Article pubs.acs.org/Macromolecules
Physical Networks of Metal-Ion-Containing Polymer Brushes Show Fully Tunable Swelling and Nanomechanical and Nanotribological Properties Ella S. Dehghani, Vikrant V. Naik, Joydeb Mandal, Nicholas D. Spencer, and Edmondo M. Benetti* Laboratory for Surface Science and Technology, Department of Materials, ETH Zürich, Vladimir-Prelog-Weg 5, CH-8093 Zurich, Switzerland S Supporting Information *
ABSTRACT: The interaction between weak polyacid brushes and metal ions can lead to the formation of a wide variety of complex structures across the polymer grafts. In the case of poly(hydroxyethyl methacrylate) brushes derivatized with succinate side groups (PHEMA-SA), the coordination with Zn2+ or Ca2+ species can be tuned by varying the solution pH, below and above the pKa of the polyacid brush. Attenuated total reflection Fourier transform infrared spectroscopy (ATR-FTIR) indicates that Zn2+ ions generate relatively weak, localized monodentate bridges along PHEMA-SA grafts at basic pH. These Zn2+−brush conjugates swell profusely in water, are compliant and very lubricious, as observed by combining variable angle spectroscopic ellipsometry (VASE), quartz crystal microbalance with dissipation (QCM-D), and atomic force microscopy (AFM) methods. In contrast, incubation of PHEMA-SA brushes with Ca2+ or Zn2+ at acidic pH leads to the formation of more extended, bidentate linkages forming a physical network between the metal centers and the surrounding grafts. This type of coordination causes brush dehydration and the stiffening of the films, as well as high friction, due to the energy dissipation required to perturb the dense, brush physical network by the shearing AFM probe. Regulating the interaction between metal ions and ionizable polymer brushes emerges as a versatile and easily accessible tool to control the interfacial properties of grafted polymer films. The achieved modulation of nanomechanical and nanotribological characteristics is technologically relevant, in that it allows both function and performance to be tuned for widely applicable polymer coatings.
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INTRODUCTION Densely grafted, weak polyelectrolyte (wPE) brushes are extremely efficient boundary lubricants in aqueous environments because of their intrinsically high hydration and their marked entropic resistance toward compression.1,2 In contrast to strong polyelectrolyte brushes, the charge density of wPE brushes can be tuned as a function of the pH of the medium in which they are immersed, making their hydration properties switchable between swollen and partially collapsed states. This distinctive property has enabled the application of wPE brushes as stabilizers for colloidal dispersions,3 coatings for fabricating pH-responsive membranes,4 and tunable polymer matrixes for the synthesis of metal nanoparticle (NP)-based hybrids.5,6 In addition, the postfunctionalization of wPE brushes with protein cues and peptide sequences has demonstrated their suitability in the design of cell-sensitive platforms.7,8 The swelling properties of wPE brushes can be additionally modulated by adding monovalent, divalent, or multivalent metal ions to the medium, leading to different swelling transitions due to e.g. electrostatic screening, ion pairing, or water exclusion from the wPE brush films.9−12 These saltinduced effects can alter the nanomechanical and nanotribological properties of wPE brushes significantly. For © XXXX American Chemical Society
example, Duner et al. reported a relevant increase in friction on poly(acrylic acid) (PAA) brushes in the presence of calcium ions, which was attributed to the strong interaction between the metal ions and the carboxylate groups along the polymer grafts.13 Furthermore, Liu et al. demonstrated how the tribological properties of similar wPE brushes could be repeatedly switched by exchanging the counterions within the films,14 finally suggesting that brush dehydration due to ion pairing interactions could lead to brush stiffening and a concomitant increase in friction. Despite these initial studies, a comprehensive investigation focusing on the combined effects of salt addition and pH variation on the nanomechanical and nanotribological properties of wPE brushes is currently missing. In particular, the formation of different types of conjugates between precisely chosen metal ions and wPE grafts is expected to generate diverse macromolecular architectures within the brush films. The formation of metal ion−wPE brush complexes can be regulated by varying the pH of the medium, i.e., the wPE charge Received: December 10, 2016 Revised: February 4, 2017
