Gradient Poly(styrene-co-polyglycidol) Grafts via Silicon Surface

Apr 14, 2015 - Gradient copolymer grafts of styrene and α-tert-butoxy-ω-vinylbenzyl-poly(glycidol ethoxyethyl ether) (PGLet), a precursor of ...
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Gradient Poly(styrene-co-polyglycidol) Grafts via Silicon SurfaceInitiated AGET ATRP Monika Gosecka,† Joanna Pietrasik,‡ Philippe Decorse,§ Bartosz Glebocki,† Mohamed M. Chehimi,§,∥ Stanislaw Slomkowski,† and Teresa Basinska*,† †

Center of Molecular and Macromolecular Studies, Polish Academy of Sciences, Sienkiewicza 112, 90-363 Lodz, Poland Technical University of Lodz, Technical University of Lodz, Institute of Polymer and Dye Technology, Stefanowskiego 12/16, 90-924 Lodz, Poland § ITODYS, University Denis Diderot & CNRS (UMR 7086), 15 rue Jean de Baïf, 75013 Paris, France ∥ Université Paris Est, ICMPE, SPC, PoPI Team, UPEC, 2-8 rue Henri Dunant, 94320 Thiais, France ‡

S Supporting Information *

ABSTRACT: Gradient copolymer grafts of styrene and α-tertbutoxy-ω-vinylbenzyl-poly(glycidol ethoxyethyl ether) (PGLet), a precursor of α-tert-butoxy-ω-vinylbenzyl-polyglycidol macromonomer (PGL), were prepared on silicon wafers via a surface-initiated activator generated by electron transfer radical polymerization (AGET ATRP). Silicon plates with previously attached 2-bromoisobutyrate served as a macroinitiator for the AGET ATRP (activator generated by electron transfer) of styrene and PGLet. The copolymers’ gradient P(Sco-PPGL) of composition and thickness was obtained by a simple method where the plates were slowly removed from reaction mixture using a step motor. PGLet was added continuously (dropwise) into the reactor during withdrawal of the plates from solution in order to increase the relative concentration of PGLet in polymerization mixture. A range of strategies of making grafts was tested. The plates with copolymers grafts were analyzed by various techniques, like XPS, ellipsometry, and FTIR spectroscopy. The results indicate that the AGET ATRP process is dependent on the styrene/PGLet macromonomer ratio in the polymerization mixture. Under optimal conditions, the addition of PGLet during polymerization and subsequent deprotection of hydroxyl groups of PGLet permit to obtain plates with a novel copolymer layer with composition, thickness, and wettability gradient. Plates with chemical composition of copolymer grafts gradient served as versatile supports with controlled hydrophilic/ hydrophobic area and were suitable for tailored deposition of particles.

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silicon wafer,12 formation of initiator monolayer at the water/ air interface (using Langmuir−Blodgett trough) and its subsequent immobilization on an oxidized silicon substrate,13 or eventually by chemical incorporation of self-assembled benzyl chloride monolayer on silica gel using trichlorosilyl derivative to build up into silica with the network formation.14 The goal is to produce a high-density polymer brush in the final step. The preceding stepinitiator graftingis however difficult to control, particularly if the surface with hydrophilicity gradient should contain regions composed of hydrophilic and hydrophobic polymer chains. For instance, hydrophilicity arranged in a gradient mode along the plate can be obtained by dense grafting of copolymer chains on the surface composed of hydrophilic−hydrophobic copolymers. The edges of substrate should contain close to 100% of hydrophilic or hydrophobic chains, respectively, with intermediate regions

INTRODUCTION Supports with gradient of chemical composition controlling their interactions with environment and/or their response to external stimuli, like pH, temperature, electrical charge, and sensitivity to wetting, have received significant attention over the past years. In principle, such materials could be used for obtaining in a single experiment, using one plate, a wide range of information on interactions of biomolecules (proteins, antigens, oligosaccharides, and cells).1−5 There are examples of solid supports with gradients of immobilized biological compounds like heparin6 and various proteins and peptides.7−9 However, prior to the attachment of polymer chains or biologically active molecules to the surface, initiators, receptors, or other anchor molecules have to be covalently bound to the surface of substrate. These small molecules can be deposited by a variety of methods, e.g., plasma treatment in a special chamber equipped with a mobile holder of the substrate,6 microlithography,10 controlled electron-beam-induced deposition of initiators,11 or controlled addition of initiator solution to the vessel containing a vertically oriented chemically modified © 2015 American Chemical Society

