is desirable, but in others it becomes problematic as it is more likely to lead to adhesive failure of the system. Under unfavorable conditions, the films can be subject to destruction by the Big Four Ds :

8 8 Polymer Brushes: On the Way to Tailor-Made Surfaces. desorption during solvent exposure;. displacement by molecules which have stronger interaction with the surface;. dewetting (for films above the glass transition temperature, T g ); and. delamination (for films below T g ). Desorption and displacement are especially important, as coatings are usually not prepared and kept under ideal (i.e., ultrahigh vacuum) conditions, but rather are exposed to environments containing all sorts of contaminants. In these real-life environments, contaminants are present on every surface, and/or competing adsorbates will fight for surface sites during or after the coating process. Examples of molecules which are present in many different environments, and which typically compete quite efficiently with coating materials for surface sites, include water, ions, polyelectrolyte molecules, or oils. The contaminants or displacing agents might have such a strong interaction with the surface that the molecules of the coating can no longer remain in contact with the substrate, but eventually will instead be located on top of a thin layer consisting of contaminant/displacer molecules. In this respect, polar surfaces which absorb ambient water are especially problematic, as water has a strong affinity for such surfaces and exhibits a very high adsorption enthalpy. Accordingly, water functions as a very efficient competitor for surface sites and easily displaces adsorbed molecules from such high-energy surfaces. What renders the situation even worse is that under these conditions the surface properties of the material become strongly dependent on the history of the sample that is, which environment the sample has been exposed before use and this is, potentially, a very problematic situation. Dewetting occurs in all systems, where the surface tension of the substrate is lower than that of the coating material if the molecules are allowed to reach an equilibrium. This may be achieved either by heating the film above T g, or by exposure to molecules which can act as a plasticizer for the polymers of the coating. In contrast, delamination occurs if the films are in the glassy state and subjected to wide temperature swings, or if the coating swells in the environment to which it had been exposed, while the substrate does not swell. In such cases, strong mechanical stress develops at the interface, and this may cause the entire film to peel off, leading to large-scale adhesive failure. An alternative to the above-mentioned procedures which allows improvement in the long-term stability of coatings even in very adverse environments, is to attach the molecules of the coating to the surface of the substrate through chemical bonds. The price which must be paid for an enhanced stability of the system is a more complicated coating procedure and/or the requirement to choose the coating conditions more carefully, so that the surface reaction proceeds in high yield and with limited side reactions. A current, very frequently employed strategy for the preparation of well-controlled surface layers is the use of small molecules with a reactive head group that is amenable to form a covalent bond with a corresponding chemical moiety on the surface of the substrate, which is to be modified. As this process is selflimiting that is, the surface-attachment reaction stops when all the reactive surface groups have been consumed or are no longer accessible such layers are commonly

9 3 Surface-Coating Techniques 9 Figure 7 Schematic of the self-assembly process and examples of anchor groups used for the modification of surfaces with selfassembled monolayers (SAMs) of organic molecules. called self-assembled monolayers (SAM) [25]. Examples are silanes on oxide surfaces, phosphates or phosphonate on metal(oxide)s, and thiols or disulfides on noble metal surfaces (Figure 7). In this way, surface coatings can be obtained which are very stable and may even have a strong degree of positional and orientational order. In some cases, even crystalline packing of the surface-attached molecules has been observed. If molecules are assembled that carry at their tail end a specific chemical moiety or a biochemically active group, it is possible to obtain a more or less strict 2D arrangement of these functionalities (Figure 8) [26]. Examples are molecules which contain fluorocarbon segments in the assembling units [27 29], and can convert a hydrophilic surface into a highly water-repellent hydrophobic one, or the introduction of ligands as recognition sites in bio-affinity assays. In this way, surfaces can be generated for example, on top of the transducer of a biosensor that very specifically bind proteins from solution [30,31]. Figure 8 Example of a structure prepared via soft lithography from acid and methyl-terminated thiols. First, the Me-terminated thiol was stamped onto a gold surface; second, the unmodified areas were backfilled from a solution containing the acid-terminated thiol. (Reprinted with kind permission from Ref. [26]; Wiley-VCH, 1998.)