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DOI: 10.1021/acs.macromol.6b02673 Macromolecules XXXX, XXX, XXX−XXX
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out in accordance with the already-reported protocol by Sanjuan et al.16 Before the formation of the initiator self-assembled monolayers (SAMs), silicon oxide wafers were cleaned with piranha solution (3:1 H2SO4 (98%, Sigma-Aldrich)/H2O2 (30%, Sigma-Aldrich)) and placed overnight under 10 mM solution of BPCS in toluene. QCM-D sensors were sonicated in ethanol and toluene twice, each step for 10 min. Then the sensors were cleaned in a UV/ozone cleaner (UV/ozone ProCleanerTM, USA) for 40 min, prior to overnight functionalization with BPCS (from toluene solution). In a typical procedure for the polymerization of HEMA, nitrogen was bubbled through a HEMA:water mixture (4:4 mL) containing 244 mg of bipyridine for half an hour, after which the solution was transferred to a flask containing 55 mg of CuCl and 36 mg of CuBr2 (molar ratios of HEMA:CuCl:CuBr2 = 50:1:0.25).17 This solution was stirred for 30 min until complete dissolution of the catalyst. Then the polymerization solution was transferred to a flask containing the BPCS-modified silicon oxide substrates. PHEMASA brushes were obtained by overnight reaction of PHEMA films with succinic anhydride in a 0.5 M pyridine solution. Characterization. The dry thickness of PHEMA and PHEMA-SA brushes was measured with a variable-angle spectroscopic ellipsometer (VASE) (M-2000F, LOT Oriel GmbH, Darmstadt, Germany), using a three-layer model (SiO2/Cauchy-BCPS layer/Cauchy-polymer layer) with known thicknesses and refractive indices for Si, the SiO2, and the initiator layer (software WVASE32, LOT Oriel GmbH, Darmstadt, Germany). A Cauchy model, n = A + B/λ2, where A is defined as offset (A = 1.45) and B as wavelength dispersion (B = 0.01), was used to describe the refractive index of the PHEMA and PHEMA-SA brush films. The swollen thickness and swelling ratio of the films were measured by using a homemade liquid cell and applying a four-layer model (SiO2/Cauchy-BCPS layer/Cauchy-polymer layer/aqueous layer), keeping the refractive index of water set as 1.33. The swollen PHEMA-SA thickness in the presence of different metal salt solutions was modeled with an effective-medium approximation (EMA) model, provided by the instrument software, featuring both Cauchy and water components. Dry thickness (Tdry), swollen thickness (Twet), and swelling ratio Sr (%) = 100(Twet − Tdry) Tdry−1 of the different brush films were obtained by VASE. FTIR spectra of the dry PHEMA/PHEMA-SA films were recorded in transmission mode employing an infrared spectrometer (Bruker, IFS 66 V) equipped with a liquid-nitrogen-cooled MCT detector. A background spectrum was collected from a freshly cleaned, bare silicon wafer. Attenuated total reflection Fourier transform infrared (ATRFTIR) spectroscopy was carried out with a Cary 670 FTIR spectrometer (Agilent Technologies) equipped with a SPECAC ATR diamond accessory at room temperature in 100 mM buffer solution and 10 mM Zn(NO3)2 and Ca(NO3)2 solutions. ATR-FTIR spectra were recorded between 700 and 4000 cm−1. Water contact angle (CA) measurements were performed with a drop-shape analysis system (DSA 10, Krüss, Germany). For each sample, individual measurements at three different spots were performed and averaged. Contact-angle measurements were performed at room temperature using Milli-Q water, while the CA values were obtained using the tangent-fitting method. X-ray photoelectron spectroscopy (XPS) on a Theta-Probe X-ray photoelectron spectrometer (ARXPS, monochromatic Al Kα source with beam diameter of 300 μm, Thermo Fisher Scientific, Waltham, MA) was used to study the chemical composition of the interfacial layer of the brush. The pass energy of the survey spectrum was 200 eV, while the pass energy was 100 eV for the high-resolution spectra used for carbon peak-shape analysis. A combined electron/argon-ion flood gun was used to compensate for the charging occurring at the interface. Three measurements were performed in the standard lens mode with an emission angle of 53° to the surface and an acceptance angle of ±30°. The elemental composition determined from the survey scan (69.9% carbon and 30.1% oxygen) reflected, within experimental error, the composition expected for a PHEMA film (66.7% carbon and 33.3% oxygen, 4% error).
