Conjugated polymers with pendant iniferter units: Versatile materials for grafting

Article abstract of DOI:10.1021/ma102692h

A novel compound N-(N′,N′- diethyldithiocarbamoylethylamidoethyl)aniline (NDDEAEA) was synthesized and fully characterized. Conjugated poly(NDDEAEA), consisting of N-substituted polyaniline (PANI) backbones with dithiocarbamate ester pendant groups (which

Full text of DOI:10.1021/ma102692h

ARTICLE
pubs.acs.org/Macromolecules
Conjugated Polymers with Pendant Iniferter Units: Versatile Materials
for Grafting
Petya K. Ivanova-Mitseva,† Vasiliki Fragkou,† Dhana Lakshmi,*,† Michael J. Whitcombe,† Frank Davis,†
Antonio Guerreiro,† Joseph A. Crayston,‡ Diana K. Ivanova,† Petar A. Mitsev,§ Elena V. Piletska,† and
Sergey A. Piletsky†
†Cranfield Health, Vincent Building, Cranfield University, Cranfield, Bedfordshire, MK43 0AL, U.K.
‡School of Chemistry, University of St. Andrews, North Haugh, St. Andrews, Fife, KY16 9ST, Scotland, U.K.
§School of Applied Sciences, Cranfield University, Cranfield, Bedfordshire, MK43 0AL, U.K.
S Supporting Information
b
ABSTRACT: A novel compound N-(N0,N0-diethyldithiocarbamoylethy-
lamidoethyl)aniline (NDDEAEA) was synthesized and fully character-
ized. Conjugated poly(NDDEAEA), consisting of N-substituted
polyaniline (PANI) backbones with dithiocarbamate ester pendant groups
(which can act as iniferters), was synthesized by both chemical and
electrochemical polymerization. UV-initiated living polymerization was
utilized to graft styrene, methacrylic acid (MAA), lauryl methacrylate, and
acrylamido-2-methylpropanesulfonic acid (AMPSA) onto the conjugated
macroiniferter which had previously been deposited on various surfaces
(glass, polypropylene, polystyrene, and gold electrodes). The resultant
polymeric surfaces were characterized by static contact angle measurements, XPS, SEM, and AFM. This versatile new material can
be used for creating materials with integrated functionalities (e.g., conductivity, molecular recognition, catalysis and controlled
transport properties, etc.) for application in sensors and microfluidic devices and for the construction of patterned surfaces.
’ INTRODUCTION
dithiocarbamate salt,13,15 by use of a polymer bearing dithiocar-
bamate ester groups,18ꢀ21 by attachment of a surface-reactive
dithiocarbamate ester, such as a silane derivative,16,22 or by
photochemical activation of surface-confined double bonds with
a soluble dithiocarbamate ester.23,24 In this article we report the
application of a novel conjugated polymer with pendant iniferter
groups synthesized from a polyaniline precursor bearing a
dithiocarbamate ester.
Conducting polymers (CPs), especially polyanilines (PANIs),
have received much attention as a result of their promise in
optical and electronic applications such as solar cells, sensors,
and light-emitting diodes.25ꢀ27 The design of new monomers as
a means of introducing functional groups into conjugated
polymers has led to improvements in surface hydrophilicity,
biocompatibility, and adhesion properties. Improvements of this
type allow the polymer films to be used in many potential
applications such as transparent electrodes, chemical and biolo-
gical sensors, organic electrical conductors, optically active
materials, and others by combining the appropriate functional
groups with a conjugated polymer backbone. Recently, we
reported one example in which a conjugated polymer backbone
was combined with the double-bond functionality of a
Surface grafting of polymers is an important process for
introducing interfacial and functional properties to both micro-
scopic and macroscopic objects. Examples include modifying the
hydrophilicity/hydrophobicity, adhesive, electronic, or electrical
properties of the surface and the introduction of features, such as
molecular recognition or fluorescence. In this context, the so-
called “living” polymerization processes1ꢀ3 (atom-transfer radi-
cal polymerization (ATRP),4,5 reversible additionꢀfragmenta-
tion chain transfer (RAFT),6ꢀ9 nitroxide-mediated10,11 and
iniferter-initiated polymerization12ꢀ16) offer a number of advan-
tages. These include control of the grafting process due to the
stepwise addition of monomers, resulting in a more homoge-
neous growth of polymer, and the possibility to prepare block
copolymer architectures by reinitiation in the presence of new
monomers.17 Of these, the use of photochemically activated
iniferters, based on dithiocarbamate esters, is especially conve-
nient for selective grafting to surfaces, since the process can
readily be controlled and confined by the selective application of
long wavelength UV light.
Grafting by any of these methods however requires the
immobilization of a suitable initiator species on the surface to
be grafted. In the case of iniferters, this requires the in situ
formation or attachment of a suitable dithiocarbamate ester. This
has been achieved in the past by polymer analogous reaction of
side-chain residues, such as chloromethylstyrene groups, with a
Received: November 25, 2010
Revised:
February 17, 2011
Published: March 14, 2011
r
2011 American Chemical Society
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Scheme 1. Schematic Representation of the Synthesis of N-(N0,N0-Diethyldithiocarbamoylethylamidoethyl)aniline (NDDEAEA)
(2) from S-(Carboxypropyl)-N,N-diethyldithiocarbamic Acid (1) and Its Polymersa
a Poly(NDDEAEA) (3) formed via electrochemical polymerization or chemical oxidative polymerization giving rise to polyaniline chains with pendant
dithiocarbamate moieties. Addition of monomer, M, by UV grafting giving rise to poly(M)/poly(NDDEAEA) graft copolymers.
methacrylamide group in the form of the novel monomer N-
phenylethylenediamine methacrylamide (NPEDMA).28 NPED-
MA undergoes chemical (oxidative) or electrochemical polym-
erization to form PANI-based materials derivatized with double
bonds. Furthermore, the same material can be utilized as the
basis for the construction of an electrochemical sensor (for
catechol) by grafting a layer of a catalytically active molecularly
imprinted polymer (MIP) over an electropolymerized layer of
poly(NPEDMA), deposited on a gold electrode24 or prepared as
nanotubes.29 The sensors proved to be superior to electrodes
prepared by immobilization of MIP particles of essentially the
same composition at the surface of a screen-printed electrode.
