Diazonium salts for surface-confined visible light radical photopolymerization

Article abstract of DOI:10.1002/pola.28241

The surface chemistry of aryl diazonium salts has progressed at a remarkable pace in the last two decades, and opened many avenues in materials science. These compounds are excellent coupling agents for polymers to surfaces via several surface-confined polymerization methods. For the first time, we demonstrate that diazonium salts are efficient for surface initiating radical photopolymerization in the visible light of methyl methacrylate (MMA) and 2-hydroxyethyl methacrylate (HEMA) taken as model monomers. To do so, 4-(dimethylamino)benzenediazonium salt was electroreduced on gold plates or flexible ITO sheets to provide 4-(dimethylamino)phenyl (DMA) hydrogen donor layers; while excited state camphorquinone acted as the free hydrogen abstractor. In the same way, we co-polymerized HEMA and MMA with ethylene glycol dimethacrylate in order to obtain crosslinked polymer grafts. We demonstrate by XPS that gold was efficiently screened by the polymer layers and that the wettability of the surfaces accounts for the hydrophilic or hydrophobic characters of the tethered polymers. Homo- and crosslinked PMMA grafts were found to resist removal by the paint stripper methyl ethyl ketone. The grafted DMA/camphorquinone system operating in the visible light holds great promises in terms of adhesion of in situ designed continuous or patterned polymer coatings on various substrates.

Full text of DOI:10.1002/pola.28241

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Diazonium Salts for Surface-Confined Visible Light Radical
Photopolymerization
1
,2
3
1
1
1
Idriss Bakas, Gorkem Yilmaz, Zouhair Ait-Touchente, Aazdine Lamouri, Philippe Lang,
1
4
1,4
3
Nicolas Battaglini, Benjamin Carbonnier, Mohamed M. Chehimi, Yusuf Yagci
Univ Paris Diderot, Sorbonne Paris Cit eꢀ , ITODYS, UMR CNRS 7086, 15 Rue J-A De Ba €ı f, Paris, 75013, France
1
2
Equipe Mat eꢀ riaux Photocatalyse Et Environnement, Facult eꢀ Des Sciences, Universit eꢀ Ibn Zohr,
B.P. 8106, Cit eꢀ Dakhla, Agadir, Morocco
3
Department of Chemistry, Istanbul Technical University, Maslak, Istanbul 34469, Turkey
4
Universit eꢀ Paris Est, ICMPE (UMR7182), CNRS, UPEC, Thiais 94320, France
Correspondence to: M. M. Chehimi (E-mail: chehimi@icmpe.cnrs.fr) or Y. Yagci (E-mail: yusuf@itu.edu.tr)
Received 31 March 2016; accepted 20 July 2016; published online 00 Month 2016
DOI: 10.1002/pola.28241
ABSTRACT: The surface chemistry of aryl diazonium salts has
progressed at a remarkable pace in the last two decades, and
opened many avenues in materials science. These compounds
are excellent coupling agents for polymers to surfaces via sever-
al surface-confined polymerization methods. For the first time,
we demonstrate that diazonium salts are efficient for surface ini-
tiating radical photopolymerization in the visible light of methyl
methacrylate (MMA) and 2-hydroxyethyl methacrylate (HEMA)
taken as model monomers. To do so, 4-(dimethylamino)benze-
nediazonium salt was electroreduced on gold plates or flexible
ITO sheets to provide 4-(dimethylamino)phenyl (DMA) hydrogen
donor layers; while excited state camphorquinone acted as the
free hydrogen abstractor. In the same way, we co-polymerized
HEMA and MMA with ethylene glycol dimethacrylate in order to
obtain crosslinked polymer grafts. We demonstrate by XPS that
gold was efficiently screened by the polymer layers and that the
wettability of the surfaces accounts for the hydrophilic or hydro-
phobic characters of the tethered polymers. Homo- and cross-
linked PMMA grafts were found to resist removal by the paint
stripper methyl ethyl ketone. The grafted DMA/camphorquinone
system operating in the visible light holds great promises in
terms of adhesion of in situ designed continuous or patterned
polymer coatings on various substrates. V
2016 Wiley Periodi-
C
cals, Inc. J. Polym. Sci., Part A: Polym. Chem. 2016, 00, 000–000
KEYWORDS: aryl diazonium salts; camphorquinone; polymer
grafts; surface-initiated radical photopolymerization; visible
light
INTRODUCTION The surface chemistry of aryl diazonium
possible to design efficient and robust systems for removal of
2
0
21–23
24
salts has progressed at a remarkable pace since the last two
