Article pubs.acs.org/Macromolecules
Postpolymerization Modification of Poly(glycidyl methacrylate) Brushes: An XPS Depth-Profiling Study Raphael Barbey,†,∥ Vincent Laporte,‡ Saad Alnabulsi,§ and Harm-Anton Klok*,† Institut des Matériaux and Institut des Sciences et Ingénierie Chimiques, Laboratoire des Polymères, École Polytechnique Fédérale de Lausanne (EPFL), Bâtiment MXD, Station 12, CH-1015 Lausanne, Switzerland ‡ Centre Interdisciplinaire de Microscopie Électronique, Surface Analysis Facility, École Polytechnique Fédérale de Lausanne (EPFL), Bâtiment MXC, Station 12, CH-1015 Lausanne, Switzerland § Physical Electronics, Inc., 17825 Lake Drive East, Chanhassen, Minnesota 55317, United States †
S Supporting Information *
ABSTRACT: Surface-initiated polymerization represents a versatile strategy to modify a diverse range of materials with thin, functional polymer coatings. While in many cases the desired functional groups can be directly incorporated via (co)polymerization of the appropriate monomer(s), other functional groups are incompatible with the polymerization strategies that are commonly used to grow polymer brushes and can only be introduced by postpolymerization modification. Determining the local concentration and spatial distribution of these functional groups in postmodified brushes is a challenging task but could help to optimize the design and properties of these polymer coatings. This article reports on the use of X-ray photoelectron spectroscopy (XPS) combined with C60 cluster ion sputtering to address this challenge. Poly(glycidyl methacrylate) (PGMA) brushes prepared via surface-initiated atom transfer radical polymerization (SI-ATRP) were used as a model platform and were postmodified with propylamine and bovine serum albumin (BSA). The XPS depth-profiling experiments showed that the small propylamine molecules were essentially homogeneously distributed throughout the brush, with the exception of the top few nanometers, which were enriched in propylamine moieties. It was also demonstrated that the amount of propylamine introduced within the polymer brush increased with increasing the postpolymerization reaction time, while no concentration gradients could be observed, indicative of a fast diffusion of the propylamine through the polymer brush layer. On the other hand, XPS depth-profiling experiments performed on polymer brushes that were postmodified with BSA revealed that this protein was only localized in the topmost layers of the polymer coating, which reflects the steric hindrance by the dense polymer brush that prevents efficient diffusion of these large molecules. Together, the results of these experiments demonstrate that XPS depth-profiling combined with C60 cluster ion sputtering is an efficient and powerful means to study the distribution of functionalities incorporated within a polymer brush layer by postpolymerization modification reactions.
■
INTRODUCTION
functional moieties that cannot be introduced via direct surfaceinitiated polymerization of the corresponding monomer.1 In spite of the fact that postpolymerization modification reactions are widely used to functionalize polymer brushes, only relatively little is known about the location and distribution of these functional groups in the final polymer brush. It seems very likely, however, that steric constraints, e.g. due to the high grafting density of the brushes combined with the size and/or polarity of the reagents that are used during postpolymerization modification, can lead to an inhomogeneous distribution of functional groups and concentration gradients throughout the brush layer. Insight into the localization and distribution of functional groups in postmodified polymer brushes could help to optimize the design of the polymer brushes (e.g., by engineering grafting density) and/or adjust the postpolymerization modification reaction parameters and ultimately control
Polymer brushes, which consist of a dense arrangement of individual polymer chains that are tethered by one of their chain ends to a substrate, represent a very versatile platform to engineer the surface properties and functionality of a wide range of materials.1 Among the different methods that exist for the preparation of these thin polymer coatings, surface-initiated controlled radical polymerization techniques have attracted a lot of attention over the past two decades, as they allow precise control over brush thickness, composition, and architecture.1−7 A particular advantage of these radical-based strategies is that they are relatively tolerant to a broad range of functional groups. This allows the direct preparation of polymer brushes bearing in their side chains reactive pendant moieties, such as e.g. hydroxyl,8−17 carboxylic acid,18−20 epoxide,21−27 active ester,28−31 α-hydroxyalkyl ketone,32 or isocyanate33 groups, which can be either directly utilized as chemical handles to introduce specific functional groups or further transformed using appropriate postpolymerization modification into reactive © 2013 American Chemical Society
