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Research ArticleReceived: 13 August 2007 Revised: 6 November 2007 Accepted: 7 November 2007 Published online in Wiley Interscience: 11 January 2008

(www.interscience.com) DOI 10.1002/sia.2687

Interfacial aspects of polymer brushesprepared on conductive substrates by aryldiazonium salt surface-initiated ATRPMinh Ngoc Nguyen,a Tarik Matrab,a Chantal Badre,b Mireille Turmineb andMohamed M. Chehimia∗

This article describes the use of aryl diazonium salts to attach halogenated functional groups that initiate atom transfer radicalpolymerization (ATRP) at the surface of conductive substrates. The interest of this procedure lies in the fact that aryl diazoniumsalts permit the grafting of high-density initiators within a few minutes of electrochemical surface treatment, and subsequentlypromote the growth of very compact polymer chains.

Several brominated aryl groups were tested for surface-initiating ATRP of styrene and methacrylates. The chemical structureof the polymer brushes was characterized by high-resolution XPS. Particularly, it is shown that grafting of poly(tert-butylmethacrylate), PtBMA, from the surface of glassy carbon plates followed by hydrolysis resulted in carboxylated, tethered chainsthat were characterized in terms of hydrophilicity and propensity to uptake silver cations. Copyright c© 2008 John Wiley &Sons, Ltd.

Keywords: diazonium salts; ATRP; polymer brushes; XPS

Introduction

Atom transfer radical polymerization (ATRP) is the most investi-gated method of controlled radical polymerization (CRP) owingto its tolerance to a variety of functional monomers, operation atroom temperature in either aqueous or organic solvents.[1] Sincethe independent pioneering works of Wang and Matyjaszewski in1995[2] on the one hand, and Sawamoto et al.[3] on the other, ATRPhas experienced a quantum jump in the number of publicationsover the past 10 years.

This CRP approach can be surface-initiated on either planaror particle surfaces[1,4] for several purposes such as thermo-responsive nanoparticles[5]; materials surfaces with controlledcell adhesion properties[6] and separation materials based onnanofilms of molecularly imprinted polymers,[7] to name buta few.

Provided that the initiator is grafted to the substrate ofinterest, ATRP can be surface-confined by a variety of ways.[1,4,8,9]

Particularly, halogenated silanes and thiols are massively used toattach initiators to surfaces.[8]

We have recently shown that controlled ATRP of vinylicmonomers can be initiated on metals and carbon by electro-grafted brominated aryl species based on diazonium salts.[10,11]

The use of aryl diazonium salts is indeed an elegant way ofmodifying conducting substrates, particularly carbon.[12] Theinterest of using aryl diazonium salts to surface-initiate ATRP (SI-ATRP) is that this class of organic salts leads, after electrochemicalreduction, to the covalent attachment of functional aryl groups,[12]

thereby, ensuring a covalent link between the polymer chains andthe support. Taking into account the chemistry of diazonium saltsand the principles of surface-confined ATRP, we have designedBF4

−, +N2-C6H4-CH(CH3)-Br (D1), a diazonium salt which is ananalog for 1-phenylethyl bromide, a bulk solution ATRP initiator.[10]

D1 can be readily electrochemically reduced on glassy carbon (GC)and iron to yield the attached -C6H4-CH(CH3)-Br (1) aryl group forsurface-initiated ATRP. We have also designed another salt, BF4

−,+N2-C6H4-CH2-CH2-Br (D2), and found it suitable for modifyingthe surface of multi-walled carbon nanotubes.[13]

The aim of this article is to present new possibilities offered bythe ‘Diazo/ATRP’ approach in controlling the surface propertiesof iron and carbon substrates by aryl diazonium salt-initiatedATRP. To D1 and D2 we have added another salt, BF4