This phenomenon was explained by the conjugated polymer
acting as a “molecular wire” resulting in more intimate contact
between the catalytically active imprint sites and the conjugated
polymer layer than was achievable by immobilization of an
electrically insulating polymer onto an unmodified electrode.
Grafting of the MIP layer was achieved by irradiating the double-
bond-bearing poly(NPEDMA) layer in the presence of N,N-
diethyldithiocarbamic acid benzyl ester. This resulted in the
partial conversion of the pendant double bonds into groups
capable of initiating further polymerization. It would be desirable
however to avoid the use of an additional activation step. This
could be achieved if the conjugated polymer precursor was
already derivatized with dithiocarbamate ester groups, rather
than double bonds. Such a material would allow for the forma-
tion of conjugated PANIs, derivatized with dithiocarbamate ester
groups and capable of direct UV-activated grafting of addition
polymers.
In this work therefore we describe the synthesis, characteriza-
tion, and polymerization behavior of a new bifunctional mono-
mer, N-(N0,N0-diethyldithiocarbamoylethylamidoethyl)aniline
(NDDEAEA) (Scheme 1). NDDEAEA can undergo electropo-
lymerization and chemical oxidative polymerization in a similar
manner to NPEDMA to give a conducting form of polyaniline,
bearing a high density of dithiocarbamate ester pendant groups.
Poly(NDDEAEA) can be formed as thick or thin films, powder,
particles, microparticles, or nanoparticles, all of which can act as a
macroiniferter in UV-activated graft polymerization. In addition,
we report the use of polymers of NDDEAEA as a means of
activating a range of surfaces toward grafting of polymer and
block copolymers and their characterization.
’ EXPERIMENTAL SECTION
Materials. Carbon disulfide (99%) and sodium hydroxide (97%)
were purchased from Acros Organics. 1-(3-(Dimethylamino)propyl)-3-
ethylcarbodiimide hydrochloride (98%) was purchased from Alfa Aeser.
Acrylic acid (99%) and N-phenylethylenediamine (98%) were pur-
chased from Sigma-Aldrich, and diethylamine (98%) was obtained from
Fisher Scientific. All other chemicals and solvents used within this work
were purchased from Sigma-Aldrich and were of analytical grade and
used as received. Milli-Q distilled water was used in all experiments.
Apparatus. NMR measurements were made using a JEOL ECX 400
MHz NMR, and FT-IR spectra were recorded using KBr disks on a
ThermoNicolet Avatar-370 spectrometer (Nicolet, US). For UV irra-
diation, CERMAX xenon arc lamp (Perkin-Elmer Optoelectronics, Inc.)
fiber-optic light source (300 W) or a Philips type HB 171/A self-tanning
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UV lamp, fitted with four CLEO 15 W UVA fluorescent tubes (Philips)
with continuous output in the region 300ꢀ400 nm, delivering 0.09
W cmꢀ2 at a distance of 8 cm, were used. Sessile water contact angle
(CA) measurements were made using a Cam 100 optical angle meter
(KSV Instruments Ltd., Finland) along with the software provided.
Elemental analyses were provided by Medac Ltd., Egham, UK. Optical
densities were recorded using a microplate reader (for microtiter plates)
or a UV spectrophotometer (UV-1800 Schimadzu, Japan). An Autolab
PSTAT-10 instrument (Eco-Chemie BV, Utrecht, Netherlands) was
utilized for all electrochemical experiments. AFM experiments were
performed using a DI3000 AFM (Digital Instruments, Tonawanda, NY)
in tapping mode in air. SEM images were recorded with an FEI XL30
SFEG (scanning field emission gun) microscope. Mass spectra were
obtained using a Waters LCT Premier XE mass spectrometer. X-ray
photoelectron spectroscopy (XPS) measurements were carried out
using a VG ESCAlab (East Grinstead, UK) Mark-2 X-ray photoelectron
spectrometer. The X-ray gun was operated at 14 kV and 20 mA. Spectra
were collected at 20 eV pass energy with Mg KR 1253.6 eV radiation at
an analysis chamber pressure of 10ꢀ9 mbar. Elemental composition was
calculated using peak areas and tabulated atomic sensitivity factors. A
confocal laser scanning 3D color microscope Olympus LEXT (model
OLS3000) was used for the imaged layers of chemically polymerized
NDDEAEA, and a Dektek 3 surface profiler (Veeco, New York) was
used for thickness measurements of grafted films.
