Topology Effects on the Structural and Physicochemical Properties of

Article pubs.acs.org/Macromolecules

Topology Effects on the Structural and Physicochemical Properties of Polymer Brushes Mohammad Divandari,† Giulia Morgese,†,§ Lucca Trachsel,†,§ Matteo Romio,∥ Ella S. Dehghani,† Jan-Georg Rosenboom,‡ Cristina Paradisi,∥ Marcy Zenobi-Wong,§ Shivaprakash N. Ramakrishna,† and Edmondo M. Benetti*,† †

Laboratory for Surface Science and Technology, Department of Materials, and ‡Department of Chemistry and Applied Biosciences, Institute of Chemical and Bioengineering, ETH Zürich, Vladimir-Prelog-Weg 1-5/10, 8093 Zürich, Switzerland § Cartilage Engineering + Regeneration Laboratory, Department of Health Sciences and Technology, ETH Zürich, Otto-Stern-Weg 7, 8093 Zürich, Switzerland ∥ Department of Chemical Sciences, University of Padova, via Marzolo 1, 35030 Padova, Italy S Supporting Information *

ABSTRACT: The application of polymer “brushes”, with their unique physicochemical properties, has led to a radical change in the way we functionalize biomaterials or formulate hybrids; however, their attractive traits can be largely surpassed by applying different polymer topologies, beyond the simple linear chain. Cyclic and loop brushes provide enhanced steric stabilization, improved biopassivity, and lubrication compared to their linear analogues. Focusing on poly(2-ethyl-2-oxazoline) (PEOXA), an emerging polymer in nanobiotechnology, we systematically investigate how topology effects determine the structure of PEOXA brushes and to what extent technologically relevant properties such as protein resistance, nanomechanics, and nanotribology can be tuned by varying brush topology. The highly compact structure of cyclic PEOXA brushes confers an augmented entropic barrier to the surface, efficiently hindering unspecific interactions with biomolecules. Moreover, the intrinsic absence of chain ends at the cyclic-brush interface prevents interdigitation when two identical polymer layers are sheared against each other, dramatically reducing friction. Loop PEOXA brushes present structural and interfacial characteristics that are intermediate between those of linear and cyclic brushes, which can be precisely tuned by varying the relative concentration of loops and tails within the assembly. Such topological control allows biopassivity to be progressively increased and friction to be tuned.

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INTRODUCTION The coupling of synthetic polymers to inorganic surfaces yielding polymer brush1 interfaces has represented a revolution in many aspects of materials science and established a new field of expertise, focusing on polymer chemistry applied to surface science. While this research area has been progressing during the past two decades, attaining fundamental surface properties such as steric stabilization, lubrication, and biopassivity has become simultaneously an accessible task through the chainend grafting of functional polymers2 or by performing polymerizations from initiator-decorated supports.3−6 Polymer brushes comprising linear grafts of various compositions and molecular weights provide stable dispersions of inorganic colloids in both aqueous and physiological media.7,8 Hydrophilic and overall neutral brushes are applied on supports for tissue engineering and biosensors to efficiently prevent unspecific biological contamination.9−17 Moreover, similar grafts applied onto inorganic surfaces served as boundary lubricants, dramatically reducing friction.18−21 The inquiry motivating our study is whether we can surpass the unique physicochemical properties of polymer brushes © XXXX American Chemical Society

without exploring new chemistries or proposing sophisticated and technically demanding surface-modification protocols. We believe that the answer to this intriguing question relies on the fabrication of brushes presenting grafts of different polymer topologies, beyond the simple linear chain. In particular, we recently demonstrated that cyclic polymer adsorbates deposited on metal oxide surfaces yield polymer brushes displaying augmented steric stabilization and improved protein repellency with respect to their linear analogues of comparable molar mass.22 Additionally, cyclic brushes show a superlubricious behavior when sheared against topologically identical films. When similar grafts were applied as stabilizers for metal oxide nanoparticles (NPs), the generated cyclic-brush shells impart long-lasting colloidal stability and efficiently prevent the interaction of the inorganic cores with serum proteins, while linear brush analogues fail in hindering protein physisorption quantitatively.23 All these attractive traits derived from the Received: August 8, 2017 Revised: September 18, 2017

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DOI: 10.1021/acs.macromol.7b01720 Macromolecules XXXX, XXX, XXX−XXX

