1

Grafting polymer films onto materials surface: the one-step redox

processes

Guy Deniau, Serge Palacin, Alice Mesnage, Lorraine Tessier

CEA, IRAMIS, SPCSI Chemistry of Surfaces and Interfaces Group,

F-91191, Gif-sur-Yvette, France.

If we consider that the properties of materials are often directly expressed by

their surface, organic coatings and more particularly thin polymer films are nowadays

very attractive research subjects with potential fallouts in many fields of application

such as depollution, microelectronics, automobile, biomedical engineering tools, etc.

Indeed, surface modification by resistant polymer coatings confer to materials many

properties targeted by those industries (antifouling, antisoiling, adhesion, lubrication,

biocompatibilization). In consequence, a large range of coating methods, including

“physisorption” or “chemisorption” techniques, has been developed for the synthesis

of organic or composite layers.

Physisorption techniques such as painting, spin coating, and vacuum

evaporation, have no limitation in the choice of the substrate-layer couple, but the

weak interactions involved in the interfacial zone result in fragility of the coating and

possible loss of the desired properties with time.

On the contrary, chemisorption techniques, including plasma polymerization

[1,2] self-assembly [3,4] and in situ surface polymerizations, seem by far the most

convenient to get stable films. Among them, the latter appears to be a method of

choice for a strong covalent attachment of polymer chains to the surface forming

ultrathin films. Among in situ surface polymerizations, it is important to distinguish

“grafting to” from “grafting from” pathways. The first ones involve the bonding of

preformed end-functionalized polymer chains to the surface, and the second ones,

also called surface-initiated polymerizations [5-7] correspond to the polymer growth

(initiation and propagation) from the surface. It is possible to adapt any classical bulk

polymerization to form grafted polymer brushes by modifying in a first step the

substrate with initiator-bearing layers [8-14]. Obviously, living polymerizations such

as controlled radical ones [15,16] achieve maximum control over brush density, chain

polydispersity, composition, molecular weight and thickness of the grafted polymer

brushes [17-19]. Especially surface-initiated atom transfer radical polymerization

(SI-ATRP) [5,20] (Chapter 6) has become the most popular route, mostly because of

its tolerance to a wide range of functional monomers and its possibility to form block

copolymers and several architectures [21].

In parallel, electrochemical reduction of diazonium salts has been widely

investigated for the past ten years and this method is now almost ubiquitous for easy

surface modification [22] (Chapter 1). That method delivers only very thin polyaryl

coatings and was already used with halogenated (brominated) aryldiazonium salts

[23] that eventually act as initiators in ATRP. Therefore, by choosing the appropriate

diazonium salt, “diazonium-based” ATRP was performed with many monomers

(styrene, methylmethacrylate, butylmethacrylate) on several substrates [24-29].

2

Unlike the previous processes, direct cathodic electrografting of activated vinylic

monomers (CE) [30] is a one-electrochemical-step process relying on the direct

electroinitiation of vinylic monomers, reduced under a cathodic current in very

unstable radical-anions which immobilize on the electrode. Then, the chain growth

proceeds from the surface by purely anionic propagation [31,32].

CE and SI-ATRP techniques were recently compared [33] and although they

are valuable tools for the synthesis of grafted organic coatings on conducting

surfaces, both processes have drawbacks: high monomer concentration, long

polymerization time and heating are required for SI-ATRP processes; drastic

anhydrous conditions, and a narrow choice of monomers (restricted to (meth)acrylic

derivatives [34]) limit the expansion of the cathodic electrografting process. CE and

SI-ATRP drawbacks could be overcome with an alternative electrografting method

based on a radical mechanism.

Aryldiazonium salts have been shown to be good initiators for radical

polymerization [35,36]. SEEP (Surface Electroinitiated Emulsion Polymerization)

[37-39] and GraftfastTM [40,41] are two grafting processes recently developed which

precisely rely on diazonium salts by a chemical redox mechanism to initiate the

radical polymerization of vinylic monomers. Both proceed in aqueous media and lead

to similar strongly adherent films. Although SEEP (as CE) is restricted to conducting

materials, GraftfastTM as a purely chemical process can be applied to any type of

surfaces from conductors to insulators. After a short description of the original CE

process, this chapter will detail both newly grafting processes, SEEP and GraftfastTM.

1. Cathodic electrografting (CE) in organic medium

1.1 . Direct cathodic electrografting of vinylic polymers

As originally shown by Lécayon [42,43], strongly adhesive polymer films are

formed on any conductive surface by cathodic electrodeposition from anhydrous

solutions of vinylic monomers such as methacrylonitrile or methylmethacrylate. It is

not the purpose of the present review to give a detailed description of that process

which was already reviewed a couple of years ago [30]. Therefore, the reader may

refer to that review for details and references.

