Controlled Crosslinking Is a Tool To Precisely ... - ACS Publications

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Controlled Crosslinking Is a Tool To Precisely Modulate the Nanomechanical and Nanotribological Properties of Polymer Brushes Ella S. Dehghani, Shivaprakash N. Ramakrishna, 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: Covalent crosslinking of weak polyelectrolyte brushes widens the tuning potential for their swelling, nanomechanical, and nanotribological properties, which can be simultaneously adjusted by varying the crosslinker content and the pH of the surroundings. We demonstrate that this is especially valid for poly(hydroxyethyl methacrylate) (PHEMA) brushes and brush hydrogels, and their ionizable, succinate-modified derivatives (PHEMA-SA), covalently crosslinked with different amounts of di(ethylene glycol) dimethacrylate (DEGDMA) during surface-initiated atom transfer radical polymerization (SIATRP). Atomic force microscopy (AFM) methods highlight how pristine PHEMA films are stiff and display high coefficients of friction in water. Their succinate derivatives swell profusely in aqueous media. Under acidic conditions they are neutral, compliant, and lubricious, with apparent Young’s moduli (E*) lying between 10 and 30 kPa. Their contact mechanical behavior can be described by either the Johnson−Kendall−Roberts (JKR) or the Derjaguin−Müller−Toporov (DMT) model, depending on crosslinker content. In contrast, under basic conditions, brushes and brush hydrogels become charged, expand, and present a rigid, electrostatic barrier toward the AFM probe. Friction is extremely low at relatively low applied loads, whereas it increases at higher loads, to an extent that is regulated by the number of crosslinks within the films.

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INTRODUCTION Covalently crosslinked, polymer brushes have been emerging as highly versatile coatings for a variety of applications in surface and biomaterials science.1−5 When compared to “linear” polymer brushes, brush gels or brush hydrogels allow the modulation of interfacial, physicochemical properties over a very wide range.6 Of these, the swelling, nanomechanical, and nanotribological properties are among the most technologically relevant.7 Within a brush network, the presence of covalent crosslinks, introduced by means of opportunely chosen, difunctional monomers during the surface-initiated polymerization (SIP) process, and the control of their concentration, enable the precise modulation of the film characteristics, while maintaining the interfacial chemical composition nearly constant.7,8 The presence of the grafting surface, which effectively represents a common crosslink to all the grafted chains, fundamentally alters the distinctive properties of the polymer network and enhances their variation in response to a structural change (e.g., an increase of crosslink concentration).6 For example, polyacrylamide (PAAm) brushes show a 10fold increase in Young’s modulus and a similar increment in the coefficient of friction (μ) upon introduction of just 5 mol % of bis-acrylamide crosslinker during SIP, whereas the interfacial, chemical composition of the films did not show a significant variation.7,9 © XXXX American Chemical Society

Additionally, it has been observed that poly(hydroxyethyl methacrylate) (PHEMA) brushes crosslinked with di(ethylene glycol) dimethacrylate (DEGDMA) or tetra(ethylene glycol) dimethacrylate (TEGDMA) present protein-repellent properties that can be precisely varied by changing the crosslinker type and its relative concentration within the films.8 The unique and fully tunable physicochemical properties of brush hydrogels have stimulated their application within more complex polymer architectures. Multilayered films constituted of block copolymers, including brushes and brush hydrogels, have been applied to accurately vary the swelling and mechanical properties of PAAm coatings across the direction perpendicular to the grafting surface.5,10 Interestingly, when, within similar multilayered films, a “linear” brush is replaced by thermoresponsive, poly(N-isopropylacrylamide) (PNIPAM) segments, brush hydrogel/brush films display a composite-like mechanical behavior above the lower critical solution temperature (LCST) of PNIPAM, the brush hydrogel block acting as a reinforcing element during the collapse of the multilayered structure.11 With the objective of further exploring the characteristics of brush hydrogels displaying stimuli-responsive properties, we Received: November 7, 2016 Revised: March 8, 2017