Article

Macromolecules translation of well-known topology effects24−26 by cyclic polymers into a marked alteration of relevant surface properties; namely, (i) the smaller hydrodynamic radius of cyclic macromolecules enables the fabrication of denser brushes, (ii) their ringlike morphology provides more compact assemblies where chain stretching is enhanced, and (iii) the absence of chain ends at the brush interface suppresses interdigitation when two brush-functionalized surfaces are placed in close contact. In what way these different contributions act together or independently in determining technological relevant properties, such as nanomechanics, friction, and biopassivity of the entire brush, has never been systematically explored. In order to get a fundamental and comprehensive insight into the effect of chain topology on the interfacial properties of surface-grafted polymer assemblies, we fabricated topologically different but chemically identical brushes, ranging from linear grafts, loop brushes presenting different mixtures of loops and linear tails, and cyclic brushes. We gave a special focus on how the progressive variation of polymer topology from linear to cyclic, through a mixture of the two, defines the structural and physicochemical characteristics of the generated films, including swelling, nanomechanical and nanotribological properties, and bioinertness toward complex protein mixtures. These were thoroughly studied by a combination of variable angle spectroscopic ellipsometry (VASE), X-ray photoelectron spectroscopy (XPS), and atomic force microscopy (AFM). The chemical composition of the brushes was based on poly(2ethyl-2-oxazoline) (PEOXA), which represents an emerging polymer in biomaterials and drug delivery applications, and it is envisioned to replace poly(ethylene glycol) (PEG) and its derivatives in several formulations.27−29 Linear and cyclic PEOXA adsorbates were synthesized by cationic ring-opening polymerization (CROP) and postfunctionalized with nitrocatechol groups (NC), which allowed them to be efficiently grafted on TiO2 surfaces,30,31 yielding sub-10 nm thick films. Alternatively, PEOXA brushes presenting mixtures of loops and tails were fabricated from random PEOXA copolymers featuring a variable concentration of surface-reactive, NC side groups. The results originated from this study highlight how the progressive disappearance of linear chains coupled to the increasing concentration of loop grafts stiffen the assemblies and provides a simultaneous increase of steric stabilization, lubrication and bioinertness. In the extreme case of cyclic grafts all these effects are further emphasized. The high density and increased stretching by cyclic brushes reduce swelling and concurrently provide the highest steric stabilization to the surface among the different brush topologies studied. The unique structural characteristics of cyclic brushes, in combination with the intrinsic absence of interfacial chain ends, generate superlubricious films that additionally show excellent biopassive properties toward the adsorption of full human serum. The distinctive properties of polymer brushes can be enhanced by replacing linear grafts with cyclic chains, while between these two limits, the relative content of loops and tails can be varied to largely modulate the interfacial physicochemical properties.

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Aldrich. 2-Ethyl-2-oxazoline (EOXA) was distilled over KOH. Methyl triflate and propargyl p-toluenesulfonate were distilled over CaH2. 2Azidoethylamine and nitrodopamine hemisulfate (NC) were synthesized according to previously reported procedures.22 The detailed synthesis of PEOXA adsorbates is reported in the Supporting Information.32 Grafting-To of PEOXAs on TiO2 Substrates. Silicon wafers purchased from Si-Mat (Landsberg, Germany) were coated by reactive magnetron sputtering (Paul Scherrer Institute, Villigen, Switzerland) with a 19 nm thick TiO2 layer and used as substrates for grafting topologically different PEOXAs. Prior to polymer adsorption, the substrates were cleaned by ultrasonication in toluene (2 × 10 min) and isopropanol (2 × 10 min) and subsequently dried under a stream of N2. Then, they were treated with UV-ozone (UV Clean Model 135500 from Boekel Industries, Inc.) for 40 min. The cleaned substrates were subsequently incubated in 2 mg mL−1 solutions of the polymers in 0.1 M 3-(N-morpholino)propanesulfonic acid (MOPS) buffer at pH 6, containing different concentrations of K2SO4 (summarized in Table S2), at the specific cloud point temperature (Figure S18 and Table S2). After 1 h, the samples were removed from the solution, and left rinsing in ultrapure water until a constant dry polymer thickness was attained. Proton Nuclear Magnetic Resonance Spectroscopy (1H NMR). A Bruker Avance III 500 MHz spectrometer was used to record spectra at room temperature using D2O or CDCl3 as solvents. Gel Permeation Chromatography (GPC). An Agilent 1100 series HPLC was used to perform analysis over 2× PSS PFG Linear M columns. The samples were dissolved in hexafluoroisopropanol (HFIP) + 0.02 M potassium trifluoroacetate (KTFAc) and eluted at 1 mL min−1. Poly(methyl methacrylate) (PMMA) standards were used to determine the molar masses of the different PEOXAs. Fourier Transform Infrared Spectroscopy (FT-IR). A Nicolet 5700 FT-IR spectrometer (Thermo Fisher Scientific, Switzerland) was used to record spectra in transmission mode in the 400−4000 cm−1 range at a resolution of 2 cm−1 and 64 scans. Atomic Force Microscopy (AFM). AFM height micrographs of the brush films were acquired in water using a Dimension Icon AFM (Bruker, Santa Barbara, CA) in peak-force tapping mode and using SNL-10 cantilevers with a spring constant of ∼0.06 N m−1. Colloidal probe AFM and LFM measurements were performed with a MFP3D AFM (Asylum Research, Oxford Instruments, Santa Barbara, CA) equipped with a liquid cell using 0.1 M pH 6 MOPS buffer. The normal (KN) and the torsional (KT) spring constants of the tipless cantilevers (Table S3) (NSC-38, Mikromash, Bulgaria) were measured by the thermal noise33 and Sader’s34 method, respectively, prior to the attachment of the colloid to the cantilevers (following the procedures reported in detail in the Supporting Information). The colloidal probes were fabricated in-house by applying a microdroplet of two component Araldite glue to the end of the cantilever followed by attaching a silica microparticle (EKA chemicals AB, Kromasil R, Sweden) of 20 μm diameter, using a custom-made micromanipulator. FS profiles recorded against a “bare” countersurface were collected using colloidal probes coated with a 20 nm thick Au layer (via vapor deposition). In order to functionalize the colloidal probes, a 20 nmthick TiO2 layer was deposited by reactive magnetron sputtering (Paul Scherrer Institute, Switzerland), and PEOXA brushes were later on grafted following the same procedures applied on the flat TiO2 substrates. Lateral force calibration was carried out by the test-probe method described by Cannara et al.35 The detailed protocol used for lateral force calibration is described in the Supporting Information. Friction measurements were carried out by acquiring 10 “friction loops” along the same line for each applied load over three different positions on each sample (scan rate: 0.5 Hz; sliding distance: 5 μm), from which the average friction forces and the standard deviations were calculated. The true friction values were obtained by multiplying the measured cantilever torsional signal with the calibration factor (α) calculated for each measurement. UV−Vis Spectroscopy. The CP of the different PEOXA solutions used for grafting were determined with an Agilent Cary 600