However, some main characteristics of CE should be reminded here, before

switching to the more recent processes:

• CE only works in dry organic solvents on a few vinylic monomers able to be

electroreduced (acrylates, acrylonitrile, vinylpyridine exhibit a reduction peak

around -2.5 V vs Ag+/Ag).

• CE proceeds in three main steps: (i) the monomer is reduced at the cathode to

form a radical-anion; (ii) the radical-anion grafts onto the electrode; (iii)

another monomer reacts on the grafted anion (typical propagation of an

anionic polymerization) [31]. As the grafted anion can reasonably be

considered as a metastable moiety, the polymerization from the substrate can

be seen as a stabilization process for the grafted species, since the negative

charge is driven away for the charged cathode during propagation.

3

• CE gives truly grafted polymer films. Indeed, the carbon-to-metal bond was

evidenced by XPS [32] (Figure 1).

• CE also produces large amounts of non-grafted polymer chains, mainly arising

from radical-anions that did not permanently graft on the electrode. The non-

grafted chains can be easily washed away from the substrate with a suitable

solvent.

• The propagation step is independent from the applied tension; hence the final

thickness of the grafted film only depends on the experimental conditions:

concentration, solvent, temperature… CE is then not an electro-driven one (as

the anodic polymerization of conducting polymers) but an electro-induced

process.

• Thanks to the local generation of active and short-living species, CE can be

localized providing that the cathode-substrate exhibits areas of different work

functions, as for composite surfaces or locally doped semiconductors [44,45].

• CE is a quite fast process when compared to other grafting-from processes

such as for example surface-initiated ATRP, [34].

Figure 1 X-ray photoelectron spectrum of C1s core level of a nickel surface obtained by

electrochemical polarization of 2-butenenitrile in acetonitrile .Reproduced with permission, from

reference [32]. Copyright 2006, Elsevier Science & Technology Journal.

The above list clearly emphasizes the ability of CE to produce truly grafted

polymer films onto any conducting substrate. However, CE suffers from several

drawbacks which obviously limit its practical use, particularly in industrial

conditions:

• Due to its anionic mechanism, CE requires strictly anhydrous conditions.

• The applied potential is highly cathodic (around -2.5 V/ Ag+/Ag), which might

be detrimental for substituted monomers bearing fragile groups.

4

For those reasons, CE cannot be considered for real applications, unless very

specific. However, it is a very valuable tool for studying organic-to-metal interfaces in

highly controlled conditions, as already evidenced for the carbon-to-metal signal

observed in XPS [32]. There is thus a clear need for an alternative method which

could provide similar grafted polymer films in less demanding conditions.

1.2. Indirect cathodic electrografting

To avoid the main drawbacks of CE (high cathodic potential and anionic

polymerization), the idea was to switch from a direct and purely anionic mechanism

to an indirect and radical one while preserving the covalent grafting of the resulting

coating. This was simply obtained by adding diazonium salts in the classical CE

solution (acetonitrile, quaternary ammonium and vinylic monomer). As recently

reviewed, diazonium salts are very good radical sources for many radical processes

including radical polymerization [35,36]. It was thus interesting to investigate

electroreduced diazonium salts as precursors for radical polymerization, and the

behaviour of the resulting radical oligomers towards the electrode surface.

Figure 2 Cyclic voltammetry in acetonitrile of (a) tetraethyl ammonium perchlorate TEAP (5.10-2 M),

(b) TEAP + vinylic monomer (VM) and (c) TEAP + VM + nitrobenzene diazonium tetrafluoroborate

NBDT (10-3 M). Scan rate 50 mV s-1.

As shown in Figure 2, displaying cyclic voltammograms of a supporting

electrolyte alone (quaternary ammonium) (a), electrolyte + vinylic monomer (b) and

electrolyte + diazonium salt (NBDT) (c), the reduction potential of the electroactive

species increases from (a) to (c) (-3V to 0V/ Ag+/Ag). The reduction reactions are

given here after in Scheme 1:

-3,0 -2,5 -2,0 -1,5 -1,0 -0,5 0,0 0,5

-9

-8

-7

-6

-5

-4

-3

-2

-1

0

1

nitro group reduction

diazonium salt reduction

I (m

A)

E (V) vs Ag+/Ag

quaternary

ammonium

vinylic monomer reduction

(a)

(b)(c)

5

Scheme 1 Electrochemical reduction of the studied species.