−,+N2-C6H4-CH2-CH2-OH (D3), which can be electrochemicallyreduced to graft -C6H4-CH2-CH2-OH groups. The latter were furtheresterified to obtain grafted -C6H4-CH2-CH2-O-C(O)-C(CH3)2Brgroups. The monomers explored in this work are styrene,methyl methacrylate and tert-butyl methacrylate. A GC/poly(tert-butyl methacrylate) hybrid was further hydrolyzed in order toobtain poly(methacrylic acid) brushes that were characterizedin terms of hydrophilic/hydrophobic character and uptake ofAg+ cations from aqueous test solutions of silver acetate. XPSand contact angles measurements were used to characterizesubstrate/polymer hybrids.

∗ Correspondence to: Mohamed M. Chehimi, ITODYS, Universite Denis Diderot- Paris 7 & CNRS (UMR 7086), 1 rue Guy de la Brosse, 75005 Paris, France.E-mail: chehimi@paris7.jussieu.fr

a ITODYS, Universite Denis Diderot - Paris 7 & CNRS (UMR 7086), 1 rue Guy de laBrosse, 75005 Paris, France

b Laboratoire d’Electrochimie et de Chimie Analytique, Universite Pierre et MarieCurie & CNRS (UMR 7575), 4 place Jussieu 75252 Paris cedex 05, France

Surf. Interface Anal. 2008; 40: 412–417 Copyright c© 2008 John Wiley & Sons, Ltd.

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Diazonium salts for surface-initiated ATRP

Experimental

Synthesis of the diazonium salt BF4−, +N2-C6H4-CH(CH3)-Br

(D1)

The synthesis of the starting diazonium salt D1 was described inRef. [10]. 1H NMR (200 MHz, CDCl3), δ ppm : 3.67 (q, 1H), benzylicprotons; 2.07 (d, 3H), methylic proton; 7.90 (d, 2H) and 8.50 (d, 2H),aromatic protons β to the diazonium function and to the benzyliccarbon, respectively.

Synthesis of the diazonium salt BF4− +N2-C6H4-CH2-CH2-Br

(D2)

The synthesis of the starting diazonium salt D2 was described inRef. [13]. 1H NMR, 200 MHz, δ ppm: 3.35 (t, 2H), benzylic protons;3.47 (t, 2H) ethylic proton; 7.87 (d, 2H) and 8.59 (d, 2H), aromaticprotons β to the diazonium function and to the benzylic carbon,respectively.

Synthesis and electrografting of the diazonium salt BF4− +N2-

C6H4-CH2-CH2-OH (D3)

0.69 g (5 mmol) of 2-(4-aminophenyl)ethanol was dissolved in3 ml of acetonitrile and 2 equiv of tetrafluoroboric acid (48%in diethylether) and kept in a freezer. 0.63 g (5.5 mmol) of tert-butylnitrite was dissolved in 3 ml of acetonitrile and kept in afreezer. After the solutions were cooled to −20 ◦C, they weremixed and kept overnight at 4 ◦C. The solution was dried undervacuum to evaporate the solvents. The diazonium salt was keptin a desiccator at −20 ◦C. 1H NMR, 200 MHz, δ ppm: 3.0 (t, 2H),benzylic protons; 3.51 (m, 1H) alcohol proton; 3.72 (t, 2H) ethylicproton; 7.85 (d, 2H) and 8.59 (d, 2H), aromatic protons β to thediazonium function and to the benzylic carbon, respectively.

After electrochemical reduction of D3 (see below), the grafted-C6H4-CH2-CH2-OH group was treated by 2-bromoisobutyryl bro-mide to yield the initiator group -C6H4-CH2-CH2-OC(O)-C(CH3)2Br,hereafter abbreviated by 3. To do so, the hydroxylated GC platesand 20 ml of dichloromethane were added in a double-walledcylindrical tube. Then, 1.0 ml of triethylamine was added followedby 1.5 g (6.5 mmol) of 2-bromoisobutyryl bromide. The reactionmixture was stirred and heated at reflux for 24 h. The function-alized plates (GC-3) were removed from the tube and rinsedconsecutively with dichloromethane, a mixture of water/acetone(50/50) and acetone prior to surface-initiated ATRP of tert-butylmethacrylate.