Received: May 5, 2014 Revised: March 26, 2015 Published: April 14, 2015 4853

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by surface initiated atom transfer radical polymerization during which blocks of P(PEGMA) and PNIPAAm were obtained by sequential polymerization. The continuous injection of the second comonomer to the polymerization vessel in which a plate was fixed vertically allowed obtaining plates with gradients of grafted copolymers thickness, chemical composition, and thermosensitivity along the plate. In this process thermosensitivity depending on the local concentration of PNIPAAm was obtained.20 Despite the significant attention already paid to supports with attached hydrophilic polymers and/or copolymers new precursors and methods tailored to particular applications are still needed. The aim of the studies described in this paper was to elaborate a method for preparation of silicon wafers with tethered polystyrene chains containing polyglycidol grafts arranged to create a gradient of chemical composition and wettability along the surface. The above-mentioned brush structure was formed by copolymerization grafting of styrene and α-tert-butoxy-ω-vinylbenzyl-poly(glycidol ethoxyethyl) ether (denoted as PGLet), the precursor of α-tert-butoxy-ωvinylbenzyl-polyglycidol macromonomer (PGL),21 of the general formula shown in Figure 1.

containing varied fractions of chains enriched in hydrophobic or hydrophilic units. The polymer brushes on substrates can be obtained by “grafting to” or “grafting from” the surface. In the latter approach the grafted polymer is obtained as a result of polymerization (e.g., radical controlled polymerization) initiated at the surface, whereas in the former the already synthesized oligomers with reactive end groups react with the surface chemical groups. During the “grafting from” process the initiator is grafted on the substrates uniformly or in a gradient way. Polymerization then also leads to density gradient of attached polymer chains. Mei et al. developed a procedure of initiator grafting on the substrate in a gradient way, followed by SI-ATRP polymerization of poly(2-hydroxyethyl methacrylate) with gradually increasing surface concentration of polymer chains between the edges from very low to dense grafting.12 This procedure allowed to obtain a monotonic gradient of incorporated polymer chains with their conformation gradually changing from “mushroom” to “brush”. The gradient brushes with steep slopes at length scales down to 100 nm were manufactured by combining interference lithography and surface initiated polymerization in a parallel way. The first UV-light exposure partially deactivates or destroys the grafted photoinitiator by the UV-interference pattern. The surface is then exposed to light several more times each with 90° rotation of the sample. The result is a plate with checkerboard-like topography of polymer chains of three different heights at regions which were irradiated once, twice, or not at all.15 Substrates with density gradient of the attached hydrophilic chains were used not only for adsorption of various biomolecules but also for attachment of the whole cells.16 Poly(ethylene glycol) (PEG) was often used for modifications of surface for these studies. For instance, gold substrates with PEG (Mw = 5600) gradient coverage were obtained in a process in which the substrate was first coated with a cystamine selfassembled monolayer and then covered with an agarose film. PEG oligomers containing N-hydroxysuccinimide ester end groups in aqueous solution were introduced from the capillary point source into the hydrogel, diffused through it, and subsequently covalently bonded to the cystamine selfassembled monolayer on the gold substrate. The gradient of immobilized PEG oligomers was formed due to the divergent diffusion of polymer chains from the point source.17 Supports with two or more kinds of grafted polymer chains with gradients of chain density or chemical composition are interesting materials for fabrication of arrays suitable for oneassay detection of different compounds or parameters. Ionov et al. reported on silicon wafers with density gradient of immobilized poly(acrylic acid) and poly(2-vinylpyridine) brushes with PV2-COOH (PV2-COOH) end groups.18 The authors noticed that pH changes induced switching within the wafer interfacial layer resulting in an exposure to the exterior of poly(2-vinylpyridine) and poly(acrylic acid) chains at low and high pH, respectively, as well as variation of the thickness of interfacial layer. Ekblad et al. prepared glass slides with uniformly modified surfaces (e.g., with gold and polystyrene) and covered with poly(2-aminoethyl methacrylate hydrochloride) and poly(2carboxyethyl acrylate) arranged in a way producing gradient of the charge density.19 Surface-grafted block copolymer brushes composed of poly(poly(ethylene glycol) monomethacrylate)) (P(PEGMA)) and poly(N-isopropylacrylamide) (PNIPAAm) were produced

Figure 1. Chemical structures of α-tert-butoxy-ω-vinylbenzyl-polyglycidol macromonomer (denoted as PGL) and α-tert-butoxy-ωvinylbenzyl-poly(glycidol ethoxyethyl ether) (denoted as PGLet) precursor of PGL.

