Catalyst-free synthesis of oligoanilines and polyaniline nanofibers using H2O2

Article abstract of DOI:10.1021/ja905014e

(Chemical Equation Presented) Nanofibers of polyaniline and oligoanilines of controlled molecular weight, e.g., tetraaniline, octaaniline, and hexadecaaniline, are synthesized using a versatile high ionic strength aqueous system that permits the use of H<

Full text of DOI:10.1021/ja905014e

Published on Web 08/17/2009
Catalyst-Free Synthesis of Oligoanilines and Polyaniline Nanofibers
Using H2O2
Sumedh P. Surwade, Srikanth Rao Agnihotra, Vineet Dua, Neha Manohar, Sujit Jain,
Srikanth Ammu, and Sanjeev K. Manohar*
Department of Chemical Engineering, UniVersity of Massachusetts Lowell, Lowell, Massachusetts 01854
Received June 18, 2009; E-mail: sanjeev_manohar@uml.edu
Bulk quantities of polyaniline nanofibers and oligoanilines of
controlled molecular weight, e.g., aniline tetramer, octamer, and
hexadecamer, are obtained in one step using a simple and versatile
high ionic strength aqueous system (HCl/NaCl) that permits the
use of pure H2O2 as a mild oxidant. Polyaniline nanofibers are
highly conducting, σ ∼1-5 S/cm, and spectroscopically similar to
conventional polyaniline synthesized using stronger oxidants.
Solution cast films of oligoanilines on flexible plastic substrates
are used as chemiresistors to reversibly detect chemically aggressive
vapors like NO2 in the 100-5 ppm concentration range in ambient
air.
Polyaniline is typically synthesized by oxidative polymerization
of aniline in dilute acids using peroxydisulfate oxidant using a
variety of approaches.1 The reaction is characterized by an induction
period followed by bulk precipitation of a dark-green electrically
conducting powder called emeraldine salt. Nanofibers can be
obtained by altering the reaction conditions during the induction
period using a variety of methods.2 There are many recent attempts
to synthesize polyaniline using milder oxidizing agents, e.g., benzoyl
peroxide and metal/enzyme catalyzed H2O2.3 Although nanofibers
have been obtained in some Fe2+/H2O2 systems,3f in many instances
there is loss of nanofiber morphology and a significant reduction
in bulk conductivity as a result of defects introduced along the
polymer backbone (∼10-2 S/cm).3 Pure H2O2 is considered to be
too weak to oxidatively polymerize aniline to polyaniline; i.e., at
room temperature there is no observable reaction, and at higher
temperatures the reaction yields a dark brown insulating solid. In
this study, we demonstrate that by simply carrying out the reaction
at high ionic strength, H2O2 can be used as an effective oxidant to
achieve the desired linear head-to-tail coupling of aniline units
resulting in polyaniline that is not only highly conducting but also
composed entirely of a nonwoven mesh of nanofibers. Importantly,
the high ionic strength system is used to synthesize oligoanilines
having preselected molecular weights that show very interesting
sensor properties.
Figure 1. Polyaniline synthesized using the H2O2/HCl/NaCl system: (A)
SEM image of as-synthesized polyaniline nanofibers. Inset: Free-standing
film of emeraldine base cast from solution in NMP. (B) Potential-time
profile of aniline polymerization using H2O2 (solid line) and (NH4)2S2O8
(dotted line). Inset: Cyclic voltammogram in aq. 1.0 M HCl, SCE reference.
(C) UV-vis spectrum in NMP solvent. (D) Reaction scheme.
such as the walls of the reaction flask, etc.4 Under the right
conditions, aniline dimer spontaneously dimerizes to aniline tetramer
nanofibers that subsequently act as seeds that promote bulk
nanofiber formation. We believe the high ionic strength in the H2O2/
HCl/NaCl system favors ionic over free radical pathways, e.g.,
radical cation coupling (head-to-tail) to yield aniline dimer over
free radical pathways that would yield undesired low molecular
weight products. Suppressing free radical pathways is particularly
important when employing H2O2 because of the myriad pathways
available that would not yield polyaniline. Therefore, the high ionic
strength in the reaction simultaneously promotes linear chain growth
and fibrillar morphology while eliminating the need for added
catalysts to activate H2O2.
Polyaniline nanofibers are synthesized using the H2O2/HCl/
NaCl system by adding 2.0 mL of aniline to 100 mL of aqueous
1.0 M HCl saturated with NaCl followed by addition of 5.0 mL
of aqueous 35 wt % H2O2. The reaction is initiated by heating
gently to ∼50 °C and cooling to room temperature (see
Supporting Information (SI) for details). After ∼2 h the dark
green emeraldine salt (∼200 mg) is filtered, washed, and dried
under dynamic vacuum (σRT ∼1-5 S/cm, pressed pellet).
The scanning electron microscopy (SEM) image of the dark-
green emeraldine salt shows that it is composed entirely of
micrometers long, 60-80 nm diameter nanofibers (Figure 1A).
Nanofibers are obtained without the aid of added seed fibers,
oligomers, and other templates, and even when the ionic strength
is high. We have recently shown that nanofiber formation in
polyaniline is governed by aniline dimer formation on inert surfaces,
Potential-time profiling shows that the H2O2/HCl/NaCl system
does not proceed via the intermediacy of polyaniline in the highly
oxidized pernigraniline oxidation state that is typical of the classical
1
2-
b
S2O8 system (Figure 1B). The gradual fall in potential during
the course of the reaction suggests that only the chain ends are
oxidized and couple with new aniline units while the main chain
remains in the half-oxidized emeraldine oxidation state.3b The new
polyaniline is spectroscopically and electrochemically similar
to conventional polyaniline (Figure 1C, SI). Free-standing films
(σ ∼1 S/cm after HCl doping) are readily obtained from NMP
solutions which can be made more robust when a small amount of
epoxidized linseed oil is used as a biodegradable plasticizer (Figure
1A, inset).
The H2O2/HCl/NaCl system can also be used to synthesize aniline
tetramer, octamer, and hexadecamer (Figure 2). For example, when
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12528 J. AM. CHEM. SOC. 2009, 131, 12528–12529
10.1021/ja905014e CCC: $40.75  2009 American Chemical Society

