Article pubs.acs.org/Langmuir

Polymer Brush Gradients Grafted from Plasma-Polymerized Surfaces Bryan R. Coad,*,† Tugba Bilgic,‡ and Harm-Anton Klok*,‡ †

Mawson Institute, University of South Australia, Mawson Lakes, SA 5095, Australia Laboratoire des Polymères, Institut des Matériaux and Institut des Sciences et Ingénierie Chimiques, École Polytechnique Fédérale de Lausanne (EPFL), Bâtiment MXD, Station 12, CH-1015 Lausanne, Switzerland

‡

S Supporting Information *

ABSTRACT: A new method for generating a surface density gradient of polymer chains is presented. A substrateindependent polymer deposition technique was used to coat materials with a chemical gradient based on plasma copolymerization of 1,7-octadiene and allylamine. This provided a uniform chemical gradient to which initiators for atom transfer radical polymerization (ATRP) were immobilized. After surface-initiated atom transfer radical polymerization (SI-ATRP), poly(2-hydroxyethyl methacrylate) (PHEMA) chains were grafted from the surface and the measured thickness profiles provided direct evidence for how surface crowding provides an entropic driving force resulting in chain extension away from the surface. Film thicknesses were found to increase with the position along the gradient surface, reflecting the gradual transition from collapsed to more extended surface-tethered polymer chains as the grafting density increased. The method described is novel in that the approach provides covalent linkages from the polymer coating to the substrate and is not limited to a particular surface chemistry of the starting material.

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INTRODUCTION One of the most interesting energetic properties of surfaces grafted with terminally attached polymer chains (“polymer brushes”) is the ability to influence the entropic properties at the surface interface. This is a consequence of unfavorable chain interaction energies arising when neighboring chains are brought into close proximity. On a surface, two closely interacting polymer chains with N repeating units separated by a distance s will experience increasing unfavorable interaction energies when N increases and s decreases. This is caused by a reduction of available conformations due to potential overlap of chain segments and results in chains being forced to extend and stretch away from the surface to access free solution volume.1,2 In good solvents, the new equilibrium chain conformation possesses a more ordered state (compared to a random walk conformation expected in free solution) and contributes to an overall lower entropy state. The increased thickness and lower entropy characterize the “polymer brush” conformation.3 While increasing N contributes to the formation of polymer brushes, lowering s (or increasing the surface density, σ) gives a more responsive way to do so because the height of the polymer brush H is linearly proportional to N but scales with σ1/3. It is for this reason that considerable research effort has been invested into methods for manipulating the grafting density.4 A dramatic way to visualize the effect of increasing the grafting density and its effect on polymer morphology is to prepare gradient grafted materials. The best examples of these come from platforms where polymer chains have been grafted from surfaces possessing an increasing surface density of © 2014 American Chemical Society

initiating sites in one dimension along the surface. After surfaceinitiated polymerization, grafted polymer chains ranging from one side of the sample to the other may show evidence of increased surface crowding giving an increasing polymer brush height and, possibly, the mushroom to brush transition. On the general subject of gradient polymer surfaces, there are several reviews providing a broad overview of the research.5−8 As mentioned, for surface density gradients of tethered polymers, the best way to ensure that polymer chains possess a nearly uniform degree of polymerization and can be grafted at high densities is to use controlled polymerization initiated from surface sites. Therefore, the first step is to prepare gradients of chemical functionality, which dictate the spatial arrangement of initiators. Because of its many advantages in preparing polymer brushes, surface-initiated atom transfer radical polymerization (SI-ATRP) has been a popular choice.9 A number of methods for creating gradients of surface-bound ATRP initiators have already been developed. In general, these methods use silicon or gold substrates because methods for coupling ATRP initiators onto these surfaces are well-known. One method to generate surfaces that present gradients of surface-bound ATRP initiators is based on the coupling of chlorosilanes onto hydroxyl-functionalized surfaces. In one of the earliest reports, Genzer’s group allowed the vapors of a volatile silane compound 1-trichlorosilyl-2-(m-p-chloromethylphenyl)ethane to diffuse across the surface of a silicon substrate to form a Received: August 1, 2013 Revised: June 24, 2014 Published: June 26, 2014 8357