EXPERIMENTAL SECTION

Materials. All chemicals with the exception of 0.5 M NH3 in tetrahydrofuran (THF) (Acros Organics) were purchased from SigmaB


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Figure 1. Topologically different brushes were fabricated by grafting PEOXAs bearing NC anchors on TiO2 surfaces. L-PEOXA (x ∼ 110) and lPEOXA (x ∼ 70) were obtained by grafting monofunctional PEOXAs. Reaction conditions: CROP (i) using methyl triflate, 2-ethyl-2-oxazoline, dry ACN, 80 °C, 40 h, Ar atmosphere (X = CH3); termination by NH3, dry THF, 20 °C, 48 h, Ar atmosphere (Y = NH2); (ii) succinic anhydride, TEA, dry ACN, reflux, overnight; (iii) HOAt/HBTU, dry DMF, 20 °C, 30 min, Ar atmosphere, nitrodopamine hemisulfate, DIPEA, dry DMF, 0 to 20 °C, 6 h. C-PEOXA (x ∼ 120) and c-PEOXA (x ∼ 50) were obtained from monofunctional cyclic PEOXAs. Reaction conditions: (i) CROP using propargyl p-toluenesulfonate as initiator (X = HCC), 2-ethyl-2-oxazoline, dry ACN, 80 °C, 40 h, Ar atmosphere; termination by 2azidoethylamine, dry ACN, 20 °C, 48 h, Ar atmosphere (Y = −NHCH2CH2N3); (iv) intramolecular cyclization: CuI, sodium ascorbate, water, 20 °C, 48 h, Ar atmosphere; (v) succinic anhydride, TEA, dry ACN, reflux, overnight; (vi) HOAt/HBTU, dry DMF, 20 °C, 30 min, Ar atmosphere, nitrodopamine hemisulfate, DIPEA, dry DMF, 0 to 20 °C, 6 h. Loop PEOXA brushes were obtained by grafting random PEOXA copolymers with different content of NC groups (y = 4 or 7 mol %). Reaction conditions: (i) CROP using methyltriflate as initiator, 2-methoxycarboxyethyl-2oxazoline, 2-ethyl-2-oxazoline, dry ACN, 80 °C, 40 h, Ar atmosphere; termination by piperidine, dry ACN, 20 °C, 40 h, Ar atmosphere; (vii) deprotection of methyl ester by NaOHaq (pH ∼ 14), overnight, 20 °C; (viii) HOAt/HBTU, dry DMF, 20 °C, 30 min, Ar atmosphere, nitrodopamine hemisufate, DIPEA, dry DMF, 0 to 20 °C, 6 h. (ix) Surface grafting of PEOXAs on TiO2 were performed under CP conditions. lPEOXA: 0.3 M K2SO4 in 3-(N-morpholino)propanesulfonic acid (MOPS) buffer (pH = 6), 62 °C. L-PEOXA: 0.3 M K2SO4 in MOPS buffer (pH = 6), 62 °C. c-PEOXA: 0.25 M K2SO4 in MOPS buffer (pH = 6), 45 °C. C-PEOXA: 0.25 M K2SO4 in MOPS buffer (pH = 6), 53 °C. PEOXA-4:0.2 M K2SO4 in MOPS buffer (pH = 6), 51 °C. PEOXA-7:0.2 M K2SO4 in MOPS buffer (pH = 6), 51 °C. spectrophotometer (Santa Clara, CA) using 10.00 mm-optical path quartz cuvettes (Hellma 114-QS, Germany). The polymers were dissolved in the solutions summarized in Table S2 at a concentration of 2 mg mL−1. The absorbance was monitored at a wavelength of 500 nm in the 20−80 °C range of temperature with a 1 °C min−1 rate. The CP for each solution was chosen as the onset of the sharp increase in absorbance (Figure S18). Variable Angle Spectroscopic Ellipsometry (VASE). A Woolam ellipsometer (J.A. Woolam Co. U.S.) equipped with a custom-built, liquid cell was used to measure the dry and swollen thickness of the polymer films. Ψ and Δ were acquired as a function of wavelength (350−800 nm) and analyzed employing the package CompleteEASE (Woollam). For dry measurements, fitting was performed based on a layered model using bulk dielectric functions