PGL is well-known as a macromonomer suitable for synthesis of many functional homo- and copolymers with composition tailored to a variety of applications.22−24 PGL was successfully used for preparation of particles with cores enriched in polystyrene and hydrophilic shells enriched in polyglycidol25 as well as for flat polymeric coatings26 with highly hydrophilic character that imparts resistance to biofouling.27 It is however worth noting that surfaces covered with polyglycidol are interesting not only because of their antifouling properties but also due to the presence of reactive hydroxyl groups in each of the monomeric units of the polymer. Indeed, PGL-based microspheres and thin planar coatings were proved to be reactive vis-à-vis trichlorotriazine28 and carbonyldiimidazole (CDI),26 therefore providing active sites for the covalent immobilization of antigens suitable for recognition of the corresponding antibodies.26,28 It was shown that PGL provides convenient pathways to functionalization of the curved and planar surfaces. As far as the latter are concerned, ATRP has proved efficient to modify glassy carbon plates by copolymers of styrene and PGL (P(Sco-PPGL)).29 It was found that copolymerization of styrene with only 2% of PGL reduced the hydrophobic character of the 4854

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Langmuir Table 1. Conditions for Preparation of the P(S-co-PPGL) Grafts on the Silicon Plates sample 1/1a 2/2a 3/3a 4/4a 5/5a

b

feeding into reaction medium

removal of solution

polym timec (h)

Mn of PGL-ether (PGLet)

Mw/Mn

DP, (GL)n

Mn of PGLa

yes yes yes yes yes

no no yes yes no

8 16 16 16 16

6990 6990 6990 3361 3361

1.18 1.18 1.18 1.05 1.05

46 46 46 22 22

3670 3670 3670 1820 1820

a

Mn of PGL after deprotection. bParallel plates were prepared for each set of experimental conditions. cTotal time of polymerization (incubation + withdrawal of the plate). Toluene was dried by refluxing with sodium and destillation and stored in an ampule under vacuum. Water was deionized using the ADRONA filtration system. Silicon wafers type-orient: N+/Sb ⟨100⟩ single side polished, with thickness 675 ± 25 μm (Si-MAT Silicon Materials) were cut into small pieces (approximately 4.5 cm × 0.8 cm) and used after surface cleaning performed according to the procedure described by Szelag et al.39 Methods. All methods applied in these studies are described in the Supporting Information (SI1−SI2).

surface and thus also the extent of protein adsorption in comparison to adsorption on the pure PS grafts. Hydrophilic character of polyglycidol does not preclude a possibility to synthesize its copolymers with hydrophobic monomers. For such copolymerizations one can use its hydrophobic precursor α-tert-butoxy-ω-vinylbenzyl-poly(glycidol ethoxyethyl ether) (PGLet) shown in Figure 1, which is soluble in many organic solvents and suitable for the homogeneous radical copolymerization with styrene. After synthesis the ethoxyethyl groups in poly(glycidol ethyl ethoxy ether) units can be hydrolyzed yielding the polyglycidol segments (see Figure 1). In the past years the so-called AGET-ATRP (activator generated by electron transfer-ATRP) method was explored.30−32 This process is less prone to oxidation then classical surface initiated ATRP. From the point of view of equipment used (the system is open to air but it does not go inside since argon is blown through the reaction mixture) the main advantage of AGET initiation is that the reducing agent can also be used to remove dissolved oxygen from the system, and hence the reaction can be conducted in the presence of limited amounts of air. The strategy was found suitable for grafting of 2(dimethylamino)ethyl methacrylate,33 glycidyl methacrylate,34 and poly(ethylene glycol) methacrylate35 from solid substrates. SI-ATRP is an elegant and convenient process for attaching various polymers or copolymers to surfaces via stable, covalent bonds, which imparts a dense coverage of the surface.36−38 In this work, we explore a new facet of the surface chemistry and particularly of the SI-ATRP of PGL in view of making surfaces with arrangement of P(S-co-PPGL) copolymer grafts. The paper bridges the gap between making polymer grafts and surface-initiated AGET-ATRP of PGL and styrene in order to design P(S-co-PPGL) coatings of various local compositions. The strategy involves ATRP on silicon grafted initiator in the presence of one comonomer, while the other one is continuously loaded into the reaction medium. Using this approach, we wanted to characterize the composition and polymer/copolymer thickness of the grafts along the silicon support with respect to molecular weight of PGLet macromonomer and various experimental conditions (i.e., time of copolymerization and feeding to copolymerization mixture).