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self-assembled monolayer (SAM) of ATRP initiators.10 This reaction requires “backfilling” or blocking of unfunctionalized silicon areas with an inert or dummy chlorosilane such as, e.g., n-octyl trichlorosilane. This two-step procedure can also be reversed by letting the inert chlorosilane diffuse over the surface and then backfilling with the ATRP initiator.11 This backfilling procedure can be advantageous in allowing double density gradients to be surface-initiated by two different controlled radical polymerization techniques, namely, ATRP and nitroxide mediated polymerization.12 Another method that has been used to generate gradients of surface-bound ATRP initiators involves dip-coating a silicon wafer with a poly(glycidyl methacrylate) solution and then rest the surface along a temperaturecontrolled stage with a temperature gradient along the surface.13 Differential temperature annealing of this polymer provides a thickness gradient where the density of the remaining epoxide groups is varied according to the temperature gradient. Linking a carboxy-ATRP derivative to this surface provides a surface from which polymers can be grafted. Electrochemical potential gradients are a powerful way to generate surface gradients of ATRP initiators on gold substrates.14 In a first step, an alkanethiol, such as, e.g., hexadecanethiol, is adsorbed on the gold surface and subsequently selectively desorbed in a gradient fashion by applying a lateral electrochemical potential. Into these empty areas, a brominated disulfide initiator molecule is adsorbed to form the gradient initiator surface. Nanolithography also provides many opportunities to etch or write nanoscale gradients of ATRP initiators onto surfaces.15 These include methods for controlling the placement of initiators through scanning probe lithography,16 dip pen nanodisplacement lithography,17 and feature density method.18 These nanolithography methods, however, generally produce only relatively small-scale density gradients on the order of nanometers. While the techniques outlined above have all been successfully used to generate surface gradients of ATRP initiators, they all in some way or another are substratespecific. Plasma polymerization is an alternative, substrateindependent method for generating surfaces with gradients of chemical reactivity used to functionalize different substrate materials apart from silicon and gold. The general method involves bringing the vapors of organic precursor molecules (monomers) into a vacuum chamber and exciting them by use of a radiofrequency generator producing a plasma. These highenergy moieties bombard underlying surfaces and deposit into a cross-linked polymer coating containing covalent linkages which also form chemical bonds to the substrate material itself. Continued deposition continually overlays and links new plasma polymer into the network meaning that the overall functionality of the deposited plasma polymer film is similar regardless of the chemical nature of the underlying substrate.19 Reactors with a moving aperture allow deposited plasma polymer to be spatially located along one dimension along the surface.20 Further advancements in reactor design allow two monomers to be deposited in gradient fashion in a controlled way by the opening and closing of valves.21 Choice of the reaction conditions (flow rate, plasma power, time) controls the thickness of the deposited layer and allows functional groups present in the precursor molecules to be retained intact in the plasma polymer. An example of this has been to use 1,7octadiene and allylamine precursors to create amine surface density gradients, which have been used to control protein22

and nanoparticle23 adsorption to surfaces in a gradient fashion. The reactivity of amine groups on the surface can be exploited in the fabrication of gradients of ATRP initiator molecules by following a two-step procedure of gradient plasma polymer deposition of a compound containing a desired chemical functionality followed by chemical derivatization with a reactive ATRP agent. Previously we have shown how plasma polymerized surfaces can be modified with macroinitiators of both fixed24 and tunable amounts25 of ATRP groups to make grafted polymer surfaces in a substrate independent fashion. We have also very recently demonstrated a gradient plasma technique for functionalizing surfaces with ATRP initiators directly, without the need for a chemical derivatization step.26 These and other reports27 show the utility of how different substrate materials (many different plastics, within confined geometries of polystyrene multiwell plates, and on metals, semiconductors, etc.) can be grafted using SI-ATRP with plasma polymer interlayers with identical polymer layers regardless of the chemical nature of the underlying substrate material. In the present work we demonstrate a gradient plasma technique to fabricate variable surface-density samples where a gradient of concentrations was present and differed across lateral position on a single sample.

