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POLYMER BRUSHES Introduction Polymer brushes can be defined as polymer materials prepared and assembled from end-tethered or surface-initiated polymer systems where the average distance between grafting points can be much smaller than the radius of gyration (Rg ). This tethering can be of sufficient density such that under equilibrium conditions, these polymer chains stretch along the direction normal to the grafting surface both in the presence or absence of external fields or stimuli (Fig. 1) (1). They can be quite different from that of flexible polymer chains in solutions where the chains are free to adopt a random-walk configuration. In the presence of a good solvent, polymer chains avoid contact with each other in order to maximize polymer–solvent interaction. Also, in the absence of solvent or in melt conditions, polymer chains can stretch away from the interface to avoid overfilling incompressible space (Fig. 2). Moreover, analogous macromolecule systems can be studied and compared to polymer blends (qv), block copolymers (qv), complex polymer architectures, and other functional polymers where confinement in surfaces and interfaces gives unique behavior and properties. In general, the interface to which polymer chains can be tethered may be solid–gas, solid–liquid, liquid–liquid, and liquid– gas. Furthermore, in solid surfaces, polymer chains can be chemically bonded (chemisorption) or may be just physically adsorbed onto the surface (physisorption) of the substrate. Recently, the investigation of polymer brushes has been focused on the synthesis of new tethered polymer systems primarily through surface-initiated polymerization (SIP). Previously, the term polymer brushes has been limited to the investigation of block copolymers (qv) or end-functional linear polymers that have been physically or chemically adsorbed to surfaces, respectively (3,4). Recent synthetic efforts using different polymerization mechanisms have resulted in the discovery of many novel properties of polymer brushes. This has been aided no less than the use of innovative and unique surface-sensitive analysis methods as applied to flat substrates and particles. The study of polymer brushes has benefited from improved dielectric, optical, spectroscopic, and microscopic characterization methods. Understanding the chemistry of these grafting reactions and how Encyclopedia of Polymer Science and Technology. Copyright John Wiley & Sons, Inc. All rights reserved.
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Fig. 1. The polymer brush approach and their classification: grafting to vs grafting from approaches and surface-initiated polymerization (SIP). The influence of grafting density and the enthalphic and entropic factors are clearly present on the “grafting onto” approach. Higher grafting density is expected with the SIP.
Fig. 2. The two limiting cases of a polymer brush regime, showing (a) tethered chains having the critical grafting density σ ∗ = 1/π Re 2 and (b) chains having σ > σ ∗ and forming the stretched brush. Other types of brush regime terminologies have been reported in literature. In this case, a characteristic length in the uncompressed brush (b) is given by 1 = σ − 1/2 . From Ref. 2.
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changes in monomer type, polymerization conditions, and interfacial properties affect the polymerization kinetics and molecular weight control is essential. It is ideal to have a polymerization technique which affords control over the molecular weight, molecular weight distribution, grafting density, and microstructure of the resulting polymers. In principle, an increase in the molecular weight of polymer chains with monomer conversion and a narrow molecular weight distribution for the grafted chains will give a uniform brush length similar to that observed in living polymerization in solutions. Thus, it is essential to understand the protocols and mechanisms of free-radical, living free radical, anionic, cationic, metathesis polymerizations on surfaces. A number of these polymerization mechanisms have been reviewed in the past by the Tsubokawa and co-workers, including the use of irradiation and plasma ionization techniques to functionalize and polymerize at particle surfaces (5,6). However, of great interest are the recent and current studies on the different polymerization mechanisms on “flat” surfaces and the characterization techniques that can be applied to these systems.