density, while the morphological arrangements of the created, hybrid brush can be exploited to tune the film’s stiffness and friction at the nanoscale. In order to demonstrate these hypotheses, we first synthesized poly(hydroxyethyl methacrylate) (PHEMA) brushes by surface-initiated atom transfer radical polymerization (SI-ATRP), followed by their derivatization with succinic anhydride, to yield PHEMA succinate (PHEMA-SA) brushes, displaying a weak polyacid behavior. We subsequently investigated the properties of PHEMA-SA brushes in response to a pH variation and in the presence of Ca2+ and Zn2+ ions, by a combination of attenuated total reflection Fourier transform infrared spectroscopy (ATR-FTIR), variable angle spectroscopic ellipsometry (VASE), quartz crystal microbalance with dissipation (QCM-D), atomic force microscopy (AFM), nanoindentation, and lateral force microscopy (LFM). ATRFTIR indicated that succinate functions along the brush backbones chelate Ca2+ ions regardless of the pH of the medium, leading to the formation of a physical brush network and the consequent collapse of the film, as confirmed by VASE and QCM-D. This phenomenon causes a stiffening of the PHEMA-SA brushes and a simultaneous increase in friction, as measured by AFM and LFM, respectively. In contrast, complexation of Zn2+ ions by the PHEMA-SA brushes can be regulated by varying the characteristics of the surrounding medium, in response to a pH variation the brush structure switching between bidentate bridging and monodentate complexes. Above the pKa of PHEMA-SA brushes, soft brush films featuring monodentate metal−grafts linkages were formed, showing high water content and extremely low friction. In contrast, acidic pH values triggered the formation of bidentate complexes within the brush, giving rise to stiff and dehydrated PHEMA-SA/Zn2+ networks that display significantly high friction. The precisely tunable, physicochemical characteristics of the fabricated metal ion-PHEMA-SA brush hybrids are enabled by the capability of weak polyacids to interact in different and tunable ways with metal ions, forming diverse complex architectures: stronger, weaker, more or less localized, and leading to single or multiple interactions. On the larger scale of a polymer brush assembly, this translates into different architectures that are capable of regulating technologically relevant properties such as film swelling, mechanical characteristics, and friction.
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EXPERIMENTAL SECTION
Materials. 10-Undecen-1-ol (Aldrich-Fine Chemicals, Switzerland), dry THF (99.5% extra dry, Acros, Germany), α-bromoisobutyryl bromide (Aldrich-Fine Chemicals, Switzerland), dimethylchlorosilane (Aldrich-Fine Chemicals, Switzerland), hydrochloric acid (Sigma-Aldrich, Germany), succinic anhydride (Aldrich-Fine Chemicals, Germany), pyridine (Sigma-Aldirch, Germany), zinc nitrate hexahydrate (Sigam-Aldrich, Germany), and calcium nitrate tetrahydrate (Alfa Aesar, Germany) were used as received. Hydroxyethyl methacrylate was purified according to the procedure described by Baker et al.15 Water was deionized with a GenPure filtration system (18.2 MU cm, TKA, Switzerland). 0.1 M tris(hydroxymethyl)aminomethane base buffer solution (Alfa Aesar, Germany) and 0.1 M bis-tris methane base buffer solution (SigmaAldrich) were prepared with Milli-Q water, and the pH was adjusted to 9 and 5, respectively, with addition of 0.1 M HCl (Aldrich-Fine Chemicals). Surface-Initiated Atom Transfer Radical Polymerization (SIATRP). The synthesis of the silane-based initiator for SI-ATRP, 11-(2bromo-2-methylpropionyl)dimethylchlorosilane (BPCS), was carried B
DOI: 10.1021/acs.macromol.6b02673 Macromolecules XXXX, XXX, XXX−XXX