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MATERIALS AND METHODS

Materials. Water was purified (>18 MΩ.cm) by use of either a P.Nix UP 900 purification system (Human Corp., S. Korea) or a Millipore Direct-Q 5 ultrapure water system. 1,7-Octadiene (98%) and allylamine (>99%) were obtained from Sigma-Aldrich, Australia. Polished silicon wafers (⟨100⟩ orientation, from Gritek, China) were cut into 1.5 cm × 1.0 cm rectangles and marked in one corner to define directionality. Wafer pieces were cleaned by sonicating with 2% RBS 35 detergent solution (Thermo Scientific, USA), rinsing and sonicating in water, followed by rinsing with ethanol and blown dry with purified nitrogen in a laminar flow hood. Solvents used in plasma polymer soaking and stability studies were reagent grade solvents supplied by Sigma-Aldrich, Australia. Dichloromethane (DCM) and methanol used for washing were technical grade (Aldrich). 2Hydroxyethyl methacrylate (HEMA) (97%) was received from Aldrich. Prior to polymerization, the monomer was passed through a column of activated basic aluminum oxide to remove the inhibitor. 2,2′-Bipyridine (bipyr, ≥99%, Aldrich), copper (I) chloride (CuCl, 99.99%, Acros), copper (II) bromide (CuBr2, 99.999%, Aldrich), and 2-bromo-2-methylpropionyl bromide (BiBB, 98%, Aldrich) were used as received. Triethylamine (≥98.0%) was purchased from Fluka and distilled over KOH. Toluene was purified and dried using a solventpurification system (PureSolv). The ATRP initiators 6-2-(2-bromo-2methyl)propionyloxy)hexyldimethylchlorosilane (1)28 and 2-bromo-2methyl-N-{3-[chloro(dimethyl)silyl]-propyl}propanamide (2)29 were prepared as previously reported. Plasma Polymerization. Gradient copolymerization was performed using a plasma apparatus with a 13.56 MHz radiofrequency generator as previously described.20,23,26,30 The choice of run conditions allows customizable control of the length of the gradient as well as the slope by varying the instrumental parameters as follows. The length of the gradient (over 1.5 cm) was dictated by the sample size and the translational speed of the moving mask. These were also chosen such that the chemical composition of the gradient did not vary too much under the ellipsometry beam area (roughly 0.15 cm diameter) and XPS beam area (roughly 0.05 cm diameter). The mixture of chemical vapors forming the plasma phase and, therefore, the relative composition of functional groups is controlled by programmed opening and closing of needle valves a certain number of turns, calibrated to measured flow rates. A 0.1-cm-wide slit mask was moved over the substrates while simultaneously changing the vapor composition of both 1,7-octadiene and allylamine using 8358

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Scheme 1. SI-ATRP of HEMA from ATRP Initiator Anchored Gradient Plasma Polymer Substrates

programmed valves. 1,7-Octadiene vapor was introduced at 10 standard cubic centimeters per minute (sccm) and the plasma was ignited at 10 W (as read from the digital power meter readout) as the substrate mask began to move in steps of 0.025 cm every minute. 1,7Octadiene vapor was linearly reduced to 0 sccm when the mask was positioned from 0.5 to 1.2 cm along the substrate; concurrently, the allylamine vapor flow rate was linearly increased from 0 to 10 sccm. Finally, the flow of allylamine was maintained at 10 sccm from positions 1.2 to 1.5 cm. The defined chemical species within the plasma are deposited onto the unmasked portion of the substrate and generally reflect the vapor composition of the chemical precursors. The matching is not absolute as energetic species within the plasma have been known to diffuse underneath the stainless steel mask and deposit a few millimeters into masked areas. This phenomenon, however, is reproducible from batch to batch and it is straightforward to work out empirically run conditions that reproduce gradients in the desired way. For example, in this work, the opening of the allylamine valve was triggered after the mask had traveled 0.5 cm along the sample knowing that amine groups would be present at positions less than 0.5 cm. Grafting of ATRP Initiator on Gradient Plasma Polymer Substrate. Gradient plasma polymer substrates were immersed into dry toluene (30 mL) and freshly distilled triethylamine (0.6 mL) was added and mixed in the reactor. The system was cooled to 0 °C in an ice bath; thereafter, BiBB (0.6 mL) was added into the system dropwise. The reaction was kept at 0 °C for 1 h; then, the ice bath was removed and the reaction continued at room temperature overnight. The ATRP initiator anchored plasma polymer modified substrates were washed with DCM and methanol twice and dried under nitrogen. Uniformly ATRP-Initiator Functionalized Silicon Substrates. Silicon wafer pieces (0.8 cm × 1.0 cm) were sonicated for 5 min in acetone, 5 min in ethanol, and 5 min in deionized water and dried under a stream of air. Subsequently, the silicon wafers were exposed to a microwave-induced oxygen plasma system (200 W, Diener electronic GmbH, Germany) for 15 min. Next, the silicon wafers were immersed into a 2 mM solution of SI-ATRP initiator (1) or a 10 mM solution of SI-ATRP initiator (2) in dry toluene for 16 h at room temperature under an inert atmosphere. After that, the slides were rinsed extensively with dichloromethane (DCM), and methanol. Finally, the initiator-functionalized slides were dried under a flow of nitrogen. Micropatterned initiator modified surfaces were prepared using a protocol previously reported in the literature.31 SI-ATRP of HEMA. SI-ATRP of HEMA from the plasma polymer modified gradient substrates as well as from the uniformly modified silicon substrates was accomplished following a published procedure.32