Classification Grafting onto and Grafting from. The first type of classification that can be used to distinguish polymer brushes is by their source and direction of grafting technique (see GRAFT COPOLYMERS). Grafting refers to the tethering or point of anchor for polymer chains. In the case of preformed polymers where an anchoring group was added by design, the polymer is expected to find its way to the surface or interface, where preferential adsorption can take place. This grafting onto of preformed functionalized polymers onto the surface could be further divided as a type of physisorption or chemisorption (another classification). During the grafting procedure, it is expected that the polymer chains will find its way to the surface where enthalpic and entropic considerations will determine the degree of grafting, eg diffusion, steric crowding, solvent quality. On the other hand, in a grafting from scheme, the polymer is formed in situ from grafting sites (initiators), where the growth of the polymer is limited primarily by reactivity of the active center during propagation, monomer diffusion or availability, and other peculiarities of the interface region, affecting the overall polymerization mechanism. However, to construct a highly dense polymer brush, the grafting from technique offers the most promise. This is because the kinetics of adsorption and reactivity is more favorable for smaller molecules, such as the initiator or the monomer (7). Therefore, to achieve a macromolecular design of a dense polymer brush, the SIP technique must be applied. Physisorption vs Chemisorption. Another important classification involves distinguishing the adsorption process. The physical adsorption or physisorption of homopolymers or block copolymers have been well reported (4,8). Physical Adsorption (qv) means that noncovalent forces, eg ionic or electrostatic attraction, dipole forces, H-bonding, hydrophobic effect, and van-der Waals forces, are responsible for grafting the polymer to surfaces. A number of macromolecular systems involving the use of linear homopolymers, end-group functional homopolymers, anchor-buoy designated block copolymers, amphiphilic block copolymers, and even star copolymers have been investigated (4). In this case, the polymer is attracted to the surface (both flat or particle) owing to a more
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favorable free energy gained by adsorption process (9). However, the problem is that such adsorbed layers are still susceptible to removal, eg by exposure to a thermodynamically good solvent or any subphase conditions that can weaken physical adsorption. Furthermore, it is often difficult to precisely control the physical structure of such adsorbed layers. On the other hand, adhesion between the polymer chains and the substrate may be greatly enhanced if the chains are tethered covalently to organic or inorganic surfaces by chemical adsorption or chemisorption. This approach involves the reaction of a preformed end-functionalized polymer with appropriate reactive sites on the inorganic surfaces (10). The preformed polymer is synthesized to have a reactive end group that is complementary to another reactive functional group already tethered to a surface (usually by forming selfassembled monolayers or SAM). The polymer end group then finds its way to the surface and a covalent bond is formed between the end-functional polymer and surface-bound complementary functional group. Thus, this method is more thermodynamically favorable than physisorption since the polymer forms a covalent bond anchor, which can only be removed by another chemical reaction. Still, the disadvantage of this method of grafting is that the chains, which are attached at the beginning of the reaction, sterically shields the remaining reactive sites on surfaces (11). Also, not all chemical reactions have the same rates in surfaces as compared to solution and bulk. Thus, both physisorption and chemisorption have their drawbacks for grafting onto surfaces. To this respect, grafting from surfaces by SIP has important advantages in terms of polymer brush densities and other physical properties. The rest of this article deals primarily with the subject of polymer brushes prepared by SIP. Other polymer brushes prepared primarily by physisorption or chemisorption is best referred to other review articles and books (12).