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RESULTS AND DISCUSSION Preparation and Characterization of Polymer Grafts. On the basis of the experimental setup displayed in Supporting Information SI1 and the operating conditions, one would expect to obtain copolymer gradients on the substrate surface. Thus, in subsequent studies we determined the composition, thickness, and wettability along the polymer layer. The copolymer-coated plates were prepared using slightly varied procedures; nevertheless, all of them involved the progressive feeding of PGLet to the reaction medium while withdrawing the plates from the polymerization mixture. It is worth noting, however, that due to the slow rate of polymerization inherent to ATRP, the plates were initially left for 60 min in the reaction medium before the start of withdrawal. The influence of following parameters/conditions on the copolymer coating formation was investigated: (i) time of polymerization, (ii) molecular weight of PGLet, and (iii) optional removal of some polymerization solution simultaneous to the feeding of PGLet. The latter procedure was adopted in order to keep the total volume of reaction mixture constant. Table 1 describes conditions at which the copolymer grafts were formed on the substrates. XPS Characterization of P(S-co-PPGL) Grafted Silicon Plates. a. Grafting of ATRP Initiator. The plates cleaned of the silicon oxide layer with HF solution were immediately used for grafting of the initiator. Figure 2 displays Br 3d and C 1s narrow regions from Si−Br. A line scan of Si−Br is displayed to account for homogeneity of the initiator grafting (bottom). The Br 3d feature and the C 1s O−CO component centered at 289 eV prove the grafting of ATRP initiator. The grafting was uniform along the displacement axis of the plate (the relative intensities of Br 3d (71 eV), Si 2p (99 eV), Si 2s (150 eV), C 1s (285 eV), and O 1s (533 eV) were constant). The average thickness of the initiator layer was calculated using the Beer−Lambert equation

EXPERIMENTAL PART

Materials. Styrene, chloromethylstyrene (Aldrich) were purified by distillation. Glycidol (Aldrich), 1-ethyl vinyl ether (Fluka), potassium persulfate, aluminum chloride hexahydrate (Sigma-Aldrich), copper(II) bromide, ethyl 2-bromoisobutyrate (EBIB), tris[2(dimethylamino)ethyl]amine (Me6TREN), tin(II) 2-ethylhexanoate (Sn(EH)2), allyl 2-bromo-2-methylpropionate, anisole, N,N-dimethylformamide, chloroform (CHCl3), and methanol (MeOH) (all from Sigma-Aldrich) were used as received.

I = I ° exp( −d /λ cos θ )

(1)

where I° is the intensity of a core level peak from the bare substrate (we selected Si 2p from Si° centered at 99 eV), I is the core level peak from the substrate after coating by the initiator, d is the coating thickness, λ is the attenuation length of 4855

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survey XPS spectra (Supporting Information, Figure SI3). In these profiles, the fractions of Si0 and Siox atoms were determined from the high resolution Si 2p spectra (see Si 2p spectra from plate 5 in Supporting Information SI4). It is worth noting that for the plate 5 (Figure 3a) the atomic percentage of silicon Si0 and Siox approaches 0 at a distance of 0.6 cm from the edge, shorter than for plate 2 (Figure 3b). The carbon content increases gradually as the moving plate is grafted with higher amount of organic materials. For the plate 1 (Figure 3c) the atomic percent contents of Si0 and Siox signals never are equal zero, which means that the plate is covered with a polymer layer the thickness of which is smaller than the sampling depth of the silicon peaks (see also Supporting Information SI4). After fast initial changes of the C 1s peak intensity (at short distance from start) the subsequent change in the carbon atom content remains constant or increases very slowly. Thus, at the longer distance from the edge the thickness of the coating does not increase substantially and remains below the sampling depth (∼12 nm) of the silicon substrate. For this reason the detection of the Si 2p peaks from the silicon substrate is persistent. It is worth noting that the silica and silicon XPS signals do not change similarly (see Figure 3a). At the least coated part of the plate, the Si0/Siox atomic ratio is larger than 2 and for the parts with increasing coating the ratio decreases. This observation conforms to the layered structure of the plates consisting of silicon−silica−organic coating: the silicon substrate is screened both by the silica overlayer and organic layer. It follows that it undergoes additive attenuations due to silica and to the PGL grafts, whereas the silica is subjected to screening only by the gradient organic layer (see Si 2p spectra inSupporting Information SI4). For plate 2, the pattern in Figure 3b indicates that the total attenuation of silicon is very unexpected and likewise the increase in carbon percentage. Selected C 1s regions from plate 5 (Table 1, entry 5) corresponding to organic coating are displayed in Figure 4. The spectra account for progressive screening of the underlying silicon substrate. One can see that the top of the plate exhibits an O−CO component (at 289 eV) assigned to the initiator. As the ATRP process progresses and affects the bottom of the plate incubated for longer time, the ester peak is gradually screened until it vanishes. This occurs because the grafted initiator forms a layer between the substrate and the growing PGL grafts. Interestingly, the shakeup satellite which accounts for aromatic species is more visible at the side of the plate incubated in the styrene/macromonomer mixture for the longer time. Since only pure PS exhibits a strong π−π* shakeup satellite P(S/PGL)