The reaction conditions were as follows: CuCl (55 mg, 0.55 mmol), CuBr2 (36 mg, 0.16 mmol), and 2,2′-bipyridine (bipyr) (244 mg, 1.56 mmol) in a mixture of HEMA/water (8 mL). Polymerizations were carried out using constant molar ratios of CuCl/CuBr2/bipyr/HEMA: 3.5/1/10/200. The amount of water in the polymerization solution was adjusted to generate monomer concentrations of 1.0, 1.5, and 2.0 M. The reaction time and concentration of the monomer were varied to study the effect on the polymer thickness. The system was not compatible with HEMA concentrations greater than 2.0 M as patchy coatings resulted with some evidence of layer delamination. After the polymerization, PHEMA grafted substrates were thoroughly washed with water, methanol, and ethanol and finally dried under a flow of nitrogen. Washed samples showed no residual traces of copper or other catalyst species in XPS spectra. Surface Chemical Characterization. Coated samples were analyzed for their surface chemical composition using a Kratos Axis Ultra DLD X-ray photoelectron spectrometer (XPS) equipped with a monochromatic Al source. Charging of the samples during irradiation was reduced by an internal flood gun. Each sample was analyzed at an emission angle normal to the sample surface. Survey spectra were acquired at 120 eV pass energy and high-resolution C 1s spectra were recorded at 20 eV pass energy. Data were processed with CasaXPS (v 2.3.16 Pre rel. 1.4, Casa Software Ltd) with residuals for curve fits minimized with multiple iterations using simplex algorithms. Spectra were corrected for charge compensation effects by offsetting the binding energy relative to the C−C component of the C 1s spectrum, which was set to 285.0 eV. Uncertainties in peak fittings were determined using a Monte Carlo procedure. The relative surface composition of elements was determined by quantifying the peak areas from survey spectra and accounting for the relative sensitivity factors of the instrument in the elemental library. Thickness Measurements. All film thicknesses reported in this manuscript were determined under ambient condition and are referred to as “dry” thicknesses. Due to their hydrophilic nature, however, polymer films such as PHEMA contain some water that is absorbed from the atmosphere and contributes to some degree of swelling.33−35 Layer thicknesses from gradient samples on silicon wafers were determined through the use of a variable-angle spectroscopic ellipsometer (VASE, J.A. Woolam Co. Inc. NE, USA). Data were acquired at 65°, 70°, and 75° with light wavelengths scanned from 250 to 1100 nm in 10 nm intervals. A translational stage moved the sample in defined increments along the gradient. Data at each point were fit to a 2 layer model for Ψ and Δ comprising an infinitely thick silicon base layer plus a Cauchy overlayer. Thus, the absolute thickness determinations include a contribution from the native oxide layer 8359