Surface-Initiated Polymerization Methods Polymerization at the interface is governed by thermodynamic and kinetic considerations not necessarily present in bulk or solution. There is a need to understand how these factors affect what is traditionally known about polymerization mechanisms in solution and in bulk. SIP promotes polymerization of monomers directly from initiator sites attached to surfaces, in which case activation of initiator and diffusion of monomers to the reactive sites become primary factors (13). The interface is a highly anisotropic environment where a variety of conditions exists, eg surface energy, polarity, and electrical double layer, not to mention the various geometries, size, shape, and surface properties of the solid-support substrate to which the polymer is bound (14). With the recent popularity of the SIP protocols, unique parameters and conditions toward the various polymerization mechanisms (initiation, propagation, termination, etc) warrant comparison with solution or bulk polymerization methods (15). In terms of chemistries involved at the interface, the homogeneity or heterogeneity of the system and the differences between the bulk phases defines the lifetime of the initiator, the flux of the monomer, and the rate of termination. Because of the high density of grafting sites, it is possible to prepare grafted polymers were the average distance between grafting points is much smaller than Rg . Selective spatial polymer growth is possible by both lithographic and nonlithographic patterning techniques (16). The
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behavior of polymer brushes under electric field, temperature gradient, solvent polarity, photochemical effects, and flow gradients can lead to novel types of field responsive polymers (17). Polymer Brushes on Flat Surfaces. By focusing on SIP in flat ideal surfaces, it is possible to take advantage of a wide range of surface-sensitive spectroscopic and microscopic analytical techniques. Flat substrates that have been investigated include glass, quartz, Si wafer, Au-coated glass, and Al-coated glass. There is potential for tailored polymer brush applications via surface modification and patterning (18). Theoretical predictions have been used to calculate molecular weight and polydispersity of polymer brushes grown by SIP on flat surfaces (19). Other theoretical predictions have been made on conformation and dynamic behavior of tethered polymers at flat surfaces (20). For example, it is always interesting to observe theoretical predictions on block copolymer brush behavior with respect to the Flory–Huggins (χ ) interaction parameter, Kuhn length, block volume fraction, and substrate surface energies (21). The number of different polymerization mechanisms reported has increased. Polymer brushes by SIP can be prepared using free-radical (13,22), cationic (23), ring-opening metathesis polymerization (ROMP) (24), atom transfer freeradical polymerization (ATRP) (25), polymerizations using 2,2,6,6-tetramethyl1-piperidyloxy (TEMPO) (26), and anionic polymerization (27). All these methods are suitable for polymerizing different types of monomers on a variety of flat surfaces, resulting in different polymer growth kinetics and chain densities (Fig. 3). Different polymer brushes by composition (homopolymers, copolymers, mixed polymers) and architecture (block copolymers, hyperbranched, graft, etc) have also been achieved (28). A number of polymers with distinct wetting and polyelectrolyte behavior have been prepared (29). Since the interface is a highly anisotropic environment, it is typical to use surface-sensitive analytical methods. Innovative use of these analytical tools has allowed elucidation of polymerization mechanisms and physical properties of these tethered polymers in situ on flat surfaces. Surface-sensitive spectroscopic and microscopic techniques like atomic force microscopy (AFM), ellipsometry, X-ray reflectometry, etc, have allowed detailed investigations on polymer brush behavior and growth kinetics under glass, melt, or swelling condition. Several articles have focused on reviewing polymer brushes grafted from flat surfaces, including SiO2 , Au, and glass (31). It should be emphasized that for each polymerization mechanism, peculiar conditions and parameters have to be optimized for controlled polymer brush growth (length, molecular weight, density, etc). There are parallels or contrasts for the SIP process when applied to flat geometry and particle surfaces. Polymer Brushes on Particles. SIP from particle surfaces also involves growth of end-tethered polymer brushes under different mechanisms. A variety of initiators, reaction conditions, monomers, and nanoparticles can be employed. Important differences to solution and bulk polymerization can be observed where the nanoparticles with grafted initiators behave as macroinitiators. In turn, the development of these materials allow the preparation of thermodynamically and kinetically stable nanocomposites and colloids. The preparation of polymer brushes grafted on particles and even nanoparticles is by far the more widely studied systems in polymer grafting methods
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Fig. 3. Protocol for free-radical SIP using AIBN analogue initiators attached to silica nanoparticles. (a) general initiator attachment, polymerization, and cleavage scheme; (b) AIBN surface initiator. From Ref. 30.