for Si, SiO2, and TiO2. The analysis of the polymer brush layers was done on the basis of the Cauchy model: n = A + Bλ−2, where n is the refractive index, λ is the wavelength, and A and B were assumed to be 1.45 and 0.01, respectively, as values for transparent organic films.36 Measurements in ultrapure water were performed by equilibrating the samples for 30 min before acquiring the spectra. The recorded VASE data were fitted with a two-component effective medium approximation (EMA) model, consisting of a Cauchy and a water component using the Maxwell−Garnett equations.37 In this case, the thickness (T) and water content (w) were set as fitting parameters. Raw data and fittings are reported in Figure S20. Angle-Resolved X-ray Photoelectron Spectroscopy (ARXPS). Depth-dependent elemental analysis of the different PEOXA films was carried out by ARXPS using a Theta-Probe X-ray photoelectron C


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Macromolecules Table 1. Characterization of Topologically Different PEOXA Brushes brush (topology)

Mna

PDI

L-PEOXA (linear) l-PEOXA (linear) C-PEOXA (cyclic) c-PEOXA (cyclic) PEOXA-4 (loop) PEOXA-7 (loop)

11000 7000 12000 5000 11800 12300

1.3 1.3 1.2 1.1 1.3 1.4

Tdryb 3.2 3.5 5.1 4.5 3.8 4.2

± ± ± ± ± ±

0.1 0.2 0.6 0.2 0.2 0.2

Twetc

SRd

σe

nEOXAf

Rgg

L/2Rgh

± ± ± ± ± ±

1.6 0.9 0.7 0.8 1.1 0.9

0.20 0.34 0.29 0.61 0.22 0.23

22 24 35 30 26 28

5.5 4.7 3.9 2.9

0.22 0.19 0.25 0.24

8.3 6.7 8.9 7.9 7.9 8.1

2.1 1.5 3.3 2.4 3.2 2.5

a Molar mass of the polymer adsorbates expressed in [g mol−1] and measured by GPC. bDry thickness of the polymer films expressed in [nm] and measured by VASE. cSwollen thickness of the polymer films expressed in [nm] and measured by VASE in water. dSwelling ratio calculated as (Twet − Tdry)/Twet from the values obtained by VASE. eGrafting density expressed as [chains nm−2] calculated using the equation σ = ρTdryNAMn−1, where ρ is the density of the dry polymer layer (1.14 g cm−3), Tdry is the dry thickness measured by VASE, NA is the Avogadro number, and Mn is the average molar mass of the adsorbate measured by GPC. fSurface density of EOXA units expressed as [monomers nm−2]. gRadius of gyration of the polymer adsorbates; for linear PEOXA in water it was calculated according to Chen et al.41 with the equation Rg = [(Mwη)/(Φ63/2)]1/3, where η is the intrinsic viscosity for PEOXA (46.5 cm3 g−1 at 30 °C), Φ = 2.68 × 1023 mol−1, and Mw is the weight-average molar mass of the different PEOXAs measured by GPC and reported in Table S1. For cyclic PEOXAs, Rg was estimated according to Rg(linear)2/Rg(cyclic)2 = 2, according to Hadziioannou et al.42 and Arrighi et al.43 hThe degree of chains overlapping is expressed as the ratio of the distance between grafting points (L) and 2Rg.