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vice versa.14 Allylamine-derived plasma polymers possess primary amine groups that can be used in nucleophilic substitution reactions.37,38 Previous work has also demonstrated the reactivity of these groups with successful covalent attachment of molecules possessing aldehyde functionality to similar allylamine/octadiene gradients.22 The plasma polymerized allylamine/1,7-octadiene interlayer was characterized with ellipsometry and XPS. Figure 1 shows

which is typically approximately 2 nm for the wafers used in this study. A minimum number of uncorrelated fitting parameters (optical constants in the Cauchy layer) were used to fit both the optical properties of the film and the film thickness. The thickness of the PHEMA brush layer was determined by subtracting the thickness of a measured control sample composed of the initiator-functionalized plasma polymer, which had been soaked in water overnight and then dried. Film thicknesses of polymer brushes grown from micropatterned substrates were determined by measuring the step-height of the polymer layer using atomic force microscope profilometry, which was carried out with Veeco multimode Nanoscope IIIa instrument operated in tapping mode and using NSC14/no Al (MikroMasch) cantilevers. Figure S1 in the Supporting Information compares thicknesses of PHEMA brushes grown from silicon substrates as determined by AFM and ellipsometry and indicates that the results of these two techniques are in relatively good agreement with the ellipsometry data being ∼10% higher as compared to the AFM results.

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RESULTS AND DISCUSSION Gradient plasma polymerized surfaces are new conjugation platforms for preparing variable-surface density grafting environments for polymer brushes prepared via surfaceinitiated polymerization. They can be relied upon to demonstrate how surface crowding of linear polymer chains causes morphological changes resulting in a mushroom to brush crossover observable on a single sample. Apart from demonstrating an interesting energetic property based on entropic crowding, density-gradient surfaces supply a stiffness gradient and would be an interesting platform on which to measure cell response. The novelty of this work comes from the plasma polymer interlayer, which allows covalent attachment to potentially any substrate. This is advantageous and removes restrictive limitations in the choice of substrate with, for example, a specific set of paired, complementary linking chemistries required for systems such as silicon/silane or gold/thiol. Scheme 1 shows the steps in our synthesis. First, gradient plasma copolymerization of 1,7-octadiene and allylamine allows covalent attachment of the plasma polymer interlayer to the silicon substrate. By gradually varying the feed composition of the 1,7-octadiene/allylamine monomer feed, a chemical gradient with an increasing surface density of amine groups is generated, which allows conjugation of the BiBB ATRP initiator via an amide bond via acid-halide coupling. Finally, HEMA is polymerized from the surface bound initiators on the gradient by ATRP. Characterization of the Plasma Polymer Gradient. A plasma polymer gradient of allylamine was generated by a method previously described.20,22,23 Silicon was chosen as the substrate in this study because its optical properties make it ideal for ellipsometry studies; however, the plasma technique is substrate independent19 with gradient plasma polymerization, in particular, shown to be effective on many substrate materials including glass23 and plastic coverslips.21,26,30 The choice of monomers for the generation of the plasma polymer interlayer (allylamine and 1,7-octadiene) was motivated by a desire to modulate the surface density of reactive amine groups among a background of neutral reactivity. The deposition of chemically reactive amine functionalities simultaneously with unreactive hydrocarbon functionalities provides a chemical gradient in one step and overcomes disadvantages of two-step techniques where functional moieties are first deposited onto a substrate and empty space in the interstitial spaces within the gradient are taken up by “backfilling” chemically inert moieties10,36 or

Figure 1. Thickness of the plasma polymerized copolymer layer as a function of position along the substrate measured by ellipsometry (blue ◆). The thickness of the same layer with coupled ATRP initiator is overlaid (red ■). Values are averaged from 4 experiments with standard error (95% confidence limit). Thickness values include a ∼2 nm contribution from the native oxide layer on silicon.

that the ellipsometric thickness of the generated plasma polymer interlayer as a function of linear distance along the substrate remained fairly uniform and did not deviate by more than about 6 nm end-to-end. To assess the surface chemical composition of the gradient samples, the films were analyzed with XPS. Figure 2 illustrates the evolution of the N/C ratio as a function of the position along the substrate. In addition, Figure 2 presents survey and high-resolution C 1s scans recorded at 0.1 cm, respectively, 1.3 cm along the substrate. In addition, Figure S2 in the Supporting Information presents an overlay of C 1s high resolution scans recorded at different positions along the gradient. Peak fitting routines on the octadiene-rich end of the gradient (Figure 2B) were done as reported by Whittle et al.,38 while spectra recorded at the allylamine rich side (Figure 2C) were analyzed as discussed by Hook et al.37 As shown in Figure 2B and C, the high resolution C 1s peaks from the octadiene and allylamine rich sides of the gradient could be fitted with 3 or 4 components, respectively. The C1 component was assigned to the aliphatic hydrocarbon peak (C−C/C−H) at 285 eV. For the spectrum recorded at 0.1 cm, C2 was assigned to C−O (286.4 eV). In the C 1s spectrum recorded at 0.1 cm, C2 could include contributions from both C−O and C−N. The C3 component around 287.8 eV can be assigned to CO and NCO binding environments. As shown in the survey scan results inserted in Figure 2B, XPS analysis at 0.1 cm also reveals the presence of nitrogen, likely due to some allylamine polymer diffusing under the mask. Finally, in the C 1s scan obtained at 1.3 cm, C4 at 289.1 eV was assigned to O−CO. Quantification of these different components for the spectra shown in the main text Figure 2 8360