Received: April 21, 2013 Revised: June 20, 2013 Published: July 15, 2013 6151
dx.doi.org/10.1021/ma400819a | Macromolecules 2013, 46, 6151−6158
Macromolecules
Article
(SI-ATRP) were performed on silicon wafers cut in pieces of 2 cm × 0.8 cm. Analytical Methods. Brush thicknesses were determined by means of a Sopra GES 5E spectroscopic ellipsometer working at an angle of incidence of 75°. The calculation method was based on a three-layer silicon/polymer brush/ambient model, assuming the polymer brush to be isotropic and homogeneous. The polymer brush layer was modeled using a Cauchy model with an initial An parameter value of 1.45. All reported ellipsometric film thicknesses represent an average over 5 data points taken from the same substrate and are corrected for the approximately 2 nm-thick native oxide layer on the silicon substrates. X-ray photoelectron spectroscopy (XPS) data were collected at Physical Electronics headquarters (Chanhassen, MN) using a PHI VersaProbe instrument equipped with a microfocused monochromatic Al Kα X-ray source (1486.6 eV). The area illuminated by the X-ray beam was a 100 μm in diameter rastered over a 1400 μm line. The takeoff angle between the surface and the direction in which the photoelectrons were analyzed was 45°. Dual beam charge neutralization was employed. To minimize chemical damage that may occur upon prolonged X-ray exposure during a depth profile, short acquisition times were employed and the source power was maintained at 100 W. The high-resolution spectra were thus collected at low energy resolution (117.4 eV) and were then mathematically deconvoluted to resolve the subtle chemical structure. This provided the ability to acquire spectra with high counting statistics in less time with low X-ray dose. C60 sputtering with XPS analysis provided quantitative chemical state information as a function of depth with minimal ion beam induced chemical damage. A 10 keV, 10 nA C60 ion source was rastered over a 2 mm × 2 mm area to create the depth profile (with an observed sputter rate of ∼8.5 nm min−1 for PGMA). To minimize sputtering artifacts and improve interface definition, Zalar (azimuthal) rotation and 70° incidence angle for C60 sputter depth-profiling were used for this work. In addition, relative sensitivity factors (RSF) of 58.808 (C1s), 137.432 (O1s), 93.508 (N1s), and 80.117 (Si2p) were used to correct area ratios. Procedures. ATRP Initiator-Functionalized Substrates. Silicon substrates were functionalized with the ATRP initiator according to a published procedure.22 After successful functionalization, the substrates were stored until needed for a polymerization. Preparation of Poly(glycidyl methacrylate) (PGMA) Brushes. The polymer brushes were prepared following a published procedure.22 Briefly, the reaction mixture (GMA/CuICl/CuIIBr2/2,2′-bipyridyl in a molar ratio of 2000/20/1/50) was canula-transferred to nitrogenpurged reaction vessels containing the initiator-functionalized silicon substrates. After a polymerization time of 6 h at room temperature, the substrates were removed from the ATRP solution, rinsed thoroughly with methanol, and then consecutively washed in methanol (1 h), dichloromethane (30 min), and acetone (30 min). Finally, the polymer brush-coated silicon substrates were rinsed with ethanol, dried under vacuum, and characterized by ellipsometry and XPS. Postpolymerization Modification of PGMA Brushes. After the determination of the dry ellipsometric thicknesses of the pristine, unmodified samples, the polymer brush-coated substrates were incubated in aqueous solutions of either propylamine (1 M in water) or bovine serum albumin (BSA) (50 mg mL−1 in PBS). At predefined reaction times, the substrates were taken out from the reaction solutions, thoroughly rinsed with water, and dried under a stream of compressed air. The postmodified samples were analyzed by XPS, and their dry thicknesses were determined by ellipsometry.