10 10 Polymer Brushes: On the Way to Tailor-Made Surfaces In some of these applications the intrinsic limitations of this strictly 2D arrangement of the functional groups are evident: the maximal surface density of the functional moieties is limited by the surface area cross-section of the assembled unit. In some cases it is even lower than the arrangement of the individual functional units at such high packing densities in some cases leads to a mutual blocking or, at least, to a limited accessibility. One obvious solution to the above problem is the extension into the third dimension that is, the use of polymers carrying the functional groups along the chain, thus generating higher cross-sectional densities of these groups and simultaneously guaranteeing good accessibility. 4 Surface-Attached Polymers Most approaches which aim at attaching polymers to a surface use a system where the polymer carries an anchor group either as an end group or in a side chain. This anchor group can be reacted with appropriate sites at the substrate surface, thus yielding surface-attached monolayers of polymer molecules (termed grafting to ) (Figure 9) [32 37]. While the attachment of terminally functionalized polymers to the surface leads to layers, where one group is connected to the surface, side chain attachment usually leads to multiple attachment points and, accordingly, a rather flat conformation of the polymer molecules. In the latter case, the functional groups of different molecules compete for reactive sites on the surface, and accordingly the amount of polymer which can be immobilized depends strongly on the reaction conditions, and especially on the concentration of the polymer in solution. This chemical linking of polymers to a substrate surface is, in principle, closely related to the formation of self-assembled monolayers of low molecular-weight compounds described above. Accordingly, if such (end)functionalized polymers are avail- a) b) c) Figure 9 Schematic illustration of different processes used for the attachment of polymers to surfaces: (a) grafting to ; (b) grafting via incorporation of surface-bound monomeric units; (c) grafting from/surface-initiated polymerization.

11 4 Surface-Attached Polymers 11 able (which is a nontrivial condition, as the synthesis of polymers with reactive end groups is far from being trivial), the attachment of the polymers is, from a chemical point of view, rather simple. Another straightforward technique for the attachment of polymers to surfaces which allows the generation of a great variety of functional surfaces is to carry out a polymerization reaction in the presence of a substrate onto which monomers had been attached [32,38 41]. In such a polymerization reaction, the surface-attached monomers are incorporated into growing polymer chains in the very same way as their peers in solution (Figure 9). However, once one or more surface-attached monomers are incorporated, the polymer is glued firmly to the surface. During the process, a macroradical initially attacks the monomers on the surface, while in a second step further monomers units are added, so that the chain grows again, away from the surface. However, careful studies of the polymerization mechanism have shown that the grafting to step represents the bottle-neckof the reaction and thus limits the polymer immobilization [42,43]. Accordingly, very similar layers are obtained by using immobilized monomers as by the chemisorption of preformed chains. A general limitation of the technique is that the substrate must be immersed in a polymerization solution, but if this poses no problem it is one of the simplest techniques to generate surface-attached layers, especially as there is no need to synthesize a polymer functionalized with an anchor group. Although grafting to reactions are easy to perform, it should be noted that certain rather strict limitations apply to the structures which can be realized by use of a grafting to strategy. First, the use of reactive anchor groups for the surface-attachment of polymers imposes some rather strict limitations on the choice of functional groups available for incorporation into the polymer. One of the reasons for this is that the functional groups on the polymer can compete with the anchor moieties for surface sites. Especially if the aim is to immobilize functional polymers containing highly polar or charged groups onto polar surfaces, the adsorption of functional groups to the surface can be very strong and compete very effectively with the chemisorption process. Such competition between anchor and functional groups has been observed, for example, in the case of the attachment of a low molecular-weight alkoxysilane containing amine groups to a silicon oxide surface [44 46]. In such a system, interactions between the basic amine groups of the SAM-forming silane and the rather acidic silanol groups of the silicon (oxide) substrate can strongly compete with the condensation reaction of the alkoxysilyl moiety with the substrate silanol groups. As result, layers are obtained which contain both physisorbed molecules due to acid base interactions and chemically attached molecules. Second, in order to obtain a fast and complete surface attachment reaction with a high surface density of chains covalently bound to the substrate, rather reactive anchor groups are required. These groups, however, tend not to tolerate the simultaneous presence of a large variety of functional groups in the polymer. For example, if a chlorosilyl group is chosen as an anchor group for the attachment of the polymer to a silicon oxide surface, this choice excludes the incorporation of many functional groups into the polymer, including amine-, hydroxyl- or carboxylic acid moieties, as these would react with the chlorosilyl groups.