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Figure 2. (A) Nitrogen to carbon atomic ratio from survey spectra obtained by XPS along different positions along the plasma polymerized octadiene/allylamine gradient copolymer film; (B and C) High resolution C 1s spectra with survey spectrum (inset) at gradient positions 0.1 and 1.3 cm, respectively, indicated by gray dashed arrows.

is given in Table S1 in the Supporting Information. The presence of oxygen functional groups in thin films generated by plasma polymerization of non-oxygen-containing monomers is well-known and has been ascribed to the presence of water and residual oxygen in the reactor or post-plasma oxidation.38 On plasma polymerized allylamine, Hook et al. found a C2/Ctot value of 23.8% and N/C = 0.14, which are close to the same numbers calculated on the gradient samples at 1.3 cm (29.1%, respectively, ∼0.16). Hook et al. also found that the C2 carbon signal revealed a large proportion of C−N species, which were attributed to primary amine groups.37 The authors showed that this abundance was sufficient to allow a high density of polymers bearing aldehyde groups to be conjugated to the surface resulting in the conclusion that the aldehyde plasma polymer contained a relatively large surface density of primary amine groups that could participate in nucleophilic substitution reactions. Taken together, however, Figure 2A nicely illustrates that changing the composition of the allylamine/1,7-octadiene feed during the plasma polymerization process allows generation of a gradient surface that displays a gradual increase in the nitrogen content along the substrate, as determined from the survey XPS scans. The increase in nitrogen content on the substrate is also nicely reflected in the high resolution C 1s spectra in Figure 2B and C, which shows an increase in the C2

and C3 components when moving from 0.1 to 1.3 cm along the substrate. Post-modification of Plasma-Polymerized Gradient Surfaces and Surface-Initiated Atom Transfer Radical Polymerization. The amine gradient was transformed into an initiator gradient through acid-bromide coupling with BiBB. Nucleophilic attack of primary amine groups toward BiBB requires the use of anhydrous solvents; therefore, the plasma polymer stability was investigated with a selection of common solvents. Ellipsometric analysis indicated that some reordering and minor swelling of the plasma polymer occurs in various solvents, but does not result in drastic thickness changes, except in DMSO, which was found to lead to delamination (Supporting Information, Table S2). As a consequence, chemical modification of the plasma polymerized interlayer can be performed in a wide range of organic solvents; for the coupling of BiBB, toluene was selected. As indicated in Figure 1, coupling of BiBB to the plasma polymerized octadiene/ allylamine gradient films did not appreciably change the overall layer thickness except perhaps a slight (but not significant) change in thickness due to swelling and drying as mentioned. The BiBB post-modified plasma-polymerized interlayers were further analyzed by XPS. These XPS analyses were performed on a second, separately prepared set of plasma polymerized interlayers, which were polymerized and functionalized using 8361

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Figure 3. (A) Bromine relative atomic percent from survey spectra obtained by XPS along different positions along the BiBB-conjugated gradient surface. (B and C) High resolution C 1s spectra at gradient positions 0.2 and 1.4 cm, respectively, indicated by dashed arrows.