(6). The ease of preparation and analysis of such systems by simple gravimetric methods is one of the most common reasons. Also, particles are readily available and their dispersion properties have been highly studied in industry. In addition, there is great interest in much smaller “nanoparticle” systems as host for the polymerization process (32). The synthesis, characterization, and development of
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Fig. 4. The protocol for polymer brushes by SIP ring opening metathesis polymerization (ROMP), with initiators attached to Au nanoparticles. From Ref. 39.
new nanoparticle materials have both scientific and technological significance. Primarily through the quantum size effect, a number of these nanoparticle hosts have interesting electrooptical and magnetic properties (33). Different mechanisms are possible on a variety of initiators, reaction conditions, monomers, where a continuum of properties can be possibly observed on going from flat surfaces to high surface-to-volume nanoparticles. A number of polymerization mechanisms for particles have also been reported. This includes free-radical (22), cationic (34), TEMPO (35), anionic (36), ATRP (37), and metathesis (38) (Fig. 4). Polymerization from particle surfaces is challenging from the perspective of doing surface chemistry with colloids (14). The particle size and geometry vary. Also, the surface energy and solvent polarity changes, which can make a difference in terms of forming stable dispersions in selective solvents at each stage of the polymerization process. There are a number of analytical techniques which have been used to investigate this hybrid systems in situ. One of the compelling reasons to do SIP on particles is that it offers the advantage of preparing large quantities of SIP-grown polymers that can be degrafted and extensively analyzed ex situ. This is because of the high surface-to-volume ratio afforded by these substrates. However, it is also advantageous to characterize these systems with the polymers still grafted on the particles for the sake of learning more about certain fundamental properties of colloids (14).
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Polymerization Mechanisms for Polymer Brushes on Particles A number of SIP methods on flat surfaces have been the subject of recent reviews (1). More detailed description can be found in this article regarding the various mechanisms used and investigated on flat substrate surfaces. The formation of polymer brushes with various SIP mechanisms on particle systems is the focus of this section. However, as representative polymerization mechanisms, it gives insight into the peculiar aspects for each type of reaction when confined to surfaces. Surface-Initiated Free-Radical Polymerization. Free-radical SIP of polystyrene from nanoparticle surfaces have been investigated in detail by Ruhe and Prucker (12,32). Using AIBN-type free-radical initiators (qv), they have found that initiation and growth of the polymer at low conversion of a surface-attached initiator are comparable to polymerization in solution. The differences observed were attributed mainly to termination reactions. Another difference between solution polymerization and SIP is that all transfer reactions to either solvent or monomer or transfer agent can lead to termination of growing surface-attached chains via SIP. The consequence is a decrease in the radical efficiency of the initiator in surfaces compared to solution. Similar work on grafting to particles has been reported by Suter and co-workers (40) who tethered azo initiators by ion exchange to mica. A styrene-like initiator has been used to grow polymer brushes from clay surfaces by free-radical SIP (41). Other groups have also reported free radical polymerization (qv) from nanoparticle surfaces (42,43). Surface-Initiated Atom Transfer Radical Polymerization. Living free radical ATRP show the most promise for polymerization control on surfaces (44,45). It has been shown that typical ATRP conditions proved insufficient for controlled polymerization from flat surfaces (46). The use of a deactivator (Cu(II)) and a “sacrificial” free initiator to the polymerization system afforded molecular weight control. ATRP initiators based on 3-(2-bromopropionyloxy)propyl dimethylethoxysilane have been used to do SIP on surfaces (47). Controlled “living” radical polymerization (qv) was possible through changes in the polymer’s grafting density, composition, structure, and molar mass. This was manifested in the linear kinetic plots, linear plots of molecular weight (M n ) versus conversion, increased hydrodynamic diameter with conversion, and narrow molecular weight distributions (M w /M n ) for the grafted polymer materials. Hybrid nanoparticles possessing well-defined tethered block copolymers are accessible using ATRP (48). Recently, surface-initiated ATRP on silica particles has been demonstrated in aqueous media (49). Other relevant works related to surfaceinitiated ATRP have been reported (50). Another living free-radical polymerization process called reversible addition-fragmentation chain transfer or RAFT has been applied to surfaces (51). Grafting of polymers to carbon black has been reported using TEMPO (52). Living Anionic Surface-Initiated Polymerization. Anionic polymerization (qv) is a most versatile method to make well-defined architectures of polymers. It has been employed to grow polymer brushes from various silica gels (53), graphite and carbon black (54), and also flat surfaces (27,55). Results on living anionic polymerization on clay nanoparticles have been reported (56). Special conditions are required since difficulties arise owing to the effects of moisture and other impurities on anionic polymerization. A major limitation of the prior work
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by Oosterling and co-workers is that the choice of tert-butyllithium (t-C4 H9 Li) as the initiator for styrene polymerization from silica in the presence of toluene is inefficient (54). Yet, despite these difficulties, anionic polymerization remains attractive for the synthesis of complex and well-defined macromolecular architectures (57,58). The Advincula group has recently reported results on living anionic polymerization on clay (59), SiO2 nanoparticles, and flat surfaces using the initiator precursor 1,1-diphenylethylene (Fig. 5) (27,36,57).