functional group density and placement. Information about the overall concentration of functional groups in a polymer brush can usually be relatively readily obtained using, for example, UV−vis or Fourier transform infrared (FTIR) spectroscopy or X-ray photoelectron spectroscopy (XPS). While the former two techniques provide information about the overall functional group concentration in the polymer brush, XPS probes the chemical composition in the top 1−10 nm of the brush only. Assessing the distribution of functional groups throughout a postmodified polymer brush is more complex and requires much more advanced analytical tools. In a previous study, neutron reflectivity was used to probe the localization and distribution of deuterated functional groups in postmodified poly(2-hydroxyethyl methacrylate) brushes.14 This article explores the use of XPS depth-profiling analysis to study the localization and distribution of functional groups in a postmodified polymer brush. XPS allows the quantitative determination of the elemental surface composition of a material by irradiating the substrate with an X-ray beam and simultaneously measuring the kinetic energy and amount of electrons escaping from the top 1−10 nm of the material. The use of sputtering techniques in combination with XPS permits to study the chemical composition as a function of film thickness (sputtering depth). In contrast to argon ion sputtering, which is known to cause chemical damage to soft materials (such as polymer films),34 this study uses a C60 cluster ion source, which has been shown to induce less damage during sputtering.35−37 Analysis of the XPS spectra obtained at different sputtering times provides insight into the chemical brush composition as a function of film thickness. In this study, the postpolymerization modification of poly(glycidyl methacrylate) (PGMA) brushes with propylamine and bovine serum albumin (BSA) was used as a first proof-of-concept to explore the feasibility of XPS depthprofiling to investigate the functional group distribution in postmodified polymer brushes. PGMA represents an attractive platform for a postpolymerization modification study as it can be utilized, without any prior activation, to immobilize (bio)molecules via the nucleophilic ring-opening of its oxirane side-chain functional groups.21−24 The results presented in this article indicate that the small propylamine molecules are distributed homogeneously within the polymer layer, while BSA molecules are only grafted at the top few nanometers of the polymer brush and thus do not take advantage of the 3D sampling volume that is offered by the polymer film. In addition, the obtained data show that the amount of incorporated propylamine increases homogeneously with increasing the postpolymerization modification reaction time but that the amount of BSA immobilized onto PGMA brushes remains constant as a function of reaction time.
■
EXPERIMENTAL SECTION
■
Materials. Unless stated otherwise, all chemicals were obtained from Sigma-Aldrich and were used as received. Ultrahigh-quality water with a resistance of 18.2 MΩ·cm (at 25 °C) was obtained from a Millipore Milli-Q gradient machine fitted with a 0.22 μm filter. Phosphate buffered saline (PBS, pH = 7.4) was prepared from PBS tablets (Sigma). The inhibitor in glycidyl methacrylate (4-methoxyphenol) was removed by passing the monomer through a column of activated, basic aluminum oxide. The atom transfer radical polymerization (ATRP) initiator, 6-(chloro(dimethyl)silyl)hexyl 2-bromo-2methylpropanoate, was synthesized according to a published protocol.22 Surface-initiated atom transfer radical polymerizations
RESULTS AND DISCUSSION To investigate the feasibility of XPS combined with C60 ion sputtering to characterize polymer brushes, this technique was first evaluated for the analysis of unmodified PGMA brushes and subsequently used to study the postpolymerization modification of these brushes with propylamine and BSA. The next two sections will successively present and discuss the results of these experiments. 6152
dx.doi.org/10.1021/ma400819a | Macromolecules 2013, 46, 6151−6158
Macromolecules
Article
Scheme 1. Preparation of PGMA Brushes via SI-ATRP from ATRP Initiator-Functionalized Silicon Wafers
Unmodified Poly(glycidyl methacrylate) Brushes. PGMA brushes were grown from silicon substrates via a strategy involving the immobilization of an ATRP initiator (6(chloro(dimethyl)silyl)hexyl 2-bromo-2-methylpropanoate), followed by the room temperature SI-ATRP of glycidyl methacrylate in a methanol/water solvent mixture using a CuICl/CuIIBr2/2,2′-bipyridyl (bpy) catalytic system (Scheme 1). After 6 h of polymerization, the brush layer thickness was determined by ellipsometry to be 100.6 ± 0.5 nm. The chemical composition of the PGMA brushes was confirmed by XPS analysis. Figure S1 shows the XPS highresolution C1s and O1s scans of a 100 nm-thick PGMA brush, and Table S1 summarizes the different binding energies and area ratios determined for each of the different carbon and oxygen atoms of the PGMA brush. In agreement with the literature,22,38 the high-resolution C1s spectrum can be fitted with the expected area ratios with five Gaussian/Lorentzian curves, corresponding to the carbons atoms from the polymer backbone (C−C/C−H, 284.8 eV), the carbon atoms adjacent to the ester groups (C−CO, 285.1 eV), the C−O moiety (286.3 eV), the carbon atoms of the oxirane rings (C−O−C, 286.6 eV), and those of the ester groups (O−CO, 288.8 eV), whereas the high-resolution O1s signal can be fitted with the correct area ratios with three curves, which correspond to the oxygen atoms from the O−CO (533.4 eV), C−O−C (532.9 eV), and O−CO (532.1 eV) groups. Next, as a first evaluation of the feasibility of combining C60 ion sputtering and XPS analysis for the depth-profiling of polymer brushes prepared by SI-ATRP, the chemical composition of a 100 nm-thick PGMA brush was probed as a function of layer thickness (Figure 1). As observed in Figure 1A, the chemical composition remains stable throughout the polymer brush layer, which is good evidence that C60 ions only induce negligible chemical damage during sputtering, as compared to the significant changes in polymer film composition that were observed when argon sputtering was used for depth-profiling (Figure S2). The variation in C1s and O1s concentrations as a function of C60 ion sputtering time is minimal. The C1s concentration only increased from 71.5% to 74.3% over the first 10 min of sputtering, whereas the O1s concentration decreased from 28.5% to 25.7% over the same time period. Therefore, the C1s/O1s ratio remains very close to its theoretical value of 7/3 throughout the polymer brush layer, which confirms this method is suitable for depth-profiling study of polymer films. Figure 1A also indicates that about 12 sputtering cycles (of 60 s each) using 10 keV C60 ions are needed to remove a 100 nm-thick layer of PGMA brush and reach the silicon substrate, which allows to determine an approximate sputtering rate for PGMA of 8.5 nm min−1. This value is in good agreement with the sputtering rate of 13 nm min−1 determined for a PMMA film sputtered with 20 keV C60 ions.39
Figure 1. C60 depth profiles of unmodified PGMA brushes. C1s, O1s, N1s, and Si2p atomic concentrations as a function of sputtering time and film thickness (if the error bars are not visible, they fall into the data point) (A); XPS high-resolution C1s (B) and O1s (C) signals as a function of sputtering cycle.