t-butyl bromide surface functional groups could be inferred from their chemical nature, viz., their unique ability to generate radicals and growing chains in SI-ATRP. After exposure to CuCl/CuBr2, ligand, and HEMA, polymer chains were grown from the surfaces, which led to an appreciable increase in thickness of the polymer film. XPS confirmed the presence of grafted polymer with all positions along the gradient displaying a high resolution C 1s spectrum typical of PHEMA (Supporting Information, Figure S4). Figure 4 plots the thickness of the PHEMA brush layer as a function of position along the substrate for surface-initiated atom transfer radial polymerizations that were carried out using different reaction times and monomer concentrations. The results indicate that the PHEMA brush thicknesses depend on both the duration of the polymerization and the monomer concentration. Separately, increasing both the polymerization time and the monomer feed concentration allowed for greater degrees of polymerization, which is evident from the increasing thicknesses observed by ellipsometry. Except for polymerizations performed at a monomer concentration of 1 M for 1 and 2 h and for SIATRP reactions carried out for 1 h with a HEMA concentration of 1.5 M, the thickness of the PHEMA brushes grafted from the plasma-polymerized interlayer corresponds relatively well with those that were obtained by SI-ATRP of HEMA from silicon wafers that were uniformly modified with (and assumed to be at maximum grafting density) 6-2-(2-bromo-2-methyl)-

the same procedures as described above with the only difference being a slightly higher amount of nitrogen observed on the surface due to variabilities in the plasma polymerization including the need to recalibrate flow valves. The evolution of the N/C ratio versus position along the substrate for this precursor allylamine/1,7-octadiene film is included in the Supporting Information (Figure S3) and is qualitatively similar to the profile shown in Figure 2. Results of the XPS analysis of the plasma-polymerized film after post-modification with BiBB are summarized in Figure 3. As illustrated in Figure 3A, postmodification of a gradient allylamine/1,7-octadiene plasma polymer film with BiBB generates a surface that displays a gradual bromine surface concentration gradient along the substrate. The high-resolution C 1s spectrum on the initiatorrich side of the gradient (Figure 3C) is similar to a similar position on the amine gradient (Figure 2C) except for slight a slight decrease in area of components C2 and small increases to C3 and C4 which are the result of initiator coupling. The presence of CO in the surface-bound initiator likely increases higher-energy carbon/oxygen photoelectrons (around 287.9 eV, C3 component), but definitive assignment of this and C− Br environments is difficult owing to the thinness of the initiator overlayer against the background of the plasmapolymerized surface. Surface-Initiated Atom Transfer Radical Polymerization. Empirical evidence for the presence of ATRP active 8362

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Figure 5. Thickness of PHEMA brushes grown from silicon substrates that were uniformly modified with SI-ATRP initiator 1. SI-ATRP was carried out using three different monomer concentrations, viz. (blue ◆) 1.0 M, (red ■) 1.5 M, (green ▲) 2.0 M. Thickness given is a function of monomer concentration and polymerization time and was determined by AFM.

octadiene) starting when the slit mask was at a position of 0.5 cm along the sample, diffusion under the mask allowed nitrogen-containing species to be deposited toward the very beginning of the sample. Since both the nitrogen content and the bromine content at this end of the gradient are very small, the surface concentration of initiator and thus the grafting density (and thus the film thickness) at this end of the gradient are predicted to be low. As illustrated in Figure 4, performing the SI-ATRP of HEMA at a monomer concentration of 2 M and a polymerization time of 4 h results in a density gradient surface, where the polymer brush film thickness at the low density side of the gradient is about 2.5 times less compared to the film thickness measured at the higher grafting density side of the gradient. As all surfacetethered polymer chains have grown for the same time, this observed increase in film thickness reflects the increased extension of the polymer chains due to the increased steric repulsions they are exposed to upon increasing the grafting density along the gradient substrate. There is good agreement between the evolution of the surface concentration of nitrogen (and therefore assumed initiator content) and the shapes of the height profiles of the grafted samples as a function of gradient position along the surface (Figure 6). The sigmoidal shape present in both data overlays shows that there is good fidelity from the plasma polymer chemical composition giving rise to the anchored initiator and the thickness of the resulting grafted polymer. This profile is similar to what has been reported previously.14,26,36 Finally, a set of experiments was carried out to evaluate the possible influence of the nature of the ATRP initiator on the surface-initiated PHEMA growth profiles. This was thought to be relevant since the PHEMA brush thicknesses reported in Figure 5 (which largely correspond well with the data on the gradient surfaces in Figure 4) were obtained from silicon oxide surfaces modified with the ester-linked ATRP initiator 1. BiBB attachment to the amine-functionalized plasma-polymerized interlayers, however, generates an amide coupled ATRP initiator. To compare the possible influence of initiator attachment on polymer brush growth, PHEMA brush films were grown from silicon oxide substrates modified with 6-2-(2bromo-2-methyl)propionyloxy)hexyldimethylchlorosilane (1) and 2-bromo-2-methyl-N-{3-[chloro(dimethyl)silyl]-propyl}propanamide (2). As can be seen from Supporting Information