In the case (b), the radical anion was able to graft to the metallic surface and

initiates anionic polymerization from the surface. In the case (c), the aryl radical

obtained at very low potential (about 0 V/Ag+/Ag) is able to (i) graft on the surface

and (ii) to initiate a radical polymerization in solution. The result of this experiment is

a grafted thin polyvinylic film which resists to all the methods tested to pull it out

from the substrate as Soxhlet extraction or sonication bath. In Figure 3 is presented

the infrared spectra of grafted coatings obtained at various potentials, less cathodic

than the one necessary for the direct vinylic electron transfer. The main absorption

bands around 1730 cm-1 prove in each case the presence of the polyvinylic moieties

(in that example polybutylmethacrylate, PBMA) onto the metallic gold surface. The

comparison of the spectra indicate that a control of the film thickness can be obtained

by choosing the applied potential. The higher the cathodic potential, the more intense

the main absorption band and consequently the thicker the coating.

Figure 3 IR-ATR spectra of the grafted coatings obtained after 10 voltammetric cycles at 50 mV s-1

scan rate: final potential (a) -1.0, (b) -1.6, (c) -2.2. V /Ag+.

One drawback of this method, when it comes to real applications, is the poor

stability of the working solution: diazonium compounds are well known to be fragile

(N2 is one of the best leaving groups in organic chemistry) and distillated vinylic

a

b

c

6

monomers easily polymerize in solution (by thermic and/or UV activation). We

however demonstrated that those limitations could be bypassed by:

- forming the diazonium salt in situ from stable precursors [46]. The

diazonium salt can be obtained in a ‘one pot’ process by mixing in situ the

amine with the diazotizing reagent (e.g. NOBF4) without any visible

consequence on the grafting process.

- using vinylic monomers as received from their commercial sources. The

radical inhibitor of the commercial vinylic monomer (in most of the case

hydroquinones) can be “quenched” by reaction with an anhydride to form

an ester.

Thus, the indirect cathodic electrografting process simply works by mixing

two stable solutions (nitrosonium in ACN and aniline derivative + commercial butyl

methacrylate BMA in ACN) in an electrochemical cell, adding the anhydride and

immediately polarizing the working electrode until approximately -0.5 V/Ag+/Ag.

Strongly grafted PBMA thin films were obtained by this method and used in

order to biocompatibilize vascular prosthesis (stents). This study was at the origin of

the creation of the start-up Alchimer (http://www.alchimer.com/) in 2001 (Chapter

14).

2. Surface Electroinitiated Emulsion Polymerization (SEEP)

Another key parameter towards real applications of cathodic electrografting

consisted in discarding the use of organic solvents and employing aqueous precursor

solutions, as it is well known that radical polymerization can be performed in water

[47]. The challenge was also to avoid restrictions of the process to water soluble

vinylic monomers (acrylic acid, hydroxyl (acry) methacrylates and acrylonitrile). We

thus got inspiration from emulsion polymerization commonly using non water

soluble monomers by adding a surfactant in the solution to obtain an emulsion [48].

Such polymerization leads to latex polymers in solution. What happens when a

monomer emulsion (or miniemulsion) containing a diazonium salt is in contact with a

cathode?

For example, a miniemulsion can be obtained from 0.7 M BMA in acidic

solution (water solubility 2.5 x 10-3M) and sodium dodecyl sulfate (SDS, 9 x 10-3 M =

1.125 CMC (Critical Micelle Concentration) [49]) submitted to ultrasonication [50].

Then, nitrobenzene diazonium tetrafluoroborate (NBDT) (2 x 10-3 M) was added

under magnetic stirring. The resulting complex biphasic mixture (Scheme 2) is stable

with time and can be used for electrografting of vinylic monomers for a few hours.

The miniemulsion is then transferred in a three electrode cell. The working

electrode (gold) polarization is achieved under cyclic voltamperometry from 1 to 10

cycles at 10 mV s-1 scan rate from the rest potential (≈0.5V/SCE) to the final potential,

usually -1.0 V/SCE. Such electrochemical set-up leads to the formation of a visible

coating onto the electrode surface after ultrasonication in DMF (performed to remove

any physisorbed polymer chains and which shows the strong attachment of the film.

7

Scheme 2 Composition of the initial miniemulsion system in the SEEP electrochemical cell. Adapted,

with permission, from [39]. Copyright 2009 American Chemical Society.

