Poly(ethylene glycol) brushes grafted to silicon substrates by click chemistry: influence of PEG chain length, concentration in the grafting solution and reaction time

Abstract

The grafting of poly(ethylene glycol) (PEG) brushes to silicon substrates by the copper(I)-catalyzed Huisgen 1,3-dipolar cycloaddition, also coined as click chemistry, was studied in detail. First, the grafting kinetics of an alkyne-functionalized dimethylchlorosilane SAM from a toluene solution or in the vapor phase was monitored by water contact angle measurements. α-Methoxy-ω-azido-PEGs with M_w of 5, 20, and 50 kDa were then grafted to the alkyne functionalized SAMs via click chemistry in THF using Cu(PPh₃)₃Br/DIPEA as the catalytic system. The influence of polymer concentration in the grafting solution (Φ = 0.01–50 wt%) and reaction time (t = 0–72 h) on the thickness, morphology and wetting properties of the PEG brushes was investigated by ellipsometry, scanning probe microscopy and water contact angle measurements. PEG brushes up to 6 nm thick with homogeneous surface coverage and morphology as well as surface roughness on a nanometric scale were thus obtained using mild and robust grafting conditions.

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1. Introduction

The grafting of polymer brushes¹ represents an efficient method to control and precisely tune the surface properties of a wide range of 2D solid substrates and finds interest in a broad range of applications such as low friction surfaces with lubrication properties,² biomimetic and non-biofouling coatings,³ chemical or electrochemical gates,⁴ biotechnologies,⁵ stimuli-responsive surfaces,⁶ as well as patternable layers for microelectronics and lab-on-chip nanotechnologies.⁷ A broad array of neutral, charged, responsive or functionalized polymer brushes has thus been developed using either the “grafting-to” or the “grafting-from” approaches.⁸

The chain-end tethering of poly(ethylene glycol) (PEG) brushes is of significant importance as the peculiar physico-chemical properties of PEG afford surfaces with interesting non-biofouling properties such as resistance to protein adsorption or bacterial adhesion.⁹ A large amount of studies varying by the grafting method and the nature of the solid substrate has thus been reported. Several examples of living anionic “grafting-from” polymerization of ethylene oxide have been reported.¹⁰ However, the harsh reaction conditions involved (in terms of temperature, pressure, complex experimental setup as well as the demanding purification of the solvents and monomers) restrain the large development of such approach and the versatile access of these materials to a broad range of researchers. Conversely, the “grafting-to” method is a more versatile process that takes advantage of the well-known characteristics of the brush linear precursors. Several tethering reactions performed in solution or in bulk have thus been reported for the grafting of PEG brushes to 2D solid substrates. Most encountered methods include the condensation of silane-functionalized PEGs to silanol groups from the surface of silicon substrates,¹¹ the reaction between amine- or acid-terminated PEGs to poly(glycidylmethacrylate) pseudo-brushes,¹² the amidation reaction between amine-terminated PEGs and N-hydroxysuccinimide SAMs as well as the complementary approach based on amine-functionalized substrates,¹³ the reaction between tresyl-terminated PEGs and amine-functionalized SAMs,¹⁴ the tethering of thiol-terminated PEGs to gold substrates,¹⁵ the reductive amination between aldehyde-terminated PEGs and amine-functionalized SAMs,¹⁶ and the reaction between vinyl-terminated PEGs and silanols from the surface of silicon substrates.¹⁷ Most of these tethering pathways are subjected to side reactions, lack of chemical orthogonality and their application scope are therefore limited, especially when considering the grafting of brushes carrying chemical functionalities.

More recently, the robust, efficient and orthogonal nature of the click chemistry philosophy¹⁸ has confirmed its astonishing potential for the manipulation of the chemical composition of surfaces.¹⁹ Particularly, the copper(I)-catalyzed azide-alkyne cycloaddition (CuAAC), the most studied and reliable example of click chemistry so far, offers several advantages such as tolerance to oxygen, moisture and surrounding functionalities, high yields under mild reaction conditions, selectivity, efficiency and processing in protic or aprotic media. Several examples have paved the way for the CuAAC grafting of hydrophilic, hydrophobic or charged polymer brushes to 2D solid substrates.²⁰ We report herein a detailed study on the influence of experimental parameters such as reaction time, polymer chain length and concentration in the grafting solution, on the thickness (h), grafting density (σ), morphology and wetting properties of PEG brushes obtained by this general click chemistry “grafting-to” approach.