heavy metals, bioactive surfaces,
electrochemical and
25
1
–3
12
decades,
and opened so many new and innovative ave-
optical sensors, and supercapacitors. In these systems,
diazonium salts have a central role as they bridge the support
to the reactive and functional polymers. They appear, thus, as
new items of major importance for the toolbox of the modern
surface and materials chemist; this is testified by the exis-
tence of industrial products such as diazonium-modified drug
eluting stents, carbon blacks and magnetic nanoparticles for
nues in surface science in general and in materials engineer-
ing, in particular. Of relevance to the present paper, these
compounds were demonstrated to be efficient coupling
agents and offered various pathways to graft polymers
through a broad range of substrates using a variety of
surface-confined polymerization methods, namely: atom
4
–7
transfer radical polymerization (ATRP),
reversible addi-
26
theranostics. The success of such multicomponent materials
8
tion–fragmentation chain transfer polymerization (RAFT),
depends on the way polymers are attached. In this regard,
surface-confined polymerization is attractive because it
ensures grafting polymers at relatively high density, approach-
9
,10
radical photopolymerization,
iniferter (initiator-transfer-
1
1,12
13
terminator),
anionic polymerization, electropolymeriza-
1
4–16,
17,18
19
tion,
and chemical
or photochemical
oxidative
2
ing near 1 chain/nm which is actually the case for bridging
polymerization of conjugated monomers.
polymers to surfaces through aryl layers from aryl diazonium
2
7
Through accurate combinations of substrates, diazonium salts,
polymerization methods and functional monomers, it became
salts. In this case, polymer adhesion is very good and the
1
0
layers withstand attack by paint removers.
Additional Supporting Information may be found in the online version of this article.
2016 Wiley Periodicals, Inc.
V
C
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Among the various methods applied to graft polymers on
various substrate surfaces, photochemicals protocols serve
significant advantages due to a wide range of ecological, ener-
2
8–32
getical and financial expectations.
These advantages
include temporal control, low-energy requirements, high rates
of polymerizations at ambient temperatures without the
necessity of using volatile organic solvents giving rise to envi-
ronmental and health-care benefits. There has been a continu-
ous effort for the development of new photoinitiating systems
with visible light sensitivity to fulfil such requirements.
Among them, camphorquinone/amine system is specifically
used in dental applications, as congregates nontoxicity and
high initiating efficiency characteristics under low-energy
2
9
radiations. In this virtue of view, herein we report the prep-
aration of ultrathin polymer grafts on gold surfaces under vis-
ible light irradiation for the first time, by utilizing the
camphorquinone (CQ)/amine as the initiating system.
FIGURE 1 Cyclic voltammogram of a gold-coated glass slide
grafted with 5 mM diazonium salt in acetonitrile 10.1 M
21
In this paper, we shall describe a new route for polymer
grafts through surface-confined radical photopolymerization
using Type II photoinitiating system: DMA groups were elec-
trografted by electrochemical reduction to serve as hydrogen
donors for the visible light excited CQ. Methyl methacrylate
4 4
NBu BF , v 5 0.2 V s . Reference SCE.
in acetone. The mixture was kept in an ice bath for 10 min,
then 1 mL of tert-butylnitrite was added dropwise at 0 8C.
The solution was kept under stirring at 0 8C for 15 min.
Diethyl ether was added to precipitate the diazonium salt
and finally the product was recovered by filtration. The fil-
trate was washed with copious amounts of cold ether.
(
MMA) and 2-hydroxyethyl methacrylate (HEMA) were pho-
topolymerized and the resulting coatings characterized by X-
ray photoelectron spectroscopy (XPS) and polarization
modulation-infrared reflection-absorption spectroscopy (PM-
IRRAS). To account for the stability and robustness of the
coatings, we prepared crosslinked copolymers of MMA or
HEMA and ethylene glycol dimethacrylate (EGDMA) and
dipped the polymer-grafted gold plates in methyl ethyl
ketone (MEK).