spectrometer (Thermo Fisher Scientific, USA), equipped with a monochromatic Al Kα source and a beam diameter of 400 μm. Highresolution spectra of N 1s and survey spectra were recorded with a pass energy of 100 and 200 eV, respectively. In order to compensate for the charging of the modified surfaces, an electron-argon-ion flood gun was used. Theta-Probe XPS allowed the simultaneous acquisition of spectra at different emission angles, alternatively sampling the interfacial and the bulk elemental composition of the brush films. In particular, three measurements were performed for each sample at eight different emission angles to the surface, ranging from 26.8° to 79.2°. XPS curve fitting was performed using the CasaXPS software. Protein Adsorption Tests. Polymer-coated substrates were immersed in phosphate buffer saline (PBS) at pH 7.4 for 5 min to rehydrate; subsequently, they were exposed to full human serum (FHS) (Precinorm U, Roche Diagnostics GmbH, Mannheim, Germany) for 30 min. Then, the samples were rinsed with ultrapure water, dried under a stream of N2, and analyzed with VASE. Bare TiO2 substrates were used as positive controls. At least three samples were tested for each experimental condition.

conditions (Table S2), thus assuring the formation of highgrafting density layers.40 The structural properties of PEOXA brushes presenting different topologies were investigated by VASE performed in air and in ultrapure water. As reported in Table 1, cyclic PEOXA brushes showed the highest dry thickness among the different films. This corresponded to a lower content of water and a comparably higher surface density of grafted chains (σ) with respect to linear PEOXA brushes presenting similar molar masses. The denser character of cyclic PEOXA brushes is an intrinsic effect of their fabrication by grafting-to, and derived from the smaller hydrodynamic diameter of cyclic PEOXAs with respect to linear analogues (as measured by dynamic light scattering, DLS in Table S3), coupled to the formation of smaller aggregates at CP.44,45 These topology effects reduced the steric hindrance between surface-immobilized chains and the adsorbates approaching the surface during the grafting process,46 resulting in the formation of denser assemblies. The degree of overlapping between neighboring chains presenting different topologies could be estimated from the ratio between the interchain distance (L) and 2 times the radius of gyration (Rg) for each brush type. Because of the smaller Rg for cyclic PEOXA adsorbates with respect to linear analogues,42,43 chain overlapping resulted similar among linear and cyclic grafts, being in all cases included between 0.19 and 0.25. Under these conditions, with similar values of L/2Rg ≪ 1, both linear and cyclic PEOXA grafts are in a comparably dense-brush regime.1 We can thus conclude that at a similar degree of chain overlapping cyclic brushes are significantly more stretched along the normal direction to the grafting surface with respect to their linear counterparts, as witnessed by their higher equilibrium swollen thicknesses (Twet), compared to their smaller values of Rg in water. Remarkably, this representation of the equilibrium morphology of cyclic brushes agrees well with previous theoretical predictions by Su-Zhen et al., who illustrated cyclic brushes as more oriented and compact with respect to similar films generated by linear grafts presenting the same chain length.47 It is also noteworthy to mention that the nanomorphology of linear and cyclic PEOXA brushes did not show any relevant influence of chain topology, as recorded by atomic force microscope (AFM) peak-force imaging (Figure S19). Random PEOXA copolymers presenting different content of NC groups formed loop brushes featuring mixtures of loops

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RESULTS AND DISCUSSION Topologically different brushes were fabricated by grafting-to2 of PEOXA-based adsorbates (Figure 1). Cationic ring-opening polymerization (CROP) coupled to controlled termination and postfunctionalization were employed to synthesize monodisperse, monofunctional, and heterobifunctional PEOXAs, alternatively presenting lower (5000−7000 Da) and higher (11 000−12 000 Da) molar masses (Table 1 and Supporting Information). Monofunctional PEOXAs presenting a nitrocatechol (NC) chain-end strongly bound to the TiO 2 surface,30,31 yielding linear brushes (L-PEOXA and l-PEOXA of 11 and 7 kDa, respectively). Cyclic PEOXAs were obtained by Huisgen cycloaddition from heterobifunctional polymer precursors,22,32 which were subsequently coupled to NC yielding two types of TiO2-grafted cyclic brushes: C-PEOXA and c-PEOXA, presenting Mn of 12 and 5 kDa, respectively. Brushes presenting mixtures of loops and tails (named as loop brushes) were fabricated starting from random PEOXA copolymers featuring 4 and 7 mol % of 2-methoxycarboxyethyl2-oxazoline (MestOx) comonomer,38,39 while presenting the same Mn of ∼12 kDa. Subsequent deprotection of the methyl esters and further coupling with NC generated PEOXA adsorbates capable of forming loop brushes on TiO2 surfaces (PEOXA-4 and PEOXA-7). In all cases, PEOXA adsorbates were grafted on TiO2 surfaces under cloud point (CP) D


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Figure 2. (a) ARXPS provided the composition of PEOXA brushes at different depths, corresponding to different angles of emission (θ), included between 27° (in the proximity of the grafting surface) and 79° (brush interface), and confirmed how all the NC groups are bound to TiO2. (b) The parameter χ, expressing the relative concentration of NO2 groups across the films, was estimated for different values of θ. (c) Schematic representation of the chemical appearance of loop and linear PEOXA brushes close to the TiO2 surface. (d) High-resolution N 1s spectra recorded by ARXPS at four different angles θ on L-PEOXA brushes. The signal at ∼406 eV, correlates to NO2 groups from NC, and the N 1s signal centered at ∼400 eV originatesfrom the EOXA repeating units. .