Synthesis of N-(N0,N0-Diethyldithiocarbamoylethylami-
doethyl)aniline (NDDEAEA). S-(Carboxypropyl)-N,N-diethyl-
dithiocarbamic acid (CNDDA) (1) was synthesized using a method
adapted from the patent disclosed by Hook et al.30 Acrylic acid (0.5 mol,
36.0 g, 1 equiv), diethylamine (0.5 mol, 36.5 g, 1 equiv), and carbon
disulfide (0.6 mol, 41.5 g, 1.1 equiv) were added dropwise in that order
to a cooled (0 °C) solution of sodium hydroxide (0.5 mol, 20.0 g, 1
equiv) in 200 mL water. The mixture was stirred for 30 min at ambient
temperature and then for 30 min at 60 °C (bath temperature). After
cooling in ice, the solution was acidified with hydrochloric acid to pH
5.5. The oil formed in the reaction solidified on vigorous stirring. This
solid was filtered off and washed well with distilled water. The pale
N(CH2CH3)2). 1H NMR (400 MHz, DMSO-d6, 130 °C): δ 7.51 (1H, s,
NHC6H5), 7.00 (2H, t, J = 7.68 Hz, C6H5), 6.52 (2H, d, J = 7.68 Hz,
C6H5), 6.48 (1H, t, J = 7.34 Hz, C6H5), 5.04 (1H, s, NHCO), 3.88ꢀ3.74
(4H, m, N(CH2CH3)2), 3.40 (2H, t, J = 6.99, CH2S), 3.24ꢀ3.20 (2H,
m, CH2NHCO), 3.20ꢀ3.00 (2H, m, CH2NHC(O)), 2.47 (2H, t, J =
6.99 Hz, NHC(O)CH2), 1.18 ppm (6H, t, N(CH2CH3)2).
13C NMR (100 MHz, DMSO-d6, 25 °C): δ 193.86 (C(S)S), 170.40
(C(O)), 148.57, 128.89, 115.69, 111.91 (C6H5), 49.60 (CH2CH3),
46.80 (CH2CH3), 42.10 (C5H6NHCH2), 39.00 (CH2NHC(O)), 34.30
(NHC(O)CH2), 32.40 (CH2SC(S)), 12.23 (CH2CH3), 11.36 ppm -
(CH2CH3). 13C NMR (100 MHz, DMSO-d6, 130 °C): δ 194.33
(C(S)S), 169.75 (C(O)), 148.18, 128.11, 115.40, 111.77 (C6H5),
48.50 (CH2CH3), 42.10 (C5H6NHCH2), 38.00 (CH2NHC(O)),
34.00 (NHC(O)CH2), 32.00 (CH2SC(S)), 11.21 ppm(N(CH2CH3)2).
(The assignments were based on COSY, DEPT, and HMQC experi-
ments made in DMSO-d6.)
HRMS (ES): Calculated mass for C16H25N3NaOS2 [M þ Na]þ: m/
z: 362.13. Found: 362.14. Elemental analysis (C16H25N3OS2): C, 56.60;
H 7.42; N 12.38, O 4.71, S 18.89. Found 1: C 57.85, H 7.65, N 13.01, S
18.25; Rf = 0.24 (50% ethyl acetate/hexane). All spectra are provided in
the Supporting Information, Figures S1ꢀS5.
Electropolymerization of the Aniline Group of NDDEAEA.
Screen printed gold electrodes (SPE) (4.0 mm diameter, from
Dropsens) were used, with gold working and counter electrodes and
Ag/AgCl reference electrode. Before polymerization each new electrode
was cleaned and pretreated by cycling the potential between 0 and þ0.7
V, 50 mV sꢀ1 scan rate in 1.5 M HCl, five cycles. For the deposition step,
electrodes were cycled (20 times) between ꢀ0.2 and þ0.9 V at a scan
rate of 100 mV sꢀ1 (step potential 7 mV) in a 0.2 M solution of
NDDEAEA in 0.56 M HCl in a 3:1 waterꢀacetonitrile mixture. Stock
solution was prepared by adding 1.25 mL of acetonitrile solution of 0.8
M NDDEAEA into a 5 mL beaker containing 1.87 mL of 1.5 M HCl and
1.9 mL of water. Solutions were mixed well and degassed for 10 min by
purging with argon or nitrogen, and the solution was kept covered with
aluminum foil to protect from light before deposition. CV measure-
ments were performed by placing 50 μL of the test solution of
NDDEAEA onto the surface of the SPE for electropolymerization.
Electropolymerization was carried out under argon and in the dark.
Electropolymerized films were rinsed with deionized water (once), dried
in a stream of nitrogen, and stored dry and in the dark. A blue-green layer
of poly(NDDEAEA) was observed to have formed on the working
electrode.
yellow crystals (30% yield) were dried. UV (acetonitrile): λmax
=
275 nm, (ε = 4280 Mꢀ1 ꢀ1); 334 nm, (ε = 73 Mꢀ1 ꢀ1). Further
L
L
analytical characterization of this material is presented in the Supporting
Information.
A solution of 1 (0.004 mol, 0.922 g, 1 equiv) in 20 mL of anhydrous
acetonitrile was prepared in a dried 50 mL round-bottomed flask, under
argon atmosphere with exclusion of light. To this mixture, N-pheny-
lethylenediamine (0.004 mol, 0.545 g, 1 equiv) and 1-(3-(dimethy-
lamino)propyl)-3-ethylcarbodiimide hydrochloride (0.008 mol, 1.6 g, 2
equiv) were added in that order. After 3 h of stirring, the solvent was
removed in vacuo. The crude mixture was dissolved in 40 mL of ethyl
acetate and extracted five times with 40 mL of distilled water, the organic
layer was dried with anhydrous sodium sulfate and filtered, and the
solvent was evaporated in vacuo to obtain an oily product that solidified
to a white waxy material in 50% overall yield (98% purity by NMR).
The product N-(N0,N0-diethyldithiocarbamoylethylamidoethyl)aniline
(NDDEAEA) (2) (see Scheme 1 and Supporting Information Schemes
S1 and S2) was used without further purification.
Surface-Confined Photografting of Various Addition
Polymers onto Electropolymerized Films of NDDEAEA. An
electropolymerized poly(NDDEAEA)-modified SPE was placed hor-
izontally in a glass vial and the working electrode covered with 50 μL of a
0.1 M solution of unsaturated monomer in acetonitrile under dark
conditions. The monomer solutions had been purged with nitrogen for
10 min to remove oxygen prior to irradiation. The vial was sealed with
Parafilm and continuously purged with nitrogen to maintain an inert
atmosphere over the solution during irradiation. The electrode surface
was then UV-irradiated for 20 min with a fiber-optic light source
(CERMAX xenon arc lamp). The photografted electrodes were then
rinsed in a mixture of 50:50 v/v methanol:deionized water and dried in a
stream of nitrogen. Contact angle measurements were used to char-
acterize changes that had occurred in the functionality and hydropho-
bic/hydrophilic nature of the polymeric film after grafting.
Several polymers were grafted onto poly(NDDEAEA) following the
protocol described above. Solutions (0.1 M) of methacrylic acid,
AMPSA, or styrene were utilized to produce grafted films of the
respective polymers via UV irradiation. As well as grafting single
polymers onto poly(NDDEAEA), layered (block copolymer grafted)
structures, MAA followed by styrene, could also be constructed. For the
layer-by-layer grafting experiment, a poly(MAA) grafted SPE electrode,
Spectroscopic Data (NDDEAEA) (2). IR (KBr): 1647.26
(CdO), 1603.77 ((S)CꢀN), 1508.10 (NꢀC(O)), 1351.55 (CdS),
1268.93 (CꢀNC6H5), 981.92 (CꢀN), 755.80 ((S)CꢀS), 694.92 cmꢀ1
(SꢀCH3).