A

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described by Baker et al.16 Diethylene glycol dimethacrylate (DEGDMA) (Aldrich Fine Chemicals, Switzerland) was passed through a column of basic alumina prior to use. Water was deionized with a GenPure filtration system (18.2 MΩ cm, TKA, Switzerland). Tris(hydroxymethyl)aminomethane (Alfa Aesar, Germany) basic buffer solution (0.1 M) and bis(2-hydroxyethyl)aminotris(hydroxymethyl)methane (Sigma-Aldrich) base buffer solution (0.1 M) were prepared with Milli-Q water, and the pH was adjusted to 9 and 5 by the addition of HCl (0.1 M) (Aldrich Fine Chemicals, Switzerland). Surface-Initiated, Atom-Transfer Radical Polymerization (SIATRP). The synthesis of the silane-based initiator (11-(2-bromo-2methyl-propionyl)-dimethylchlorosilane (BPCS) for SI-ATRP was carried out in accordance with the already reported protocol by Sanjuan et al.8,17 BPCS monolayers on silicon oxide substrates were obtained by wet deposition. Namely, silicon substrates were first treated with piranha solution (warning: piranha solution is very reactive and corrosive; use extreme caution!) and subsequently immersed overnight in a 10 mM dry toluene solution of BPCS. PHEMA brushes and brush hydrogels were grown from BCPS SAMs using SI-ATRP of different HEMA/DEGDMA mixtures, applying a catalytic system comprising CuCl (55 mg, 0.55 mmol), CuCl2 (36 mg, 0.26 mmol), and 2,2-bipyridine (244 mg, 1.56 mmol) in a mixture of monomer:water (4:4 mL).8 The monomer mixture, solvent, and ligand were bubbled with nitrogen for half an hour to remove dissolved oxygen, after which the solution was transferred to a flask containing the CuCl/CuCl2 species and kept under nitrogen. The resulting solution was stirred for 30 min following the complete formation of the catalyst complex (dark-brown solution). The polymerization solution was then transferred to the flask containing the BPCS-functionalized silicon oxide substrates, and the reaction was allowed to proceed for 1 h. Following the polymerization, the samples were removed from the solution and rinsed extensively with ethanol and water. In order to introduce carboxylic acid functions along the PHEMA grafts, the modified wafers were immersed in 0.5 M solution of succinic anhydride in dry pyridine overnight. Characterization. The dry thickness of brushes and brush hydrogels was measured with a variable-angle spectroscopic ellipsometer (VASE) (M-2000F, LOT Oriel GmbH, Darmstadt, Germany). The measurements were carried out under the assumption that the dry polymer film has a refractive index of 1.45 and applying a three-layer model (SiO2/Cauchy (initiator)/Cauchy (PHEMA)) with SiO2 and initiator layers presenting fixed thicknesses and refractive indices (software WVASE32, LOT Oriel GmbH, Darmstadt, Germany). A Cauchy model, n = A + B/λ2, was used to describe the refractive index of the PHEMA films by means of two fitting parameters: offset (A = 1.45) and wavelength dispersion (B = 0.01 μm2). For measuring the swelling ratio, ellipsometry measurements in Milli-Q water were performed, using a custom-built liquid cell and applying a four-layer model (SiO 2 /Cauchy (initiator)/Cauchy (PHEMA)/ambient (water)), with the refractive index of water set at 1.33. The swollen PHEMA thickness was analyzed with an effective-medium approximation (EMA) model, featuring both Cauchy and water components. FTIR spectra were recorded in transmission mode on dried samples by 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 silicon wafer. Attenuated total reflection Fourier-transform infrared (ATR-FTIR) spectroscopy was carried out with a Cary 670 FTIR spectrometer (Agilent Technologies) equipped with a SPECAC ATR diamond accessory, at room temperature, recording ATR-FTIR spectra within the range 700−4000 cm−1. The chemical composition of brushes and brush hydrogels was analyzed by X-ray photoelectron spectroscopy (XPS). XPS measurements were performed using a Theta-Probe X-ray photoelectron spectrometer (ARXPS, Thermo Fisher Scientific, Waltham, MA), with a monochromatic Al Kα source with a beam diameter of 300 μm. High-resolution spectra of carbon and oxygen with a pass energy of 100 eV were collected. Three measurements were performed for each sample, all containing the survey spectra. The

report the fabrication of covalently crosslinked, weak polyacid brushes and the characterization of their pH-dependent, physicochemical properties. We have especially concentrated on PHEMA-based brush hydrogels, synthesized by surfaceinitiated atom transfer polymerization (SI-ATRP) in the presence of different concentrations of DEGDMA, which was used as crosslinker. PHEMA brush hydrogels were subsequently derivatized with succinic anhydride to yield ionizable, succinate-bearing polymer backbones within the films (PHEMA-SA). The swelling properties and the nanomechanical characteristics of brushes and brush hydrogels were first addressed by a combination of variable angle spectroscopic ellipsometry (VASE) and atomic force microscopy (AFM), which revealed the influence of crosslinking on water incorporation and film stiffness. These properties were then compared for the pristine PHEMA films and PHEMA-SA polyacid layers, which were immersed in water at different pH values. Having established how the interplay between crosslinker concentration and pH determined swelling, stiffness, adhesion, and charge on the different brush layers, we focused on understanding how these parameters regulate the nanotribological behavior of the films. Lateral force microscopy (LFM) measurements highlight how friction is generally determined by the particular brush architecture and its pH-dependent swelling properties. The introduction of crosslinks and the increase in their concentration cause a decrease of conformational freedom in the grafts and reduce the amount of fluid lubricant within the films. Both these effects cause an increase of friction. In contrast, the introduction of hydrophilic, carboxylic acid functions produces an increase in the amount of swelling water incorporated in the brush hydrogel films, translating into a decrease of film stiffness and an improvement in its lubricating properties.12 The pHresponsive nanotribological behavior of brushes and brush hydrogels can be further described by applying continuum contact-mechanics models. In the neutral form, PHEMA-SA brushes and brush hydrogels display frictional properties that are typical of compliant, swollen films. Alternatively, the presence of negative charges at high pH values induces a further decrease of friction, the charged films increasing their swollen thickness due to the electrostatic tension between neighboring grafts. However, as a distinctive attribute of brush hydrogels, all the parameters regulating the nanotribological character of the films can also be adjusted by modifying the concentration of crosslinks within the polymer architecture. pH-responsive brush hydrogels emerge as functional coatings that are readily accessible via relatively simple fabrication procedures and display fully tunable, interfacial physicochemical characteristics. The precise and nearly independent modulation of their swelling, mechanical, and tribological properties, allowed by tuning the unique structure of surfacegrafted networks, is a highly attractive feature for a variety of applications in surface and materials science.13−15