Surface-Initiated
Ring-Opening
Metathesis
Polymerization.
Nanoparticles can be coated with alkanethiol-containing and silane-containing initiators for ROMP (39,60). ROMP on surface-functionalized particles makes use of organometallic catalysts to allow metathesis type of polymerization methods. Since this is also a living polymerization process, the advantages of this strategy are numerous, including exceptional control over polymer length and chemical composition as well as particle size, solubility, and shape. Other groups have also reported strategies for ROMP on surfaces (61). Surface-Initiated Living Cationic Polymerization. Cationic ringopening surface initiated polymerization reactions can be directed to particle surfaces (34). Using appropriate initiation procedures with 2-oxazoline monomers, a ring-opening cationic polymerization process was observed to proceed in a living manner. Hydroxyundecane-1-thiol-functionalized gold particles were converted to triflate functional groups by exposure to trifluoromethanesulfonic anhydride. Then 2-oxazoline monomer was added and polymerization proceeded under reflux conditions. Termination was done by adding N,N-di-n-octadecylamine. Other groups have reported SIP by living cationic mechanism on a variety of surfaces (62,63). In summary, different mechanisms are possible and have been successfully demonstrated for grafting polymer brushes. Comparison can be made to their solution or bulk polymerization counterparts. These systems differ in (1) rate of polymerization, (2) mechanisms and methods of termination, (3) consideration of interfacial properties and reaction conditions, and (4) methods of analysis and correlation with theoretical trends. Again, different polymerization mechanisms can also be found with polymer brushes on flat surfaces.
Analytical and Characterization Methods The study of polymer brushes has been of great interest not only for their synthesis but also for their unique properties and characterization approaches. The analysis of polymer brushes is a challenge. Because of their reduced dimensionality, assumptions based on analogous solution or bulk systems do not necessarily hold. There is a desire to understand fundamental properties at interfaces and in developing new applications. Since the interface is a highly anisotropic environment, surface energy, polarity, electrical double layer, etc, needs to be taken into account (14). In general, the methods of analysis can be divided into spectroscopic, microscopic, and optical in approach although other classifications can be mentioned. Both in situ and ex situ methods are possible (see SURFACE ANALYSIS). Characterization of Polymer Brushes on Flat Substrates. Ultrathin organic and polymer films have been the subject of extensive investigations for
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Fig. 5. Protocol for living anionic surface initiated polymerization (LASIP), with initiators attached to silica nanoparticles or clay: (a) grafting of initiator and general polymerization scheme; (b) nanoparticle grafting procedure. From Ref. 36.