6153
dx.doi.org/10.1021/ma400819a | Macromolecules 2013, 46, 6151−6158
Macromolecules
Article
Scheme 2. Aqueous, Room Temperature Postpolymerization Modification of PGMA Brushes with Propylamine or Bovine Serum Albumin (BSA), Including Possible Side Reactionsa
a
Note: cross-linking can occur via either intra- or interchain reactions.
Figure 2. C60 depth profiles of PGMA brushes reacted with a 1 M propylamine solution for 1 h. C1s, O1s, N1s, and Si2p atomic concentrations as a function of sputtering time and film thickness (if the error bars are not visible, they fall into the data point) (note: the N1s signal intensity has been multiplied by 10) (A); XPS high-resolution C1s (B), O1s (C), and N1s (D) signals as a function of sputtering cycle.
ions36,40 and is responsible for the slight, but continuous, increase in the C1s atomic concentration and the variation of the C1s/O1s ratio as a function of sputtering time. Figure 1B also illustrates the rapid decrease in the C1s concentration when the silicon substrate is reached (after 12 min of sputtering). The signal that appears at a binding energy of approximately 283 eV once the polymer has been completely removed (≥14 min of sputtering) is characteristic of silicon carbide (Si−C) that is formed upon bombardment of the bare silicon substrate
The XPS high-resolution C1s (Figure 1B) and O1s (Figure 1C) spectra of the PGMA brushes illustrate in more detail the evolution of the chemical composition of the brush layer as a function of the number of sputtering cycles (i.e., depth). Figure 1B shows a small variation in the C1s composition of the film with increasing sputtering cycles with an increase of the signal attributed to aliphatic carbon atoms at 284.8 eV. This surface contamination, although minor, is due to carbon atoms introduced during the sputtering of the film with C60 cluster 6154
dx.doi.org/10.1021/ma400819a | Macromolecules 2013, 46, 6151−6158
Macromolecules
Article
Figure 3. Evolution of the N1s atomic concentration as a function of sputtering time and approximately dry ellipsometric film thickness of PGMA brushes after incubation in a propylamine solution (1 M in water) for different postpolymerization modification reaction times (ranging from 5 min to 48 h). The dashed lines represent the N1s atomic concentration averaged over the sputtering times ranging from 60 to 720 s.