12 12 Polymer Brushes: On the Way to Tailor-Made Surfaces At first view, there is a tendency to consider that resorting to less reactive anchor groups for example, using a less reactive alkoxysilane instead of a highly reactive chlorosilane would solve the problem. This, however, is incorrect as a more indepth analysis shows. If the nucleophilicity of the anchor group is reduced, this affects both the undesired side reaction, which is the reaction of the functional group with the anchor group, and which leads to loss of anchor moieties, and the desired reaction of the anchor group with a group at the surface of the substrate, which results in a successful chemisorption reaction. Accordingly, both reactions are slowed down at the same time, and the ratio between the rates of the two reactions remain the same in all cases. Another complication inherent to grafting to processes is an intrinsic limitation of the film thickness, and accordingly the number of functional groups per surface area which can be obtained by using such an approach. Films generated by chemisorption from solution are limited to (dry) thicknesses of typically 1 to 5 nm. This limitation has both kinetic and thermodynamic origins. With increasing coverage of the surface with attached chains, the polymer concentration at the interface quickly becomes larger than the concentration of polymers in solution. Additional chains, which are to become attached to the surface, must diffuse against this concentration gradient that ever increases with increasing grafting density of the attached polymer (Figure 10). This diffusion slows down the immobilization reaction at the surface further and further as the reaction proceeds. Thus, the rate of the attachment reaction levels off rather quickly and further polymer is linked to the substrate only at an extremely slow rate due to this kinetic hindrance. Indeed, it has been shown [42,43] both theoretically and experimentally that once the surface-attached coils overlap, the attachment of further polymer molecules takes place on a logarithmic time scale, and already at rather low graft density time frames of thousands or even millions of years would be required to add a few more nanometers of polymer to the layer. Accordingly, as far as practical reaction times are concerned, films generated by this technique are intrinsically limited with regard to the film thickness. Further- a) b) Figure 10 Schematic illustration of the grafting to process. (a) Chains that are to be attached to the surface can easily reach the surface at low graft densities. (b) The attachment process comes to a virtual halt as soon as the surface is covered with polymers, as the already attached chains form a kinetic barrier against which incoming chains have to diffuse to reach the surface.

13 5 Polymer Brushes: General Features 13 more it should be noted that, even if this kinetic limitation is somehow circumvented, the attachment of chains to a strongly covered surface becomes unfavorable also for thermodynamic reasons. At high grafting densities the surface-attached polymer chains are in a rather stretched conformation due to the presence of strong segment segment interactions, as will be discussed in more detail below. A chain, which is now becoming attached to the surface, must change from a coil conformation in solution to a stretched ( brush-like ) conformation at the surface. The entropy loss during this process, however, is only compensated by the establishment of one chemical bond, namely the one connecting the polymer to the surface. Hence, the higher the graft density of the chains at the surface, the stronger will be the entropy penalty, and this rapidly precludes the attachment of further chains. 5 Polymer Brushes: General Features As mentioned briefly above, the term polymer brush refers to a system in which chains of polymer molecules are attached with one or with a few anchor points to a surface in such a way that the graft density of the polymers is high enough that the a) b) c) Figure 11 Artist s perception of the terms (a) mushroom, (b) pancake and (c) brush used for the different possible conformations of surface-attached polymers.