Figure 2. Typical narrow regions from the macroinitiator Si−Br: Br 3d (right) and C 1s (left).

Si 2p in the initiator layer, and θ is the analysis angle relative to the surface normal (θ = 90°, so cos θ = 1). λ is estimated to be 4.1 nm in the organic top layer using the well-known Seah and Dench equation.40 Given a photoelectron emission normal to the surface, the thickness d computed from

d = λ ln(I °/I )

(2)

was estimated to be 5.2 ± 1 nm. This value is consistent with thickness of the initiator layer determined by ellipsometry (4.6 ± 0.9 nm). These values albeit small are too large for initiator monolayer. Possibly, the overestimation is due to the roughness of HF treated silicon surface. b. Gradient P(S-co-PPGL) Grafts. During withdrawal of the plates the polymerization (ATRP) and feeding of PGL proceed simultaneously. Thus, it would be reasonable to expect that the lower side of the plate (see Supporting Information SI1), remaining in the polymerization mixture during the whole process, will be coated by the longest grafts forming the thickest layer. It is also part of the plate exposed eventually to the highest concentration of PGL macromonomer and therefore the most hydrophilic one. The chemical composition profiles along plates 1, 2, and 5 are displayed in Figure 3. These profiles are based on the 3D

Figure 3. Lateral composition profile (in at. %) for plates 5 (a), 2 (b), and 1 (c) corresponding entries 5, 2, and 1 in Table 1. 4856

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In Figure 5, the fractions of PGL and its corresponding glycidol monomeric units are plotted as a function of distance

Figure 4. C 1s spectra from selected sites along the main axis of plate 5 (Table 1, entry 5).

Figure 5. Fractions of PGL and glycidol monomeric units determined at various spots of plate 5 and copolymer thickness vs distance from edge using Tougaards QUASES software (the method of copolymer thickness determination is described in Supporting Information SI1). The distance on a plate is measured from the start of polymerization. The lines are drawn for guiding the eye.

core/shell particles41 or copolymer brushes,29 to much lesser extent, and homopolyPGL ((PPGL) grafts showing no satellite at all26), one may assume that bottom of the plate is rich in polystyrene units. We also examined pure PPGL-grafted plate and found no shakeup satellite but a relatively intense C 1s component centered at ∼286.5 eV due to the carbon atoms in C−O groups from the pendant chains in PPGL (see Supporting Information SI5). In addition to the comparison of the C 1s narrow regions from copolymer grafts and PPGL, for selected sites on plate 5 we estimated the fractions of corresponding glycidol monomeric units. The fraction of PGL macromonomer units can be estimated using the following formula: fPGL = [(O/C)exp(grafts) ]/[(O/C)th(PGL macromonomer)]