1H NMR (400 MHz, DMSO-d6, 25 °C): δ 7.96 (1H, t, NHC6H5),
7.01 (2H, t, J = 7.68 Hz, C6H5), 6.51 (2H, d, J = 7.68 Hz, C6H5), 6.30
(1H, t, J = 7.34 Hz, C6H5), 5.49 (1H, t, J = 5.27, NHCO), 3.90ꢀ3.87
(2H, m, CH2CH3), 3.65ꢀ3.61 (2H, m, CH2CH3), 3.35 (2H, t, J = 6.99,
CH2S), 3.20ꢀ3.12 (2H, m, CH2NHC(O)), 3.09ꢀ2.90 (2H, m,
C6H5NHCH2), 2.44 (2H, t, NHC(O)CH2), 1.18ꢀ1.1 ppm (6H, m,
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prepared as described in above, was treated with a solution of styrene
(50 μL of 0.1 M in acetonitrile) and irradiated under the same conditions
as described above for the grafting of polymer layers. The contact angles
of the grafted films were measured and their thickness determined by
Dektek analysis. All surfaces were characterized using SEM and AFM
(Supporting Information Figures S6ꢀS8).
glass slide was then masked with aluminum foil, and UV photografting of
a hydrophobic monomer (styrene) was performed on the unmasked
half. The sample was then rinsed, the styrene-modified area was masked,
and the other half was grafted with a hydrophilic monomer (AMPSA) as
described above. Again contact angles were measured for the coatings.
Surface-Confined Photografting of Various Monomers on
Poly(NDDEAEA)-Modified Microtiter Plates. Poly(NDDEAEA)-
coated polystyrene microtiter plates were placed horizontally in a Petri
dish. A solution of monomer (200 μL, 0.1 M, degassed with nitrogen for
10 min) was placed in each of the wells in which grafting was to be carried
out, in the dark. The Petri dish and microplate were placed inside a sealed
homemade box purged with nitrogen. Monomer solutions were made
up in water (methacrylic acid or AMPSA) or acetonitrile (lauryl
methacrylate). The contents of the box were irradiated for 45 min using
a Philips UV lamp, mounted at a distance of 8 cm from the well bottoms.
The photografted surfaces were rinsed with deionized water and dried in a
stream of nitrogen. Contact angle measurements were performed on the
grafted films. Control experiments were performed in the same manner
using unsubstituted polyaniline in place of the poly(NDDEAEA) films.
Spatially Confined Grafting of Poly(N-(3-aminopropyl)-
methacrylamide) over Chemically Polymerized Poly-
(NDDEEA) Using a TEM Grid as a Photomask. A solution
comprising NDDEAEA (0.08 M), HCl (0.6 M), and APS (0.05 M) in
8 mL of 25% acetonitrile/water was allowed to react for 1 h in the dark.
The resultant polymer formed as a precipitate at the bottom of the vial
was washed several times by the addition and decantation of water. The
washed residue was dissolved in 1 mL of methanol. One drop of this
solution was placed on a precleaned gold-coated glass slide (2 ꢁ 2 cm),
and the solvent was allowed to evaporate to deposit a cast film of
poly(NDDEAEA). A TEM grid (G213, Bal-Tec, 200 mesh copper with
100 μm ꢁ 100 μm square holes) was placed on the surface of the
polymeric film, and then a drop of a previously degassed solution of
N-(3-aminopropyl)methacrylamide (0.43 M in 20% acetonitrileꢀwater
mixture) was placed onto the grid. The grid was covered with a standard
microscope cover glass slide, and the assembly was then placed in a
plastic bag with a nitrogen atmosphere. After 5 min purging with
nitrogen, the assembly was UV irradiated for 20 min with the Philips
UV lamp. The slide was rinsed with water and left to dry for 12 h at room
temperature. Confocal microscopy (10ꢁ magnification) was used to
record 3D images of grafted poly-N-(3-aminopropyl)methacrylamide
structures, before and after removal of the TEM grid.
Polymerization of NDDEAEA by chemical oxidation could also be
used for the formation of thick or thin (transparent) coatings on a range
of supports (polystyrene microtiter plates, cuvettes, and polypropylene
filtration membranes). Experimental parameters were optimized in
order to obtain films of the desired thickness, which could then be used
as substrates in a number of grafting procedures (see Supporting Infor-
mation, method A (Figure S9) and method B (Figures S10ꢀS12) for the
effect of varying the experimental parameter on film growth). Polymer-
ization resulted in the formation of green coatings of poly(NDDEAEA)
on the surfaces of the microplate wells. The washed wells were filled with
dilute acid (0.01 M HCl, pH = 1), and the absorbance was measured at
405 nm. Thin transparent layers (absorbance = 0.17, Figure S9) were
formed under the conditions: NDDEAEA (25 mM), HCl (225 mM)
and ammonium persulfate (18.3 mM), polymerized for 2 h in 25%
acetonitrile:water. Similarly thicker films were obtained (absorbance
(measured in water at 550 nm) = 3.5, Figures S10 and S11) under the
conditions: NDDEAEA (80 mM), HCl (0.6 M), and ammonium
persulfate (50 mM), polymerized for 1 h in 25% acetonitrile:water.
Chemical Oxidative Polymerization of NDDEAEA on Poly-
propylene (PP) Ultrafiltration Membranes (Thinner Films).
PP membranes (with a nominal cutoff pore diameter of 0.2 μm and
thickness of 150 μm, PP 2E-HF, Membrana, Germany) were cut into
small disks of 25 mm diameter. Membranes were pretreated with
methanol for 1 h in a Petri dish. Oxidative polymerization was performed
using ammonium persulfate (APS) under the following conditions:
0.018 M APS, 0.025 M NDDEAEA, 0.225 M HCl, 25% acetonitrile in
water, 1.45 h polymerization in the dark at room temperature. The
reaction mixture was prepared by combining 0.75 mL of NDDEAEA
(0.1 M in acetonitrile) with 0.6 mL of an aqueous solution of APS
(0.0915 M), 675 μL of HCl (0.1 M), and 0.98 mL of water. Polymer-
ization resulted in the deposition of a thin, green layer of functionalized
PANI coating over the membrane surfaces. Poly(NDDEAEA)-coated
PP membranes were washed in three cycles of water, followed by 0.1 M
HCl. The degree of grafting was determined gravimetrically and
expressed as a percentage change in mass. The following equation was
used:
’ RESULTS AND DISCUSSION
DG ð%Þ ¼ ðW1 ꢀ W0Þ=W0 ꢁ 100%
A new monomer, N-(N0,N0-diethyldithiocarbamoylethylami-
doethyl)aniline (NDDEAEA), was prepared by coupling
S-(carboxypropyl)-N,N-diethyldithiocarbamic acid, prepared by
reaction of acrylic acid, CS2, and diethylamine in the presence of
base,30 with N-phenylethylenediamine (Scheme 1). NDDEAEA
Here W0 and W1 represent the weights of the membrane before and after
grafting, respectively. Contact angles were also measured.