2.1. Poly(butyl methacrylate) films characterization

This section gives a brief reminder of the major characteristics of SEEP dealing

with a particular example of electrografted PBMA films on gold surface from a

miniemulsion. The IR-ATR spectrum of a typical thin PBMA film (30 – 50 nm thick) on

gold is shown in Figure 4. It confirms the presence of PBMA together with nitro

moieties originating from the electroreduction of the diazonium precursor. No

important change was observed in the IR intensity of the main band of the PBMA

coating upon ultrasonication (cf. insert Figure 4), which confirms the robust grafting.

In conclusion, this spectrum presents all the features of a poly(nitrophenylene)

(PNP)–PBMA copolymer.

8

Figure 4 IR-ATR spectrum of the grafted PBMA coatings on gold obtained by SEEP after 5

voltamperometric cycles at 10 mV s-1 scan rate from rest potential to -1.0 V vs SCE. Insert compares the

spectra obtained before and after ultrasonic treatment in DMF.

The XPS survey spectrum (not presented here) of a 10 nm thick PBMA film

obtained by SEEP displays the characteristic carbon and oxygen peaks of PBMA [51]

respectively centred at 285 eV and 532 eV. The O1s and C1s core levels are in very

good accordance to those of PBMA found in literature [52]. Gold peaks are barely

visible indicating that the organic layer thickness is close to the sampling depth of the

technique (10-15 nm). The nitrogen region (around 400 eV) is detailed in Figure 5.

The highest energy peak at 406 eV is obviously attributed to the N1s in nitro groups

(-NO2). According to the literature, the first component at 399.3 eV can be attributed

to amino groups (-NH2) and the second one at 400.4 eV to azo groups (-N=N-).

Hence, the reduction of BMA/NBDT mixtures leads to strongly grafted PBMA

films that contained nitrophenyl groups, azophenyl groups and aminophenyl groups,

all coming from the reduction of NBDT.

9

Figure 5 N1s core level XPS spectrum of PBMA film (10 nm thick) grafted by SEEP (5 cycles/-1V/10

mV s-1 scan rate). Adapted, with permission, from [39]. Copyright 2009 American Chemical Society.

2.2. Determination of the film structure

In order to understand the mechanism of the SEEP process, it is essential to

determine and establish the structure of the films. The main point of this task is to

localize the nitrophenyl groups (C6H4-NO2) in the film. We already know that

nitrophenyl radicals from NBDT reduction form a polynitrophenylene-like layer

according to the mechanism described in the literature [51-55] and also that they are

able to initiate radical polymerization [35,36]. So, the question is: are nitrophenyl

groups only concentrated at the gold interface in that copolymer film? ToF-SIMS

analyses provide an answer.

ToF-SIMS allows a detailed analysis of the whole depth of the film, from the

upper (superficial) part to inner part (interface area). Indeed, the coating profile

(Figure 6a) gives the normalized intensity of the characteristic collected ionized

fragments resulting from ion bombardment of a PBMA grafted film versus depth

profiling time. Time zero corresponds to the top of the film and at the final time, the

substrate is reached. For the sake of clarity, only one fragment profile from

nitrophenyl moieties (CNO; m/z = 42.0) and two from PBMA (C2H, m/z = 25.01;

C4H5O, m/z = 85.04) are represented. The last line is the gold substrate profile. The

intensity of the PBMA fragments remains high (70 – 90 %) during almost all the

depth profiling time and starts to decrease only when gold is reached. This is

consistent with the chemical composition of the grafted films, which is almost pure

PBMA. On the contrary, the intensity of nitrophenyl fragments (CNO line) increases

when reaching the substrate with a slope almost similar to the gold one which is

consistent with the presence of a very thin PNP sub-layer, whose thickness is related

to the shift in abrasion time between the green and grey lines. However, nitrophenyl

fragments are also extracted from the top part of the film: indeed, the CNO intensity

recorded at low abrasion times is far from negligible. If nitrophenyl moieties were

only located in a sub-layer, the CNO signal should be close to zero at low abrasion

time and should increase progressively in parallel to the gold one. The significant

signal, observed at low depth profiling time, undoubtedly demonstrates that

5500

5600

5700

5800

5900

6000

6100

6200

394396398400402404406408410412

Binding energy (eV)

CP

S

NH2

N=N

NH3+

NO2

10

nitrophenyl moieties are present throughout the entire film. Therefore, nitrophenyl

groups are located in high concentration close to the substrate, forming a PNP sub-

layer near the interface with gold, and are also spread at lower concentration

throughout the full polymer thickness up to the top of the film (Figure 6b).