with C60 ions.41,42 Figure 1C shows that the O1s chemical composition remains essentially unchanged upon sputtering with C60 ions until an abrupt increase in the O1s content, which is due to the native oxide layer at the interface between the polymer brush and the silicon substrate, is observed. After that, the O1s signal continuously decreases while additional sputtering cycles are performed. It is worth mentioning that the sputtering rate of the hard silicon substrate is a lot slower (i.e., approximately 0.4 nm min−1) than that of the soft, polymeric film. In conclusion, this first set of experiments confirms that XPS depth-profiling using C60 cluster ion sputtering is a suitable method for determining the chemical composition of polymer brush films as a function of depth. Sputtering with C60 ions has been proven a relatively mild process for the depth-profiling of PGMA brushes with only very limited surface contamination and artifacts. This depth-profiling strategy is therefore a useful technique to quantify and determine the distribution of (bio)molecules that are immobilized within polymer brush layers that have been modified via postpolymerization modification reactions. Postpolymerization Modification of PGMA Brushes. In a next set of experiments, XPS analysis combined with C60 cluster ion sputtering was used to evaluate the distribution of functional groups after postpolymerization modification of the PGMA brushes with propylamine or BSA (Scheme 2). To this end, the PGMA brushes were incubated at room temperature for different, predefined reaction times in solutions of propylamine (1 M in water) or BSA (50 mg mL−1 in PBS). These amino-containing compounds are readily immobilized via the nucleophilic ring-opening of the oxirane side chain functional groups of the polymer brushes. These two sets of experiments allow the investigation of the influence of the size of the molecules on their final distribution within the polymer brush layer. For the postpolymerization modification with propylamine, reaction times were varied ranging from 5 min to 48 h. Figure 2 illustrates the evolution of the C1s, O1s, N1s, and Si2p atomic
concentrations as well as the C1s, O1s, and N1s XPS highresolution scans as a function of sputtering time for a 100 nmthick PGMA brush that was modified for 1 h in a propylamine solution (1 M in water). (Figures S3−S8 in the Supporting Information show the results for the other postpolymerization modification reaction times that were investigated.) Similar to what was observed during the XPS depth-profiling of the unmodified PGMA brushes, which was discussed in the previous section (vide supra), the C1s and O1s atomic concentrations remain stable as a function of sputtering time (Figure 2A), indicating minimal surface contamination due to the sputtering with C60 ions. It is worth noting, however, that the overall shape of the XPS high-resolution C1s spectra (Figure 2B) indicates the presence of a slightly increased amount of aliphatic carbon atoms as compared to the unmodified PGMA brushes, which is expected from the successful incorporation of propylamine into the polymer layer. As in Figure 1C, the XPS high-resolution O1s signals of the postmodified PGMA brush remain unchanged until the native oxide layer interface layer is reached, after which the O1s content decreases constantly. The appearance of a clear N1s signal at around 399.4 eV proves that the incorporation of propylamine via nucleophilic ring-opening of side chain epoxide groups of the PGMA brushes was successful. Moreover, the evolution of the N1s atomic concentration as a function of sputtering time indicates that the distribution of the immobilized propylamine is largely homogeneous throughout the modified brush. Only the topmost layer of the polymer brush seems to be enriched with propylamine moieties as indicated by the high intensity of the N1s signal recorded before the first sputtering cycle. Figure 3 illustrates the evolution of the nitrogen distribution profiles (i.e., N1s atomic concentration vs sputtering time) as a function of reaction time for the postpolymerization modification of a 100 nm-thick PGMA brush with propylamine. At any given reaction time, except for the topmost layer, the N1s atomic concentration is relatively constant throughout the polymer film, which confirms that the propylamine is evenly distributed within the brush. It is interesting to note that even 6155
dx.doi.org/10.1021/ma400819a | Macromolecules 2013, 46, 6151−6158
Macromolecules
Article
at relatively short reaction times essentially no concentration gradients throughout the polymer film are observed, which suggests that propylamine diffuses in a fast and homogeneous manner through the polymer brush layer. Increasing the postpolymerization modification reaction time results in a gradual increase in the average, almost depth-independent, N1s concentration in the polymer brush films. Using the dashed lines indicated in Figure 3 to represent the average N1s atomic concentration in a polymer brush at a given postpolymerization modification reaction time, a kinetic plot can be established for the postpolymerization modification of a PGMA brush with propylamine as illustrated in Figure 4. Figure