Photografting by UV Polymerization of Monomers onto
Chemically Oxidized Poly(NDDEAEA)-Modified PP Mem-
branes and Glass Slides. The preweighed poly(NDDEAEA)-coated
membranes were immersed in a Petri dish containing 3 mL of 0.1 M
monomer solution (methacrylic acid or AMPSA was dissolved in water
and styrene in acetonitrile) which had been degassed by bubbling with
nitrogen for 10 min. The Petri dish was then placed inside a homemade
box supplied with an inlet for continuous flow of nitrogen throughout
the period of polymerization and irradiated (Philips UV lamp) for 45
min. After drying at 45 °C overnight, the degree of grafting was
calculated. Contact angles were measured before and after grafting.
Similarly, chemical oxidative polymerization of NDDEAEA was per-
formed onto glass microscope slides to obtain thicker films The slide was
placed in a 500 mL glass beaker, and 30 mL of a solution containing 0.08
M NDDEAEA, 0.6 M HCl, and 0.05 M APS in 25% acetonitrile/water
was added. The polymerization was allowed to proceed at ambient
temperature for 1 h. The glass slide was then rinsed with water and dried
at ambient temperature for 12 h. Half of the poly(NDDEAEA)-modified
1
was fully characterized (see Supporting Information). The H
NMR spectrum of NDDEAEA, recorded at room temperature
(Figure S1, Supporting Information), showed that some signals
were split due to the existence of two conformers as a result of
hindered rotation about the NꢀC(=S) single bond.21 NMR
spectra recorded at 100 and 130 °C (Figures S2 and S3) showed
coalescence of these signals to single peaks. This can be explained
by overcoming the torsional barrier at higher thermal energy,
resulting in rapid interconversion of the two forms on the NMR
time scale.21
Polymerization of the aniline functionality of NDDEAEA
should result in the formation of a conjugated polymer bearing
a high density of dithiocarbamate ester (iniferter) as pendant side
chains. The ability of polyaniline to form stable coatings on both
hydrophilic and hydrophobic surfaces31ꢀ33 suggests that this
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Figure 1. Cyclic voltammogram obtained during deposition of electropolymerized N-(N0,N0-diethyldithiocarbamoylethylamidoethyl)aniline
(NDDEAEA) films on gold screen-printed electrodes. Electropolymerization of NDDEAEA (0.1 M in 0.75 M HCl/25% acetonitrile/water) on a
screen printed gold electrode by cyclic voltammetry (potential range: ꢀ0.2 to 0.9 V vs Ag/AgCl, at 100 mV sꢀ1 scan rate, 20 sweeps), under a nitrogen
atmosphere and in the dark, giving rise to poly(NDDEAEA).
approach would allow a range of articles with different surface
characteristics to be coated with “living” initiator species. The
conjugated nature of the polymer may also have a number of
advantages in the construction of electrochemical sensors24 and
other electronic devices. While there are examples of styrene- and
methacrylate-based polymers bearing dithiocarbamate side
chains, there are practically no examples of conjugated polymer
backbones derivatized with iniferter groups. The polymeric
macroinifeters of Lutsen and Vanderzande,34 consisting of poly-
(vinylene phenylene) with pendant dithiocarbamate ester side
groups, formed by the partial elimination of dithiocarbamate
ester groups from a precursor polymer are a marked exception.
However, these polymers neither carry a high density of dithio-
carbamate ester groups nor do they have a continuously con-
jugated structure since each remaining iniferter group is a result
of incomplete elimination from the precursor.
The aniline functionality of NDDEAEA has the potential to be
polymerized both electrochemically and using chemical oxidants
such as ammonium persulfate. It was important to ascertain
whether either route could be employed with this monomer and
to show whether the integrity of the dithiocarbamate groups was
compromised by either method of polymerization.
Electropolymerization of NDDEAEA. Electropolymerization
of NDDEAEA was carried out in cyclic voltammetry (CV) mode.
Figure 1 shows the CV obtained during the electropolymeriza-
tion (20 cycles) of a solution of NDDEAEA in a 3:1 v:v mixture
of HCl (0.75 M) and acetonitrile. The CV clearly shows
oxidation and reduction peaks at þ0.65 and þ0.52 V (vs Ag/
AgCl), respectively. The observed electrochemical behavior is
quite similar to that obtained during the polymerization of aniline
and ring-substituted anilines dissolved in acidic solutions.24,35ꢀ39
The large currents observed at the maximum positive potential
are due to the superposition of two distinct processes: one is
electron transfer from poly(NDDEAEA), corresponding to the
oxidation of the PANI film, and the other is electron transfer
from the monomer, NDDEAEA, to the electrode, corresponding
to the oxidation of NDDEAEA to produce a precursor of
polyaniline formation. In order to investigate the stability of
poly(NDDEAEA) films, the electropolymerized electrodes were
dipped into a solution of HCl (0.75 M), and the CV was
measured. The CV showed quasi-reversible peaks: one at
þ0.12 V, which corresponds to oxidation of the leucoemeraldine
to protonated emeraldine form,40 and also a peak at þ0.65 V,
resulting from the oxidation of emeraldine and deprotonation of
the polymer.40 Both these peaks are quite stable. It has been
reported that during the electropolymerization of substituted
anilines in acidic aqueous solutions an intermediate with high
stability is formed.41,42 It is therefore possible to perform both
kinetic and mechanistic studies of the polymerization reaction of
poly(N-alkylaniline) derivatives.