Figure 6 a) ToF-SIMS profile of a PBMA film grafted by SEEP from BMA in miniemulsion (5 cycles / -

1.0 V / 10 mV.s-1): CxHyOz fragments from PBMA, CNO fragments from nitrophenyl groups, Au profile.

b) Deduced structure of PBMA films obtained by SEEP. Adapted, with permission, from [39]. Copyright

2009 American Chemical Society.

The structure of the grafted films provided by SEEP is thus well established.

However, in order to propose a detailed mechanism for their formation, we still need

to understand which reactions are involved.

11

2.3. Reduction of protons and role of hydrogen radicals

Several experiments were carried out to elucidate the role played by protons

reduction in the SEEP mechanism. Details are given in the work of Tessier et al. [39].

• Experiments without diazonium salt did not provide any grafted polymer on

the electrode (gold), but polymer chains were found in solution. This strongly

suggests that (i) radical species issued from the electroreduction of protons

are able to initiate the radical polymerization of the vinylic monomers in

solution; (ii) the PNP primer layer is essential to graft the vinylic polymer onto

the substrate.

• Experiments were performed by varying the final cathodic potential and

measuring the corresponding thicknesses of the films: the more cathodic the

final potential, the thicker the grafted film. As the amount of reduced protons

also increases when the potential become more cathodic, it was easily

suggested by this experiment that hydrogen radicals resulting from protons

reduction play an important role. Moreover, this experiment shows that when

the final potential in the cyclic voltamperometry is stopped before the proton

reduction regime (i.e. above ca. -0.4 V vs SCE), the grafted film is thinner than

when protons are reduced.

• SEEP experiments on metallized quartz of an electrochemical microbalance

(EQCM) allow simultaneous measurements of electrochemical parameters and

mass changes at electrode. The EQCM experiment shows that during the

protons reduction regime, there is an important mass increase at each

voltammetric cycles whereas the mass variation due to diazonium reduction is

only significant during the first two cycles.

• SEEP is typically a one-pot process. However, when the grafting is performed

in two separate steps (first, electrografting of a polynitrophenyl-like (PNP)

film by reducing a diazonium alone and then use this PNP-modified substrate

as working electrode in a SEEP medium without diazonium), the result shows

unambiguously that a polymer layer (PBMA in that case) is formed and grafted

on the intact PNP sub-layer, provided the proton reduction regime is reached.

Consequently we claim that hydrogen radical (H•) is able to initiate radical

polymerization under these conditions. Indeed although the major product of protons

reduction is dihydrogen, H• intermediates have been shown to act as radical

polymerization initiators [56-60].

2.4. Mechanism of SEEP

From all the experimental results given above it was possible to elaborate a

SEEP mechanism including classical radical polymerization steps, namely, initiation,

propagation and termination. If initiation and propagation are quite similar to those

in a bulk radical polymerization, termination reaction leads, in SEEP, to the polymer

grafting. As usual, initiation involves two reactions; first, primary radical generation

by electrochemical reduction of the diazonium salt and then initiation by reaction

with a vinylic monomer.

12

Termination corresponds to the “grafting to” step of PBMA chains on the

substrate. Moreover, SEEP mechanism includes an additional reaction at the

beginning, which is the PNP-like layer formation. This sub layer is obtained by an

addition-elimination mechanism as shown in Scheme 3a. Such mechanism belongs to

‘grafting from’ methods.

As an example, the SEEP mechanism given here (cf. Scheme 3b) corresponds to

the one obtained with NBDT, BMA and SDS in acidic water solution.

Scheme 3 a Chemical mechanism of the formation of the polyaryl sub-layer by an addition –

elimination reaction on the aromatic ring. b Chemical mechanism of PBMA chains linkage on the

polyaryl sub-layer by an addition – elimination on the aromatic ring. Adapted, with permission, from

[39]. Copyright 2009 American Chemical Society.

The SEEP process can be applied to any conducting substrate, with any

monomer able to polymerize through a radical mechanism. Moreover, almost all the

tested diazonium salts were successfully grafted [37-39], and cationic, anionic or

neutral surfactants can be used to form the emulsion. Most importantly, SEEP is a

one-pot process that works in aqueous dispersed media from available reagents

without any catalyst and with low reaction times, which makes it perfectly acceptable

from an industrial point of view. Only a few competing methods exist such as the one

described in the next section and the one described by Jerôme et al. [61] (on very

specific synthesized amphiphilic monomers that play the role of surfactant, initiator,

and monomer at the same time).

To conclude this electrochemical part, we delivered very significant

improvement from CE process to CE/diazonium and finally to SEEP. The key point of

13

that improvement was to shift from a direct anionic induction in anhydrous organic

medium to an indirect one in water as solvent. This was done by adding in the

medium only one simple molecule: the diazonium salt. More details on the SEEP

process are given in Lorraine Tessier’s thesis [62].