and tails. Their dry thickness slightly increased with the reactivity of the random PEOXA copolymer adsorbates, PEOXA-7 generating thicker films with respect to PEOXA-4 (Table 1). However, σ did not show a remarkable variation (0.22 and 0.23 chains nm−2 for PEOXA-4 and PEOXA-7, respectively), and it was also comparable to the corresponding value for L-PEOXA brushes, which feature similar molar mass (0.20 chains nm−2, as reported in Table 1). The increase of NC concentration on PEOXA adsorbates, and thus the higher tendency to form loops,48−50 caused a progressive reduction in the amount of water within the assemblies, with swelling ratios (SR) decreasing from 1.6 of L-PEOXA to 1.1 and 0.9 for PEOXA-4 and PEOXA-7, respectively (i.e., increasing the loops-to-tails ratio). In order to further prove the formation of loops, angleresolved XPS (ARXPS) was applied to monitor the vertical distribution of NC groups across the grafted films. Namely, the effective formation of polymer loops involves the grafting of nearly all the NC functions to TiO2, i.e., their accumulation in the proximity of the substrate with respect to the film’s interface.51 Hence, the chemical composition of PEOXA-4 and PEOXA-7 brushes was analyzed at different depths (corresponding to different emission angles, θ) (Figure 2a−c). We especially focused on the high-resolution N 1s spectra, where the contribution of the NO2 groups from NC is highlighted through a signal centered at 406 eV (Figure 2d and Figure S20). The parameter χ correlates the area of the NO2 peak (INO2 1s) with that of the N signal from the EOXA repeating units, centered at 400 eV (IEOXA 1s), thus providing the relative concentration of NO2 groups throughout the films:

χ = 100 ×

INO2 1s INO2 1s + IEOXA 1s

(1)

The values of χ for PEOXA-4 and PEOXA-7 calculated from spectra recorded at different emission angles were compared to those obtained by analyzing L-PEOXA, a linear brush presenting NC groups uniquely bound to TiO2 and an interfacial composition comprising just EOXA units (Figure 2b). At low angles of emission (25°−50°), i.e., by sampling the composition of the brushes in the proximity of the TiO2 substrate, the χ values for PEOXA-4 and PEOXA-7 were significantly lower than that calculated for L-PEOXA, nearly half, suggesting that the relative concentration of EOXA units close to the anchoring groups of loop brushes is higher than that recorded on linear grafts. This result validates the proposed design of PEOXA loops, with each NC anchor supporting two PEOXA segments (Figure 2c), and confirms how copolymers featuring strong reactive functions toward the substrate are generally capable of forming loops at the surface.52−57 A further decrease of the emission angle from ∼40° to ∼80° (i.e., by gradually analyzing the interfacial composition of the films) was accompanied by a progressive decrease of χ, which in the interfacial region showed very similar values among loop and linear brushes, finally reaching ∼2% at the highest emission angle tested. Hence, the interfacial composition of loop brushes was comparable to that displayed by L-PEOXA, implying the virtual absence of unbound NC units near the loop brush interface. The formation of loops was also accompanied by the development of a textured nanomorphology, as witnessed by AFM (Figure S19), with L-PEOXA showing a smooth polymer E


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Figure 3. (a, b, d, e) FS profiles recorded on topologically different PEOXA brushes by AFM, in MOPS buffer (pH 6) using a TiO2-coated colloid with a diameter of 20 μm, and functionalized with identical PEOXA brushes (normal spring constant of the cantilever = 0.1 N m−1, indentation speed = 2 μm s−1). In (c) a schematic representation of cyclic PEOXA brushes compressed between a functionalized AFM colloidal probe and a TiO2 surface is provided. (f, g) FHS adsorption on the topologically different PEOXA brushes as measured by VASE. Bare TiO2 substrates were used as control (∗ = P < 0.05, # = P < 0.01, ## = P < 0.001). (h) Compression of cyclic PEOXA by approaching proteins causes the lateral extension of the grafted polymer rings through the neighboring assembly, which is highly entropically unfavorable.