The electrochemistry of poly(N-alkylanilines) is less compli-
cated by comparison with PANI due to the absence of the pH-
sensitive emeraldine base-emeraldine salt transition. Polymeri-
zation of NDDEAEA, which contains both aniline and dithio-
carbamate ester groups, may offer more advantages, provided the
latter are stable under the conditions of polymerization. Dithio-
carbamate anions are unstable under aqueous acidic con-
ditions,43 undergoing protonation and subsequent decomposi-
tion into carbon disulfide and amine, so there was some initial
concern about the stability of the esters under the conditions
necessary to polymerize NDDEAEA. It was also not clear
whether the iniferter groups would be affected by the electrical
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Table 1. Static Water Contact Angles in Air for Various Substrates, Substrates Modified with Poly(NDDEAEA) and after Surface-
Grafting of Polymers
nature of the surface
water contact angle (deg)
pretreated screen-printed gold electrode (SPE)
electropolymerized poly(NDDEAEA)
66.0
29.4
45.1
33.0
64.3
64.4
132.3
85.2
57.6
37.2
72.8
27.9
55.6
88.3
35.3
poly(MAA) surface-grafted to electropolymerized poly(NDDEAEA) film
poly(AMPSA) surface-grafted to electropolymerized poly(NDDEAEA) film
polystyrene surface-grafted to electropolymerized poly(NDDEAEA) film
polystyrene over poly(MAA) surface-grafted to electropolymerized poly(NDDEAEA) film (layer-by-layer grafting)
bare polypropylene membranes
poly(NDDEAEA) grafted layer over PP membrane by chemical oxidative polymerization
poly(MAA) surface-grafted to chemically polymerized poly(NDDEAEA) film onto PP membrane
poly(AMPSA) surface-grafted to chemically polymerized poly(NDDEAEA) film onto PP membrane
polystyrene surface-grafted to chemically polymerized poly(NDDEAEA) film onto PP membrane
glass microscope slides
poly(NDDEAEA) chemically polymerized over glass surface
polystyrene surface-grafted to chemically polymerized poly(NDDEAEA) film onto glass slide
poly(AMPSA) surface-grafted to chemically polymerized poly(NDDEAEA) film
Figure 2. Electropolymerized poly(N-(N0,N0-diethyldithiocarbamoylethylamidoethyl)aniline) (poly(NDDEAEA)) films UV irradiated for 20 min in
the presence of styrene or methacrylic acid (0.1 M, acetonitrile), leading to surface-confined grafted polymer layers. Also the layer-by-layer (block
copolymer) grafting of poly(styrene) over a grafted poly(methacrylic acid) layer is displayed. The static water contact angle shows changes consistent
with the uppermost surface layer of the grafted polymer, such that polystyrene grafted over poly(MAA) shows the same contact angle as a single
polystyrene grafted layer.
potential needed for electropolymerization of the monomer. It
appears however that these fears were unfounded and suitable
conditions were discovered for electropolymerization of
NDDEAEA, in which the integrity of the pendant groups were
preserved (Figure 1). The existence of a high density of iniferter
groups was confirmed by XPS analysis and through grafting
experiments. NMR measurements on NDDEAEA and its poly-
mer also show no significant spectral changes with time under
acidic conditions.
Surface-confined grafting of polymers was readily initiated
upon UV irradiation of electropolymerized poly(NDDEAEA)
layers in the presence of deoxygenated solutions of monomers
such as AMPSA (Table 1), methacrylic acid (MAA), or styrene
(Figure 2). This allowed precise control of the macromolecular
architectures of the grafted surfaces (Figure 2). Grafting was
readily achieved despite the highly colored nature of the polyani-
line backbone, presumably due to the high density of dithiocar-
bamate ester groups available at the surface. XPS analysis and
water contact angle measurements (Table 1) performed before
and after grafting provided evidence that polymerization pro-
ceeded at the surface only during photoirradiation. The results
of XPS measurements (Figure 3 and Table 2) revealed an
appreciable amount of sulfur in the electropolymerized poly-
(NDDEAEA) surfaces even after 20 min of UV irradiation and
that irradiation in the presence of monomers can be used to graft
any other layer onto the poly(NDDEAEA). SEM and AFM
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Figure 3. XPS spectra of electropolymerized film of poly(NDDEAEA) on a gold screen-printed electrode before UV irradiation (solid line) and after
UV irradiation (dotted line).
Table 2. XPS Element Composition (wt %) of the NDDEAEA Monomer and Its Polymers
compound
S
N
O
C
Au
Bi
total
S 2p intensity
N 1s intensity
ratio S/N
NDDEAEA monomer
5
4
3
8
7
7
11
10
13
28
76
79
77
60
100
100
100
100
3.83
3.26
2.332
4.74
4.5
0.80
0.73
0.49
electropolymerized NDDEAEA
UV irradiated electropolymerized NDDEAEA
bare electrode
4.79
7
5
observations showed the surface morphology of grafted surfaces
(Supporting Information Figures S6ꢀS8).
poly(AMPSA) was more hydrophilic (CA of 33.0°) due to the
sulfonic acid moiety. On the other hand, polystyrene grafting led
to increased hydrophobicity with a CA of 64.3°. A contact angle
consistent with grafting of the second monomer (styrene, CA =
64.4°), on top of an initial methacrylic acid grafted layer (CA =
45.1°), was found, as expected (Table 1). The thickness of the
poly(NDDEAEA) layer on the modified SPEs was measured as
∼150ꢀ200 nm, and the grafted poly(methacrylic acid) and
poly(AMPSA) layers were ∼50 nm as revealed by Dektek
analysis.
From SEM and AFM images it is possible to observe grafting
of monomers onto electropolymerized poly(NDDEAEA)
(Supporting Information Figures S6ꢀS8). In general, the
uniformity of the grafted polymer on the modified electrode
surface is usually crucial to control the grafting efficiency. SEM
reveals a globular structure of the pretreated SPE surface which
displays an obvious change (some holes appear) associated with
the electropolymerization of NDDEAEA. After grafting of
hydrophilic monomers, MAA and AMPSA, onto electropoly-
merized poly(NDDEAEA), the films show rather smooth
surfaces and compact film structures compared to the surface
with grafted styrene which displayed some uneven hollow
structures. These results imply that the phase separation and
structural heterogeneity are dependent on the monomer used.