21
1
2
1
IR, cm : (N , 2291), (C@C , 1598). HNMR (DMSO), d
Ar
ppm: 8.28 (d, 2H, j 5 8 Hz), 7.07 (d, 2H, j 5 8 Hz), 4.3 (s,
2H), 3.26 (s, 6H).
Electrochemical Grafting of Aryl Layers from the
Diazonium Salt
EXPERIMENTAL
The gold electrodes were ultrasonically rinsed with acetoni-
trile, water and ethanol, then dried in a stream of argon,
cleaned in a UV–ozone cleaner and rinsed with ACN in order
to remove the organic residues from the surface. The electro-
grafting of Au slides was performed with N,N-dimethylbenze-
nediazonium salt (DMA) by scanning 30 times the potential
between 0.2 and 21 V. The reduction peak position obtained
in the first cycle is 2300 mV (Fig. 1).
Chemicals and Materials
Gold-coated glass slides (8 3 15 mm; gold thickness 55 nm)
were purchased from SSens (The Netherlands), and the UV-
ozone cleaner was from Boekel, Model 135500. Flexible elec-
trodes of 300 nm-thick indium-tin-oxide (ITO) coated on
125 mm-thick poly(ethylene naphthalate), PEN, were pur-
chased from Peccel (Kanagawa, Japan). For electrografting,
we used Biologic potentiostat (model SP-150). The mono-
mers methyl methacrylate (MMA, 100.11 g mol , Aldrich),
We note that after one cycle the surface becomes almost pas-
sivated by the grafted aryl groups on the electrode surface.
After 30 cycles, the electroreduction current vanishes and a
quasi-flat response is recorded. The modified surface is not-
ed Au-DMA. After electrografting, the surface was sonicated
in ACN for 3 min, DCM and distilled water and dried under
argon.
2
1
2
1
2-hydroxyethyl methacrylate (HEMA, 130.14
g
mol
,
Aldrich), and ethylene glycol dimethacrylate (EGDMA,
2
1
1
98.22 g mol , Aldrich) were purified by filtering through
basic alumina column to remove the inhibitor, and then
stored in the fridge before use. N,N dimethylaniline (DMA,
Fluka), tert-butylnitrite (Aldrich) and camphorquinone (CQ,
Ciba Specialty Chemicals product) were used as received.
Acetic acid and solvents including acetonitrile (ACN), metha-
nol (MeOH), ethanol (EtOH), dichloromethane (DCM) were
from VWR Prolabo.
Surface-Initiated Photopolymerization (SIPP)
The polymerization reagents (Table 1) were put into a
Schlenk tube (inner diameter 5 9 mm) then the electro-
grafted plates were dipped in the polymerization mixture
and the reaction mixture was degassed under a stream of
argon. The mixture was irradiated using a Ker–Vis blue pho-
toreactor equipped with six lamps (Philips TL-D 18 W) emit-
ting light nominally at 400–500 nm at room temperature for
Synthesis of Diazonium Salt
The diazonium salt was synthesized by adding 1 mL of HBF4
to the solution of 1 g of N, N-dimethyl-4-phenylenediamine
2
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a
TABLE 1 Materials Used for the Preparation of Polymer Grafts Through Aryl Layers
Modified Gold plates
Monomers
HEMA (1.7 mL, 1.4 3 10 mol)
Crosslinker (EGDMA)
–
24
Photosensitizer (CQ)
22
26
Au-PHEMA
1.5 mg, 9 3 10 mol
26
22
Au-P(HEMA/EGDMA)
Au-PMMA
HEMA (1.7 mL, 1.4 3 10 mol)
0.028 g, 1.4 3 10 mol
1.5 mg, 9 3 10 mol
26
22
MMA (1.7 mL, 1.6 3 10 mol)
–
1.5 mg, 9 3 10 mol
26
22
24
0.028 g, 1.4 3 10 mol
Au-P(MMA/EGDMA)
MMA (1.7 mL, 1.6 3 10 mol)
1.5 mg, 9 3 10 mol
a
All polymer grafts were prepared on Au-DMA. For the sake of simplicity, “DMA” has been removed from the abbreviations of polymer-coated gold
plates.