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the last few decades (64). They have finite thicknesses of a few to several hundred nanometers, on a variety of solid substrate supported systems. The quasi two-dimensional ordering within layers can be extended into stacked structures perpendicular to the solid substrate. In the investigation of polymer brushes, surface-sensitive spectroscopic and microscopic analytical methods can be applied that have been typically used for investigating other types of organic and polymer ultrathin films which includes spin coating, alternate polyelectrolyte (65), Langmuir–Blodgett technique (66), and self-assembled monolayers (67,68) (see LANGMUIR-BLODGETT FILMS). Spectroscopic methods such as UV–vis absorbance, fluorescence, and other intensity-sensitive spectroscopic techniques have been used to investigate changes in absorption increase, energy transfer processes, and monitor film growth in situ for polymer brushes. The formation of uniform polymer films can also be verified and observed by X-ray reflectivity and other scattering methods (69). Neutron diffraction and reflectometry can be used, relying on deuterated species within layers (70). These techniques are important in probing the mobility of polymer chains, diffusion within layers, and short-range and long-range order parameters that can be correlated with other spectroscopic and microscopic techniques (71). Techniques for determining the chemical functional group and molecular (elemental) species also include FT-IR and X-ray photoelectron spectroscopy (XPS) (72). XPS in particularly useful for monitoring the presence of different oxidized states of atoms, relative abundance of atomic species, and the presence of the substrate. FT-IR and IR-RAS has been used to monitor specific IR-sensitive functional groups even at monolayer thicknesses (73). Optical techniques can be used to monitor optical thickness and dielectric constant parameters. This includes ellipsometry, multiple reflection interferometry (74), evanescent wave (75), and surface plasmon resonance spectroscopy techniques (43). Ellipsometry has been used widely and routinely to investigate film thickness of polymer brush films (76). For optical properties of films, it is important that the average film roughness and uniformity is specified. Often, sampling is localized by the “spot” size, such that it is necessary to probe and average different areas of a sample. Microscopy is very useful especially in monitoring lateral morphologies, layer roughness, domains, or patterns. This includes AFM, scanning electron microscopy (SEM), optical microscopy, and other surface probe microscopy methods (77,78). SEM can be used for relatively thick films combined with microtoming and cryogenic techniques (79). It is very useful for obtaining three-dimensional morphologies and characterization of features of these films. AFM has been used to investigate polymer brushes in a number of ways. This includes (a) general mapping of topology or investigating morphologies (80), (b) identifying features resulting from phase segregations (28), (c) measuring the surface forces involved in different brush geometries (81), (d) estimating the molecular weight of brushes (82), and (e) investigating patterned polymer brushes (Fig. 6) (see ATOMIC FORCE MICROSCOPY). Other surface-sensitive methods can and have been used (1) quartz crystal microbalance methods have been used to investigate the deposition process especially in situ (84). The value of in situ
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Fig. 6. Formation of a nanopattern from a PS-b-PMMA brush with 23-nm PS and 14-nm PMMA as investigated by AFM. From Ref. 83.
(2)
(3)
(4) (5)
monitoring methods is in determining the rate functions of the adsorption process. Contact angle measurements or surface tensiometry allow determination of surface energy or surface tension on polymer brushes (85). It also gives information on differences in morphology and functional group distribution on surfaces. Electrochemical methods involve redox behavior of probes or the polymers themselves. It is possible to utilize this technique to investigate permselectivity (86). It can be utilized to probe any redox active species in the polymer films. Streaming potential measurements with ellipsometry can possibly be applied to polyelectrolyte polymer brushes (87). A very important technique is the use of a surface force apparatus (SFA) (88). It is capable of measuring the surface forces directly between two molecularly smooth surfaces, eg mica, with a sensitivity of a few millidynes (10 nN) and a distance resolution of about 0.1 nm. These flat smooth surfaces of mica can be covered with polymer brushes to obtain tribological properties between different polymer brush materials. Direct measurement of surface forces is also possible by AFM (89).
Other methods will likely be reported in the future and find unique utility in characterizing and analyzing polymer brushes. It should also be mentioned that
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computational and simulation methods are important “tools” for predicting and scaling the observed behavior in polymer brushes and should go hand-in-hand with experimental results (90).