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EXPERIMENTAL SECTION

Materials. 10-undecen-1-ol (Aldrich Fine Chemicals, Switzerland), dry THF (99.5% extra dry, Acros, Germany), bromoisobutyryl bromide (Aldrich-Fine Chemicals, Switzerland), dimethylchlorosilane (Aldrich Fine Chemicals, Switzerland), succinic anhydride (Aldrich Fine Chemicals, Germany), pyridine (Sigma-Aldrich, Germany), and hydrochloric acid (Sigma-Aldrich, Germany) were used as received. Hydroxyethyl methacrylate was purified according to a procedure B

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Scheme 1. Fabrication of PHEMA-Based Brushes and Brush Hydrogels by SI-ATRP (a); Three Different PHEMA Films Were Obtained: PHEMA Brushes (b), PHEMA-DEG1 (c), and PHEMA-DEG2 Brush Hydrogels (d); Derivatization of the Hydroxyl Groups of HEMA Units with Succinic Anhydride Yielded Ionizable, Succinate Functions along the Grafts (e); pH-Responsive Brushes and Brush Hydrogels Were PHEMA-SA (f), PHEMA-DEG1-SA (g),and PHEMA-DEG2-SA (h)

system was equipped with a combined low-energy electron/ion flood gun for charge compensation. The measurements were performed in standard lens mode with an emission angle of 53° to the surface and an acceptance angle of ±30°. The elemental composition from the survey spectrum (69.9% carbon and 30.1% oxygen) was within the error of the expected composition for the PHEMA film (66.7% carbon and 33.3% oxygen, 4% error). The C 1s peak for PHEMA was resolved into five component peaks: a hydrocarbon (C−C) peak at 285 eV, a quaternary carbon (q-C) peak at 285.6 eV, a hydroxyl (C−OH) peak at 286.4 eV, an ether (C−O) peak at 287 eV, and a carbonyl (O−C O) peak at 289.1 eV (see Supporting Information). The binding energies and the relative areas of all these peaks were consistent with the structure of PHEMA, as previously reported.18 In order to quantify the crosslinker concentration in the brush hydrogel films, the C 1s peak recorded for brush hydrogel coatings were also resolved with the same component peaks. To estimate the crosslinker concentration, we employed eq 1, where A is the calculated area of the C−OH peak from XPS C 1s (summarized for each film in Table S1).4

mol % of cross‐linker = (A brush − A brush hydrogel )/A brush

normal spring constants of the cantilevers were measured by applying the thermal-noise method19 before attaching the microspheres. The nanomechanical and adhesive properties of brushes and brush hydrogels were measured from the approaching and the retracting profiles of the recorded force vs distance (F−D) curves (35 force curves over 10 μm × 10 μm area, at a minimum of three spots). The apparent Young’s modulus (E*) of the different films was calculated using a Hertzian model,20 provided by the instrument software, by fitting the approaching profiles of the F−D curves between the contact point and a penetration depth corresponding to less than 5% of the films’ swollen thickness with the following equation:

F=

4E*R0.5δ1.5 3 − 3ν 2

(2)

where F is the applied load, R is the radius of the colloid used as a probe, ν is the Poisson’s ratio (considered to be 0.5), and δ is the deformation of the polymer films (calculated from the relative piezoextension z and relative deflection of the cantilever). The lateral force calibration of the cantilevers was performed according to the method described by Cannara et al.21 The torsional spring constants of the cantilevers were measured by using Sader’s method.22 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. This smooth silicon plane was used as vertical “wall” for measuring the lateral sensitivity. A test probe (cantilever with a silica colloidal sphere of diameter around 40 μm glued to it) was 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, and this was

(1)

The nanomechanical and nanotribological properties of brushes and brush hydrogels were investigated under Milli-Q water by means of atomic force microscopy (AFM) (Asylum Research MFP-3D, Santa Barbara, CA). The probes used for the measurements were modified by gluing silica microparticles with a diameter of 16 μm (EKA chemicals AB, Kromasil, Sweden) to the end of Au-coated, tipless cantilevers (NSC36, Micromasch, Bulgaria) using a home-built micromanipulator and a UV-curable glue (Norland Optical Adhesive 63, Cranbury, NJ). The C