14 14 Polymer Brushes: On the Way to Tailor-Made Surfaces surface-attached chains become crowded and are stretched away from the surface (Figure 11). From the stretching of the polymer chains perpendicular to the surface, several new physical phenomena arise. Examples are ultralow friction surfaces [11,12] obtained through coating of two surfaces that slide against each other with polymer brushes, or the so-called autophobic behavior [47 50], in which materials coated with surface-attached polymer chains do not become wetted by free polymer, even if the surface-attached and the free chains are chemically identical (Figure 12). Figure 12 Optical micrograph of a dewetted polystyrene layer (initial thickness 60 nm) on top of a polystyrene brush (6 nm; prepared via grafting from). This picture was taken after annealing the sample for 40 h at 180 C (scale bar = 200 lm). (Reprinted from Ref. [50], with kind permission; American Chemical Society, 1996.) In the following discussion, the focus will be placed on polymer brushes at solid surfaces, although brush-like chain conformations can also be obtained at the boundary between phases in blockcopolymers [8] or in so-called molecular bottlebrushes [51 53]. In the latter system, polymers are attached as side chains to the backbone of a polymer molecule, so that every segment of the backbone carries such a polymeric side chain. Although the overall physical picture for the different systems is very similar, here only chains attached to solid surfaces at one end will be described and discussed. When polymer molecules are tethered to a surface, two basic cases must be distinguished depending on the graft density of the attached chains [8 10]: 1. If the distance between two anchoring sites is larger than the size of the surface-attached polymers, the segments of the individual chains do not feel each other and behave more or less like single chains nailed down onto the surface by one end. Depending on the strength of interaction of the polymer segments with the surface, again two cases must be distinguished [10]. If the interaction between the polymer and the surface is weak(or even repulsive), the chains form a typical random coil that is linked to the surface through a stem of varying size. For such a situation, the term mushroom conformation has been coined (Figure 11). However, if the segments of the surfaceattached chains adsorb strongly to the underlying surface, the polymer molecules obtain a flat, pancake -like conformation (Figure 11). 2. A completely different picture is obtained if the chains are attached to the surface at such short distances between the anchor points that the polymer molecules overlap. In this case, the segments of the chains try to avoid each other as much as possible and minimize segment segment interactions by

15 6 Theory of Polymer Brushes 15 stretching away from the surface (Figure 11). This chain stretching, however, reduces the number of possible polymer conformations, which is equivalent to a reduction in the entropy of the chains. This loss of entropy gives rise to a retracting force trying to keep the chains coiled, as occurs in a stretched piece of rubber. Thus, a new equilibrium at a higher energy level is obtained in which the chains are stretched perpendicular to the surface. 6 Theory of Polymer Brushes The theoretical description of polymer brushes attached to surfaces of different topologies that is, planar and curved surfaces is well developed [7 9]. However, as in this bookthe main focus is set on new developments concerning the chemical methodology, only a very brief outline of the theory of brushes is provided here. For a more detailed discussion, the reader is referred to reviews recently published on this subject [7 9]. The key idea behind the theoretical description of polymer brushes is that the free energy F of the chains is obtained from a balance between the interaction energy between the statistical segments F int and energy difference between stretched and unstretched polymer chains F el (elastic free energy) caused by the entropy loss of the chains: F = F int + F el (1) The most important parameters, which are of interest for a description of brush systems, are the segment density profile (u(z)) of the surface-attached chains and/or the brush height h as a function of the graft density r, the molecular weight (/degree of polymerization) of the surface-attached chains, and the solvent quality of the contacting medium (Figure 13). The first description of such a brush system has been attempted by Alexander [54] for monodisperse chains consisting of N segments, which are attached to a flat, non-adsorbing surface with an average distance of the anchor points d much smaller Figure 13 Two hundred chains of a polymer brush (chain length N = 100) under good solvent conditions. (Reproduced with kind permission from Ref. [11]; Springer, 1998.)

16 16 Polymer Brushes: On the Way to Tailor-Made Surfaces Figure 14 Schematic illustration of the Alexander model for the theoretic description of polymer brushes. The chain segments with the blobs (indicated by the circles) behave as random ( Gaussian ) coils. (d represents the average distance between anchor points.) than the radius of gyration of the same unperturbed chains not in contact with the surface (Figure 14). If both the interaction energy resulting from binary monomer monomer interactions and the elastic energy of a Gaussian chain are calculated and minimized in respect to the brush height h, the following equation is obtained for brushes in a good solvent: h ~ N r 1/3 (2) In a poor solvent that is, close to H conditions the exponent describing the influence of the grafting density is slightly different and h ~ N r 1/2 (3) is obtained. It should be noted, that in both cases the brush height scales linearly with the degree of polymerization/molecular weight of the polymer molecules, which is a much stronger dependency than that of the size of a polymer coil in solu- Figure 15 Schematic illustration of segment density profiles for surface-attached polymers in different regimes. For details, see the text (adopted from [9]).