Characterization and Analysis Methods for Polymer Brushes on Particles. There are several important reasons for in situ analysis of polymer brushes on particles: (1) to monitor grafting of initiator to particle surface, (2) to monitor surface functional group conversion, (3) to investigate the polymerization mechanism (initiation, propagation, termination), and (4) to investigate the properties of a hybrid organic–inorganic particle or nanocomposites. Properties that can be measured include grafting density, polymer shell thickness, swelling and contraction of shell, and polydispersity of polymer brushes. The nature of the particle substrate is important. The analysis of these materials can be further complicated by the fact that different particle and nanoparticle substrates have varying dispersion properties. The particles are governed by size, shape, size distribution, surface charge, hydrophilicity, etc. Each has to be handled properly such that the material is not lost or precipitated with the different polymerization methods. Direct or in situ methods allow monitoring of differences in initiator/monomer composition ratios, time of polymerization, solvent, and temperature conditions. Classical methods for analyzing polymer brush–particle systems include gravimetric (91,92) thermal analysis (93–95) light scattering (96), NMR (97), zeta potential (98,99) and rheology (100). Microscopy methods such as TEM, SEM, and AFM can also be employed, giving direct visualization of core–shell composite architectures (101). An interesting method for investigating polymer– particle dynamics using fluorescence correlation spectroscopy has recently been reported (102). It will be interesting to see in the future how SNOM techniques can be used to probe specific particle–polymer interactions (103). Once polymers have been detached from the substrate, they can be analyzed like any normal polymer product in an ex situ manner.
Investigating Polymer Brush Systems on Flat Surface Substrates Polymer Brush Regimes on Surfaces. In the presence of an interface, the configurational space of polymer chains is limited. The deformation of polymer chains is generally a contribution of both interaction and elastic free energies. The interplay of these two terms determines the equilibrium thickness and brush regimes of tethered polymers. The internal structure and deformation of polymer brushes has also been investigated by numerical and analytical self-consistent field (SCF) calculations, and by computer simulations (104). For example, in densely tethered polymer chains, a strong overlap among the undeformed coils exists at the interface. This increases the polymer-to-polymer contacts and the corresponding interaction energy. The polymer chains are then forced to stretch away along the direction normal to the grafting sites. Stretching lowers the interaction energy per chain, at the price of a high elastic free energy. Thus, different conformational regimes are possible with polymer brushes by the interplay of these two terms. Polymer brushes have equilibrium thicknesses that vary linearly with the degree of polymerization. In contrast, free polymer chains in a theta solvent possess an unperturbed configuration. Research work employing theory, scaling theories, simulation, and surface probe techniques have shown that the
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Fig. 7. Investigating phase segregation and formation of patterns in multicomponent polymer brushes as observed by AFM. In this case, glassy and rubbery binary components of grafted polymethacrylate (PMA) and fluorinated polystyrene copolymer (PSF) were sensitively imaged and differentiated between topography (left), phase imaging (right), and glassy (top) and rubbery (bottom) states of the binary brushes. Dimensions at 5 × 5 µm. From Ref. 107.
different regimes can result from solvent swelling, differences in molecular weight, and differences in grafting density as the tethered polymer chains change their conformation (105). Depending on the equilibrium conditions, densely tethered polymer chains can be deformed and result in a variety of brush regimes which includes “mushroom,” “pancakes,” “micelle,” and “dimples.” While such studies have been recently applied to SIP type of polymer brushes, the investigation of brush regimes has long been associated with physisorbed polymers. Phase Segregation in Multicomponent Polymer Brushes. Phase segregation and ordering in multicomponent polymer brushes have also been extensively studied (Fig. 7) (106,107). In the presence of two tethered immiscible