2. Experimental section

2.1 Materials and methods

α-Methoxy-ω-azido-PEGs were prepared as described previously from 5, 20, and 50 kDa monomethoxy-PEGs purchased from Polymer Source (USA).²¹Ethynyldimethylchlorosilane (EDMS),²² and copper bromide tris-(triphenylphosphine) (Cu(PPh₃)₃Br),²³ were synthesized using previously described procedures. All other reagents and solvents were purchased with the highest purity available from Aldrich and used as received. Silicon wafers (orientation 100, 525 μm thick with a SiO₂ layer of ca. 2 nm) were purchased from Siltronix (France). Before deposition of the SAM, silicon wafers were immersed for 15 min in a piranha solution (7 : 3 H₂SO₄/H₂O₂) maintained at 80 °C (Caution! Piranha solution should be handled with extreme care, as it reacts violently with most organic materials), thoroughly rinsed with ultrapure water and dried with a stream of nitrogen. Film thicknesses were measured with a spectroscopic ellipsometer UVISEL (Horiba Jobin Yvon, France) at a fixed angle of incidence of 70°, on at least five different places on the sample. A three layer model (Si + SiO₂ + PEG brushes) was used to simulate experimental data. Thickness values were obtained from the best Cauchy fits using the refractive index of bulk PEG (n = 1.45). For the calculation of the grafting densities, the bulk density of PEG was used (ρ = 1.13 g/cm³). The wetting properties of the polymer brushes were analyzed by the dynamic measurement of the advancing (θ_a) and receding (θ_d) water contact angles (CA) using an Easydrop contact angle system (KRUSS GmbH, Germany). The reported experimental values are averages of at least five measurements carried out at room temperature on different parts of each sample. Scanning probe microscopy (SPM) images were obtained under ambient conditions in both height and phase contrast modes using a Veeco Nanoscope IIIa in the tapping mode.

2.2 Deposition of EDMS SAMs in the vapor phase

A few drops of EDMS (bp ∼ 37–42 °C) were deposited in a vial next to a freshly cleaned silicon substrate (or several of them) placed in a flat-bottom reactor under argon. The reactor was then maintained under static vacuum (ca. 0.8 atm) in order to evaporate EDMS and to promote its condensation with the silanol groups from the surface of the silicon wafer yielding a SAM of EDMS. The reaction was performed at the solid/vapor interface without any contact between EDMS and the silicon wafer, for reaction times up to 25 h at room temperature. The chemically modified silicon wafers were then rinsed with chloroform and dried under a nitrogen stream before being involved in the characterization or grafting procedures.

2.3 Deposition of EDMS SAMs in toluene solution

A freshly cleaned silicon wafer was introduced into a schlenk reactor and was dried further under vacuum during ca. 14 h at 60 °C. Anhydrous toluene (10 mL) was then added under an argon atmosphere in order to cover the entire silicon substrate. Triethylamine (73 mg, 0.72 mmol) and EDMS (80 mg, 0.84 mmol) were then sequentially injected under argon before heating the reaction media at 70 °C for reaction times up to 25 h. A white ammonium salt rapidly precipitates as a result from the trapping by the tertiary amine of hydrochloric acid released by the condensation reaction. The chemically modified silicon wafers were then sequentially rinsed with methanol and chloroform, dried under a nitrogen stream before being involved in the characterization or grafting procedures.

2.4 Grafting of PEG brushes to EDMS SAMs by CuAAC

Grafting of PEG brushes to alkyne functionalized silicon wafers was performed in sealed reactors specially designed for the chemical modification of thin planar substrates. This setup allows performing the grafting reaction using low volumes (ca. 100–500 μL depending on the size of the substrate) thus preventing the use of high quantities of polymers when targeting the grafting of brushes using concentrated polymer solutions. In a typical procedure alkyne passivated silicon wafers were covered by a glass slide supported by two 150 μm thick kapton wedges. Independently of PEG molar mass, a solution of α-methoxy-ω-azido-PEG (e.g. for a polymer concentration in the grafting solution of 10 wt%: 50 mg, 0.01 mmol of a 5 kDa PEG), Cu(PPh₃)₃Br (0.2 mg, 0.002 mmol) and N,N-diisopropylethylamine (0.8 mg, 0.006 mmol) in 450 mg of tetrahydrofuran, THF) was introduced by capillarity between the silicon and glass substrates, and the reactor was sealed under ambient atmosphere. The bottom of the reactor contained a circular channel filled with THF in order to saturate the atmosphere thus preventing solvent evaporation during the grafting reaction. After a reaction time ranging from 1 to 72 h at 60 °C, the grafted silicon substrate was sequentially sonicated and rinsed by a ca. 6 h soxhlet extraction using chloroform in order to remove the catalytic system and noncovalently adsorbed polymer chains. The substrate was then dried under a stream of nitrogen before being involved in the characterization procedures.