17 6 Theory of Polymer Brushes 17 tion on the molecular weight, where the radius of gyration R g, scales with R g ~ N 0.59 for a polymer in a good solvent and R g ~ N 0.50 for solutions close to H conditions. Although the Alexander model is very simple, it predicts the experimentally observed scaling behavior more or less correctly and allows an understanding of some of the most striking properties of polymer brushes, such as lubrication and the wetting behavior. More sophisticated models have been developed to describe the segment density profile of the brushes (Figure 15). To this numerical and analytical self-consistent field (SCF), theories [55 57] for such systems have been proposed based on the assumptions that, for strong stretching and high molecular weights of the brushes, fluctuations around the most favorable configuration of the polymer chain diminish. A general result of the SCF calculations is, that the segment density profile is more or less parabolic as long as the grafting density is moderate and the molecular weight of the brush chains is high. At very high grafting densities the SCF assumptions are no longer valid, as three body interactions between the polymer segments become significant. The results of the SCF calculations have been verified both experimentally and in simulations. For the latter, molecular dynamics and Monte Carlo methods have been employed [58]. If smaller differences in the numerical coefficient are neglected, then the SCF results are in good agreement with the results from simple scaling arguments. In addition to these somewhat straightforward calculations, more complicated situations have also been tackled where the polymer chains have a distinct polydispersity [59], are in specific topologies such as attached to small particles [60], which exhibit a significant curvature also on the molecular scale, and to brushes which carry charges along the polymer chain [61] (Figure 16). In particular, the latter case can become very complicated if the polymer chains interact specifically with ions in the surrounding medium, as under these circumstances the situation can no longer be described by simple mean field approaches, but specific complex formation and (local) changes in the solubility of the polymer play a key role in describing the swelling behavior of such brushes. Figure 16 Schematic illustration of a polyelectrolyte brush (PEL brush).

18 18 Polymer Brushes: On the Way to Tailor-Made Surfaces 7 Synthesis of Polymer Brushes An obvious requirement for forcing polymer molecules into brush-like conformations is that the strength of anchoring of the molecules to the interface is sufficiently high that the molecules are connected irreversibly to the surface of the substrate. A second requirement is that the synthetic strategy allows for the generation of grafting densities high enough to cause sufficient repulsive segment segment interactions within the surface-attached chains to induce significant chain stretching. In particular, the latter condition imposes some strict limitations onto the appropriate synthetic strategy for brush formation as the chains lose a considerable amount of entropy when stretched into an elongated form. In the following section, four different approaches to reach these goals will be briefly discussed. A complete review of the published literature on this subject would clearly be beyond the limits of this introductory chapter. Approach 1 In the first approach, amphiphilic blockcopolymers consisting of a water-soluble blockand a water-insoluble blockare spread at the air-water interface [62,63]. The water-soluble blockattempts to dissolve into the aqueous subphase, but is anchored to the air-water interface by the hydrophobic block. Upon compression of the thus obtained Langmuir monolayer, the distance between the anchor points of the polymer chains decreases and the hydrophilic blockis stretched away from the surface into the aqueous subphase. A prerequisite for this is that the hydrophobic blockis in the molten state, because only is it possible for a rearrangement of the chains within the film to occur upon compression. Furthermore, it is important that the hydrophilic balance is chosen in such a way that the loss of chains to the subphase and the formation of micelles can be avoided. The thus obtained films can be crosslinked through photochemical reactions and transferred to a solid substrate. Approach 2 In the second case, blockcopolymers or end-functionalized polymers are physisorbed to a solid surface [7]. End-functionalized polymers can be discussed together with blockcopolymers as they are structurally very similar to such systems in terms of their essential physics of adsorption to a solid surface. In some ways they can be viewed as blockcopolymers with a very short block, consisting only of one unit. In the blockcopolymer concept, one blockadsorbs strongly at the surface and acts as an anchor for the polymer chains. The other blockadsorbs only weakly at the surface that is, the interactions of the polymer with the solvent are stronger than those with the surface and so the blockfloats in the solvent like a buoy. Although during the past, many different polymer layers have been prepared by this route, the