Electrochemical treatment of iron and glassy carbon plates

Electrochemical grafting of the diazonium salts was achievedon polished and cleaned iron disks (1 cm diameter, WeberMetaux) or GC plates (1 × 2 cm). The grafting reaction wasperformed in acetonitrile medium (ACN + 0.1 M of NBu4BF4) bychronoamperometry for 300 s, at a potential of 300 mV negativeto the peak potential (measured on GC). The samples were thenthoroughly rinsed under sonication in ethanol, dichloromethaneand acetone.

Surface-initiated ATRP of vinylic monomers

The general procedure for grafting polystyrene (PS) andpoly(methyl methacrylate) (PMMA) chains from the surface ofiron disks and GC plates modified by electrochemical reduction ofD2 is adapted from Refs [10, 13].

For the modification of GC by tethered poly(tert-butylmethacrylate) chains, the monomer tert-butyl methacrylate(tBMA) (Aldrich products) was distilled prior to polymerization.N,N,N′ ,N′′,N′′-pentamethyldiethylenetriamine (PMDETA), CuCl andCuCl2 (Aldrich) were used as received. The detailed procedure forpreparing the polymer brushes is as follows. First, a 100-ml double-walled cylindrical tube (similar to a Schlenk flask) equipped witha magnetic stir bar and sealed with a rubber septum was de-oxygenated under vacuum followed by back-filling with nitrogenthree times. The CuCl (45 mg, 4.5 × 10−4 mol) and CuCl2 (6.8 mg,5.05 × 10−5 mol) powders, and the substrates (iron disks or GCplates) were introduced into the cylindrical tube under a nitro-gen flow. A mixture containing tBMA (14.32 g, 0.1 mol), PMDETA(87 mg, 5.02 × 10−4 mol), and toluene (3 g) previously degassed,was added to the polymerization tube using a syringe (previouslypurged with nitrogen) under a nitrogen flow. The tube was placedin an oil bath at 90 ◦C for appropriate polymerization time. Thepolymerization was stopped by cooling and opening of the tubein order to expose the catalyst to air.

The GC-3-PtBMA hybrids were taken out for different periodsof time, rinsed successively under sonication, with acetone/water,acetone and dichloromethane.

Preparation of poly(methacrylic acid) brushes

Poly(methacrylic acid) brushes on the surface of GC (GC-1-PMAA)were obtained by acidic hydrolysis of GC-1-PtBMA. Typically,the latter plates were put into a double-walled cylindrical tubecontaining 30 ml of 10% aqueous hydrochloric acid. The mixturewas heated at 115 ◦C for 12 h. Then, the plates were takenout, rinsed successfully with copious amounts of water andtetrahydrofuran, and dried under argon at room temperature.This simple procedure for preparing PMAA brushes requires astable initiator layer; for this reason, we used D1 instead of D3for the ATRP of tBMA. Indeed, -C6H4-CH2-CH2-OC(O)-C(CH3)2Br (3)can be cleaved due to the acid hydrolysis, resulting in removal ofentire chains from the surface.

Interaction of Ag+ with PtBMA and PMAA brushes

The GC-1-PtBMA and GC-1-PMAA plates were incubated in 40 ml ofa 10 mM aqueous solution of silver acetate (66.8 mg). Under theseconditions, the silver acetate solution has a pH of 4.9, sufficientlyhigh to de-protonate over 50% of the carboxylic acid groups of GC-1-PMAA; the pKa of PMAA being 4.66.[14] The reaction was carriedout under stirring at 40 ◦C for 24 h. Then, the plates were takenout, thoroughly rinsed with distilled water and dried in argon.