from the top edge of plate 5. Obviously, there is less PGL at the bottom of the plate, and this accounts for the more visible shakeup satellite of the styrene units (in Figure 4 at the distance of 2.61 cm). However, because each PGL macromonomer chain contains on average 22 glycidol monomeric units, the fraction of glycidol monomeric units remains within the narrow 0.88−0.99 range. It is nevertheless worth stressing that indeed the gradient of polyglycidol was achieved, from the total 100% of polystyrene grafts (at the start of polymerization process), through high surface fraction of polyglycidol chains, and then to decreasing surface concentration of hydrophilic polyglycidol. To finish the XPS part of this study, we would like to address the determination of copolymer grafts thickness. It is known that for the appropriate range of film thickness polymer brushes can achieve maximal surface hydration, which is important for antifouling properties of grafts.42 We noticed (see Supporting Information SI3) a gradual change in the C 1s peak intensity alongside the main axis of the plate; for silicon the Si 2p might remain the same or simply completely vanish from a certain position onward. Determining the thickness in such a case is problematic and for this reason the organic coating thickness determination we have adopted QUASES, the Tougaard’s peak shape analysis software (see Supporting Information SI1). Using the procedure, we can deduce the thickness of the organic layer, assuming similar mean free path for C 1s. The peak shape analysis was repeated for different spots alongside the main axis and the profile is also shown in Figure 5. For short incubation time, thickness is 18.2 nm for the top layer. Taking into account the initiator thickness (∼5.2 nm), it appears that the copolymer layer is about 13 nm thick. For the longest incubation time in the ATRP medium, we estimated the total thickness at 3.157 cm from start, equal to 26.7 nm, corresponding to 21.5 nm after correction of the initiator layer thickness. We could not use the traditional Beer−Lambert equation for the organic layer thickness determination because it is based on the attenuation of silicon elastic peaks which for long incubation time are not detected (see the survey spectra in Supporting Information SI3). It is worth noting that ellipsometry is a very good technique for determination of thickness of polymer layers or adlayers. Therefore, we attempted to measure brush thickness using also

(3)

For the pure PGL macromonomer used in our studies, the average DP of the glycidol monomeric unit was 22, and therefore the calculated O/C atomic ratio should be equal to 0.57 (45 oxygen atoms for 79 carbon atoms). The experimental (O/C)exp for the copolymer (styrene-coPPGL) was calculated by considering the contributions of silica and oxygen atoms in the ester group of the initiator to O 1s. The C 1s region should contain a contribution of the initiator which has 1 ester carbon atom among 7. Therefore, the experimental (O/C)exp ratio corresponding to the copolymer was calculated as follows: (O/C)exp = [(O‐2Si103‐2Cester ]/[(C‐7Cester )]

(4)

where O and C are the total atomic fractions of these elements, overall fraction of carbon comprises carbon deriving from styrene and glycidol monomeric units, Si103 is the atomic percent content of silicon in the SiO2 determined from the peak area of Si 2p centered at nominal value 103 eV corresponding to silica, and Cester is the surface concentration of ester carbon atoms (in at. %) determined from peak fitting in the C 1s regions. From f PGL one can estimate the fraction of glycidol monomeric units by taking into account the DP value of the macromonomer. For example, for plate 5, we used PGL with n = 22, which implies that the fraction of glycidol monomeric units is equal to the value calculated from the formula FPGL = fglycidol × 22/(fglycidol × 22 + fstyrene )

(5)

where the fraction of styrene units in the copolymer is given by fstyrene = 1 − fglycidol

(6) 4857

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Langmuir this method. However, in the case of copolymer grafts with complex chemical structure, topology (polystyrene grafts from the surface with polyglycidol grafts from polystyrene grafts) and presumably a patchy structure on submicrometer level the ellipsometric measurements of thickness suffered from a large error (>50%). Therefore, in spite of efforts the attempts to determine the polymer brush layer thickness with gradient arrangement on the plates, using parameters for pure polystyrene (PS) and pure poly(polyglycidol) (PPGL) grafted supports failed to provide results with acceptable accuracy. The thickness of copolymer layer increases gradually along the plate with the progress of polymerization. Nevertheless, gradual injection of PGLet solution did not lead to monotonic growth of PGLet (and eventually PGL) along the wafer. There might be various reasons for that; however, the most straightforward one should be the nonmonotonic increase of local concentration of macromonomer molecules in the vicinity of active propagating centers with the increase of PGLet concentration in bulk. Immediately before addition of PGLet the surface of silicon is covered with polystyrene grafts equipped with species suitable for propagation. Addition of PGLet results in the reaction of macromonomer molecules with these species. This step should form a certain barrier screening the nearest propagating species from bulk solution. One could expect that small styrene molecules should penetrate this barrier easier than the much larger macromolecules of macromonomer. Obviously, one cannot also exclude that the PPGL-et chains screening the surface make growing of the neighboring chains impossible due to the lack of access to the monomer molecules and thus become inactive. Therefore, at the later stages the gradient of decreasing surface content of PPGLet is formed. c. IR FTIR ATR Analysis of P(S-co-PPGL) Copolymers Gradient Grafts. The attachment of organic matter (copolymer) induces higher counts when moving from 1 to 3 cm distance from the edge. The styrene units are characterized by multiple peaks at 3600 and 3800 cm−1. The OH stretching band (∼3100 cm−1) in the spectrum recorded at 1 cm distance shows that the OH/styrene peak intensity ratio is high which suggests relatively more PGL groups at this distance. The FTIR spectra of various regions of plate 5 are displayed in Supporting Information SI6. d. Wettability of P(S-co-PPGL) Copolymers Gradient Grafts. Wettability by water has been assessed by contact angle measurements on the plates (each of two prepared in parallel) using contact angle analyzer (SEO). The wettability patterns (see Figure 6) suggest that initially hydrophilicity rapidly increases and then begins to decrease gradually. Initially after addition of PGLet the surface concentration of PPGLet (and finally PPGL) rapidly increases and then begins to slowly decrease. The explanation was proposed in the section entitled XPS Characterization of P(Sco-PPGL) Grafted Silicon Plates. It is worth adding that in each experiment the volume of water drop placed on a copolymer grafted plate was equal to 2 μL and radii of contact interface were in the range of 1.0−1.2 mm depending on the wettability angles (from 89° to 73°, respectively). The results displayed above indicate that PGL can not only provide polymer grafts as we have actually demonstrated a few years ago26 but also provide copolymer grafts in a simple manner. However, some key parameters need to be controlled in order to obtain true PGL gradients. We have demonstrated