content, i.e., the introduction of propylamine moieties. Both approaches, however, are plagued by side reactions that accompany the postpolymerization modification and which lead to an overestimation of epoxide group conversion by FTIR and an underestimation by XPS. Scheme 2 illustrates some possible side reactions. A first possible side reaction could be hydrolysis of some of the epoxide groups upon incubation in the aqueous propylamine solution. However, this seems unlikely as it has been demonstrated that hydrolysis of PGMA brushes is minimal on the time scale of the postpolymerization experiments studied here (up to 48 h).22 A second and probably more relevant side reaction involves the formation of inter- and intrachain cross-links by reaction of the immobilized, secondary amines with neighboring epoxide moieties. Given the extremely dense spatial distribution of epoxide groups within a polymer brush system, this side reaction seems more likely. Cross-linking of PGMA brushes by primary amines (but in ethanolic solutions at 60 °C) has indeed already been reported earlier.23 A second series of depth-profiling experiments was performed to investigate the postpolymerization modification of the PGMA brushes with BSA. Figure 5 illustrates the C60 depth profile, as well as the XPS high-resolution C1s, O1s, and N1s scans of a PGMA brush incubated for 8 h in a BSA solution (50 mg mL−1 in PBS) (see Figures S10−S12 for other postpolymerization modification reaction times). The depth profiles as well as the high-resolution scans underline again the nondestructive nature of the C60 cluster ion sputtering. The evolution of the N1s atomic concentration, which can be obtained from the XPS spectra, as a function of sputtering time for each of the different reaction times investigated, is illustrated in Figure 6. In contrast to propylamine, which was found to be distributed relatively homogeneously throughout the brush, the N1s depth profile and high-resolution spectra indicate that the BSA molecules are essentially immobilized in the top 10 nm of the PGMA brush. In addition to the obviously increased steric hindrance of BSA as compared to the small propylamine molecules, the multiple peripheral amine moieties of the proteins could also lead to cross-linking of the topmost layers of the polymer brush and therefore might even further hinder the diffusion of BSA into the polymer brush. These results are in agreement with earlier quartz crystal microbalance with dissipation (QCM-D) studies22 as well as with the results of ellipsometric analysis of dry polymer film thickness before and after reaction with BSA (Table S2). The size and polarity of the molecules utilized for the postpolymerization modification prove to have a clear influence over the spatial distribution of the functionalities within the resulting postmodified brush. Qualitatively similar results were previously obtained using neutron reflectivity on p-nitrophenyl chloroformate-activated poly(2-hydroxyethyl methacrylate) brushes postmodified with deuterated amino acids.14 The nonpolar leucine was found to be immobilized only in the topmost 20 nm of the brush, while the polar serine was demonstrated to be evenly distributed throughout the polymer brush.
Figure 4. Average N1s atomic concentration (as determined from Figure 3) and epoxide group conversion upon incubation of a 100 nmthick PGMA brush in 1 M propylamine solution in water at room temperature as a function of postpolymerization reaction times. The dashed line is included to guide the eye.
4 indicates a gradual decrease in the rate of the postpolymerization modification reaction with increasing reaction time, which can be attributed to the increase in steric hindrance within the brush, as well as the decrease in the number of available reactive epoxide groups over the course of the reaction. The average N1s atomic concentration also allows the estimation of the epoxide group conversion. Assuming that postpolymerization modification only involves reaction of one primary amine with one single epoxide group, it is possible to determine the percentage of reacted epoxide groups. If every PGMA unit would react with a propylamine moiety (i.e., p = n as well as q, r, and m = 0 in Scheme 2), then each repeating unit would contain 10 carbon atoms, 3 oxygen atoms, and 1 nitrogen atom. A fully postmodified PGMA brush would therefore have a N1s atomic concentration of 7.143%. This correlation permits to relate the N1s atomic content to the epoxide conversion (Figure 4). As indicated in Figure 4, a postpolymerization reaction time of 48 h results in an epoxide conversion of ∼40%. It is interesting to compare the results of the kinetic analysis of the postpolymerization modification of PGMA by XPS with data obtained by FTIR analysis, which were reported previously.22 Figure S9 compares these two sets of data and indicates that there is a clear difference between the data obtained by the two methods. Whereas XPS suggests an epoxide group conversion of ∼40%, FTIR analysis indicates that ∼60% of the epoxide groups in the brush have been modified. This discrepancy is due to the fact that the FTIR analysis is based on the disappearance of the epoxide groups, whereas with XPS analysis follows the N1s
■
CONCLUSIONS This study has investigated the potential of X-ray photoelectron spectroscopy (XPS) coupled with C60 cluster ion sputtering to determine the localization and spatial distribution of functional groups within a poly(glycidyl methacrylate) (PGMA) brush that was postmodified with propylamine and bovine serum albumin (BSA). In a first set of experiments, analysis of the N1s 6156
dx.doi.org/10.1021/ma400819a | Macromolecules 2013, 46, 6151−6158
Macromolecules
Article
Figure 5. C60 depth profiles of PGMA brushes reacted with BSA (50 mg mL−1 in PBS) for 8 h. C1s, O1s, N1s, and Si2p atomic concentrations as a function of sputtering time and film thickness (if the error bars are not visible, they fall into the data point) (note: the N1s signal intensity has been multiplied by 10) (A); XPS high-resolution C1s (B), O1s (C), and N1s (D) signals as a function of sputtering cycle.