interface and a gradual roughening occurring with the increase of loops concentration in PEOXA-4 and PEOXA-7.58−60 Having established how the application of different polymer topologies determined the structure of PEOXA brushes, we systematically investigated how the distinctive characteristics of each grafted-polymer architecture influenced technologically relevant physicochemical properties. The steric stabilization imparted by the different PEOXA brushes was tested by AFM, measuring the repulsion against compression between PEOXA grafts immobilized on flat TiO2 surfaces and topologically identical brushes grafted to TiO2coated microcolloids used as AFM probes (Figure 3). In order to highlight the effect of polymer topology on steric stabilization, the approaching profiles of force vs separation curves (FS) recorded by compressing cyclic and loop brushes were compared to those measured for linear PEOXA brushes presenting similar molar mass. As shown in Figures 3a and 3b, both c-PEOXA and C-PEOXA brushes displayed an augmented repulsion against compression when compared to l-PEOXA and L-PEOXA, respectively (stronger repulsive interactions were recorded for a given separation). This was due to the

higher density of cyclic brushes with respect to their linear counterparts coupled to their stiffer character and more stretched morphology (Figure 3c and Figure S21). Similarly to cyclic grafts, also loop brushes provided enhanced steric stabilization with respect to L-PEOXA (Figure 3d,e), validating the predictions previously reported by Cao et al., who used a polymer density fluctuation theory to demonstrate how loop brushes are expected to form films more compact than those obtained by assembling linear grafts with similar composition.61 Enhancement of steric stabilization by loop- and cyclic-brush topologies is expected to translate into an improved resistance toward surface contamination by complex protein mixtures, this characteristic being critical for the application of PEOXA films in biomaterials formulations.62,63 In particular, linear, cyclic, and loop PEOXA brushes were subjected to undiluted, full human serum (FHS), and the amount of physisorbed proteins was subsequently evaluated by VASE. As displayed in Figure 3f,g, all the PEOXA brushes considerably reduced protein adsorption compared to the bare TiO2 surface, used as control. This was due to the synergistic action of conformational entropic barrier F


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Figure 4. LFM was used to investigate the nanotribological properties of topologically different PEOXA brushes. (a) FfL profiles recorded by shearing linear, loop, and cyclic PEOXA brushes against topologically identical films (radius of the PEOXA brush-functionalized colloids = 10 μm, normal spring constant = 0.1 N m−1, torsional spring constant = 3 × 10−9 N m stroke length = 2 μm, stroke speed = 2 μm s−1). (b) Cyclic PEOXA brushes do not interdigitate during sliding due to the absence of chain ends. In contrast, linear PEOXA brushes interdigitate, causing dissipative forces and a consequent increase in the recorded friction. (c) Interdigitation between two cyclic brushes placed in close proximity is highly disfavored due to the topological necessity by the cyclic grafts to double-up within the opposing brush, forming an hairpin configuration.

friction have never been systematically investigated through experiments, although theoretical predictions were derived by several groups during the past decade,66−68 and some experimental proof of the superior lubricating properties by cyclic and loop brushes was respectively reported by us22 and by the group of Israelachvili.55,57 Although these initial reports highlighted how a variation of grafted-chain topology from linear to cyclic or “semicyclic” substantially reduced friction, a direct correlation between the structural characteristics of topologically different brushes and their nanotribological properties is missing. Relevantly, such a thorough analysis would open the way for the application of topological brushes in different coatings formulations. Friction force vs applied load profiles (FfL) were acquired by shearing PEOXA brush-coated AFM colloidal probes against topologically identical films. FfL profiles of linear PEOXA brushes were in all cases characterized by an initial low friction, which steadily increased with the applied load due to interdigitation between the two sliding brushes (Figure 4a,c).69 The denser l-PEOXA brushes, generated from adsorbates of ∼7 kDa, produced lower friction compared to L-PEOXA (11 kDa). This was presumably due to the increment of osmotic pressure exerted by more densely grafted brushes, which hinders interpenetration between two opposing layers when the load between them is progressively increased. In contrast to linear brushes, cyclic PEOXA tribopairs showed extremely low friction regardless of their grafting density, and generated very similar coefficients of friction (μ) of 0.007 and 0.008 for c-PEOXA and C-PEOXA, respectively (Figure 4a). The superlubricious character of cyclic brushes was due to the absence of interdigitation, since this would require the cyclic grafts to double-up through the opposing brush, forming a “hairpin” configuration (Figure 4d), which is entropically unfavorable.66 Moreover, cyclic brushes showed lower viscoelasticity compared to their linear counterparts, as highlighted by comparing the FfL profiles recorded between a bare AFM colloid and the two different brush types (Figure S22). The interplay between hindered interdigitation and increased rigidity thus caused a decrease of mechanical energy dissipation when cyclic PEOXA brushes were subjected to shear stress, determining a substantial reduction of the generated friction.