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Macromolecules The elemental composition from the survey scan after modification with succinic anhydride was 65% carbon and 35% oxygen, while the composition expected for a PHEMA-SA film would have been 62.5% carbon and 37.5% oxygen. This 7% difference is presumably due to a combination of surface contamination and the presence of unreacted OH groups on the surface. Also, after modification with succinic anhydride, C 1s spectra were obtained and resolved into the individual components at different binding energies, based on PHEMA and PHEMA-SA structure, and around 7 ± 2% C−OH intensity was observed over three separate measurements. A quartz-crystal microbalance with dissipation (QCM-D, E4 instrument, Q-Sense, Västra Frölunda, Sweden) was used to monitor the swelling ratio of PHEMA-SA films in different pH and salt solutions. The instrument was equipped with Q-Soft 301 software (QSense AB, Göteborg, Sweden). After polymerization, the functionalized sensors were subsequently incubated in succinic anhydride solution overnight. The sensors were rinsed with Milli-Q water, followed by ethanol, and they were finally dried with nitrogen. After mounting the cells in the instrument, Milli-Q water was injected. Once a drift-free signal was recorded for 20 min, buffers or salt solutions were injected, and while collecting the data, the instrument was allowed to equilibrate. For monitoring the swelling ratio at different pH values and with different salts, an extended viscoelastic model18,19 was used to fit the frequency and dissipation shifts (Δf and ΔD, respectively) using three overtones, to obtain the change in hydrated polymer mass and film viscoelasticity, during the swelling/collapse process.20,21 The nanomechanical and nanotribological properties of the PHEMA-SA brushes immersed in different salt solutions were measured with an Asylum atomic force microscope (AFM) (MFP3D (Asylum, Santa Barbara, CA). The normal spring constants of Aucoated, tip-less cantilevers (NSC-36, Bruker, US) were measured by the thermal-noise method.22 Silica microparticles of diameter 16 μm (EKA chemicals AB, Kromasil, Sweden) were glued to the end of the cantilevers using an UV-curable glue (Norland optical adhesive 63, Cranbury, NJ). The elasticity and adhesive properties of the brush films were analyzed from the approaching and retracting profiles of the recorded force-vs-distance (F−D) curves (30 force curves over 20 μm × 20 μm area, in a minimum of three spots). The lateral-force calibration was performed according to the method described by Cannara et al.23 To determine the lateral sensitivity of the photodetector, a freshly cleaned silicon wafer (1 cm × 1 cm) was glued “edge-on” to a glass slide. The smooth silicon plane was used as a “wall” for measuring the lateral sensitivity. A silica colloid of 40 μm in diameter was glued to a cantilever and moved laterally into contact with the silicon wall without interacting with the glass surface below. The slope of the obtained lateral deflection-vs-piezo displacement curve yielded the lateral sensitivity, which was used for the lateral force calibration. Friction force loops were recorded at a constant speed of 3 μm s−1 in at least four different spots on the samples.
The swelling properties of PHEMA-SA brushes were subsequently tested in aqueous media presenting different pH values, alternatively below and above the pKa of the succinate functions along the brush backbones. At pH 9 PHEMA-SA brushes swell profusely in water, due to the deprotonation of their acid functions, reaching 500 ± 10 nm of thickness.24 In contrast, at pH 5, i.e., below the pKa of the polyacid, PHEMASA brushes are mainly in their neutral form and substantially shrink to 292 ± 10 nm of wet thickness. This reversible transition could be demonstrated by sequentially subjecting PHEMA-SA films to aqueous media at high and low pH and simultaneously recording the hydrated brush thickness by VASE (Figure 1).