2
1
2
h. After SIPP, the plates were washed in Soxhlet extractor
8 cm by adding 2000 scans with an optical mirror velocity
2
1
for 2 h in methanol to remove the unreacted monomers, and
then washed with ethanol and acetonitrile to remove organic
species.
of 0.474 cm s
.
AFM
Additional preparations of polymer grafts and polymer
patches were conducted in the sun simulator UVA Cube 400
equipped with a mercury lamp (Dr Honle, M u€ nchen, Germa-
ny) on Au-DMA and flexible ITO electrodes grafted with
DMA (ITO Flex-DMA in short). The lamp-sample distance
was 40 cm.
AFM images were recorded with an NT-MDT Solver pro
equipment. AFM topography was performed in the intermit-
tent contact mode with standard silicon cantilevers. Image
3
3
analysis was achieved with the free software WSxM.
XPS Analysis
RESULTS AND DISCUSSION
A Thermo VG Scientific ESCALAB 250 system fitted with a
micro-focused, monochromatic Al Ka X-ray beam (1486.6 eV,
General Protocol of Surface-Confined Radical
5
00 mm spot size) was used to record the spectra. The sam-
Photopolymerization on Aryl-Modified Substrates Using
Camphorquinone as Hydrogen Abstractor
In previous publications we have employed grafted hydrogen
donors from diazonium salts for conducting radical polymer-
ples were stuck on sample holders using conductive double-
sided adhesive tapes and outgassed in the fast entry airlock
2
8
for at least 1 h at 5 3 10 mbar or better. The Avantage
software, version 4.67, was used for digital acquisition and
data processing. The pass energy was set at 40 and 100 eV
for the high resolution and the survey spectra, respectively.
The spectra were calibrated against the C 1s main peak com-
ponent CAC/CAH set at 285 eV.
ization using benzophenone as photosensitizer capable of
10,22,34
abstracting hydrogen from the aryl layers.
In this
work, we electro reduced the structurally designed diazoni-
um salt in order to attach dimethylaminophenyl (DMA)
groups as hydrogen donor species.
Contact Angle Measurements
Water contact angles were determined with a Kr u€ ss DSA
CQ was deliberately selected so as to provide absorption in
the visible range which do not interfere with the absorption
characteristics of the components of the system and damage
already formed chains. Since, upon irradiation the initiating
radicals are formed on the gold surface by hydrogen abstrac-
tion by the excited CQ, it is reasonable to predict that photo-
polymerization will be conducted at the surface gold (here
Au-DMA) according to the mechanisms displayed in Figure 2.
100 instrument (Hamburg, Germany), fitted with a drop
shape analyzer.
PM-IRRAS
PM-IRRAS spectra were recorded with a Nicolet 860 FTIR
(
Thermo- Electron) spectrometer with a resolution of
FIGURE 2 Protocol of surface-confined radical photopolymerization using grafted hydrogen donors from electroreduction of aryl
diazonium salt and camphorquinone as the hydrogen donor. [Color figure can be viewed in the online issue, which is available at
wileyonlinelibrary.com.]
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FIGURE 3 Survey spectra from (a) Au, (b) Au-DMA, (c) Au-PHEMA, (d) Au-P(HEMA/EGDMA), (e) Au-PMMA, (f) Au-P(MMA/
EGDMA).
XPS
features are drastically attenuated and so is the N1s peak
from the DMA layer which is now sandwiched between the
gold electrode and the polymer grafts.
XPS was used to monitor the changes in the surface chemical
composition of gold after electrografting DMA groups and
making polymer grafts photopolymerization. The survey
spectra are displayed in Figure 3. DMA electrografting yields
a dramatic change in the inelastic background of the gold
electrode [Fig. 3(a,b)]. As previously shown, there is a rela-
tive increase in the C1s peak relative to gold features. DMA
has a unique elemental marker, nitrogen, as testified by the
appearance of a N1s peak at ꢀ399.4 eV [Fig. 3(b)] which
accounts for a nitrogen atom in an aromatic amine. After
photopolymerization, the surfaces display sharp C1s and O1s
peaks from the polymer top layers [Fig.3(c–f)]. The gold
For PHEMA grafts, crosslinking permits to attenuate a little
bit more the underlying gold [Fig. 3(d)] as can be concluded
from the total attenuation of Au4d in addition to those of
Au4p and Au4s, whereas for the Au-PMMA and Au-P(MMA/
EGDMA) there are no significant differences between the
spectra displayed in Figure 3(e) and (f).