and enthalpic shield provided by densely grafted and highly hydrated PEOXA brushes, which hinder surface contamination by biomolecules.64 Noteworthy, cyclic PEOXA brushes showed improved biopassivity compared to their linear counterparts. In particular, c-PEOXA provided the best antifouling properties among the different brush topologies tested, reaching ∼0 nm of adsorbed proteins (full biopassivity). The highly stretched configuration of cyclic brushes granted an augmented entropic barrier against protein physisorption, in accordance with the increment in steric stabilization that was monitored on the same brush layers by AFM. Although cyclic brushes are less hydrated than linear ones, their interaction with biomolecules results highly entropically disfavored, as the consequent compression of cyclic grafts would cause their lateral extension through the neighboring, vertically stretched assembly of chains (Figure 3h). It is also important to highlight that the biopassivity of cyclic PEOXA brushes did not show a direct dependence on the surface density of monomer units (nEOXA in Table 1) in the dense brush regime, in contrast to what was previously observed for both linear PEG and linear poly(2methyl-2-oxazoline) (PMOXA) brushes.29 Namely, while nEOXA decreased from C-PEOXA to c-PEOXA brushes, the amount of adsorbed proteins was simultaneously reduced. This result further suggests that the conformation of the grafted cycles plays the major role in determining brush biopassivity rather than surface coverage. A similar, although less pronounced phenomenon determined the biopassive properties of loop brushes, when these are compared to those displayed by linear analogues. As reported in Figure 3b, both PEOXA-4 and PEOXA-7 reduced protein physisorption with respect to L-PEOXA. Thus, the presence of loops played a similar role to that induced by cyclic grafts, with their compression by adhering proteins being entropically disfavored. This finding agrees well with the previous results by Li et al., who highlighted an improvement of biopassivity when linear PEG brushes were replaced by PEG telechelics forming loops at the surface.56 The comprehensive description of the interfacial physicochemical properties of PEOXA brushes featuring diverse graft topologies was finally complemented by investigating their nanotribological properties using lateral force microscopy (LFM).65 Topology effects by loop and cyclic brushes on G


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The nanotribological properties of loop PEOXA brushes were found dependent on the relative concentration of loops and tails, which determined the contributions of interdigitation and viscoelasticity on the measured friction. As reported in Figure 3b, increasing the amount of loops caused a progressive decrease of friction compared to linear brushes featuring similar molar mass, PEOXA-7 brushes reaching a relatively low μ of 0.02. Despite this improvement in lubrication, the recorded friction was not as low as that recorded for cyclic grafts. This was presumably due to the loop-brush architecture, i.e., the intrinsic polydispersity of loops’ extension58,59 that caused the formation of heterogeneous assemblies (as highlighted in the AFM micrographs reported in Figure S19) and the presence of linear tails that could interdigitate with their countersurface. Yet, the presence of polymer loops substantially reduced friction with respect to linear grafts due to the cooperative effects of reduced interdigitation and lower viscoelasticity.54

Edmondo M. Benetti: 0000-0002-5657-5714 Notes

The authors declare no competing financial interest.

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ACKNOWLEDGMENTS We thank Prof. Krzysztof Matyjaszewski (Carnegie Mellon University) for the fruitful discussions. We thank Prof. Antonella Rossi and Mr. Giovanni Cossu (ETH Zürich) for their help with the XPS analysis,and Prof. Nicholas D. Spencer (ETH Zürich) for the intellectual contribution to this work. We acknowledge the Swiss National Science Foundation (SNSF “Ambizione” PZ00P2-790148156) and the ETH research council.

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CONCLUSIONS Topology effects by cyclic and loop-forming adsorbates fundamentally alter highly relevant physicochemical properties of the subsequently formed brushes, surpassing the attractive characteristics provided by linear grafts with the same composition. The unique structure of cyclic PEOXA brushes provides an increased steric stabilization to the functionalized surface, while the absence of chain ends and the less dissipative character of the grafts generate superlubricious surfaces. The properties of loop brushes are in between those displayed by cyclic grafts and the ones characterizing their linear analogues. In particular, when loop PEOXA brushes are assembled by grafting random PEOXA copolymers featuring a different content of surface-reactive groups, their structural, swelling, nanomechanical, and nanotribological characteristics are determined by the relative concentration of loops and linear tails. The presence of loops enhances steric stabilization, in a similar way to what was observed for cyclic brushes, reducing the amount of adsorbed proteins after incubation within undiluted serum. Although loop brushes did not reach the low friction attained by applying cyclic grafts, they are substantially more lubricious than linear brushes due to partial hindering of interdigitation and reduced viscoelasticity. Thus, we can conclude that loop brushes could approximate the properties of cyclic grafts in the limit of monodispersed loops and in the absence of linear tails. The present study highlights how, without applying tedious fabrications or complex chemistries, the interfacial, physicochemical properties of polymer interfaces could be precisely modulated by varying the topology of the grafted chains.

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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.7b01720. Chemical characterization (1H NMR, FT-IR) of all the PEOXA adsorbates, further AFM data, ARXPS spectra, and AFM calibration protocols (PDF)

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REFERENCES

AUTHOR INFORMATION

Corresponding Author

*E-mail [email protected]; phone +41446326050 (E.M.B.). H


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