Figure 6. Contact angles for water drops (2 μL each) placed along the plates from start of polymerization process (entries 1/1a-5/5a in Table 1). The data for particular plates were compared with water contact angles of plate containing grafted polystyrene. The values of contact angles represent the averages of five measurements of each copolymer grafted plate prepared in parallel. Microphotographs represent the lowest and the highest contact angles measured for 5/5a plates.

that ATRP time is an important parameter, since 8 h polymerization was not sufficient to completely screen the substrates. By increasing the time to 16 h, we proved that silicon could be screened at the sites that remained in the reactor the longest, i.e., the lower parts of the plates. This is the case of plate 2/2a and 5/5a for which water contact angles are presented in Figure 6. It is well-known that composite surfaces (e.g., with patches containing polystyrene and polyglycidol), for which the influence of differences in roughness of each component may be neglected, could be described by the simple equation43 cos θcomp = fPGL cos θPGL + (1 − fPGL ) cos θPS

(7)

where θcomp is the contact angle measured on the composite layer containing polystyrene and polyglycidol segments on the surface, f PGL is polyglycidol mole fraction at the particular location on the surface, and (1 − f PGL) represents the relevant fraction of polystyrene, whereas θPGL and θPS are contact angles for liquid drops placed on surfaces covered with polyglycidol and polystyrene grafts, equal to 24 ± 1 and 88.9 ± 0.2, respectively. The correlation between the fraction of polyglycidol determined by XPS (f PGL(XPS)) and calculated (using eq 7) from contact angle measurements (f PGL(contact angle) is presented in Supporting Information SI7. The plot in Figure SI7 reveals that correlation between f PGL (XPS) and f PGL(contact angle) is equal to 0.967 and that the fraction of polyglycidol determined by XPS is about 55% higher than that measured by contact angle. This result conforms to the findings described above (in the subsection devoted to XPS studies) suggesting that during polymerization some PGLet (converted later to PGL) is buried under the surface. e. Adsorption of Polystyrene Particles on Poly(styrene-coα-tert-butoxy-ω-vinylbenzyl-polyglycidol) Plates with Gradient of the Polymer Layer Thickness and Composition. Adhesion of various biological and artificial objects (e.g., living cells or polymer particles, respectively) at various locations along the plate can depend on the local hydrophilicity and fraction of polyglycidol in the interfacial layer. Recently, Adamczyk et al. found that polymer particles could be used as good models for studies of deposition phenomena of various biomolecules (e.g., proteins).44 The authors concluded that the theoretical and experimental results acquired for nanoparticles 4858

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Figure 7. Scanning electron microscopy (SEM) microphotographs of P(S-co-PPGL) plate (no. 5) with adsorbed polystyrene particles at various positions from the start of the polymerization (top): (a) 0.0, (b) 0.5, (c) 1.0, and (d) ≥3.0 cm.