The fitted narrow regions are displayed in Figure 4. C1s
spectra are displayed in Figure 4(a,b) for Au-PMMA and
Au-PHEMA, respectively. The spectra are fitted with three
4
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FIGURE 4 Peak-fitted C1s (a, b) and O1s (c, d) narrow regions from Au-PMMA (a, c) and Au-PHEMA (b, d). [Color figure can be
viewed in the online issue, which is available at wileyonlinelibrary.com.]
components centred at 285, ꢀ286.5, and ꢀ289 eV assigned to
CAC/CAH, CAO, and OAC@O components, respectively. The
peak components are in the 2.7/1.6/1 ratio for Au-PHEMA and
is the overlayer thickness and k the mean free path of the
photoelectron from the substrate in the top layer (about
2
2
4.1 mg m
for Au4f electrons escaping organic coatings,
2
.7/0.95/1 for Au-PMMA, which accounts for the chemical
which is equal to 4.1 nm if one assumes a density of 1 for
polymers). The homo- and crosslinked PMMA grafts are so
thick that neither Au4f peaks (from the substrate) nor N1s
peak (from the initiator) could be detected. So the Beer–
Lambert equation above would give zero counts for I and
thus an infinite value for the thickness d (d cannot be deter-
mined in this case by XPS). If one considers a rough 10 nm
thickness of the polymer grafts, this corresponds to about
structures of the polymer grafts. Upon crosslinking, the C1s
peak changes slightly for Au-P(HEMA/EDGDMA) only: the
peaks are in the 3.7/1.6/1 ratio, thus indicating an increase in
the contribution of the CAC/CAH bonds.
The fitted O1s regions are displayed in Figure 4 for Au-
PMMA and Au-PHEMA, respectively. For PMMA graft [Fig.
4
from the ester group and centred at 532.2 eV while the
OACH O1s peak is centred at 533.7 eV. The O@C/OAC peak
(c)], the doublet is fitted with two peaks assigned to O@C
2
2
1
0 mg polymer m
substrate. For a HEMA repeat unit
21
molecular weight of 130 g mol , the grafting density would
be 46 repeat units per nm which is in line with the poly-
3
2
component intensity ratio is 1/1.02 matching the expected
meric nature of the graft.
1/1. For Au-PHEMA [Fig. 4(d)], the O1s region is fitted with
three peaks centred at 532.1, 533.0, and 533.8 eV assigned
to O@C, OAH, and OAC bonds. 1/1.06/0.92 matching the
theoretical ratio 1/1/1. It should be noted that no significant
changes were observed in peak fitting the O1s narrow
regions of the crosslinked grafts.
The C/N ratio is 6.6 for Au-DMA, lower than 8 due to the
existence of azo bonds (AN@NA) within the layer. After
polymerization, the C/O ratios sharply decrease to 2.8 for
PMMA graft and 2.2 for PHEMA which accounts for the
chemical structures of the tethered polymers. Similar trends
can be noted for the crosslinked polymers.
The surface elemental compositions are reported in Table 2.
The thickness of the polymer grafts was estimated from the
Beer–Lambert equation applied to Au4f peak intensity:
PM-IRRAS Characterisation
The IR spectra (Figs. 5 and 6) of polymers normalized to the
C@O band, with or without EGDMA are close, in regard to
the low amount of EGDMA added to the two monomers.
