Chimeric polymers formed from a monomer capable of free radical, oxidative and electrochemical polymerisation

Article abstract of DOI:10.1039/b821409c

A new monomer, which incorporates both aniline and methacrylamide functional groups, was shown to possess orthogonal polymerisation behaviour to produce conjugated polyaniline suitable for a wide range of applications. The Royal Society of Chemistry 2009.

Full text of DOI:10.1039/b821409c

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Chimeric polymers formed from a monomer capable of free radical,
oxidative and electrochemical polymerisationw
Dhana Lakshmi, Michael J. Whitcombe,* Frank Davis, Iva Chianella, Elena V. Piletska,
Antonio Guerreiro, Sreenath Subrahmanyam, Paula S. Brito, Steven A. Fowler
and Sergey A. Piletsky
Received (in Cambridge, UK) 28th November 2008, Accepted 27th February 2009
First published as an Advance Article on the web 25th March 2009
DOI: 10.1039/b821409c
A new monomer, which incorporates both aniline and methacryl-
amide functional groups, was shown to possess orthogonal
polymerisation behaviour to produce conjugated polyaniline
suitable for a wide range of applications.
also be polymerised electrochemically onto a suitable electrode,
giving a conductive polymer surface with a high density of
double bonds, capable of cross-linking the polymer or being
utilised as reactive sites to enable the grafting of further
polymeric or biological materials. Potential applications may
exist in microelectronics, sensors, electrochromic devices,
biofuel cells, protective and antistatic coatings, catalysts,
photolithographic materials, molecularly imprinted polymers
and in biosensors.
Polyaniline has been widely studied as a conductive organic
polymer. Its advantages include good stability in air and in
the presence of moisture, ease of polymerisation by simple
chemical or electrochemical procedures and good electrical
conductivity.1 Modern methods of synthesis have also been
used to produce polyaniline in a number of forms including
nanowires, nanotubes, films and micropheres.2 A wide range
of potential uses for this material have been proposed and
developed, including electrochromic devices,3 and sensors for
a wide variety of targets.4 One major problem with polyaniline,
however, lies in its relative intractability; solutions of polyaniline
can only be obtained using aggressive and toxic solvents such
as N-methyl pyrrolidinone or by incorporation of bulky
counterions such as camphor sulfonic acid as dopants.5
There have been many attempts to improve the processability
of polyaniline, including the introduction of sulfonic acid
groups, to give self-doped water-soluble polymers6 or by
blending with polyvinyl alcohol7 or anionic polymers such as
polyacrylic acid or polystyrene sulfonate.8 Incorporation of
polyaniline into composites with polyacrylamide has also been
used to enhance polymer stability9 or to modify the mechanical
properties of the material.10 A major problem with blending of
polymers, however, is the possibility of phase separation.
Here we present a new difunctional monomer, N-phenyl-
ethylenediamine methacrylamide (NPEDMA), which contains
both an aniline and a methacrylamide group, capable of
polymerisation by free radical methods.z This allows the
synthesis of homo or co-polymers containing the aniline
moiety which can be subsequently polymerised to yield
polyaniline. Cross-linking agents can also be incorporated,
yielding final materials comprising interpenetrating poly-
methacrylamide–polyaniline networks, where the high degree
of cross-linking prevents phase separation. The monomer can
Some of the various chemical and electrochemical poly-
merisation sequences involving the difunctional monomer are
represented diagrammatically (Fig. 1). The following examples
demonstrate these procedures in detail. At the heart of each of
these possibilities are the unusual properties of NPEDMA
which can be polymerised using free radical, chemical
oxidative and electrochemical polymerisation methods.
Initial studies were made on polymers synthesised by free
radical methods. NPEDMA was polymerised (to PNPEDMA)
using a free radical initiator in toluene or DMF. Cross-linked
polymers were also prepared by the incorporation of ethylene
glycol dimethacrylate in the polymerisation mixture. The solid
Cranfield Health, Vincent Building, Cranfield University, Cranfield,
Bedfordshire, UK MK43 0AL. E-mail: m.whitcombe@cranfield.ac.uk;
Fax: +44 (0)1234 758380; Tel: +44 (0)1234 758329
w Electronic supplementary information (ESI) available: Additional
experimental details, including polymerisation protocols, polymer
compositions, NMR, FTIR and XPS spectra, figure showing effect
of irradiation on electrochemical behaviour of the polymer, AFM,
UV-vis spectrum of polymer. See DOI: 10.1039/b821409c
Fig. 1 Structure of the monomer NPEDMA and its polymers, the
structure and functional properties of which depend on the order of
polymerisation. NPEDMA = N-phenylethylenediamine methacryl-
amide.