Figure 1. Swollen thickness (Twet) of PHEMA-SA brushes immersed in aqueous media at different pH values and in the presence of 10 mM Ca2+ and Zn2+. The values of Twet were recorded multiple times while the swelling solutions were exchanged in the VASE liquid cell. After each measurement the PHEMA-SA-functionalized substrates were subjected to a rinsing step using Milli-Q water.
The addition of 10 mM Ca2+ and Zn2+ caused a variation of the swelling properties of PHEMA-SA brushes due to interactions of the metal ions with the carboxylate functions of the grafts. The nature of these interactions could be further regulated through a shift of the medium pH. At pH 5, the presence of either Ca2+ or Zn2+ led to a minor increase in the brush swollen thickness due to the incorporation of the metal ions within the films and the partial substitution of the protons from the carboxylic acid functions along the grafts.25,26 Namely, PHEMA-SA brushes showed a hydrated thickness of 325 ± 5 and 340 ± 10 nm when immersed in pH 5, 10 mM Zn2+ and Ca2+ solutions, respectively. Interestingly, PHEMA-SA swelling was slightly reduced in the presence of Zn2+ compared to Ca2+, presumably because of the smaller hydrated radius of the Zn2+ ions with respect to the bulkier Ca2+ species.27,28 An increase of the pH to 9 was accompanied by a concomitant increment in PHEMA-SA swelling, although ionic interactions between the added metal ions and the carboxylate functions along the grafts caused a reduction of the brush swollen thickness compared to the pure buffer solution. The presence of Zn2+ induced a reduction of 7% in the swollen thickness of PHEMA-SA brushes, which reached 464 ± 6 nm. In contrast, the interactions between the Ca2+ ions and the carboxylate functions along the PHEMA-SA grafts caused a much more marked reduction of swollen thickness (28% with respect to PHEMA-SA immersed in pure buffer at the same pH), which reached 360 ± 5 nm.29,30 In order to determine the specific types of interaction between the added metal ions and the PHEMA-SA brushes at the two different pH, attenuated
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RESULTS AND DISCUSSION PHEMA brushes were synthesized by surface-initiated atom transfer radical polymerization (SI-ATRP) of HEMA,17 yielding brush films presenting an average dry thickness of 36 ± 2 nm, after 20 min of polymerization, and characterized by an average grafting density of 0.3 chains nm −2 (see Supporting Information and Figure S1 for details). In order to transform the PHEMA brushes into weak polyacid grafts, the films were subsequently reacted with succinic anhydride (SA), thus introducing carboxylic acid-bearing, succinate functions along the polymer chains (yielding PHEMA-SA brushes). The successful derivatization with SA was confirmed by FTIR and XPS (see Experimental Section and Supporting Information, Figures S2 and S3). In addition, the formation of bulkier, succinate groups caused a substantial increase in brush dry thickness, which reached 100 ± 5 nm, as measured by VASE. C
DOI: 10.1021/acs.macromol.6b02673 Macromolecules XXXX, XXX, XXX−XXX
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Figure 2. (a−c) Different coordination structures formed between carboxylic acid ligands along the PHEMA-SA brushes and the added metal ions. (d, e) ATR-FTIR spectra of PHEMA-SA brushes incubated in 0.1 M buffer solutions at pH 9 and pH 5 in the presence of 10 mM Ca2+. (f, g) ATRFTIR spectra of PHEMA-SA brushes incubated in 0.1 M buffer solutions at pH 9 and pH 5 in the presence of 10 mM Zn2+.