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signal at 1589 cm−1, correlated to the CO stretching of the carboxylate functions in the succinate moieties. In addition, the near disappearance of the broad band between 3500 and 3700 cm−1 suggested the nearly quantitative reaction of the hydroxyl groups of HEMA within the brush films (Figure 1b). The XPS elemental analysis of the C 1s signals confirmed the efficient derivatization of the hydroxyl moieties within the brush films, indicating more than 90% of conversion of the HEMA groups into succinate functions (Figure 2). Deconvolution of the C 1s spectrum of PHEMA-SA into five individual components suggested that around 7 ± 2% of C−OH groups remained unreacted (Figure 2b). The formation of bulkier succinate groups along the polymer brush backbones translated into a marked increase in film thickness for all the brush and brush hydrogel layers studied. As reported in Table 1, PHEMA-SA showed an increase in dry thickness of around 190%, reaching 288 ± 5 nm after SA functionalization. In a similar way, the dry thickness of the brush hydrogels was incremented by around 180 and 130% for PHEMA-DEG1-SA and PHEMA-DEG2-SA, respectively (Table 1). Both brushes and brush hydrogels presenting succinate side chains swelled profusely in an aqueous environment compared to the starting PHEMA-based films. This was due to the presence of hydrophilic, carboxylic acid functions along the polymer backbones. As reported in Table 1, the swelling ratio (Sr) in water of the different brush films, measured by VASE, significantly increased after the reaction with SA and showed pH-dependent behavior.6,23 At acidic pH, PHEMA-SA, PHEMA-DEG1-SA, and PHEMA-DEG2-SA showed Sr values of 2.9 ± 0.1, 2.6 ± 0.1, and 2.2 ± 0.1, respectively, the amount of swelling water decreasing with the increase in the crosslinker content.5 Under these conditions, the brush and brush hydrogel films presented most of their carboxylic acid functions in the protonated form, as indicated by in situ attenuated total reflection Fourier-transform infrared spectroscopy (ATRFTIR) (Figure S3 in the Supporting Information). In contrast, when the pH was raised to 9, the formation of negatively charged, carboxylate functions was favored (Figure S3), which enhanced the swelling of the brush films due to the electrostatic repulsion between the charged polymer segments, leading to nearly a 2-fold increment of Sr for all the films (Table 1 and Scheme 1f−h). In accordance with several previous studies by us and others, the pKa of the polyacid backbones was not sharply defined, the transition of the brushes from charged (and swollen) to neutral (and “collapsed”) occurring across a rather broad pH range, included between 5.5 and 7.6,24−28 It should be also noted that the swelling properties of the pH-responsive brush films were determined by the crosslinker content in a similar way to those of the unfunctionalized PHEMA-based brushes. This is a relevant result, since the precise adjustment of film swelling could be accomplished by tuning the pH or the crosslinker concentration within the films, or both. In order to investigate the nanomechanical and nanotribological properties of brushes and brush hydrogels in water under different pH conditions, colloidal probe atomic force microscopy (CP-AFM) and lateral force microscopy (LFM) were performed. The adhesive properties of the different films were analyzed by recording the retraction profiles of the force−distance curves (F−D) obtained by compressing the brushes and brush hydrogels in aqueous media (Figure 3a).

used for the lateral force calibration. The nanotribological properties of the different brush and brush hydrogels were evaluated by lateral force microscopy (LFM), scanning laterally on the polymer films and applying different normal loads while maintaining a constant scanning rate of 5 μm s−1. Four different spots for each sample were tested, and on each spot at least ten friction force loops were recorded.

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RESULTS AND DISCUSSION PHEMA-based brushes and brush hydrogels were synthesized by SI-ATRP from initiator-functionalized silicon oxide substrates (Scheme 1). Partial aqueous conditions during the polymerization reactions allowed the efficient grafting of around 100 nm thick films in a relatively short polymerization time (1 h).16 In order to fabricate brush hydrogels by SI-ATRP, DEGDMA was introduced into the monomer mixture with a concentration of 1 and 2 mol % with respect to the solution concentration of HEMA, finally producing PHEMA brushes with two different contents of crosslinks (PHEMA-DEG1 and PHEMA-DEG2, as exemplified in Scheme 1). The chemical composition of the fabricated films was characterized by FTIR and XPS. The FTIR spectra of PHEMA, PHEMA-DEG1, and PHEMA-DEG2 did not show any significant differences (Figure S1, Supporting Information). In all the spectra, a strong peak centered at 1750 cm−1 was correlated to the CO stretching of the monomer units, whereas the alkane stretching vibrations and the broad alcohol stretch peak appeared in the range 2800−3000 and at around 3500 cm−1, respectively (Figure 1a). A quantitative analysis of

Figure 1. Representative FTIR spectra of PHEMA (a) and PHEMASA brushes (b).

the crosslinker concentration within the polymer brush networks was performed by XPS (as described in detail in the Supporting Information). For PHEMA, PHEMA-DEG1, and PHEMA-DEG2 films, the C 1s peaks were resolved into five component peaks, originating from the monomer and crosslinker units, and their relative ratios were applied in eq 1 to determine the concentration of crosslinker within the brush hydrogel films. The content of DEGDMA within PHEMADEG1 and PHEMA-DEG2 was determined to be 6 ± 1 and 11 ± 3 mol %, respectively, indicating nearly a 6-fold increase of DEGDMA units incorporated into the brush hydrogel structures compared to the corresponding solution concentrations during SI-ATRP. The PHEMA brushes and the PHEMA-DEG1 and PHEMADEG2 brush hydrogels were subsequently reacted with succinic anhydride (SA) to yield succinate-bearing polymer grafts presenting ionizable carboxylic acid groups. FTIR spectroscopy revealed the successful derivatization of the hydroxyl groups of HEMA, through the appearance of a D

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Figure 2. C 1s XPS spectrum recorded on PHEMA brushes and the corresponding curve fit using five spectral components (a); C 1s XPS spectrum for PHEMA-SA brushes with the corresponding curve fit obtained by applying five spectral components (b). The unreacted C−OH group is highlighted as peak 3.