XPS

The spectra were recorded using a Thermo VG Scientific ESCALAB250 system fitted with a micro-focused, monochromatic AlKα X-ray beam (1486.6 eV, 650 µm spot size). The Avantagesoftware, version 3.51, was used for digital acquisition anddata processing. Spectral calibration was determined by settingthe aliphatic C–C/C–H C1s component at 285 eV. The surfacecomposition was determined using the integrated peak areas andthe corresponding manufacturer’s sensitivity factors.

Contact angle measurements

Contact angles were measured with a DSA10 (Kruss instrument)using a CCD video camera and a horizontal light source to

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M. N. Nguyen et al.

e-

+N2 XBF4

-

SI-ATRP of

BMA, t-BMAMMA, BA, styrene,

polyglycidol monomer,

Polymer brushesoligo1

X

X

Xn

X

X

Xn

CHCH3

Br

H2C

H2C Br C

HC

CH3 O

C

CH3

Br

CH3

X = ; ; O

Su

bst

rate

Su

bst

rate

Su

bst

rate

olig

o1

Figure 1. Schematic illustration of ATRP of vinyl monomers surface-initiated by a brominated oligophenylene layer (oligo1) based on the correspondingdiazonium salt.

illuminate the liquid droplet. Water droplets were placed on thesurface under an optical vessel to minimize evaporation. The entiresystem was located in a thermostated chamber at 25.0 ± 0.5 ◦C.Contact angles are averaged over at least four measurements withan uncertainty of ±2◦.

Results and Discussion

The ‘diazonium salt/ATRP’ protocol established for the growth ofpolymer brushes is shown in Fig. 1. The electrochemical reductionof the diazonium salt is known to yield a thin oligophenylene typelayer.[12] ATRP was conducted in the absence of sacrificial initiatorbut in the presence of Cu(II) deactivator in order to control thepolymerization process.[15]

The various systems investigated in this work are shown inTable 1.

The results concerning the hybrids reported in Table 1 will besplit into two parts. The first one will be devoted to the growth ofPS and PMMA on iron and GC using D2 to attach effective ATRPinitiators; the second part will consider the synthesis of PtBMAchains and their hydrolysis on GC.

BF4− +N2-C6H4-CH2-CH2-Br (D2) for the synthesis of ATRP

polymer brushes on iron and glassy carbon

In a previous paper, we have shown that multi-walled carbonnanotubes (MWCNTs) could be modified by ultrathin layers of PSand PMMA when ATRP is initiated by groups 2. Hereafter, we showthat such groups are also effective in modifying the surfaces ofiron and GC plates.

Table 1. Glassy carbon-polymer and iron-polymer hybrids investi-gated in this work

SubstrateDiazonium salt

(Initiator) Polymer brushesSubstrate-polymer

hybrids

GC D1 (1) PtBMA GC-1-PtBMA

GC D2 (2) PS GC-2-PS

GC D2 (2) PMMA GC-2-PMMA

GC D3 (3) PtBMA GC-3-PtBMA

Iron D2 (2) PS Fe-2-PS

Iron D2 (2) PMMA Fe-2-PMMA

Figure 2 shows the survey and high-resolution C1s spectra ofthe systems Fe-2-PS, Fe-2-PMMA, GC-2-PS and GC-2-PMMA.

Both substrates are very well screened by PS and PMMA brushes.PMMA exhibits much sharper O1s peaks compared to PS, which isin line with the chemical structures of the polymers.

The high-resolution C1s regions have structures comparable tothose of pure PS and PMMA, which underlines PS- or PMMA-richsurfaces. Figure 2(b, d) show that for PMMA, the peak has threemajor components due to C–C/C–H, C–O and O–C O bonds,while the PS brushes exhibit a sharp signal at 285 eV, and itsshake-up satellite at 291.5 eV.