Working in water under ambient atmosphere allowed us to make also two

other major improvements regarding both the localization of the polymer grafting: (i)

using a gel instead of a classical electrolyte [63] and (ii) using micro-electrodes in a

scanning electrochemical microscope set-up [64]. SEEP was also used to form

micrometer-size hydrophobic/hydrophilic flat patterned surfaces, which may find

applications in various fields including microelectronics and biomedical technology

[65].

However, the previously presented processes, as electrochemical procedures,

are limited to conducting substrates. To open the route towards the grafting of any

type of surfaces from conductors to insulators, a method employing a reducing agent

in solution was developed to activate the diazonium salts.

3. Chemical grafting via chemical redox activation (GraftfastTM)

To extend the SEEP advantages to insulating substrates, an innovative

chemical synthesis method, called GraftfastTM described for the first time by Mévellec

et al.[40], has been recently developed. Indeed, this coating technology is compatible

with materials from glass to metal including Teflon® (PTFE), natural plastics like latex

or rubber, cellulose (wood, paper), artificial or natural fibers, ceramics, nanoparticles,

carbon nanotubes… This extremely simple process leads to stable, homogeneous and

covalently grafted polymer films with controlled thickness. It consists in a short one-

step reaction occurring at atmospheric pressure, ambient air and room temperature

in water (green chemistry).

The GraftfastTM reaction takes inspiration from the SEEP process, just

replacing the electrochemical reduction of aryl diazonium salts by a simple redox

activation with a reducing agent as, for instance, iron powder, hypophosphorous acid

or L-ascorbic acid in absence or presence of a vinylic monomer. Both processes can

lead to the formation of a polyphenylene-like (PNP) layer or to thin polymer films,

strongly grafted on the substrate.

To date, only a few methods comparable to GraftfastTM exist. The closest one is

a bioinspired method based on the self-polymerization of dopamine [66] which can

be followed by a classical polymerization [67]. However, unlike GraftfastTM, the

formation of strong adherent polymer films on the surface of materials leads, a priori,

to physisorbed films, requires a two-step (at least) reaction and involves long

reaction times.

Literally, the name GraftfastTM is employed to describe the anchoring process

mixing a diazonium salt, a reducing agent with or without a vinylic monomer in

solution. Therefore both processes will be discussed in the two following paragraphs.

3.1. Process without vinylic monomer

When L-ascorbic acid (Vitamin C, chosen as reducing agent for its ability to

reduce NBDT [68]) and a gold substrate are introduced in a NBDT water solution, a

polynitrophenylene-like (PNP) film is formed onto the surface as shown in the

IR-ATR spectrum presented in Figure 7. The spectrum exhibits two major absorption

bands at 1525 and 1350 cm-1 attributed, to aryl-NO2 groups. The weak peak at 1600

14

cm-1 is typical of the presence of phenyl groups. XPS analyses confirm the presence of

the PNP film [41].

Figure 7 IR-ATR spectra of PNP layer grafted on gold plates obtained by immersion of the substrates

in a 0.05 M NBDT, VC (1/10 eq) solution for 60 min. Adapted, with permission, from [41]. Copyright

2010 American Chemical Society.

To confirm the radical nature of the intermediates formed in solution, an EPR

study was performed. The EPR protocol is described elsewhere [41]. The key point

was to use a spin trapping technique. Such technique [69], widely applied in

chemistry, biology and medicine, is used for the detection and identification of short

lifetime free radicals. It consists in the addition of, commonly, nitroso or nitrone

compounds, which give rise to stable nitroxide radicals as a result of spin-trapping.

In this study, the spin-trap is the monomer of the 2-methyl-2-nitrosopropane

(MNP) dimer. Solutions of NBDT, ascorbic acid and MNP as spin-trap were prepared

and the EPR spectrum was immediately recorded. A typical EPR spectrum of the MNP

adduct is shown in Figure 8. The simulation reveals that the spin-adduct is coupled to

a nitrogen atom and two pairs of equivalent hydrogen atoms (identified as the ortho-

and meta-protons in the aromatic ring).

It is also possible to resolve an additional splitting due to paramagnetic nuclei

in the para-position in the aromatic ring. Nitrogen and proton hyperfine splittings of

the spin adducts are diagnostic parameters for the identification of the trapped-

radicals. The values found (1.284 for aN, 0.21 (a2H), 0.097 (a2H) and 0.050 (aNNO2))

are in accordance with MNP adducts of nitrophenyl radicals reported in the literature

[70]. Thus, for the first time, the presence of aryl radicals from the reduction of

diazonium salts is demonstrated.