Table 1. Dry Thickness Dt (nm), Swollen Thickness St (nm), and Water Swelling Ratio, Sr = St/Dt, of PHEMA Brushes and Brush Hydrogels, before and after Modification with SA, As Measured by VASE PHEMA pristine PHEMA-based films

SA-functionalized films

Dt St Sr Dt St (pH St (pH Sr (pH Sr (pH

5) 9) 5) 9)

100 141 1.4 288 835 1559 2.9 5.4

± ± ± ± ± ± ± ±

3 4 0.1 5 7 10 0.1 0.1

PHEMA-DEG1 101 120 1.2 280 726 1288 2.6 4.6

± ± ± ± ± ± ± ±

2 4 0.1 6 4 12 0.1 0.1

PHEMA-DEG2 120 140 1.2 272 599 1117 2.2 4.1

± ± ± ± ± ± ± ±

3 3 0.1 6 4 8 0.1 0.1

PHEMA-DEG2 displayed average adhesion values of 10 ± 4 and 5 ± 2 nN.32 The nanomechanical properties of PHEMA brushes and the corresponding brush hydrogels were subsequently tested by CP-AFM, analyzing the approaching profiles from the F−D curves. As shown in Figure 3b, the introduction of DEGDMA caused a steepening of the F−D profiles, indicating the stiffening of the films by the formation of a polymer brush network. Fitting the approach curves with the Hertz model allowed an estimate of the apparent Young’s modulus (E*), which was 1.3 ± 0.1 MPa for PHEMA brushes, whereas PHEMA-DEG1 and PHEMA-DEG2 showed values of E* of 2.4 ± 0.4 and 2.9 ± 0.6 MPa, respectively. Following derivatization with succinic anhydride, PHEMA brushes and brush hydrogels showed pH-dependent adhesive and nanomechanical properties. In general, the adhesion recorded by CP-AFM significantly decreased due to the more hydrophilic character of the succinate-bearing polymer backbones (Figure 4). At pH 5, residual weak attraction, leading to adhesion forces ranging from 3 ± 1 nN for PHEMA-SA to 1 ± 0.5 nN in the case of PHEMA-DEG2-SA, were probably caused by short-range hydrophobic interactions involving the methacrylate groups along the grafts. An increase of the pH to 9 caused the disappearance of any measurable adhesion on PHEMA-SA, PHEMA-DEG1-SA, and PHEMA-DEG2-SA due

Figure 3. (a) Retraction profiles from F−D curves recorded on PHEMA brushes and brush hydrogels immersed in Milli-Q water by CP-AFM. (b) Force vs penetration recorded by CPM (spring constant of the cantilever 0.8 N m−1, radius of the silica probe 9 μm) on PHEMA brushes and brush hydrogels.

A relatively high adhesion of 25 ± 4 nN was observed on PHEMA brushes in water, presumably due to a combination of hydrophobic and van der Waals interactions between the colloid’s surface and the brush interface, coupled with a contribution of hydrogen bonding.29−31Adhesion progressively decreased upon introducing crosslinks within the PHEMA brush structure due to the loss of chain freedom and the decrease of contact area between the colloid and the crosslinked grafts. 30 In particular, PHEMA-DEG1 and E

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Figure 4. Retraction profiles from F−D curves recorded on unmodified PHEMA brushes and brush hydrogels (black traces), and following modification with SA at pH 5 (red traces) and pH 9 (blue traces). (a) PHEMA brush, (b) PHEMA-DEG1, (c) PHEMADEG2, and (d) average values of the pull-off force measured on each film. The data for the unmodified PHEMA brushes and brush hydrogels were recorded in Milli-Q water (pH ∼ 6) and did not show a dependence from the pH of the medium (all the data were collected by CP-AFM, with a cantilever with a spring constant of 0.8 N m−1, and bearing a silica probe with a radius of 9 μm).

Figure 5. Force vs penetration profiles recorded by CP-AFM on unmodified PHEMA brushes and brush hydrogels (black traces), and following modification with SA at pH 5 (red traces) and pH 9 (blue traces). (a) PHEMA brushes immersed in Milli-Q water and PHEMASA brushes immersed in water and different pH. (b) PHEMA-DEG1 brush hydrogels immersed in Milli-Q water and PHEMA-DEG1-SA brush hydrogels immersed in water and different pH. (c) PHEMADEG2 brush hydrogels immersed in Milli-Q water and PHEMADEG2-SA brush hydrogels immersed in water and different pH. E* values obtained by Hertz model fitting for the different brush and brush hydrogel films (spring constant of the cantilever used: 0.8 N m−1, radius of the silica probe: 9 μm). The data for the unmodified PHEMA brushes and brush hydrogels did not show a dependence from the pH of the medium.