Nevertheless some slight differences were observed in the
ꢁ
I 5 I expð2d=kÞ
where I is the intensity of the substrate characteristic peak
after coating and I8 is that of the substrate before coating, d
2
1
CAH stretching region (3000 cm ). For PMMA (Fig. 5) in
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TABLE 2 XPS-Determined Surface Chemical Composition of Untreated and Modified Gold Plates
Eletrografted Surface
Au
N
C
O
Thickness (DMA or Polymer)
Au
60.4
6.73
2.54
0.47
0
–
31.2
73.7
67.0
70.1
73.4
74.0
8.4
Au-DMA
11.2
0.4
0.24
0
8.44
30.0
29.2
26.6
26.0
8.8 nm
Au-PHEMA
4 nm
Au-P(HEMA/EGDMA)
Au-PMMA
11.2 nm
–
–
Au-P(MMA/EGDMA)
0
0
2
1
the presence of EGDMA, the latter seems to be evidenced by
an additional absorption, assigned to the vibration
m (OCH ACH O) and close to the band m (OCAH ) at
The band m(C@O) with PHEMA at 1730 cm has a shoulder
2
1
at 1705 cm due to intramolecular hydrogen bonds CAO—
HAOCH . To summarize, EGDMA is well observed and has
participated to the photopolymerization process.
s
2
2
s
3
2
21
2844 cm present both in the MMA repeat units and those
mixed with EGDMA.
Hydrophilic/Hydrophobic Characteristics of Gold Slides
For the PHEMA, the intensity of this same band at
The hydrophilic/hydrophobic properties of the Au-polymer
21
2
852 cm increases slightly in the presence of EGDMA; in
slides were examined by water contact angles (h ). The
w
21
the same way, the band m (OCH CH O) at 2930 cm
as
2
2
drops were stable from the early stage of contact with the
surfaces. Figure 7 shows qualitatively and quantitatively (see
reported values in Fig. 7) that PHEMA grafts are more
hydrophilic than the Au-PMMA surfaces. The average values
are in line with the literature for PMMA grafts but slightly
higher for PHEMA probably due to a slight contribution of
DMA as gold is detected by XPS through the PHEMA top lay-
er and thus through also DMA. For UV/ozone cleaned Au
increases with EGDMA relatively to that m (CCAH ) at
as
3
2
1
2
950 cm . This suggests that the polymerized EGDMA is
observed in both cases. The absence of band m(C@C) at
2
1
1
640 cm (Fig. 6) shows likely that EGDMA has polymer-
ized with MMA and HEMA monomers. The presence of small
2
1
bands at 3100 cm
m(H AC@C) particularly with PHEMA,
2
could indicate that EGDMA promotes complete polymeriza-
3
5
tion but this is not confirmed by the variation of the
36
plate, the water contact angle was reported to be 35.58
21
absorption within the m(C@C) region at about 1640 cm
.
while for aryl layer on various substrates the contact angle
FIGURE 5 PM-IRRAS spectra of PMMA and PHEMA polymers
on gold substrate with or without EGDMA in the m(CAH)
region; the spectra intensities have been normalized to the
m(C@O) peak intensity. [Color figure can be viewed in the
online issue, which is available at wileyonlinelibrary.com.]
FIGURE 6 PM-IRRAS spectra of PMMA and PHEMA polymers
on gold substrate with or without EGDMA in the 1700–
2
1
21
1000 cm region; zoom in of the 1700 cm region has been
inserted in both spectra. [Color figure can be viewed in the
online issue, which is available at wileyonlinelibrary.com.]
6
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FIGURE 7 Drops of water gently deposited on polymer-coated
Au surfaces.
3
7
is in the 558 to 1008 range. Interestingly, for each polymer
the use of the crosslinker imparts a slight degree of relative
hydrophilicity.
Adhesion Testing for Polymer Grafts
In order to demonstrate the further primacy of the described
process, we interrogate adhesion of polymer grafts to gold in
methyl ethyl ketone (MEK), a well-known powerful solvent
FIGURE 8 Digital photographs of Au-DMA coated with a Teflon
mask (upper panel), and PHEMA patches obtained after simu-
lated sunlight-graft polymerization on Au-DMA (lower panel).
[Color figure can be viewed in the online issue, which is avail-
able at wileyonlinelibrary.com.]