3. Results and discussion

3.1 Preparation of alkyne functionalized silicon wafers

The general grafting of polymer brushes to silicon substrates by click chemistry studied herein (Scheme 1) is based on the functionalization of silicon substrates by an alkyne-functionalized SAM of EDMS that allows for an efficient “passivation” of the surface toward brushes and linear precursors adsorption.^20b This latter aspect is particularly crucial in the case of charged or polar polymers such as PEG which has a high affinity toward silicon substrates. Herein, the deposition of EDMS SAMs was carried out either in toluene solution or in the vapor phase. To the best of our knowledge, EDMS is the only intermediate that combines alkyne and chlorosilyl reactive chain ends, and is therefore a crucial milestone of this strategy.²² The evolution of the hydrophobic character of the chemically modified substrates with respect to both reaction time and deposition procedure (in toluene solution or in the vapor phase) was investigated by monitoring the dynamic advancing water CA (θ_a) and wetting hysterisis (Δθ = θ_a − θ_d).

Scheme 1 CuAAC grafting of PEG brushes to alkyne functionalized silicon substrates.

Fig. 1 clearly shows that both deposition methods yield an identical evolution of the wetting behavior as constant values of ca. 85° for θ_a and 0.5° for Δθ are obtained after ca. 3 h of reaction. The steep increase of θ_a during the early stage of the reaction is due to the progressive coverage of the silicon substrate by a hydrophobic SAM of EDMS and the concomitant disappearance of hydrophilic silanol functionalities resulting from the covalent anchorage of EDMS. The significant decrease of Δθ corresponds to the increase in surface coverage of the silicon substrates and highlights the improvement of the chemical and physical homogeneities of the uppermost surface. Even if no significant differences are observed for the wetting properties of EDMS SAMs resulting from both deposition methods, the vapor phase deposition is preferred as it is easier to process especially for large dimension substrates and number of samples that can be chemically modified in a single batch. Also, the vapor phase process limits the adsorption of contaminants often encountered in solution processes. Finally, the solution process involves the formation of triethyl ammonium salt crystals that can adsorb to the high energy silicon substrates throughout the formation of the SAM. However, ultrasonication of the samples after chemical modification generally affords desorption of such crystals and results in homogeneous SAMs, as confirmed by SEM and SPM experiments.^20b Thus, both deposition methods yield SAMs with a comparable thickness of ca. 0.4 nm that corresponds to an average grafting density (σ) of ca. 2 alkynes/nm².^20b The grafting density of the resulting SAMs are therefore above the upper range limit of grafting density expectable for polymer brushes obtained by a “grafting-to” approach. Several alkyne functionalized silicon substrates were then prepared by vapor phase deposition of EDMS and involved in the subsequent click chemistry grafting of α-methoxy-ω-azido-PEG brushes with varying chain length.

Fig. 1 Advancing water contact angles (□, ●) and wetting hysteresis (■, ○) versus reaction time for EDMS SAMs obtained in toluene solution (□, ○) or in vapor phase (●, ■).

3.2 Grafting of PEG brushes by click chemistry

3.2.1 Influence of PEG chain length and weight fraction in the grafting solution. Initially, the CuAAC grafting of 5, 20, and 50 kDa α-methoxy-ω-azido-PEGs was performed at 60 °C for 24 h in THF, using Cu(PPh₃)₃Br and DIPEA as the catalytic system (Scheme 1). The polymer weight fraction in the grafting solution (Φ) was varied from 0.01 to 50 wt% in order to investigate the effect of polymer concentration on the thickness, grafting density, wetting behavior and morphology of the resulting PEG brushes. Investigating high polymer concentrations without involving too much synthetic intermediates was possible using a sealed reactor specially designed for the chemical modification of planar substrates using the capillary flow of small volumes (ca. 100–500 μL depending on the size of the substrate) of reactive polymer solutions. Grafting polymer chains from concentrated solutions is crucial to counterbalance the osmotic pressure encountered at the solid substrate and excluded volume effects that preclude obtaining high grafting density polymer brushes in acceptable reaction times, as generally observed for “grafting-to” procedures performed under dilute conditions. After removal of the catalytic system and noncovalently adsorbed polymer chains, the resulting PEG brushes were dried and characterized by ellipsometry, SPM and water CA measurements.