C O M M U N I C A T I O N S
Figure 3. Plot of percent change in resistance vs time of (A) aniline octamer
and (B) aniline hexadecamer, when exposed to NO2 vapor (N2 carrier gas).
Vapor concn values indicated at the troughs. Insets: corresponding plots of
percent change in resistance vs vapor concn.
irradiated using a 254 nm UV light source (Figure 3). Oligoaniline
films synthesized previously using other oxidants behave similarly.
The fairly high detection limit (>5 ppm) and the difference in sensor
response between octa- and hexadecaaniline described in Figure 3
could be because film thickness and geometry are not optimized
(see SI). Although NO2 vapor is highly reactive and would be
expected to oxidize (and/or degrade) thin oligoaniline films, it is
clear that this is not taking place in the time frame of our sensor
experiments; e.g., there is no change in the UV-vis spectra before
and after a series of vapor exposure/UV irradiation cycles (SI). The
resistance change is irreversible without UV irradiation suggesting
that NO2 vapor is strongly physisorbed on the film and could
irreversibly react with the oligoaniline backbone if given sufficient
time.
In summary, we demonstrate for the first time (i) a one-step
synthesis of bulk quantities of polyaniline nanofibers using H2O2
oxidant without added catalysts; (ii) a one-step synthesis of
oligoanilines of controlled molecular weight using H2O2; (iii) a
versatile, aqueous high ionic strength system (HCl/NaCl) that
permits the use of H2O2 as a mild oxidant; and (iv) chemiresistors
made of thin oligoaniline films drop-cast on rugged and flexible
plastic substrates to reversibly detect NO2 vapor.
Figure 2. Aniline oligomers synthesized using H2O2/HCl/NaCl system:
(A) SEM image of aniline tetramer nanofibers synthesized from aniline
dimer. (B) MALDI-TOF mass spectrometry; see SI for fragmentation
pattern. (C) Reaction scheme (tetramer and octamer only).
an insoluble aniline dimer is added to the above system a
spontaneous reaction occurs resulting in the formation of microme-
ters long nanofibers of aniline tetramer in the emeraldine oxidation
state (σ ∼10-2 S/cm). We have recently reported this unusual solid-
2-
state reaction in the aniline/S2O8 system and believe this to be
the key to bulk nanofiber formation.4 The reaction stops at the
tetramer stage but can be extended to the octamer stage by reducing
aniline tetramer to the leucoemeraldine oxidation state using
ascorbic acid and repeating the reaction (Figure 2, scheme). Aniline
octamer is the sole product and is easily isolated (σ ∼10-2 S/cm).
This is a solid-state reaction that takes place in an aqueous
dispersion and is unusual in that tetramer units have to couple
specifically at the chain ends which requires a significant degree
of chain mobility. It is unclear at the present time how this occurs,
although we could not extend the reaction to the hexadecamer
suggesting that chain mobility is too low at the octamer stage.
Surprisingly, aniline hexadecamer is the sole product when a 1:10
mixture of aniline tetramer and aniline monomer is added to the
H2O2/HCl/NaCl system indicative of the versatility of the system
(see SI). It is to be noted that aniline oligomers have been obtained
using FeCl3/HCl and S2O82-/HCl,5 and the present study highlights
the use of H2O2/HCl/NaCl as an environmentally friendly, mild,
and highly versatile oxidizing system that can be used to affect a
variety of transformations.
Acknowledgment. Funding from the UMass Lowell, MTC-
funded NCOE, and NSF-funded CHN, NSF Award No. 0425826.
Supporting Information Available: Synthesis procedure, charac-
terization (UV-vis, FTIR, NMR, aq. electrochemistry), sensor fabrica-
tion. This material is available free of charge via the Internet at http://
pubs.acs.org.
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Aniline oligomers synthesized previously have been used to
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Cl2, etc. This could be because corresponding sensors made of films
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