can be used as reference systems for interpretation of molecular adsorption phenomena, inaccessible by direct measurements. Thus, polystyrene particles (PS) have been used for studies of adsorption on a surface with gradient hydrophilicity. The surface of polystyrene particles was hydrophobic. The particles were synthesized by soap-free emulsion polymerization of styrene in water initiated with potassium persulfate. The characteristics of PS microspheres are presented in Supporting Information SI2. It can be assumed that the adsorption of negatively charged polystyrene particles on neutral polymer brush with variable hydrophilicity occurs exclusively as a result of hydrophobic interactions between polystyrene particles and polystyrene patches. In this case the adsorption driving force is interaction energy between particles in suspension and the solid surface and the Brownian motion of the particles toward the solid surface driven by the interaction energy gradient.44 Moreover, one should expect that adsorption of hydrophobic microparticles from water medium depending on the local hydrophilicity/hydrophobicity of the substrate would result in differential coverage of the surface by particles in each particular region with a given ratio of the polystyrene and polyglycidol fractions. Figure 7a−d displays scanning electron micrographs of PS microspheres adsorbed on a plate coated with a gradient polyglycidol copolymer. The surface density of adsorbed PS particles expressed as an average number of particles (N) per 3104 μm2, taken from three images, at a zone with individual P(S-co-PPGL) composition and thickness is given in Figure 8. On the basis of the surface density of adsorbed particles counted from the SEM microphotographs, presented in Figure 7a−d (and Table SI8), it is evident that the local hydrophilicity of the substrate has a strong influence on particle adsorption. At the most hydrophobic parts of the plates (with 100% surface fraction of polystyrene), the particles are adsorbed irreversibly and their surface concentration is high (ca. 1030 particles per 3104 μm2) (Figure 7a,d). On the contrary, adsorption of particles on the more hydrophilic regions (at f PGL = 75 mol %)

Figure 8. Dependence of the number of polystyrene particles (N) adsorbed on various zones of plate 5 and thickness of copolymer layer versus distance from edge.

is significantly reduced, with the surface density of adsorbed particles low as ca. 157 particles/3104 μm2. The markedly lower surface concentration of polyglycidol segments (f PGL in the 26−33 mol % range) activates the more effective particles adsorption on average 670 particles/3104 μm2. Moreover, it is clear that the adsorption is related to the surface local composition and not to the thickness of P(S-co-PPGL) grafts. It is also worth noticing that the arrangement of particles shown in Figure 7a,d is typical for so-called “ballistic deposition” of PS particles on hydrophobic surfaces with absence of their lateral movement upon adsorption.45,46 Where the surface fraction of polyglycidol increases along the plate, i.e., in the more hydrophilic zones, the density of particles coverage is much lower because the hydrated and highly mobile polyglycidol chains form a protecting layer against adsorption of hydrophobic polystyrene particles (Figure 7c), and thus adsorption is reversible.25,47

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CONCLUSION Gradient copolymer grafts based on styrene and α-tert-butoxyω-vinylbenzyl-polyglycidol were prepared by surface-initiated AGET ATRP on moving silicon wafers. It was possible to 4859

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control the composition, thickness, and wettability along the main axis of the silicon wafer plates with tethered copolymer brushes. Formation of gradient coating was facilitated by (i) long polymerization time; (ii) low Mn of PGLet, and (iii) continuous feeding of PGLet to the reaction medium without removal of the solution initially introduced into the reactor. Interactions of the hydrophobic PS colloidal particles with the gradient copolymer coatings resulted in preferential adsorption of particles to the hydrophobic parts of the substrate. Results of our studies suggest that the substrates coated with PGL containing copolymers providing their tailored hydrophilicity are good candidates for preparation of objects allowing for controlled interactions with synthetic colloids and possibly also with biomolecules and cells.

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ASSOCIATED CONTENT

* Supporting Information S

SI1: methods applied in the studies; SI2: synthesis and characterization methods of polystyrene microspheres (PS); SI3: XPS line scans of plates 5, 2, and 1; SI4: high-resolution Si 2p spectra recorded for various sites aligned along the main moving axis of plate 5; SI5: typical C 1s spectrum from a silicon plate grafted with pure PPGL chains; SI6: FTIR spectra of various regions on plate 5; SI7: correlation between fraction of polyglycidol determined by XPS ( f PGL(XPS)) and calculated from contact angle measurements (f PGL(contact angle)); SI8: surface density of adsorbed PS particles on copolymer grafts. This material is available free of charge via the Internet at http://pubs.acs.org.

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AUTHOR INFORMATION

Corresponding Author

*E-mail [email protected] (T.B.). Notes

The authors declare no competing financial interest.

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ACKNOWLEDGMENTS The authors thank French Campus France (project HyPOC Campus France No. 24581 RF) and National Center of Science (NCN) in Poland for financial support (Grant 2011/01/M/ ST5/06085).

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