Figure 4. PHEMA brush thickness as a function of polymerization time and position along the substrate as measured by ellipsometry: (A) 1.0 M HEMA; (B) 1.5 M HEMA; (C) 2.0 M HEMA. (blue ◆) 1 h; (red ■) 2 h; (green ▲) 4 h. Reported thickness is of the PHEMA layer (plasma polymer and initiator layers subtracted).

propionyloxy)hexyldimethylchlorosilane (1) (Figure 5). To facilitate comparison of the growth profiles of the PHEMA films generated from the gradient substrates with those grafted from silicon wafers, Supporting Information Figure S5 shows the evolution of PHEMA films thickness at three points at the higher grafting density side of the gradient substrates as a function of polymerization time. As shown in Figure 4, PHEMA grafting was also observed on the initiator-poor side of the sample at the smaller gradient positions. The nonzero thickness of the PHEMA brush layer at these positions is due to the fact that the nitrogen content on this side as determined by XPS was also nonzero (Figure 2A) and resulted in bromine being attached (Figure 3B) acting as surface initiators for polymerization. An explanation for this is that allylamine vapors are able to diffuse underneath the stainless steel mask within a small volume above the sample where the mask sits above the silicon wafer in this reactor (approximately 0.1 cm).21,22 Even though the allylamine was only introduced and plasma polymerized (along with 8363

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*E-mail: harm-anton.klok@epfl.ch. Notes

The authors declare no competing financial interest.

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ACKNOWLEDGMENTS B.R.C. gratefully acknowledges the assistance of Ms. Jamila Joseph and Ms. Urška Ž bogar for materials preparation and thickness measurements. This work was supported partly by the NHMRC via Project grant 631931 (B.R.C.) and the European Commission (H.-A. K., T.B.: HiPerDART).

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Figure 6. Superimposed data of PHEMA brush thickness (left axis) and XPS rel. nitrogen percentage (right axis, ○) as a function of analysis position. [HEMA] = 1.0 M at (blue ◆) 1 h and (red ■) 2 h. Data are superimposed to show the congruence in shape of gradient. A smoothed dotted line has been added to join the XPS data to add clarity.

Figure S6, there is no appreciable difference between the film growth profiles for the brushes obtained from the ester and amide linked BiBB groups. It is important to note that the thicknesses represented in Figure 5 were obtained by AFM, whereas the data in Figure 4 were acquired via ellipsometry. As shown in Supporting Information Figure S1, however, film thickness analysis of PHEMA brushes using these two techniques gives consistent results.

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CONCLUSIONS We have shown a new way to generate polymer brush layers which, due to an increasing gradient in surface density, are subject to crowding. This results in an increasingly extended conformation of the surface-tethered polymer chains along the gradient surface and a concomitant gradual increase in polymer film thickness. Gradient plasma polymerization allows virtually any substrate material to be coated with gradient copolymers, which control the spatial density of functional groups allowing attachment of ATRP initiators. Polymer deposition by plasma polymerization removes the requirement of choosing a starting substrate with a specific chemistry or functionality (gold or silicon, for example) and opens the way for gradient samples prepared in a substrate-independent fashion. Importantly, the starting plasma step allows for linking of the grafted polymer layers to the substrate through covalent bonds and the good solvent compatibility of the system demonstrates the robustness of the technique. There will also be value to a wide field of research where one could envision the use of this platform to study how changing surface topology or stiffness affects, for example, biomolecule or cell interactions with surfaces.39

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ASSOCIATED CONTENT

S Supporting Information *

Additional XPS and ellipsometry data as well as a comparison of ellipsometry and AFM analysis of PHEMA brush thickness. This material is available free of charge via the Internet at http://pubs.acs.org.

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