2000 1900 1800 1700 1600 1500 1400 1300 1200

0,992

0,993

0,994

0,995

0,996

υC=C (aryl)

υsNO2

Tra

nsn

itta

nce

Wavenumber (cm-1

)

υas

NO2 Subtrate

Contamination

15

Figure 8 Typical EPR spectrum of MNP adducts of nitro aryl radicals obtained in the case of NBDT

(1 mM) reduction by VC (0.1 mM) in the presence of MNP (excess) and recorded 2 minutes after the

addition of VC. Adapted, with permission, from [41]. Copyright 2010 American Chemical Society.

3.2. Process with vinylic monomer

Experiments, in that case, are performed as previously discussed but with the

addition of a vinylic monomer in the solution. If the monomer is not water-soluble,

GraftfastTM experiments can be carried out with a surfactant (as SEEP) or in organic

solvents with hydroquinone or ferrocene as chemical reducers. The resulting

coatings, their chemical structure and their mechanism of formation are not

presented here because they are almost identical to the ones previously presented for

the SEEP process. We will only describe our EPR study of the reacting solution.

When EPR was carried out as described above, but in presence of hydroxyethyl

methacrylate (HEMA), the global spectrum is similar to the one presented in Figure 8

(diazonium alone), but with an additional component attributed to the spin adduct

formed by the reaction of the poly(HEMA) chain propagation radical with MNP [41].

Hence, EPR undoubtedly demonstrate that the polymerization of the vinylic

monomer proceeds through a radical mechanism, which validates the whole

mechanism fully detailed in part 2 for the SEEP process.

In order to understand the interest of this process, it is important to have an

overview of its potentialities. Among the available methods to functionalize materials

by polymer films, GraftfastTM appeared to be particularly powerful since it

successfully applies to a large variety of materials and, as demonstrated by previous

work [40,41], it enables to control the thickness of the films, the surface properties of

the substrates as well as the localization of the grafting.

3.2.1 Type of materials

The process has been widely applied on conducting or semi-conducting

substrates such as, for example, nickel, zinc, platinum, stainless steel, titanium, gold,

carbon fibers, aluminum (cf. Figure 9 top) and also to insulating materials from glass

to Teflon® (PTFE), including various plastics, cellulose (wood, paper) or cotton. The

process also works on nano-objects such as carpets of multiwalled carbon nanotubes

[38,71] (cf. Figure 9 bottom).

O2N N

O.

1 m T

Figure 9. (Upper panel) Conducting

GraftfastTM process. Some of the grafted films are visible to the naked eye as for

where the films are located on the darkest area of the substrate.

(Lower panel) SEM images of a carpet of carbon nanotubes

magnifications) and c) after grafting. TEM

permission, from [40]. Copyright 2007 American Chemical Society.

3.2.2 Parameters controlled in the process

• Control of the thickness

As demonstrated by Mévellec

characteristic of the grafted polymer

Therefore, thickness of the grafted films can be controlled by

Figure 10 IR-ATR spectra of polyvinylic grafted films on gold plates obtained for increasing time of

immersion of the substrates in a typical

Copyright 2007 American Chemical Society

• Control of the surface properties

One of the main purposes of surface modification is, without any doubt, to

tailor the surface properties of materials. As an example, for some industrial

applications, the production of glass with a hydrophobic surface can be a major issue.

With the GraftfastTM process

the grafting of a hydrophobic polymer (PBMA) as shown in

Conducting and semi-conducting substrates successfully grafted by the

process. Some of the grafted films are visible to the naked eye as for grafting

where the films are located on the darkest area of the substrate.

of a carpet of carbon nanotubes, a) and b) before grafting (different

ons) and c) after grafting. TEM image of the grafted nanotubes in d).

permission, from [40]. Copyright 2007 American Chemical Society.

Parameters controlled in the process

Control of the thickness

by Mévellec et al. [40], the intensity of the

characteristic of the grafted polymer increases with the reaction time

thickness of the grafted films can be controlled by the reaction time

ATR spectra of polyvinylic grafted films on gold plates obtained for increasing time of

immersion of the substrates in a typical GraftfastTM solution. Adapted, with permission,

Copyright 2007 American Chemical Society.

ce properties

One of the main purposes of surface modification is, without any doubt, to

tailor the surface properties of materials. As an example, for some industrial

applications, the production of glass with a hydrophobic surface can be a major issue.

process, this transformation of the glass surface was achieved by

hydrophobic polymer (PBMA) as shown in Figure 11

16

conducting substrates successfully grafted by the

grafting on Ni or Pt

a) and b) before grafting (different

image of the grafted nanotubes in d). Adapted, with

the intensity of the main IR band

with the reaction time (cf Figure 10).

the reaction time.