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Table 1 Conductivity of the vinyl addition polymers, following
polyaniline formation using ammonium persulfate in HCl at 0 1C,
compared with aniline polymers prepared in the same way at either
0 1C or at room temperature
measurements for PNPEDMA-50 gave a surface area of
14.6 m2 ꢁ1, average pore radius of 143 A and total pore
g
volume of 0.1026 mL gꢁ1 for pores smaller than 731.0 A
before chemical oxidation and a surface area of 38.0 m2 gꢁ1
,
Polymera
Conductivity/S cmꢁ1
average pore radius of 805.6 A and total pore volume of
0.171 mL gꢁ1 for pores smaller than 886.78 A after chemical
oxidation. PNPEDMA-10 was essentially non-porous
but following chemical oxidation had a surface area of
4.3 m2 gꢁ1. Both PNPEDMA and PNPEDMA-5 were non-
porous before and after oxidation. Oxidative cross-linking
appears to increase the porosity, perhaps by reducing the
movement of the polymer chains and thereby increasing the
free volume.
PNPEDMA
PNPEDMA-5
PNPEDMA-10
PNPEDMA-25
PNPEDMA-50
PNPEDMA-OX
Polyaniline (0 1C)
Polyaniline (RT)
6.18
12.66
7.94
4.7
0.83
8.06
6.86
3.54
0.02
Poly(N-phenylethylenediamine) (0 1C)
Poly(NPEDAc) (0 1C)
40.02
Ammonium persulfate is known not only to polymerise
anilines, but also to initiate polymerisation of acrylamide
groups,14 so it is possible that cross-linking occurs within this
system. The FTIR spectra of the monomer and polymer
obtained via polymerisation with ammonium persulfate were
recorded (Fig. S1, ESIw). Bands observed in the spectrum
of the monomer include those for aromatic N–H stretch at
3350 cmꢁ1, aromatic C–H stretch at 2930 cmꢁ1, amide CQO
at 1650 cmꢁ1 and QCH out-of-plane deformation at
930 cmꢁ1. The polymer spectrum contains bands corresponding
to most of the vibrational modes associated with conductive
a
See Table S1, ESIw.
polymers, both the homopolymer (PNPEDMA) and those
cross-linked with ethylene glycol dimethacrylate, were oxidised
using ammonium persulfate to form conductive materials
(labelled as PNPEDMA-x where x is the percentage of
cross-linker), see Table S1 (ESIw). Direct oxidation of the
monomer also led to formation of a conductive material
(PNPEDMA-OX). The conductivities of the synthesised
materials are given in Table 1. Oxidative polymerisation of
NPEDMA gave a polymer with a higher conductivity than
polyaniline itself synthesised under similar conditions; this was
not true for either poly(N-phenyl ethylene diamine) or for the
equivalent acetylated material PNPEDAc (Table 1). Initially it
was expected that PNPEDMA would have a lower conductivity
due to the presence of the N-substituted sidechain, as was
found for polymers of N-methyl aniline11,12 and for the
polymer of N-phenylethylenediamine; however, this does not
appear to be the case. Material prepared by oxidation of the
linear polymer was slightly less conductive than polyaniline,
but the incorporation of 5% of ethylene glycol dimethacrylate
(EGDMA) led to a two-fold increase in conductivity. Again a
simplistic model would predict a drop in overall conductivity
due to the presence of a non-conjugated co-monomer. At 10%
EGDMA the conductivity was slightly higher than polyaniline,
although further increases in cross-linking led to a reduction of
the conductivity. Possibly in the former case cross-linking of
the polymer led to a closer association between polymer
chains within the network, facilitating charge transport
through the polymer. Previous work has already shown that
the physical morphology of polyaniline has a great effect on its
conductivity.12,13 Cross-linking could possibly affect the
crystallinity and orientation of the polyaniline chains, leading
to the observed increases in conductivity; however, since
higher levels of cross-linking lead to a decrease in conductivity,
this hypothesis may not be the most likely explanation for this
phenomenon.
polyaniline between 2800 and 2200 cmꢁ1 and at 1150 cmꢁ1
.
A
band associated with CH out-of-plane bending of
1,4-disubstituted aromatic rings is present at 830 cmꢁ1
;
however, there was only a minimal change in the amide
CQO band, indicating no loss of conjugation due to
polymerisation of the CQC bond. This indicates that during
the polymerisation with persulfate the reaction progressed
preferentially via aniline oxidation.
As an alternative to chemical polymerisation methods, the
ability of NPEDMA to electropolymerise and to perform
subsequent reactions using free radical addition or poly-
merisation was investigated. After voltammetric sweeping of
the gold or indium tin oxide (ITO) electrode in a solution of
NPEDMA, a greenish film was clearly visible, indicating
deposition of a conjugated polymer. Fig. 2 shows the CV
obtained during the electropolymerisation (sweep number 15)
of NPEDMA in 50 mM HClO4 solution, clearly displaying
oxidation and reduction peaks at +0.7 and +0.45 V (on Au
Porosities were determined using nitrogen BET measurements.