to the electrostatic, repulsive interactions between the surface of the silica probe and the negatively charged, carboxylate functions exposed at the films’ surface. Also, for PHEMA-SA brushes and brush hydrogels immersed in aqueous media with different pH values, the measured adhesion gradually decreased upon increasing the relative content of DEGDMA within the films, confirming the effects of crosslinking on the grafts’ mobility and the probe−film contact area that were already observed on the pristine PHEMA films. The pH-dependent nanomechanical properties of PHEMASA films were strongly influenced by the degree of deprotonation of the carboxylic acid functions along the grafts. PHEMA-SA brushes and PHEMA-DEG1-SA and PHEMADEG2-SA brush hydrogels generally showed a much more compliant character with respect to the unmodified PHEMAbased films, with approaching F−D curves showing an increasing steepness with the increment in crosslinker content (Figure 5a−c). Enhanced swelling by SA derivatization thus determined the mechanical properties of these brush layers compared to the more hydrophobic PHEMA brush films. Nevertheless, the amount of charged groups within the films, at the two different pH values tested, regulated the effective stiffness recorded by the compressing AFM probe. At pH 5, PHEMA-SA brushes and brush hydrogels mainly presented protonated carboxylic acid functions within their structures. Because of the absence of electrostatic repulsive interactions between the grafts, and between the films and the negatively charged silica probe, the pressure applied by the approaching probe is opposed uniquely by the osmotic pressure exerted by the brush assemblies.33 Under these conditions, PHEMA-SA brushes and brush hydrogels are characterized by a rather compliant behavior (Figure 5a−c), with E* ranging from 11 ± 3 for PHEMA-SA to 25 ± 8 kPa for the most crosslinked brush

hydrogel, PHEMA-DEG2-SA (Figure 5d). Increasing the pH to 9, i.e., well above the estimated pKa of the polyacid grafts, generated a high concentration of negative charges, leading to a profuse stretching of the grafts and a consequent vertical expansion of the brush films. The F−D approach profiles recorded under these conditions showed a marked steepening with respect to those originating from the compression of brush layers immersed in pH 5 solution. In accordance with the theoretical and experimental results reported by Abbott et al.,33 charged polyelectrolyte (PE) brushes display higher apparent stiffness with respect to their neutral counterparts due to a combination of osmotic pressure and the accumulation of electrostatic repulsive interactions within the compressed film, which hinder brush dehydration upon compression. In addition, electrostatic repulsive forces between the compressing SiO2 probe and the charged brush films also contributed to the steeping of the F−D profiles (which was also confirmed by the absence of adhesion in the retraction curves, as displayed in Figure 4a−c).34 Hence, negatively charged brushes and brush hydrogels produce a physical and electrostatic barrier toward the silica probe, while the apparent stiffness of this barrier could be enhanced by increasing the concentration of DEGDMA within the grafted films (Figure 5c). In this configuration PHEMA-SA brushes showed E* = 180 ± 58 kPa, which increased to 288 ± 87 and 486 ± 108 kPa for PHEMA-DEG1SA and PHEMA-DEG2-SA, respectively. F

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Figure 6. (a) Ff−L profiles recorded in the load range between 7 and 126 nN on PHEMA brushes and brush hydrogels immersed in Milli-Q water, where the coefficient of friction (μ) was measured as 0.49 for PHEMA, 0.67 for PHEMA-DEG1, and 0.76 for PHEMA-DEG2. The intercepts of the Ff−L plots with the X-axis correspond to the adhesion recorded by CP-AFM on the same films. The Ff−L plots were recorded applying a load range between 7 and 126 nN using a silica probe of 18 μm in diameter attached to a cantilever with spring constant of 0.8 N·m−1. (b) A schematic representation of a PHEMA brush hydrogel immersed in water and subjected to the shearing probe is provided. The data for the unmodified PHEMA brushes and brush hydrogels did not show a dependence on the pH of the medium and were recorded in Milli-Q water (pH ∼ 6).

Figure 7. Ff−L profiles recorded by LFM on PHEMA-SA brushes and brush hydrogels immersed in water at pH 5 (a). The Ff−L plots were recorded applying a load range between 9 and 146 nN using a silica probe of 18 μm in diameter attached to a cantilever with spring constant of 0.8 N m−1. The line traces in (a) are fittings of the Ff−L data with the generalized transition equation (GTE).36 (b) A schematic representation of the brush films modified with SA immersed in pH 5 water undergoing deformation by the shearing probe is provided.

⎡ R(L + FA ) ⎤2/3 Ff = μ(F + FA ) + τπ ⎢ ⎥ ⎣ ⎦ E

Having established how the variation of pH and crosslinker content altered the adhesion and the nanomechanical properties of PHEMA and PHEMA-SA brushes and brush hydrogels, we subsequently investigated the effects of these characteristics on the nanotribological behavior of the different brush architectures studied. The friction force vs load (Ff−L) profiles, recorded by LFM on PHEMA, PHEMA-DEG1, and PHEMA-DEG2, all showed a linear trend, while the introduction of crosslinks and an increase in their relative concentration caused an increment of friction (Figure 6).7,35 After derivatization with succinic anhydride, PHEMA-SA brushes and brush hydrogels generally showed lower friction when compared to the pristine, PHEMA-based films (Figure 7). In addition, the polyacid films displayed a pH-dependent, nanotribological behavior. At pH 5 all the films showed higher friction if compared to pH 9, while under acidic conditions the Ff−L profiles displayed a sublinear trend. We could interpret the Ff−L plots considering the relationship proposed by Busuttil et al.37 and Nikogeorgos et al.,38 who expressed friction force (Ff) as originating from the sum of a load-dependent term, which is correlated to “molecular plowing” and derives from the energy dissipation produced by changes in the molecular conformation of the film, and a shear term, which depends on the shear strength τ. The friction force was thus expressed as