3
8
for polymers, able to swell and remove paints and var-
1
0
nishes. The plates were dipped for 60 min in MEK and
then washed, dried and examined by XPS (see spectra in
Supporting Information SI1). The survey spectra for the Au-
PHEMA before and after dipping in MEK are quasi identical
and account for PHEMA chemical composition. The cross-
linked PHEMA experiences a slight leaching and the Au/C
ratio increases from 0.01 to 0.03 (Table 3). For either homo-
or crosslinked PMMA, the changes are marginal.
Photopolymerization was conducted for 7 min and the plate
was thoroughly washed as indicated for homo- and cross-
linked layers.
Figure 8 shows digital photographs of an Au-DMA plate coat-
ed with the Teflon mask in the small glassy container. Bare
and DMA-grafted gold areas are indicated in the upper panel.
After polymerization, the lower panel shows clear squared,
brown patches from the Au-DMA exposed areas. The bare Au
is obviously not coated due to the absence of any initiator.
The Au-DMA side that has been coated with aluminium foil
retained its gold colour. Therefore, patches were vertically
built only where the simulated sunlight struck the Au-DMA
exposed regions.
These changes induced by the solvent are not significant and
indicate that the new surface photoinitiating system, in the
visible light, is effective in providing polymer grafting with
quasi zero leaching of polymers.
Patterned PHEMA Coating
PHEMA patches were prepared using a Teflon mask. 0.7 mm
wide squares were cut from the mask to obtain windows
through which simulated sunlight strikes the Au-DMA sur-
face in contact with the polymerization solution. The mask
was held with double sided adhesive copper tape. The upper
side of the adhesive was screened with an aluminium foil.
The patches were scanned by XPS. 30 spots were analyzed
with a 200 mm-sized X-ray spot, and the step was 244 mm.
Figure 9 shows 3D image of Au4f spectra. At the extreme
TABLE 3 Au/C Atomic Ratio Determined for Polymer-Coated Surfaces Before and After Dipping in Methyl Ethyl Ketone for 60 Min
Au-PMMA
Au-P(MMA/EGDMA)
Au-PHEMA
Au-P(HEMA/EGDMA)
Pristine
0
0
0.04
0.04
0.01
0.03
Dipped in MEK
0.005
0.0005
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FIGURE 9 3D display of Au 4f spectra taken along the main axis of the slide decorated with two PHEMA patches. X-ray
spot 5 200 mm; displacement 5 244 mm. [Color figure can be viewed in the online issue, which is available at wileyonlinelibrary.
com.]
points the Au4f peak is intense while in between moderate
intense peaks appear where the Au-DMA slide was screened
by the Teflon mask. The XPS analysis over a long distance of
Other Substrate/Polymer System: Flexible ITO Grafted
with PHEMA
So far we have considered gold as a substrate for homopoly-
mer, copolymer, and patterned homopolymer grafts. The
grafted DMA/CQ initiating system is versatile and was
extended to a flexible ITO electrode (ITO-coated PEN sheet,
hereafter ITO Flex). The ITO was electrografted with DMA
prior to photopolymerization in the UVA Cube 400 sun simu-
lator for 7 min in the conditions reported in Table 1.
ꢀ
7.1 mm indicates the possibility of patterning surfaces
with the actual process. The thickness in the middle of the
left hand side PHEMA patch in Figure 8, lower panel, was
estimated to be 23 nm using Beer–Lambert equation.
The patches were imaged by AFM to estimate both the
topography of the polymer and its thickness. We have taken
advantage of the presence of a scratch defect in Figure 10(a)
to measure the thickness. Note however that scratched con-
tinuous PHEMA patches gave similar results at about a thick-
ness of ꢀ30 nm [Fig. 10(b)]. This is in fair agreement with
XPS determined thickness of 23 nm for the left hand side
patch shown in Figure 8, lower panel.
Figure 11(a) shows survey spectra from ITO Flex, ITO Flex-
DMA, and ITO Flex-PHEMA; the latter was prepared using the
sun simulator as a source of visible light. In Figure 11(b), line
scan of the same electrode is displayed and shows survey
[
Fig. 11(b)] and narrow C1s regions [Fig. 11(c)].