19 7 Synthesis of Polymer Brushes 19 chemical variability of these systems is somewhat limited as a solvent must be available in which the blockcopolymer adsorbs to the surface without formation of micelles either in solution or at the surface. Furthermore, as the layer formation requires the diffusion of polymer molecules through the layer of already attached chains, this limits the range of graft densities that can be obtained using this technique. In addition, as the interaction with the surface is based simply on physical interactions, anchoring of the molecules to the substrate surface is relatively weak, and this further limits the graft densities available and decreases the stability of the films. Approach 3 As has been discussed above, the chemisorption of polymer molecules leads to chains which are covalently attached to surfaces [32 37]. Although situations can be envisioned in which the polymer chains are slightly stretched, such processes are strongly limited in terms of the obtainable graft density, especially for high molecular-weight polymers, and this results in only relatively weakstretched polymer chains. Approach 4 Much higher graft densities can be obtained when the polymer chains are grown at the surface of the substrate in situ (Figure 17) [5,6]. To this initiator, species are either generated or self-assembled at the surface of the substrate, followed by initiation of chain growth from these surface-attached initiators, for example by controlled or free radical chain polymerization. The surface-polymerization can be started thermally either through a chemical process or photochemically. In this way, polymer monolayers with film thicknesses of more than 2000 nm in the dry state have been obtained (Figure 18). In this case, polymer molecules with number aver- Figure 17 Common synthetic strategy for the generation of polymer brushes via surface-initiated polymerization. An initiator molecule is deposited on a surface by means of a self-assembly process via the reaction of an anchor group to suitable surface sites and, subsequently, chains are grown on the surface from the initiating sites.

20 20 Polymer Brushes: On the Way to Tailor-Made Surfaces a) b) Figure 18 (a) Optical waveguide spectrum (symbols) obtained from a PMMA brush deposited on an evaporated SiO 2 layer. The solid line was obtained from model calculations based on a Fresnel formalism assuming a 2200 nm-thick polymer layer. The sample was prepared in neat MMA at 50 C, polymerization time: 96 h. (b) Thickness of PMMA brushes as a function of monomer concentration; polymerizations were carried out at 60 C for 18 h in toluene as a solvent (if required). age molecular weights of several 10 6 g mol 1 are attached at distances of anchor points of less than 3 nm. Surface-initiated polymerization reactions workfor any polymer which can be obtained by a chain growth reaction such as free and controlled radical polymerization, carbocationic polymerization, anionic polymerization, and ring-opening metathesis polymerization (Table 1) [63 97]. The different polymerization reactions can be carried out on surfaces of very different topologies (planar, curved, and irregular surfaces), and allow for the generation of polymers from a wide spectrum of different monomers. It would be far beyond the scope of this overview to try to review all recent developments on the synthesis of such systems, and a large variety of different synthetic routes for the generation of polymer brushes through surface-initiated polymerizations will be detailed in the following chapters. However, at this point some comments should be made on controlled or living polymerization reactions for the growth of polymer molecules through surface-attached initiators. In this respect, liv-

21 7 Synthesis of Polymer Brushes 21 Table 1 Selected systems for the generation of polymer brushes via surface-initiated polymerization. The list is by no means exhaustive, and is only meant to demonstrate the wide variety of synthetic strategies that have been developed over the past decade. Mechanism Initiator/initiating species Maximum thickness (nm) Reference(s) Free radical Me O Si Me H N O CN N N Me Me CN CO 2 H ~ 100 nm 63,64 Me O Si Me O O CN Me N N Me Me CN up to 2200 nm S (CH 2 ) 11 O O CN Me N N Me Me CN O Me Si Me CN N N CN Me O O O O OH n.a. 73,74 TEMPO Me O Si Me O O N 120 nm 75,76 ATRP Si (CH 2 ) n O O Br 150 nm; 700 nm (water accelerated) O Si (CH 2 ) 2 SO 2 Cl 100 nm Me O Si (CH 2 ) 2 CH 2 Cl n.a Me Me O Si Me Me O Br O < 60 nm 82,90 Others Various systems for cationic and anionic polymerizations, RAFT and reverse ATRP < 40 nm NA = not applicable.