Fig. 2 shows the evolution of the thickness and grafting density of dry PEG brushes versuspolymer concentration for grafting reactions performed for 24 h at 60 °C in THF. Grafting densities were determined using relation 1:

σ = (hρN_a)/M_w (1)

with ρ the bulk density of PEG, N_a Avogadro's number and M_w the molar mass of PEG. For a constant reaction time all three PEGs show the same general trend of a rapid rise in thickness and grafting density at low grafting concentration, with a leveling off at higher concentration so that maximum values are attained. For each molar mass, the transition between these two regimes are related to the critical concentration to achieve chains overlapping. As observed earlier,^12b,14a the thickness and grafting density decrease with the increase in PEG chain length. This is typical of a system where the chains are random coils such that excluded volume effects affect the grafting of the chains to the surface. Higher M_w PEG chains have a greater excluded volume and therefore cannot pack as tightly on the surface as lower M_w PEGs. Also, this can be explained by the slower diffusion and reorientation of longer chains especially when entangled at high polymer concentrations. The parameters of the thicker PEG grafted layers obtained (for longer reaction time) are reported in Table 1.

Fig. 2 Thickness (a) and grafting density (b) versuspolymer weight fraction in the grafting solution for 5 (○), 20 (▲), and 50 kDa (□) PEG brushes grafted by CuAAC in THF for 24 h at 60 °C.

Table 1 Parameters of PEG layers grafted by CuAAC

M w (kDa)	h (nm)	Γ ^a (mg/m^2)	σ ^b (chains/nm^2)	D ^c (nm)	R g ^d (nm)	h/2Rg
a Γ = hρ. b σ = (hρNa)/Mw. c D = (4πσ)^1/2. d R g = a(N/6)^1/2 with a the segment length (a = 0.29 for PEG) and N the polymerization degree.^12b
5	6.3	7.1	0.86	1.2	1.3	2.5
20	5.2	5.9	0.18	2.7	2.5	1.0
50	4.5	5.1	0.06	4.6	4.0	0.6

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The grafting density and surface coverage (Γ) both decrease with the increase in PEG M_w. The denser PEG brushes (σ = 0.86 chains/nm²) were achieved for the 5 kDa PEG which is below the highest values reported by Luzinov and co-workers (σ = 1.5 chains/nm²) for the same M_w and a grafting to a pseudo-brush underlayer performed in bulk.^12a For all PEG brushes (Table 1) the distance between grafting sites (D) is comparable to the gyration radius (R_g) for PEG indicating that all layers are relatively densely grafted. Moreover the h/2R_g ratio, which can be considered as a measure of chain stretching within the grafting layer, also decreases with the increase in PEG M_w as a consequence of excluded volume effects. Neutron reflectivity experiments are under process to confirm the grafting densities of the grafted PEG chains, as well as their conformation and swelling behavior in water.

Fig. 3 shows the evolution of the water CA as a function of brush thickness for the 5, 20, and 50 kDa PEGs grafted to silicon substrates by click chemistry. CA measurements allow a versatile estimation of the surface screening by PEG anchored layers. The hydrophobic nature of the initial alkyne-functionalized SAMs is characterized by a θ_a value of ca. 85°. θ_a decreases down to 52° with the increase in PEG thickness and surface coverage of the substrate. This value is higher than those reported in the literature for highly dense PEG brushes which vary from 21 to 35°.^12a,b For a comparable thickness of the PEG layer, the lowest θ_a values are obtained for the shorter PEG chains. Indeed, the swelling and stretching of the longer PEG chains (with smaller σ for a comparable thickness) uncover the underlying hydrophobic SAMs and therefore increases θ_a. These observations correlate with the decrease of grafting density of PEG grafted chains with the increase in M_w.

Fig. 3 Advancing water contact angles versus thickness for 5 (○), 20 (▲), and 50 kDa (□) PEG brushes grafted by CuAAC in THF for 24 h at 60 °C.

The surface morphology and smoothness of the grafted PEG layers were observed with tapping mode SPM under ambient conditions. SPM topographical images of 5, 20, and 50 kDa PEG brushes grafted at different polymer concentrations are displayed in Fig. 4. Corroborating ellipsometry and CA measurements, the surface coverage of PEG brushes increases with the increase in polymer concentration in the grafting solution (for a constant reaction time of 24 h). Indeed, during the grafting process a gradual packing of nanometre sized globular domains is observed. At low polymer concentrations, the surfaces are covered only by a few globular structures, indicating a physical heterogeneity in the dry state. However, the SPM contrast phase images (not shown, example in Fig. 6) indicate that there is no contrast between the top of the features and the background suggesting that the entire surface is covered by PEG brushes varying by their morphology in the dry state. The root mean square roughness on 4 μm² of the dry PEG brushes constantly decreases with time to reach ca. 1 nm independently of PEG M_w. Besides these globular objects, no crystalline structures could be distinctly observed for the 5 and 20 kDa PEG brushes. However, for the 50 kDa PEG brushes larger size aggregates and some apparent features could be distinguished and most likely attributed to a crystaline organization of the longer grafted PEG chains. The inhomogeneity of the observed features most probably stems from the patchy character of the initial SAM and the partial adsorption of PEG chains in localized domains.