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homopolymers, different ordered phases can be described, similar to lateral microphase separation in bulk (108). The composition varies as a statistical mixture of the two components or as blocks. Theoretical SCF models have been used to examine the equilibrium properties of a binary polymer brush composed of immiscible chains under melt conditions. A number of interesting morphologies and mesophases are possible both by melt and solution conditions (109). By changing the polymer brush architecture, grafting density, chain length, interaction energy between different blocks and interaction energies between blocks and solvents, a variety of novel well-ordered structures have been predicted and observed. Theoretical results indicate that tethered copolymer brushes on a flat substrate are excellent candidates for forming nanopatterned polymer films (110). Recently, this has been a topic of great interest for investigating new morphologies in SIP systems. Different Stages of the SIP Grafting Technique. SIP on flat substrate surfaces is a step-by-step procedure which begins with the preparation of substrates and ends with characterization of post-polymerized films. Usually the chosen substrate is also dependent on the applicability of various analysis methods. Primarily, the substrate used is determined by the type of SAM technique used to tether initiators on surfaces. In principle, the characterization protocol is typical for investigating SAMs at surfaces but is extended toward macromolecular dimensions once a polymer is attached. The polymerization and postpolymerization characterization is equally important. However, very few methods are available for in situ characterization during the polymerization itself. It is difficult to monitor the growth of polymer brushes in real time because the mechanism and kinetics are not necessarily the same as in solution or bulk. The last step, post-polymerization analysis, is a stage where characterization of terminated brushes can be done after several “washing” methods to isolate grafted polymers on surfaces. Any post-polymerization treatments such as cross-linking or functional group conversion of the brush can be examined at this stage. Overall, the step-by-step analysis is useful for characterizing the polymer brush formation and comparing the films “before” and “after” each stage of the grafting or treatment. Patterning Polymer Brushes. Patterning by both lithographic and nonlithographic technique results in films with features ranging from a few nanometers (nanopatterning) to microns (micropatterning) (111,112). New and simple strategies to fabricate surface-confined patterns with lithographic and nonlithographic methods have been widely reported (113). Nonlithographic methods using microcontact printing (114) and dip-pen nanolithography (DPN) (115) are recently popular. Patterning offers a number of advantages: (1) extrinsic, stimuli control of interfacial properties at the micrometer or nanometer scale; (2) precisely localized presentation of chemical or topographical features; and (3) controlled surface densities, eg required to achieve higher throughput, as in combinatorial methods (116). Various schemes have been reported where microcontact printing and even lithographic methods (18,26) can be used to prepare microscale patterns (Fig. 8). Fabricating nanopatterns by SIP is an interesting goal. One key strategy in the nanopatterning approach is the use of SAM level of initiator self-assembly. In combination with an appropriate nanolithographic technique, such as DPN, interesting future polymeric nanostructures can be prepared.
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Fig. 8. Scheme for patterning of a polymer brush using a sacrificial photoresist layer and lithographic imaging. From Ref. 18.
Applications Many applications can be derived from polymer brushes. The most practical use of polymer brushes are modification of surfaces (32), compatible biosurfaces (117), new lubrication materials (118), separations, compatibilizers (119), new adhesives, and polymer composite preparation (120). An interesting application of polymer brushes is in the form of chemical gates (121). Ultrathin and patterned organic films can be prepared which are useful in microelectrics, cell growth control, biomimetic material fabrication, microfluidics, and drug delivery. The versatility of polymer brushes compared to other polymer coating methods is that it allows control of molecular weight, polydispersity, density, composition, block sequencing, and architecture which can be varied for each type of application. Functional polymers observed in solution or in bulk should also have interesting properties when confined to surfaces. This includes liquid crystalline polymers, rigid rods, conjugated polymers, cross-linkable polymers, and polyelectrolytes.
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In the area of particles, polymer brushes are of great interest for surface modification and composite material preparation. They have interesting applications in colloidal and interfacial phenomena. In preventing flocculation, the polymer chains, which prefer the solvent to another particle surface, resist overlapping between neighboring particles, resulting in colloidal stabilization. This repulsive force arises from the high osmotic pressure inside these brushes. Strategies have been developed to tailor nanoparticle surfaces by the use of polymer brushes (32). The purpose is to directly tether polymers to nanoparticles, resulting in a core– shell type of material architecture. This strategy has lead to the formation of “hairy nanoparticles,” which protects the nanoparticle surface and at the same time, increasing the compatibility to polymeric matrices. This is especially important for the preparation of polymer nanocomposite materials that can be processed like commercial polymers. A number of inorganic–organic hybrid and nanocomposite materials have been reported (32,118).
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RIGOBERTO C. ADVINCULA University of Houston
POLYMERIC DRUGS.
See Volume 7.