Figure 11(a) shows the efficient electrografting process as
N1s appears in the survey spectrum of ITO Flex-DMA and a
concave background appears which accounts for partial
screening of ITO by the DMA grafted layer. The intensity of
the C1s peak increases too compared to those from the
underlying substrate (In4d, In3d, Sn3d, and O1s). After SIPP,
The surface roughness (RMS value) is 22 6 2 nm measured on
a 30 mm 3 30 mm scratch-less area. This value is fairly high, as
the measure integrates the bright protrusions (probably poly-
mer particles) spread out everywhere on the patch itself.
FIGURE 10 AFM topographic image of a PHEMA patch showing densely grafted coating (a). Relative Z heights are plotted versus
2
displacement in (b). Height measurements were determined along the white dotted line in (a). Scanned region: 50 3 50 mm . [Col-
or figure can be viewed in the online issue, which is available at wileyonlinelibrary.com.]
8
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We have scanned the same substrate in XPS and the 15 sur-
vey [Fig. 11(b)] and C1s [Fig. 11(c] spectra are identical, indi-
cating a homogeneous coating over a distance of ꢀ6.3 mm.
The C/O atomic ratio along the scanned line is 2.30 6 0.05
which accounts for an excellent homogeneity of the coating
on flexible ITO. The ITO peaks were absent indicating thick
coating; however on a few spots we could record very noisy
N1s peaks from the underlying DMA initiator layer. Taking
into account a mean free path of about 3.6 nm for N1s travel-
ing through an organic top layer and taking into account the
N1s peak intensity from ITO Flex-DMA and ITO Flex-PHEMA,
we estimated the minimal thickness of PHEMA grafts to
14 nm. For other sites, N1s was not distinguishable from the
noise indicating a much higher thickness.
CONCLUSIONS
Homo- and crosslinked PMMA and poly(2-hydroxyethyl
methacrylate) (HEMA) were grafted to gold through surface-
confined radical photopolymerization in the visible light.
DMA groups were electrografted by electrochemical reduc-
tion to serve as hydrogen donors for photoexcited camphor-
quinone (CQ). The changes in the chemical composition of
gold slides were monitored by XPS and PM-IRRAS whereas
the hydrophilic character was investigated by means of con-
tact angle measurements. Both XPS and PM-IRRAS analyses
indicate a dense layer of the hydrophilic (PHEMA) and
hydrophobic (PMMA) polymers. As proved by AFM, thickness
could reach 30 nm (for PHEMA) under simulated sunlight
polymerization conditions. We have also considered adhesion
aspects of polymer grafts for coated slides dipped in the
very well-known paint remover, methyl ethyl ketone (MEK)
solvent. The grafted DMA/CQ appears as a very efficient pho-
toinitiating system as it provides polymer grafts with very
good adhesion and resistance to leaching in methyl ethyl
ketone solvent. In addition to these conceptual aspects, it is
demonstrated that the new photoinitiating system permits to
obtain patterned polymer coating on gold and that it can be
extended to other substrates as demonstrated with flexible
ITO electrodes. The results obtained so far show that this
new protocol combining diazonium salt and photopolymeri-
zation using CQ as a visible light photosensitizer is a new,
fast, and elegant way to prepare ultrathin functional polymer
grafts with excellent adhesion to various substrates, either as
continuous or patterned coatings.
FIGURE 11 (a) Survey spectra from ITO Flex, ITO Flex-DMA
and ITO Flex-PHEMA; (b) survey spectra taken from 15 points
on ITO Flex-PHEMA; (c) C1s regions taken from 15 points on
ITO Flex-PHEMA. XPS analysis conditions: X-ray spot
5
ACKNOWLEDGMENTS
4
00 mm; step size 5 452 mm; total distance for line scan 5
328 mm. [Color figure can be viewed in the online issue, which
6
All authors are indebted to the ANR for provision of financial
support through the ANR project POLARISafe (No ANR-12-
SECU-011-01).
is available at wileyonlinelibrary.com.]
the survey spectrum displays only sharp C1s and O1s peaks
from PHEMA indicating very efficient attachment of PHEMA
on flexible ITO. The C/O atomic ratio is 2.25 and matches
the expected one for PHEMA. In addition, the C1s from ITO
Flex-PHEMA was fitted with three components as that shown
in Figure 4(b) for Au-PHEMA.
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