22 22 Polymer Brushes: On the Way to Tailor-Made Surfaces ing systems with rapid initiation are of major interest as they allow, in principle, surface-attached polymer chains with relatively narrow molecular weight distributions to be obtained. This facilitates comparison with theoretical models developed for surface-attached polymer brushes, provided that the initiation process is sufficiently efficient to allow high graft densities and that the molecular weight of the surfaceattached chains is high enough to allow such a discussion. Indeed, controlled polymerization approaches are expected to become even more interesting for the synthesis of surface-attached polymer brushes, as a large variety of functional brushes can also be obtained by using these methods. At present, major efforts are made especially in the area of controlled radical polymerization to polymerize functionalized monomers to create high molecular-weight compounds with low polydispersity. 8 Polymer Brushes as Functional Materials For many applications of polymer brushes, it is not simply protection against mechanical or chemical damage that is important. Rather, where the polymer layer acts Figure 19 Examples of functional groups incorporated into polymer brushes.

23 8 Polymer Brushes as Functional Materials 23 as a barrier against contact with the environment, a more specific chemical response to the surrounding medium is desirable. Examples of this situation include layers into which DNA, protein molecules or complexing agents each of which shows a specific reaction towards certain metals are chemically incorporated [99]. To this end, polymers with desired functional groups can be formed directly from the corresponding monomers (Figure 19). For example, brushes carrying either charges ( polyelectrolyte brushes ) [71,74,75, ] or pendant mesogenic units ( LCbrushes ) [103,104] have been prepared using this direct route. An alternative would be first to generate a brush from a simple and inexpensive precursor monomer containing a reactive group, and this can then be transformed into the final moiety through a polymer analogous reaction. Examples of such compounds are monomers carrying an active ester, epoxide, azalactone or amine groups [99]. It is quite evident that, in principle, the direct approach is much simpler as the desired brush can be prepared in a one-step reaction. However, this places some rather stringent requirements on the availability of the monomer, because if an incorporation of repeat units with especially valuable groups into the polymer is desired, then the amount of the valuable monomer needed for the brush generation is rather large. The reason for this is that the molecular weight of the brushes is, for most polymerization mechanisms, directly connected to the monomer concentration; consequently, if high molecular-weight polymers are desired, then relatively large amounts of monomer are required. A second requirement is that the functional group is compatible with the polymerization process used for brush formation. Monomers containing moieties that show excessive transfer properties such as sulfur groups cannot be used in direct polymerization processes as they would lead to side reaction and/or only low molecular-weight brushes. This is especially important, as for a surface-initiated polymerization reaction any chain transfer is equivalent to a termination reaction, because after the transfer further polymer is only generated in solution and removed in a subsequent extraction of the film. In addition to this, a two-step pathway for the generation of functional brushes has the advantage that it is not necessary to study the polymerization behavior of each new monomer with a new functional group from scratch because a number of different functionalities can be incorporated using the same precursor monomer. Examples are brushes of homo- or copolymers with N-hydroxysuccinimide ester or epoxide groups through which a large variety of different functionalities can be introduced by aminolysis. For example, the preparation of brushes that carry thiol, pyrene, oligoethyleneoxide or bioactive groups such as peptides or oligonucleotide units have been reported using the same precursor monomer. If the direct polymerization procedure is applied, then each and every one of these monomers must be studied with regard to the polymerization kinetics in order to obtain an in-depth understanding of the brush-forming properties. The use of living polymerization reactions that is, reactions where the number of active or dormant and thus potentially active species remains more or less constant on the time scale of the polymerization reaction allows the generation of brushes which carry at the end pointing away from the surface a functional group, or brushes which consist of a copolymer [96,98,105,106]. The latter constitute a very