Surface characterizations of the grafted electrodes were also
performed using XPS. Figure 3 shows the XPS spectra of
electropolymerized poly(NDDEAEA) films before and after
UV irradiation.
Grafted polymeric surfaces were characterized by contact
angle measurements. Table 1 shows the static water contact
angles for various grafted polymeric surfaces. Control of the
surface contact angle through changes in the surface function-
ality, while retaining the bulk material properties, is useful for
many applications, including sensors, flow control, and cell
patterning. Layer-by-layer grafting of polymers, as shown in
Figure 2, provides evidence for “living” iniferter-initiated polym-
erization at the poly(NDDEAEA) surface. According to the
literature, initiation with this type of iniferter can result in
undesirable side reactions during photolysis of end groups on
poly(methyl methacrylate) and low reactivity with acrylate ester
monomers, potentially resulting in a less efficient grafting of a
second layer.44,45 However, in our experiments, we have a
confined two-dimensional photopolymerization processes due
to the presence of high density of DTC groups; therefore, the
surface-growing polymer end explores only a two-dimensional
space and grows in a uniform way for the first and second poly-
meric layer. As expected, the contact angle (CA) measurements
demonstrate that the electropolymerized poly(NDDEAEA) was
quite hydrophilic, with CA of 29.4° (probably because it was
deposited from an acidic solution and so exists in a protonated
form). Considering these results together with those from XPS
analysis, photoactivation of the dithiocarbamate ester unit was
confirmed.46 The poly(methacrylic acid) grafted layer was
relatively more hydrophobic with a contact angle of 45.1°, while
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The XPS spectra of NDDEAEA powder and electropolymer-
ized poly(NDDEAEA) films before and after UV irradiation
exhibit N 1s and S 2p peaks due to the presence of the polyaniline
and dithiocarbamyl groups which are of comparable intensities.
In addition, the S 2p (∼174 eV) and N 1s (∼411 eV) peaks are
still observable after electropolymerization and UV-irradiation.
This indicates that a substantial amount of dithiocarbamate ester
groups remained in the surface layer of poly(NDDEAEA) There
is also no significant change in the elemental composition (wt %)
which also provides evidence that the dithiocarbamate ester
groups are still located at the surface.
Figure 4. (a) Polypropylene membrane (left) and poly(N-(N0,N0-
diethyldithiocarbamoylethylamidoethyl)aniline) (poly(NDDEAEA))-
coated membrane (right) and (b) microtiter plate showing deposition
of poly(NDDEAEA) in the wells.
XPS measurements allow the determination of elemental
ratios within the films (Table 2). It was observed that the peak
intensities of NDDEAEA corresponding to sulfur and carbon
(296 eV) did not vary greatly before and after electropolymer-
ization. Upon 20 min UV irradiation of the poly(NDDEAEA)-
coated electrode in 0.75 M HCl solution and subsequent washing
with water, the S 2p peak was still observed, although the
intensity is reduced by about 30% of the initial value. No
appreciable spectral change is observed after extensive washing
with water.
Figure 5. Water contact angle image of chemically polymerized poly-
(N-(N0,N0-diethyldithiocarbamoylethylamido ethyl)aniline) over glass
slide after UV grafting of poly(acrylamido-2-methylpropanesulfonic
acid) (left) and polystyrene (right).
Deposition of Poly(NDDEAEA) Films Using Chemical Oxi-
dation. Polymerization of NDDEAEA can also be achieved using
chemical oxidizing agents, such as persulfate, to deposit a layer of
the conductive polymer onto a range of substrates, such as
microtiter plates, cuvettes, membranes, glass, etc. This layer of
dithiocarbamate ester-functionalized PANI can function as a
vehicle to further coat the material with a layer of grafted addition
polymer in a controlled manner.24 PANIs have previously been
employed in the development of sensor devices by deposition as
thin films over various surfaces followed by immobilization of
various species capable of sensing applications. It is a popular
material due to its good optical properties, its pH and redox
sensitivity, conducting nature, and stability. To obtain poly-
(NDDEAEA) films by chemical oxidative polymerization over
hydrophobic polypropylene membranes and on glass surfaces
(or any suitable substrate), the first step was to optimize the
various deposition parameters within polystyrene cuvettes and
microtiter plates by measuring the optical densities after chemical
oxidative polymerization of NDDEAEA with varying concentra-
tions of monomer, oxidant, pH, time of polymerization, etc. Two
protocols were identified and optimized by variation in the
concentrations of the reagents, according to whether thin
transparent films for optical measurement were to be prepared
or thicker coatings required. The details are presented in the
Supporting Information (see Figures S9ꢀS11). For thin films, a
monomer concentration of 0.025 M in the presence of HCl
(0.225 M) and ammonium persulfate (0.0183 M) for 2 h in
25% acetonitrile/water was judged to be optimum (Figures S9
and S13). Similarly, thicker films for grafting experiments were
obtained at a monomer concentration of 0.08 M in the
presence of HCl (0.6 M) and ammonium persulfate (0.05
M) in 25% acetonitrile/water polymerized for 1 h (Figures S10
and S11).