The polymer PNPEDMA-25 showed the highest porosity,
with a surface area of 22.8 m2 ꢁ1, average pore radius of
g
225.3 A and total pore volume of 0.1434 mL gꢁ1 for pores
smaller than 705.1 A, which after oxidation with persulfate
gave a surface area of 48.7 m2 ꢁ1, average pore radius of
g
Fig. 2 Cyclic voltammograms obtained during the deposition of
electropolymerised N-phenylethylenediamine methacrylamide films
on gold (solid line) and indium tin oxide (dashed line) electrodes.
529.4 A and total pore volume of 0.129 mL gꢁ1 for pores
smaller than 700.25 A (at P/P0 = 0.98608). The corresponding
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substrate), respectively (versus Ag/AgCl). Ellipsometry
measurements indicated a film thickness of 40 nm. The
polymer film exhibited a sessile water contact angle of 691
(unmodified gold electrodes gave contact angles of 771)
showing that the polymer is relatively hydrophilic.
The authors would like to thank Yildiz Uludag for the
ellipsometry results. This project was supported by the
award of a Royal Society International Incoming Fellowship
(to D.L.), Grant Reference: FI071114.
Notes and references
To demonstrate the possibility of further modification,
the double bonds of electrosynthesised PNPEDMA were
cross-linked using N,N-diethyldithiocarbamic acid benzyl
ester (iniferter) as initiator, as described in the ESIw. The
contact angle of the cross-linked material increased to 711,
indicating that grafting of hydrophobic moieties had occurred.
X-Ray photoelectron spectroscopy (XPS) measurements of
washed polymer films demonstrated the presence of sulfur
following iniferter grafting (see ESIw, Fig. S2 and S3).
Grafting of poly(methacrylic acid) brushes to the iniferter-
activated film (see ESIw) gave a contact angle of 621; in
addition the grafting of molecularly imprinted polymers to
the PNPEDMA layer has been carried out and will be
reported elsewhere. An electrode coated with an iniferter-
treated electropolymerised film was immersed in a control
solution (50 mM HClO4 solution, purged with nitrogen for
10 minutes, then irradiated for 20 minutes) to investigate the
effect of UV exposure on the electrochemical behaviour of the
film. Electrochemical characterisation was carried out using
differential pulse voltammetry and showed a noticeable in-
crease in the current–voltage characteristics with increased
irradiation (Fig. S4, ESIw). The reason for this behaviour is
not known; however, it is highly likely that the iniferter grafted
to the electropolymerised film via the vinyl moieties produces
macroradicals upon irradiation, which can combine to form
additional cross-links between neighbouring polymer chains.
As was seen in the previous case, increased cross-linking
facilitates charge transport through the film, resulting in a
higher measured conductivity.
z Synthesis of NPEDMA: N-phenylethylenediamine (1.0 g, 0.96 mL,
7.3 mmol) was dissolved in methanol (20 mL) which was cooled in ice
before the addition of methacrylic anhydride (1.1 g, 1.06 mL,
7.1 mmol). The stirred mixture was held at 0 1C for 3 h before
warming to room temperature. The solvent was removed using a
rotary evaporator and the residue dispersed in diethyl ether (25 mL).
The ether phase was washed with 0.1 M NaOH (4 ꢃ 25 mL), followed
by water (1 ꢃ 25 mL), dried over anhydrous magnesium sulfate and
evaporated to give a brown oil which formed colourless crystals on
standing in a refrigerator. The yield was 99%; 1H NMR (400 MHz,
CDCl3, d): 1.9 (s, 3H, –CH3), 3.3 (d, 2H, –CH2), 3.7 (d, 2H, –CH2), 5.3
(s, 1H,QCH2), 5.7 (s, 1H,QCH2), 6.6 (d, 2H, o-H), 6.7 (t, 1H, p-H),
7.2 (m, 2H, m-H); 13C NMR (100 MHz, CDCl3, d): 18.6, 39.3, 43.9,
112.7, 117.7, 119.9, 129.3, 139.7 147.9, 169.0; IR (KBr disc): 3345,
3000, 2920, 1655, 1603, 1514, 1322, 750, 694 cmꢁ1
.
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polymerised layer of NPEDMA (Fig. S5, ESIw). As can be
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leads to deposition of polymer on the surface, smoothing out
many of these features, but with a much greater average
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‘‘bumps’’ in the polymer film. A UV-vis spectrum of a
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This monomer adds to the range of orthogonally poly-
merisable bifunctional monomers, for example the ‘‘Jekyll
and Hyde’’ monomer glycidyl methacrylate,16 and others.17
These can be exploited via the differential reactivity of
the two monomer species. Another interesting application
of unsymmetrical monomers is their ability to form
OMNiMIPs.18
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This journal is The Royal Society of Chemistry 2009
Chem. Commun., 2009, 2759–2761 | 2761