24 24 Polymer Brushes: On the Way to Tailor-Made Surfaces interesting system, as all polymer molecules are surface-attached and accordingly large-scale, irreversible reorganizations of the chains are prohibited, and the morphology of the polymer film is directly coupled to the composition of the copolymer brush. Thus, upon exposure to an environment which is selective for one of the two components the morphology of the layers of such copolymer brushes can be easily switched from one morphology to the other, and monolayers with very unusual topographies can be obtained. Another interesting system is generated if not all of the initiator is used up during the polymerization reaction, or if two different initiators are co-immobilized on the substrate surface. In such a case, after completion of the growth of one polymer species, some initiator is still present which can be used to kick-off the polymerization of another monomer [105,107,108]. This then results in the growth of a second type of polymer in direct neighborhood to the chains already attached to the surface. Such systems which commonly are called mixed brushes seem especially attractive as the two polymers can have very different interaction strengths with the surrounding of the film. This situation is very similar to that of blockcopolymer brushes described above. If one environment strongly prefers one polymer over the other, whilst a second environment favors the reverse situation, surfaces with switchable surface chemistries are generated. When the polymer layer is alternately exposed to one or the other environment, the internal structure of the polymer changes accordingly and a system that can adapt to the substrate environment ( smart surfaces ) is obtained. 9 Microstructured Polymer Brushes The (micro-)patterning of polymer brushes is especially interesting as all the polymer molecules are permanently attached to the surface [73,109]. This is an important aspect, both for the generation of the patterns as well as for applications of the microstructured surfaces, as it allows exposure of the microstructures to good solvents for the polymers. The latter aspect is especially important for biological applications, as it allows strong swelling of the brush and provides a soft cushion for the biological system at the surface of the substrate. This is of special significance as proteins tend to denature in contact with hard, solid surfaces. Also, from the viewpoint of preparing microstructured systems, the generation of thick, surfaceattached monolayers is rather attractive as it allows the washing away of reagents after completion of a chemical reaction in the patterned structures, and hence the generation of multifunctional chemical patterns with high resolution (Figures 20 and 21). Indeed, in addition to simple chemical structures being written into the film, the use of step-and-repeat procedures allows the generation of very complicated chemical surfaces and structures. This contrasts strongly with the conventional lithographic procedures used in the semiconductor industry where, upon irradiation and solvent exposure, a relief is generated and hence topological rather than chemical structures are generated on the surface of the substrate.

25 9 Microstructured Polymer Brushes 25 a) b) Figure 20 (a) Process used by Hawker et al. for the generation of polymer brushes with spatially resolved properties. A poly(t-butyl methacrylate) brush is covered with a photoresist containing a photoacid generator. Upon illumination of the sample through a mask, protons are generated in the illuminated areas. The protons diffuse into the underlying brush and hydrolyze the ester groups. (b) Illustration of the different wetting properties of a sample prepared as described in (a). The water on the sample only wets the illuminated areas that is, the areas in which the chains were transformed to a poly-(methacrylic acid). (Reprinted with kind permission from Ref. [77]; American Chemical Society, 2000.)

26 26 Polymer Brushes: On the Way to Tailor-Made Surfaces a) b) c) Figure 21 System used by Carter, Hawker et al. for the tuning of the feature size on nanostructures via a combined process consisting of (a) nanoimprinting and (b) surface-initiated polymerization from initiator sites ( inimers ) embedded into the mold; the AFM and SEM images shown in (c) demonstrate the feature size of lines after nonoimprinting (A,B) and after the subsequent surface-initiated polymerization (C,D). (Reprinted with kind permission from Ref. [111]; American Chemical Society, 2003.) In principle, three different strategies can be followed for the generation of chemically micropatterned brushes, besides the trivial photoablation of the polymers by deep UV-irradiation: 1. Deposition of the initiator in a patterned fashion and/or spatially addressed deactivation of a complete initiator monolayer. 2. Spatially controlled growth of the polymer molecules through local addressing of the initiator and/or confinement of the monomer access. 3. Spatially addressed chemical transformations of precursor brushes.

27 9 Microstructured Polymer Brushes 27 In the first case, the initiator is deposited (by inkjet printing or stamping) in certain areas of the substrate [110,111]. In a subsequent reaction step, the polymer is generated through growth of the polymer chains. In further reaction steps, initiator can be deposited in other, still uncovered areas of the substrate. Alternatively, a complete initiator monolayer can be formed and in selected areas the initiator deactivated or photochemically destroyed and evaporated, followed by growth of the brushes. In the latter case (photoablation of the initiator), new initiator can be attached to the substrate in the thus obtained blankareas, either directly or after a short etching process. In the second case, the surface-attached polymer chains can be generated through photopolymerization reactions or other means, spatially to kick-off the polymerization reaction. An alternative, in which many different polymers can be formed in a) b) Figure 22 (a) Schematic description of the l-stamping process used for the spatially resolved deposition of laminin to a brush containing active ester groups. (b) Neuronal cells aligning along the laminin grid deposited via this process.