(3)

where F is the applied, normal load, FA is the force of adhesion, R is the probe radius, and E is the elastic modulus of the film. In the limit of no adhesion or for swollen and highly compliant films the shear term in eq 3 becomes negligible and friction scales linearly with the applied load. In the case of unfunctionalized PHEMA brushes and brush hydrogels in water, swelling is rather low due to the limited solubility of PHEMA chains in this medium (1.4 > Sr > 1.2), and the brush layers are relatively stiff, with typical values of E* ranging from 1.3 MPa for PHEMA brushes to 2.9 MPa for PHEMA-DEG2 brush hydrogel. Although hydrophobic and hydrogen-bonding interactions between the silica probe and the brush/brush hydrogel surfaces caused significant adhesion forces (pull-off forces ranging from 25 to 5 nN), the probe’s penetration within the polymer films is greatly hampered (as depicted in Figure 6b). Consequently, friction on the rigid polymer brush surfaces is mainly attributable to energy dissipation by molecular plowing and the Ff−L plots assume a linear progression, with μ values of 0.49, 0.67, and 0.76 for PHEMA, PHEMA-DEG1, and PHEMA-DEG2, respectively. Although the Ff−L profiles among the different films showed a similar trend, friction generally increased from brushes to brush hydrogels and upon increasing the relative concentration of G

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Figure 8. Ff−L profiles recorded by LFM on PHEMA-SA brushes and brush hydrogels immersed in water at pH 9 (a). The Ff−L plots were recorded applying a load range between 9 and 146 nN. In (b) a schematic representation of the brush films modified with SA immersed in pH 9 water highlights the two different regimes recorded during LFM measurements. At relatively low loads (inset of (a)), friction is extremely low due to the electrostatic repulsion between the negatively charged silica probe and the deprotonated carboxylic acid functions at the polymer surface. At loads higher than 50 nN, the probe starts to deform the brush films and the friction increases.

follow the Derjaguin−Müller−Toporov (DMT) model42,43 and correspond to α = 0. Finally, when 0 < α < 1 a transition regime between the two limiting cases describes the contact. Fitting of the Ff−L data with the eq 4 could be accomplished by replacing the occurrence of a with Ff , and a0 with Ff0 (friction force at zero load),40 providing the values of the parameter α for the frictional properties of the three different brush and brush hydrogels studied in water at pH 5. The resulting values of α were 0.7, 0.44, and 0, for PHEMA-SA brushes, PHEMA-DEG1-SA, and PHEMA-DEG2-SA brush hydrogels, respectively (Figure 7a). Hence, these results suggested that the more compliant PHEMA-SA brushes, displaying relatively high adhesion, fell in the transition regime, although indicating a behavior close to JKR mechanics.44 In contrast, the introduction of crosslinks between the grafts stiffened the polymer films and caused a decrease of FA. These phenomena translated into a progressive transition toward the DMT model, which correctly described the contact mechanics on the most crosslinked, PHEMA-DEG2-SA brush hydrogels.29 When the pH of the medium is shifted to 9, i.e., well above the pKa of the polyacid grafts, both PHEMA-SA brushes and the corresponding brush hydrogels increase their swollen thickness, more than doubling their Sr values. Simultaneously, due to the electrostatic repulsion between negatively charged, neighboring chains, the films become more rigid and display a 10-fold increase of E* values with respect to their neutral analogues (Table 5). In this condition, due to the electrostatic repulsion between the silica probe and the polymer films, no adhesion was recorded for either brushes or brush hydrogels. Thus, the second term of eq 3 could be neglected and friction increased linearly with the applied load, following Amontons’ law (Figure 8). It is noteworthy that weak polyelectrolyte brushes in their charged state present higher resistance toward the compressing probe and are capable of maintaining a relatively high content of solvent within their structure, when subjected to an applied pressure.33 Hence, due to the more incompressible character of charged brushes compared to their neutral analogues, and the retention of fluid lubricant, PHEMASA brushes and brush hydrogels at pH 9 generally showed low friction. The pH-dependent frictional behavior of PHEMA-SA brushes and brush hydrogels was somehow similar to the one

crosslinks. This was due to the introduction of lateral constraints between the grafts, which hindered their mobility and translated into an increase of dissipative forces between the shearing probe and the polymer interface.30 In addition, the progressive decrease of swelling in water upon increasing DEGDMA content lowered the lubricating properties of the polymer films by reducing the content of fluid lubricant within the layers.35,39 After the reaction with SA, the brush and brush hydrogel films show pH-responsive swelling and nanomechanical properties, these being regulated by the degree of protonation of the carboxylic acid functions along the backbones of the grafts (as exemplarily depicted in Scheme 1). At pH 5, PHEMA-SA brushes and PHEMA-DEG1-SA/PHEMA-DEG2SA brush hydrogels are neutral, incorporating water to a greater extent compared to their unfunctionalized analogues (2.9 > S > 2.2) and thus exhibiting a more compliant character (25 kPa > E* > 11 kPa). The AFM probes can thus easily penetrate into the films (Figure 7b), while the adhesion force between the probe and the polymer surface becomes relevant in the second term of eq 4. Hence, the Ff−L plots follow a sublinear trend and friction increases with the increment of crosslinker concentration within the films. In order to further illustrate the contact mechanics in the case of neutral and compliant brush films subjected to a shearing probe, we applied the generalized transition equation (GTE),36 introduced by Carpick et al., which is commonly used for describing single-asperity contacts in LFM and the surface forces apparatus (SFA).40 ⎛ α + 1 + L/F a A = ⎜⎜ a0 1 + α ⎝