complexes formed within the PHEMA-SA brush structure. Carboxylate groups can coordinate with metal ions through the formation of different conjugates, i.e., monodentate, bridging bidentate, and chelating bidentate complexes (as exemplified in Figure 2a−c).34 The formation of different coordination structures is reflected by differences in the carbonyl stretching frequencies. At pH 5 and pH 9 the ATR-FTIR spectra of PHEMA-SA incubated in 10 mM Ca2+ showed similarly strong CO bands at 1750 cm−1, which suggested that bidentate complexes were formed under both these conditions. As previously indicated by McCluskey et al., at pH 5 the presence of two bands at 1559 and 1616 cm−1 with comparable intensities indicated the coexistence of chelating and bridging bidentate complexes between Ca2+ and the carboxylate groups along the grafts.35,36
total reflection Fourier transform infrared spectroscopy (ATRFTIR) was performed in situ on PHEMA-SA brushes incubated in buffer solutions with and without added salts (Figure 1). The ATR-FTIR analysis of PHEMA-SA immersed in buffer solutions alternatively at pH 5 and 9 was consistent with the swelling behavior of the polyacid grafts below and above their pKa (Figure S4). At pH 5 most of the carboxylic acid functions are present in their protonated form, as suggested by the strong carbonyl vibration recorded at 1750 cm−1. In contrast, at pH 9 the intensity of this peak was markedly reduced, while the signal correlated to the deprotonated carboxylate groups, centered at 1650 cm−1, became stronger compared to pH 5 (Figure S4).31−33 In the presence of Ca2+ and Zn2+ ions, the ATR-FTIR analysis indicated the structure of the metal−carboxylic acid D
DOI: 10.1021/acs.macromol.6b02673 Macromolecules XXXX, XXX, XXX−XXX
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Scheme 1. (a) Fabrication of PHEMA-SA Brushes by SI-ATRP Followed by Functionalization with Succinic Anhydridea
a Schematics depicting the physical appearance of PHEMA-SA brushes immersed in buffer solutions at pH 5 (b), pH 9 (c), forming bridging and chelating complexes with 10 mM Zn2+ solution at pH 5 (d), monodentate conjugates in 10 mM Zn2+ at pH 9 (e), bridging and chelating structures with 10 mM Ca2+ at pH 5 (f), and chelating bidentate complexes with 10 mM Ca2+ solution at pH 9 (g).
relatively strong bidentate complexes between Ca2+ and the carboxylic acid functions along the grafts hindered the variation of brush swelling that was observed when PHEMA-SA brushes are subjected to solutions of different pH values (Figure 1). The ATR-FTIR spectra recorded for PHEMA-SA brushes in the presence of Zn2+ showed different coordination structures
Increasing the pH to 9 led to an increase in the relative intensity of the signal centered at 1559 cm−1, which is specific for chelating bidentate complexes, in which the Ca2+ ion is coordinated to both the oxygen atoms of the carboxylate ligand. This complex structure is the one that is formed predominantly within the brush under basic conditions. The formation of E
DOI: 10.1021/acs.macromol.6b02673 Macromolecules XXXX, XXX, XXX−XXX
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Figure 3. Δf and ΔD variations recorded by QCM-D on PHEMA-SA brushes (a) subjected to buffer solutions at pH 5 and 9 (a), containing 10 mM Zn2+ (b) and 10 mM Ca2+ (c). After each incubation step a stable baseline in Milli-Q water was recorded.
pH-induced swelling of PHEMA-SA brushes. Interestingly, after incubation of PHEMA-SA brushes in pH 9 solution of Ca2+, subsequent rinsing with Milli-Q did not bring to Δf and ΔD values corresponding to the starting baseline (Figure 3c). In contrast, Δf values were positive and constant, while the corresponding ΔD data were slightly negative, suggesting that the relatively strong bidentate bridges formed at pH 9 between Ca2+ and the brush side functions were maintained during the washing step and induced a further shrinking of the brush films. It is also worthy of note that Δf variations recorded in the presence of Zn2+ and Ca2+ species at both pH 5 and 9 were likely generated by a combination of water uptake and metalion incorporation within the brushes, this phenomenon reducing the amount of swelling solvent within the films in the presence of salts. The water-exclusion effects caused by metal-ion complexation that were monitored in situ by QCM-D were also reflected by a variation in the wettability of PHEMA-SA brushes. Following incubation of the polymer films in 10 mM solutions of Ca2+ at pH 5 and pH 9, the water contact angles (CA) were 37 ± 3° and 36 ± 1°, respectively. These were significantly higher values compared to those measured on similar brush films that had solely been exposed to buffer solutions, which were 15 ± 1° following treatment with pH 5 buffer and