Qualitative examination of the survey scans permits to indicatethat growth of PS and PMMA is very efficient on GC as this substrateis very well screened. Indeed, the survey spectra compare very wellwith those of pure PS and PMMA. In contrast, on iron, the Fe2pdoublet is clearly visible and relatively intense for Fe-2-PMMA,perhaps due to a low grafting density of PMMA chains. In thecase of Fe-2-PS, Fe2p is weak and the survey spectrum exhibits aprominent inelastic background due to homogeneous PS coating.This is consistent with a thickness that should match the samplingdepth of iron; that is near 8 nm.

Grafting PtBMA brushes from the surface of glassy carbon

The survey, C1s and valence band spectra of GC, GC-3 and GC-3-PtBMA are displayed in Fig. 3. ATRP was conducted as a functionof time. The initiator group is characterized by Br3d centered at71 eV (Fig. 3(a)). The progressive growth of the polymer chains isreflected in the survey scans: the O1s/C1s relative peak intensityis approaching that of pure PtBMA.[16] It is to note that N1sdetected at ∼400 eV is possibly due to the formation of azogroups (–N N–) derived from the diazonium salts.[17,18]

The C1s region from GC-3-PtBMA after 5 h ATRP completelydiffers from that of the untreated GC (Fig. 3(b)). Two newcomponents appear: the alkoxy C–O (∼286.5 eV), and the esterO–C O (289 eV) carbon atom type.

To account further for the presence of PtBMA chains, the valenceband is shown in Fig. 3(c). It exhibits two C2s peaks centered at∼16.9 and ∼19.6 eV, due to sσ (C2s–C2s) bonding orbital in thependant groups and the main chain (noted I and II), respectively.[19]

The I/II intensity ratio at 50◦ take-off angle is very comparable tothat obtained for pure PtBMA.[16] This fine structure is highlightedat 50◦ take-off angle rather than 90◦ (relative to the surface) asPtBMA constitutes the outermost layer of the hybrid.

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Diazonium salts for surface-initiated ATRP

(d)(c)

(b)(a)

0 200 400 600 800 1000

O1sC1s

O1sC1s

GC-2-PS

GC-2-PMMA

Fe-2-PS

Fe-2-PMMA

Inte

nsity

(a.

u.)

Inte

nsity

(a.

u.)

Inte

nsity

(a.

u.)

Inte

nsity

(a.

u.)

Binding energy (eV) Binding energy (eV)

282 284 286 288 290 292 294 296

0 200 400 600 800 1000

Binding energy (eV) Binding energy (eV)

282 284 286 288 290 292 294 296

π-π*

π-π*

C-C/C-H

C-C/C-H

O-C

O-C

O-C=O

O-C=O

GC-2-PMMA

Fe-2-PMMA

GC-2-PS

Fe-2-PS

Fe2p

Figure 2. Survey (a, c) and C1s regions (b, d) for iron-polymer (c, d) and GC-polymer (a, b) hybrids prepared using D2 as a precursor for grafted initiators.

GC-3-PtBMA and GC-1-PtBMA were subjected to hydrolysisin order to prepare the GC-3-PMAA and GC-1-PMAA hybrids.However, since the initiator 3 has an ester group, it can also behydrolyzed in the same manner as the pendant ester groups ineach repeat unit of PtBMA. A hydrolysis of the initiator site islikely to result in the cleavage of entire chains. It follows thatalthough 3 is more appropriate for grafting methacrylic polymerchains from the surface,[1] we preferred to use the initiator1 because it withstands hydrolysis. The resultant GC-1-PtBMAplates were then hydrolyzed to give GC-1-PMAA. Both hybrids(precursor and hydrolysis product) were characterized in terms ofhydrophilic/hydrophobic character ad the propensity to uptakesilver cations from aqueous silver acetate solutions.