⎞2/3 ⎟⎟ ⎠

(4)

where a and a0 are the contact radius and the contact radius at zero load, respectively, with α representing the “transition parameter”, which suggests the type of contact mechanics model better describing the probe−film interaction. Generally, α = 1 refers to the Johnson−Kendall−Roberts (JKR) model,41 which appropriately describes the contact mechanics for soft materials with high surface energy and undergoing large deformations. In contrast, stiffer materials that are resistant to deformation and present low surface energy H

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Macromolecules observed by Raftari et al.,29 in the case of polybase brushes immersed in water at different pH values. Also in the mentioned study, charged polyelectrolyte brushes showed low friction, with Ff−L profiles following Amontons’ law. In contrast, in their neutral form, polyelectrolyte brushes showed sublinear Ff−L profiles and higher friction. Interestingly, the Ff−L profiles recorded for charged PHEMA-SA brushes and brush hydrogels featured two different slopes. At relatively low loads the recorded friction is extremely low, corresponding to μ values of 0.006, 0.010, and 0.043 for PHEMA-SA, PHEMA-DEG1-SA, and PHEMA-DEG2-SA, respectively. Such a low initial friction was mainly determined by the repulsive electrostatic interactions between the shearing probe and the charged polymer interfaces, as was previously observed in the case of two negatively charged surfaces sliding against each other at low loads.45−47 When the applied load is increased above around 50 nN, friction increases, although to a different extent for each type of brush film. In particular, the μ value of PHEMA-SA increased just slightly, reaching 0.019. In contrast, both PHEMA-DEG1SA and PHEMA-DEG2-SA brush hydrogels showed a noticeable increment in the slope of the Ff−L profiles, the μ values at high applied loads (50−146 nN) being 0.058 and 0.103, respectively. We can rationalize the different frictional properties of negatively charged brushes and brush hydrogels by considering that energy dissipation during sliding is higher for crosslinked grafts compared to linear brushes. This is due to the introduction of lateral constraints between the grafts and the lower swelling water present within the brush networks.

through this study, can be exploited to design and fabricate a new generation of responsive polymer coatings, finding applications in different areas of materials and biomaterials science.

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ASSOCIATED CONTENT

S Supporting Information *

The Supporting Information is available free of charge on the ACS Publications website at DOI: 10.1021/acs.macromol.6b02409. FTIR and XPS analysis of the different brushes and brush hydrogels (PDF)

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AUTHOR INFORMATION

Corresponding Author

*(E.M.B.) E-mail [email protected]; Ph +41 (0) 44 6326050. ORCID

Nicholas D. Spencer: 0000-0002-7873-7905 Edmondo M. Benetti: 0000-0002-5657-5714 Funding

Funding from the ETH Research Commission is gratefully acknowledged. E.M.B. acknowledges financial support from the Swiss National Science Foundation (SNSF “Ambizione” PZ00P2-148156). Notes

The authors declare no competing financial interest.

■ ■

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ACKNOWLEDGMENTS The authors thank Prof. Antonella Rossi and Giovanni Cossu (ETH Zürich) for their support in the XPS measurements.

SUMMARY Brush hydrogels are characterized by highly tunable swelling, nanomechanical, and nanotribological properties, which can be precisely modulated by varying the concentration of crosslinks within the brush architecture. When PHEMA-based brushes and brush hydrogels are modified by introducing ionizable succinate functions along the grafts, all the above-mentioned interfacial properties can be further varied as a function of the pH of the surrounding medium. While unmodified PHEMA brush/brush hydrogels are rather stiff in water and show high coefficients of friction, the corresponding PHEMA-SA analogues become more compliant and swollen, showing pH-dependent nanotribological properties. Contact-mechanics models provide a comprehensive representation of this responsive behavior. In its neutral form, the brush films can be deformed by the shearing AFM probe and depending on the amount of crosslinks within their structure, either transition (close to JKR) or DMT models can describe the friction vs force profiles recorded by LFM. Above the pKa of the polyacid grafts, the films become charged and thus less penetrable, although more hydrated. Under these conditions, the friction vs load profiles recorded by LFM showed bimodal, linear progressions, at low loads electrostatic repulsions leading to very low μ values, whereas at higher applied loads the frictional properties being regulated by the crosslinker concentration within the films. In summary, responsive brush hydrogels are extremely versatile coatings, whose interfacial properties can be precisely tuned as a function of the brush architecture coupled with the characteristics of the medium in which they are immersed. The full understanding of the relationship between brush architecture and interfacial properties, which is revealed

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