ATR spectra of polyvinylic grafted films on gold plates obtained for increasing time of

Adapted, with permission, from [40].

One of the main purposes of surface modification is, without any doubt, to

tailor the surface properties of materials. As an example, for some industrial

applications, the production of glass with a hydrophobic surface can be a major issue.

was achieved by

1a. The opposite

17

surface modification consisting in changing a hydrophobic surface into a hydrophilic

one was also successfully carried out on Teflon® modified with PAA (Poly Acrylic

Acid)(cf Figure 11b).

Figure 11 Contact angle measurements of a) glass before and after modification with PBMA and b)

Teflon® before and after modification with PAA. Adapted, with permission, from [40]. Copyright 2007

American Chemical Society.

• Simultaneous and sequential grafting

Thanks to this process, it is also possible to combine the characteristic

properties of two or more monomers by either introducing them simultaneously or

one after the other in solution. In the first case, by adding simultaneously various

monomers, a non-ordered mixture of the two polymers grafted on the substrate can

be obtained, bringing different surface groups with potentially different reactivity or

properties. In the second case, a multilayer-like polymer film is built which combines

the bulk properties of the first polymer introduced and the surface properties of the

last one. The range of applications of such grafted materials is broadened by such

processes.

• Localized grafting

Another important asset of this anchoring process lies in the control of the

localization of the grafting. Indeed, as the radical moieties involved in the process are

prone to graft on any surface, transient masking methods based on poorly adherent

films can be used for preventing the covalent grafting in designated areas of the full

substrate. Figure 12a shows an example of that method, using microcontact printed

alkanethiols SAMs as sacrificial layers. After the GraftfastTM step, the thiol was

removed by sonication in DMF. The thickness profile obtained when crossing two

dark zones via a clear zone is presented Figure 12 b and gives an average coating

thickness of 20 nm. Therefore, using common lift-off techniques in addition to the

GraftfastTM process [65], the localization of polymer grafting was achieved, which

opens the route towards applications requiring patterned surfaces. For further

examples and details on patterned surfaces see Chapter 3.

18

Figure 12 The optical micrograph of a gold plate coated with a thiol mask with triangular patterns

after treatment by GraftfastTM in the presence of HEMA and removal of a mask is shown in a). Dark

zones correspond to the polymer while the clear ones correspond to non-covered gold. The graph (b)

is the AFM profile obtained between covered and non-covered zones schematically represented in

dashed line on a). Reproduced with permission, from reference [65]. Copyright 2011, Elsevier Science

& Technology Journal.

The GraftfastTM anchoring process based on the reduction of diazonium salts

has come from the evolution of the SEEP process towards the grafting of all type of

materials. On top of this remarkable property, the process has shown other very

interesting advantages such as a control of the thickness of the films, of the surface

properties or on the localization of the grafting. These chemically-initiated processes

are very promising in fields such as biomedical and biotechnologies, lubrication,

anti-corrosion and have already been used for various applications for instance:

- cation exchange membranes [72],

- self-adhesive surfaces [73],

- immobilization of DNA and proteins [74],

- effluent treatment [75],

- composite materials [76],

- cosmetics: synthesis of Ti-based modified nanoparticles (filtering both UVB

and UVA) for their use in sunscreen products [77],

- electroless plating onto polymers [78]: offering a chromium free alternative

method applicable for the formation of patterned surfaces and to polymers

usually non-accessible by traditional metallization process.

4. Summary and conclusions

The functionalization of surfaces by strongly grafted polymer films was

presented trough three distinct processes: two electro-induced processes (cathodic

electrografting and surface electroinitiated emulsion polymerization) and a purely

chemical one (GraftfastTM). They offer the possibility to functionalize different types

of material surfaces: semiconductive/conductive for the electrochemical methods but

any type of surfaces in the last case. As discussed in this chapter, those methods

present different advantages and drawbacks. However, all are interesting methods

for surface functionalization by polymer coatings due to a wide versatility of the

processes, high speed reactions and a control of the films thickness. In particular, the

latest developed technique (GraftfastTM) is, to our opinion, the most versatile process

to date and this was achieved thanks to a redox reaction performed with remarkable

coupling agents i.e. diazonium salts.

a) b)

PHEMA PHEMA Gold

19

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