throughout the polymer brush as no concentration gradient was observed, independently of the postpolymerization reaction time. Comparison of the XPS results with those obtained from earlier FTIR studies provided further information on the structure of the postmodified brushes and revealed that the postpolymerization modification process is accompanied by side reactions, which include cross-linking due to the reaction of attached, secondary amines with surrounding, unreacted epoxide groups. In a second set of experiments, the N1s atomic concentration profiles obtained from PGMA brushes postmodified with BSA showed that these proteins do not penetrate the interior of the polymer brush and are only immobilized at the topmost layer. The ability to determine the localization and distribution of molecules within postmodified polymer brushes, as exemplified in this article with two proof-of-concept experiments using XPS in combination with C60 cluster ion sputtering, is of great importance for the development and design of advanced materials, such as protein microarray substrates. The XPS method presented in this article is an attractive tool as in contrast to, for example, neutron reflectivity it does not require the use of deuterated or otherwise labeled probe molecules or solvents and can be performed on the standard substrates that are typically used for surface-initiated polymerizations.
Figure 6. Evolution of the N1s atomic concentration as a function of sputtering time and approximately dry ellipsometric film thickness of PGMA brushes after incubation in a BSA solution (50 mg mL−1 in PBS) for different postpolymerization modification times (ranging from 2 to 48 h).
■
depth profiles obtained using this method from PGMA brushes that were postmodified with propylamine permitted to obtain information on the distribution of secondary amine groups throughout the polymer brush layer. Propylamine was demonstrated to diffuse in a fast and homogeneous manner
ASSOCIATED CONTENT
S Supporting Information *
XPS high-resolution C1s and O1s spectra of PGMA brushes, XPS depth-profiling of an unmodified PGMA brush using argon sputtering, C60 depth profiles of postmodified PGMA 6157
dx.doi.org/10.1021/ma400819a | Macromolecules 2013, 46, 6151−6158
Macromolecules
Article
(23) Edmondson, S.; Huck, W. T. S. J. Mater. Chem. 2004, 14, 730− 734. (24) Xu, F. J.; Cai, Q. J.; Li, Y. L.; Kang, E. T.; Neoh, K. G. Biomacromolecules 2005, 6, 1012−1020. (25) Xu, F. J.; Zhong, S. P.; Yung, L. Y. L.; Tong, Y. W.; Kang, E.-T.; Neoh, K. G. Biomaterials 2006, 27, 1236−1245. (26) Huang, J. S.; Li, X. T.; Zheng, Y. H.; Zhang, Y.; Zhao, R. Y.; Gao, X. C.; Yan, H. S. Macromol. Biosci. 2008, 8, 508−515. (27) Iwasaki, Y.; Omichi, Y.; Iwata, R. Langmuir 2008, 24, 8427− 8430. (28) Parvin, S.; Matsui, J.; Sato, E.; Miyashita, T. J. Colloid Interface Sci. 2007, 313, 128−134. (29) Arnold, R. M.; Sheppard, G. R.; Locklin, J. Macromolecules 2012, 45, 5444−5450. (30) Choi, J.; Schattling, P.; Jochum, F. D.; Pyun, J.; Char, K.; Theato, P. J. Polym. Sci., Part A: Polym. Chem. 2012, 50, 4010−4018. (31) Günay, K. A.; Schüwer, N.; Klok, H.-A. Polym. Chem. 2012, 3, 2186−2192. (32) Mardyukov, A.; Li, Y.; Dickschat, A.; Schäfer, A. H.; Studer, A. Langmuir 2013, 29, 6369−6376. (33) Hensarling, R. M.; Rahane, S. B.; LeBlanc, A. P.; Sparks, B. J.; White, E. M.; Locklin, J.; Patton, D. L. Polym. Chem. 2011, 2, 88−90. (34) Holländer, A.; Haupt, M.; Oehr, C. Plasma Processes Polym. 2007, 4, 773−776. (35) Sanada, N.; Yamamoto, A.; Oiwa, R.; Ohashi, Y. Surf. Interface Anal. 2004, 36, 280−282. (36) Nobuta, T.; Ogawa, T. J. Mater. Sci. 2009, 44, 1800−1812. (37) Nieuwjaer, N.; Poleunis, C.; Delcorte, A.; Bertrand, P. Surf. Interface Anal. 2009, 41, 6−10. (38) Beamson, G.; Briggs, D. High Resolution XPS of Organic Polymers: The Scienta ESCA300 Database; John Wiley & Sons: Chichester, 1992. (39) Szakal, C.; Sun, S.; Wucher, A.; Winograd, N. Appl. Surf. Sci. 2004, 231, 183−185. (40) Chen, Y. Y.; Yu, B. Y.; Wang, W. B.; Hsu, M. F.; Lin, W. C.; Lin, Y. C.; Jou, J. H.; Shyue, J. J. Anal. Chem. 2008, 80, 501−505. (41) Krantzman, K. D.; Kingsbury, D. B.; Garrison, B. J. Appl. Surf. Sci. 2006, 252, 6463−6465. (42) Krantzman, K. D.; Garrison, B. J. Surf. Interface Anal. 2011, 43, 123−125.
brushes after different reaction times, epoxide group conversion obtained by FTIR compared to the percentage of epoxide converted determined in this study, and thickness variation as a function of postpolymerization modification time determined by ellipsometry. This material is available free of charge via the Internet at http://pubs.acs.org.