Fig. 4 SPM height images of 5 (top), 20 (middle), and 50 kDa (bottom) PEG brushes grafted by CuAAC for 24 h at 60 °C using different polymer weight fractions in THF. All images (2 × 2 μm, inserted values: z-scale).

Fig. 6 Evolution of SPM phase (top), height (middle), and cross section (bottom) images with reaction time for the grafting of 20 kDa PEG brushes by CuAAC at 60 °C using a polymer weight fraction of 25 wt% in THF. All images (2 × 2 μm, inserted values: z-scale).

3.2.2 Influence of reaction time. A kinetic study of the click chemistry grafting of 20 kDa PEG brushes was then conducted at 60 °C using a constant polymer weight fraction of 25 wt% and reaction times ranging from 12 to 72 h. Fig. 5 shows the evolution of PEG layer thickness and water CA as a function of grafting reaction time. It is of significant importance to note that the grafting further proceeds after 24 h of reaction but the parameters of the obtained brushes after 72 h of grafting are only slightly improved with the resulting tethered layers having the following characteristics: h = 5.6 nm, Γ = 6.3 mg/m², σ = 0.19 chain/nm², D = 2.6 nm and θ_a = 45°.

Fig. 5 Thickness (□) and advancing water contact angles (●) versus reaction time for 20 kDa PEG brushes grafted by CuAAC for 24 h at 60 °C using a polymer weight fraction of 25 wt% in THF.

The evolution of the PEG layers thickness shows a typical transition between an initial diffusion-controlled and a subsequent interface reaction-controlled processes corresponding to the creation of an initial grafted layer that further chains need to pass through. The slower dynamics of the entangled PEG chains at 25 wt% increases the time for the reactive chain end reorientation to find its alkyne counter-part. In parallel, θ_a decreases significantly with the increase in reaction time or film thickness. This decrease in θ_a from 84° (for an alkyne-functionalized substrate) to 45° (for the thickest 20 kDa PEG brushes) indicates a drastic transition of the outermost surface from hydrophobic to hydrophilic nature. Moreover, whereas the grafted PEG layer thickness is only slightly improved when increasing the grafting reaction time from 24 h (4.9 nm) to 72 h (5.6 nm), θ_a significantly decreases from 58° to 45°, respectively. These observations imply a gradual packing of the grafted PEG domains in combination with a decrease in surface roughness.

Comparable observations could be drawn from the evolution of SPM height, phase and cross-section images with grafting reaction time (Fig. 6). Following an evolution comparable as the one observed for the grafting performed at a constant reaction time and an increasing PEG concentration (Fig. 5), globular objects with decreasing size and increasing homogeneity/packing are observed. In addition, the root mean square roughness on 4 μm² of the dry PEG brushes constantly decreases from 5.4 nm to 4.4, 1.9 and 1.3 nm for 12, 24, 48 and 72 h, respectively. This evolution is consistent with the evolution of other PEG brushes parameters such as h, θ_a, as well as the flattening and the decreased roughness of the globular structures observed in Fig. 6.

4. Conclusions

The formation of an alkyne-functionalized SAM in the solution or vapor phase and the influence of PEG chain length, concentration in the grating solution and reaction time on the thickness, grafting density and morphology of PEG brushes grafted by CuAAC have been investigated in details. Relatively dense and homogeneous PEG brushes with grafting densities up to ca. 0.9 chains/nm² for 5 kDa PEG were thus obtained. Even this general grafting strategy could be adapted to a broad range of (multifunctional) polymer brushes, it yields polymer brushes with grafting densities below those obtained by a grafting performed in bulk. Indeed, this process is still limited by the diffusion of the reactive chains and catalytic system through the layer of previously grafted chains. Therefore even at relatively high polymer concentration, reaction times up to 72 h are necessary to achieve a saturation of the obtained grafted PEG layer.

Acknowledgements

Authors gratefully acknowledge the financial supports from the European Community's “Marie-Curie Actions” under contract MRTN-CT-2004-504052 [POLYFILM], and from the Agence Nationale de la Recherche (ANR) under contract ANR-07-JCJC-0020-01 [MULTICLICK].

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Footnote

† This paper is part of a Polymer Chemistry issue highlighting the work of emerging investigators in the polymer chemistry field. Guest Editors: Rachel O'Reilly and Andrew Dove.

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