Hydrolysis of PtBMA resulted in GC-1-PMAA that exhibitedsignificant hydrophilic character by comparison to GC-1-PtBMA.Indeed, the water static contact angle dropped from 87.2 to 42.3◦

upon hydrolysis of the PtBMA brushes.To further account for the effective hydrolysis of PtBMA brushes,

GC-1-PtBMA and GC-1-PMAA were incubated in silver acetatesolutions along the published procedure of Baillie et al.[20] andthen analyzed by XPS.

Figure 4 shows survey scans of GC-1-PtBMA and GC-1-PMAA,after uptake of silver cations. The Ag3d doublet is relatively muchmore intense in the case of the hydrophilic GC-1-PMMA hybrid.Ag/C atomic ratio, used as a chemical descriptor of silver uptake,is ∼0.03, that is five-fold that found for the GC-1-PtBMA precursor(Ag/C = 0.006). This significant difference in the uptake of silveris due to favorable electrostatic interactions between Ag+ andthe carboxylate (COO−) groups of PMAA. These experimentalresults constitute another proof for the effective hydrolysis ofPtBMA brushes, thereby confirming contact angles measurementsreported above.

Conclusion

ATRP of styrene, methyl methacrylate and tert-butylmethacrylatewas initiated on iron or GC substrates modified by the electrochem-ical reduction of various aryl diazonium salts of the general formulaBF4

−, +N2-C6H4-R, where R is a bromine-terminated group. Thegrafted –C6H4 –R species are effective in initiating ATRP at metallicand carbon surfaces. Aryl diazonium salts do not require the assis-tance of any sacrificial initiator in solution, and the control could beachieved using the deactivator Cu(II). The surface chemical com-position of the substrate-polymer hybrids was characterized byXPS. In the case of GC-poly(tert-butyl methacrylate), GC-1-PtBMA,hydrolysis led to the GC-poly(methacrylic acid) derivative (GC-1-PMAA). Water contact angles were found to drop from 87.2 to 42.3◦

upon hydrolysis of the PtBMA brushes. Moreover, the resulting GC-1-PMAA hybrid was found to uptake five-fold silver cations fromaqueous silver acetate solutions compared to the hydrophobic GC-1-PtBMA hybrid; this is particularly due to favorable Ag+ . . .−OOCinterfacial interactions between Ag+ and the PMAA brushes.

This article conclusively constitutes an additional and importantbrick in building the ‘diazonium salt/ATRP’ procedure that wehave designed recently for controlling the surface and interfacechemistry of materials. It particularly shows that as couplingagents, aryl diazonium salts are as important as the more classicalsilanes and thiols.

Acknowledgements

MNN wishes to thank the French CNRS for the provision of apost-doctoral grant (Post-Doc CNRS 2006 no SC15).

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M. N. Nguyen et al.

0 5 10 15 20 25 30 35

50°

90°

I

C2s

Inte

nsity

(a.

u.)

Binding energy (eV)

II(c)

280 282 284 286 288 290 292 294 296 298

GC

O-C=O

C-O-C

Inte

nsity

(a.

u.)

Binding Energy (eV)

C-C/C-H

GC-3-PtBMA 5h

(b)

0 200 400 600 800 1000

GC-3-PtBMA 5h

GC-3-PtBMA 1h

GC-3

Br3d

O1sC1s

Inte

nsity

(a.

u.)

Binding Energy (eV)

GC

(a)

Figure 3. Survey, C1s and valence band regions from GC, GC-3 and GC-3-PtBMA.

0 200 400 600 800 10000

1x105

2x105

3x105

4x105

O1s

C1s

Ag3d

GC-1-PtBMA

GC-1-PMAA

Inte

nsity

(a.

u.)

Binding energy (eV)

Figure 4. Uptake of silver cations by GC-1-PMAA and GC-1-PtBMA hybrids.

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Surf. Interface Anal. 2008; 40: 412–417 Copyright c© 2008 John Wiley & Sons, Ltd. www.interscience.wiley.com/journal/sia