■
AUTHOR INFORMATION
Corresponding Author
*E-mail harm-anton.klok@epfl.ch; Fax + 41 21 693 5650; Tel + 41 21 693 4866 (H.-A.K.). Present Address ∥
R.B.: Key Centre for Polymers & Colloids, School of Chemistry, Building F11, University of Sydney, NSW 2006, Australia.
Notes
The authors declare no competing financial interest.
■
ACKNOWLEDGMENTS This research was supported by CTI/KTI as well as the European Science Foundation’s Precision Polymer Materials (P2M) Research Networking Programme.
■
REFERENCES
(1) Barbey, R.; Lavanant, L.; Paripovic, D.; Schüwer, N.; Sugnaux, C.; Tugulu, S.; Klok, H.-A. Chem. Rev. 2009, 109, 5437−5527. (2) Pyun, J.; Matyjaszewski, K. Chem. Mater. 2001, 13, 3436−3448. (3) Edmondson, S.; Osborne, V. L.; Huck, W. T. S. Chem. Soc. Rev. 2004, 33, 14−22. (4) Tsujii, Y.; Ohno, K.; Yamamoto, S.; Goto, A.; Fukuda, T. Adv. Polym. Sci. 2006, 197, 1−45. (5) Orski, S. V.; Fries, K. H.; Sontag, S. K.; Locklin, J. J. Mater. Chem. 2011, 21, 14135−14149. (6) Azzaroni, O. J. Polym. Sci., Part A: Polym. Chem. 2012, 50, 3225− 3258. (7) Olivier, A.; Meyer, F.; Raquez, J.-M.; Damman, P.; Dubois, P. Prog. Polym. Sci. 2012, 37, 157−181. (8) Brantley, E. L.; Holmes, T. C.; Jennings, G. K. J. Phys. Chem. B 2004, 108, 16077−16084. (9) Xu, D.; Yu, W. H.; Kang, E. T.; Neoh, K. G. J. Colloid Interface Sci. 2004, 279, 78−87. (10) Sun, L.; Baker, G. L.; Bruening, M. L. Macromolecules 2005, 38, 2307−2314. (11) Tugulu, S.; Arnold, A.; Sielaff, I.; Johnsson, K.; Klok, H.-A. Biomacromolecules 2005, 6, 1602−1607. (12) Xu, F. J.; Li, Y. L.; Kang, E. T.; Neoh, K. G. Biomacromolecules 2005, 6, 1759−1768. (13) Zhai, G. Q.; Cao, Y.; Gao, J. J. Appl. Polym. Sci. 2006, 102, 2590−2599. (14) Schüwer, N.; Geue, T.; Hinestrosa, J. P.; Klok, H.-A. Macromolecules 2011, 44, 6868−6874. (15) Jain, P.; Sun, L.; Dai, J. H.; Baker, G. L.; Bruening, M. L. Biomacromolecules 2007, 8, 3102−3107. (16) Tugulu, S.; Silacci, P.; Stergiopulos, N.; Klok, H.-A. Biomaterials 2007, 28, 2536−2546. (17) Sugnaux, C.; Lavanant, L.; Klok, H.-A. Langmuir 2013, 29, 7325−7333. (18) Dai, J.; Bao, Z.; Sun, L.; Hong, S. U.; Baker, G. L.; Bruening, M. L. Langmuir 2006, 22, 4274−4281. (19) Jain, P.; Dai, J. H.; Baker, G. L.; Bruening, M. L. Macromolecules 2008, 41, 8413−8417. (20) Navarro, M.; Benetti, E. M.; Zapotoczny, S.; Planell, J. A.; Vancso, G. J. Langmuir 2008, 24, 10996−11002. (21) Barbey, R.; Kauffmann, E.; Ehrat, M.; Klok, H.-A. Biomacromolecules 2010, 11, 3467−3479. (22) Barbey, R.; Klok, H.-A. Langmuir 2010, 26, 18219−18230. 6158
dx.doi.org/10.1021/ma400819a | Macromolecules 2013, 46, 6151−6158