It was shown that deposition of poly(NDDEAEA) onto solid
surfaces is quite reproducible. IR spectra of the chemically
oxidized poly(NDDEAEA) polymers which had been deposited
onto microtiter plates, scraped off, and redispersed in KBr disks
were obtained (Figure S14) and showed two intense absorption
bands at 1208 and 1491 cmꢀ1, respectively, which are assigned to
stretching vibrations (NꢀC(S)). Other bands characteristic of
N,N-diethyldithiocarbamate were also confirmed by the pre-
sence of absorption bands at 2921, 1375, and 1175 cmꢀ1 (CH3
stretching and bending vibrations and the CdS stretching
vibration, respectively). Also, a peak at 1350 cmꢀ1 was observed
due to bending vibrations of CH2. Other peaks at 1021, 910, and
1452 cmꢀ1 which are also characteristic of dithiocarbamates were
seen. There were peaks at 3430 cmꢀ1 due to NꢀH stretching
vibrations, 700 cmꢀ1 due to aromatic CꢀH bending, 1140 cmꢀ1
due to CꢀN bending, and 1600 and 1480 cmꢀ1 due to the
presence of quinoid and benzenoid rings in the polyaniline
backbone, respectively.21,22,48
The UV spectrum of a solution of poly(NDDEAEA) (1
mg mLꢀ1 in methanol) showed absorption peaks at λmax
=
247 and 275 nm. These were attributed to the diethyldithiocar-
bamate ester group (Figure S15). The spectrum also showed
three characteristic absorption bands due to PANI at around 320,
400, and 735 nm. The characteristic peaks of PANI appear at
about 320 nm due to the πꢀπ* transition of the benzenoid ring
and at about 400ꢀ430 and 730ꢀ825 nm due to polaronꢀπ* and
πꢀpolaron band transitions, respectively, showing that PANI is
present in the doped state.49
A second substrate of choice was hydrophobic polypropylene
microfiltration membranes, onto which oxidative polymerization
of poly(NDDEAEA) was performed which resulted in the
deposition of a thin, bluish-greenish layer of functionalized PANI
coating across the membrane surfaces (see Experimental Sec-
tion, Supporting Information and Figures 4 and S13). Contact
angles were measured (Table 1) and the degree of grafting
determined gravimetrically. PP membranes showed a 13.3%
mass increase for thin films and up to 54.2% for thick film
deposition (Figure 4a).
To determine the effect of pH on contact angles, microtiter
plates coated with thin poly(NDDEAEA) films were treated with
solutions of varying pH for 1 h and their contact angles measured
after drying. Contact angles decreased with decreasing pH; thus,
at pH 12.0 the films were relatively hydrophobic with a contact
angle of 82.2° whereas at pH 1.0 the contact angle was 39.5°,
behavior consistent with that reported in the literature for
polyaniline.47
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Figure 6. Poly(aminopropylmethacrylamide) grafted over poly(N-(N0,N0-diethyldithiocarbamoylethylamidoethyl)aniline) cast film over gold
sputtered glass slide through chemical polymerization: (a) before removal of TEM grid; (b) after removal of TEM grid. Features are of the dimensions
200 μm ꢁ 200 μm.
Further grafting experiments with poly(NDDEAEA) chemi-
cally deposited onto microtiter plates (Figure 4b) showed that
MAA, AMPSA, and lauryl methacrylate could be successfully
grafted (giving rise to contact angles of 52.2°, 30.6°, and 102.5°,
respectively). Control experiments were performed using un-
functionalized polyaniline-coated microtiter plates which did not
show any change in contact angle after irradiation in the presence
of monomers, confirming the necessity of the dithiocarbamate
ester functionality for photografting experiments. Similarly,
photochemical polymerization over poly(NDDEAEA)-modified
PP membranes allowed the growth of various hydrophobic and
hydrophilic polymers (AMPSA, MAA, and styrene) as shown by
changes in the contact angles (Table 1).
Glass slides were also coated with poly(NDDEAEA) using
similar conditions, and the contact angle was measured to
confirm successful polymerization (Table 1). By successively
masking two sides of a coated glass slide (aluminum foil), it was
possible to graft poly(AMPSA) and polystyrene on adjacent
areas of the glass, producing areas with very different contact
angles on the same slide (Figure 5). The measured contact angles
are reported in Table 1.
allowed grafting of further addition polymers or block copoly-
mers. Monomers utilized for grafting were methacrylic acid,
styrene, acrylamido-2-methylpropanesulfonic acid, lauryl metha-
crylate, and 3-aminopropylmethacrylamide. That these polymers
were also capable of reinitiating polymerization upon reapplica-
tion of UV light was confirmed by the grafting of polystyrene
over a poly(MMA) grafted film. UV-initiated surface graft
polymerization was also shown to be capable of producing fine
surface designs via a masking process.
Photochemical grating of various monomers and XPS studies
(before and after polymerization) clearly indicated that the
dithiocarbamate ester groups survived oxidative polymerization
and retain their “living” behavior. This versatile new material has
the potential for the creation of materials with integrated
functionalities (e.g., conductivity and molecular recognition,
catalysis, controlled transport properties, etc.) for application
in sensors, microfluidic devices, and surface patterning. Our
results suggest that NDDEAEA is a versatile monomer for use
within the fabrication of sensors and optoelectronic devices and
in polymer and material chemistry applications.
Grafting of Poly(N-(3-aminopropyl)methacrylamide) over
Chemically Polymerized Poly(NDDEAEA) through a TEM
Grid. Poly(NDDEAEA), cast from solution in methanol onto a
gold-sputtered glass slides, was then grafted with poly(3-
aminopropylmethacrylamide) from a solution of the monomer
in 20% acetonitrile/water via UV-initiation through a TEM grid.
Confocal microscopy clearly confirms the growth of polymer in a
pattern determined by the shape of the TEM grid.
’ ASSOCIATED CONTENT
S
Supporting Information. Experimental details. This ma-
b
terial is available free of charge via the Internet at http://pubs.acs.
org.
’ AUTHOR INFORMATION
Corresponding Author
*Tel þ44 1234 758328; e-mail d.lakshmi.s06@cranfield.ac.uk.
’ CONCLUSIONS
A new bifunctional monomer, N-(N0,N0-diethyldithiocarba-
moylethylamidoethyl)aniline (NDDEAEA), incorporating both
aniline and dithiocarbamate ester groups, was synthesized and
fully characterized. Conjugated poly(NDDEAEA), consisting of
N-substituted PANI backbones bearing a high density of dithio-
carbamate ester groups (iniferter), was produced, by either
electrochemical polymerization or chemical oxidation using
ammonium persulfate. Polymerization of the aniline function-
ality of NDDEAEA was performed on a number of substrates,
namely, polystyrene (cuvettes and microtiter plates), polypro-
pylene filtration membranes, screen-printed gold electrodes, and
glass. UV-activation of the dithiocarbamate ester-based iniferter
groups of these polymeric materials deposited on solid surfaces
’ ACKNOWLEDGMENT
We thank the European Commission for funding this work
(FP7, WaterMIM, CP-FP 226524). Dr. Matthew Kershaw is
gratefully acknowledged for his assistance with XPS and SEM
analysis.
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