 J. Hot Spring Sci. 0* +-.ῌ+.. ,*+*ῌ῎Synthesis of Polyaniline in Volcanic Hot Spring Water
Hiromasa GOTO+
(Received Apr. 0, ,*+*, Accepted Jul. ,*, ,*+*

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QR SEM ">+ ESRAbstract
The conducting polymer polyaniline (PANI) was synthesized in natural volcanic hot
spring water and its chemical structure was conﬁrmed by infrared absorption spectroscopy. The PANI has electrical conductivity in the same range as that of semiconductors.
The moderate acidity of volcanic hot spring water provides a favorable pH condition forpolymerization of the aniline monomer. Scanning electron microscopy showed that thepolymer has a nanoﬁber structure, demonstrating the synthesis of conductive nanoﬁber.
This may be a new approach for the synthesis of organic electronics by employing natural resources.
Key words : Acidity, Conducting polymer, Nanoﬁber, Natural hot spring water, Polyaniline, Scanning electron microscopy (SEM), Electron spin resonance (ESR)

+

134

Graduate School of Pure and Applied Sciences, Institute of Materials Science, University ofTsukuba, Tsukuba, Ibaraki -*/ῌ2/1-, Japan. +  ῌ -*/ῌ2/1-  +ῌ+ῌ+ 0*  ,*+*

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Introduction

Since the discovery of conducting polymers, plastic electronics technology has beendeveloped based on the synthesis and processing of conducting polymers. In particular,
polyaniline (PANI), as one of the most promising organic conducting polymers (MacDiarmidand Epstein, +33. ; Kuramoto and Tomita, +331 ; Streltsov et al., ,**3 ; Nasybulin et al., ,**3 ;Immonen et al., ,**3 ; Wang J. et al., ,*+* ; Wang C.-C. et al., ,**3 ; Wei et al., ,*+*), has been
applied in industry as a bu#er layer in electroluminescence (EL) devices, sensors (Janta andJosowicz, ,**-) conductors (Ram et al., +33. ; Raghavendra et al., ,**-), solar cells (Tan et al.,,**.), for anti-corrosion (Lu et al., +33/), and in composites (Mallick et al., ,**1 ; Aengin and
Kalayci, ,*+* ; Zngin et al., ,**,) and textiles (Wu et al., ,**3). PANI is generally prepared ina water medium with ammonium persulfate (APS) as an oxidizer. Aniline is treated with sulfuric acid for the formation of aniline sulfuric salt prior to addition of APS to initiate polymerization. Formation of the aniline salt is required for the synthesis of PANI.
Volcanic hot spring water (VW), as commonly found in hot springs, has been familiar topeople in the world since ancient times. VW contains various minerals such as Mg, Ca, Zn, Al,
Sr, Na, Li, Mg, Zn in the form of ions. Although the composition is dependent on the district,
VW generally contain ions such as SO.,, Cl, and HSO., and the acidity of VW varies andis also location-dependent.
Aniline forms the aniline salt by reaction with sulfuric acid. The aniline salt can beconverted into conducting polymer PANI by an oxidative polymerization mechanism. VW
with low pH and sulfonate ions can be useful in the polymerization of aniline rather thanemploy sulfuric or hydrochloric acids. In this study, the synthesis of PANI in volcanic hot
spring water was attempted, and the resulting chemical structure, generation of charge carriers, electrochemical properties, and optical properties were investigated. This method may
provide a new objective for the application of natural resources in plastic electronics.

,.
,. +

Experimental

Materials
Aniline monomer (Wako Chemicals, Japan) was puriﬁed by distillation prior to use. VWfrom Owakudani hot spring in Kanagawa (Japan) was employed in this research. The sample
water was obtained on Feb. +0, ,**3. APS (Kanto Chemical, Japan) was used as received. An
analysis report of the VW is published via internet (Tsujiuchi, +333). Analysis data, applicant
for the analysis : Tsujiuchi, Owakudani hot spring water, 0..1, pH,.3, electrical conductivity(s),,11* mS/cm, total amount of constituents+,*+- mg/kg, main cation (Ca,, +*. mg/kg ;Na, .-.+ mg/kg ; Mg,, -3.* mg/kg), main anion (SO.,, ..1 mg/kg ; Cl, +,. mg/kg ; HSO.,+2.0 mg/kg), type of hot spring source : acid-calciumῌmagnesium-sulfuric salt-chloride fountain, analysis date : July ,3, +333. Multi-channel type induced plasma coupling (ICP) analysis
was carried out at the analysis center of University of Tsukuba (May ,1, ,*+*). The analysis
revealed that the water contains Li, Be, B, Na, Mg, Al, Si, S, Ca, Mn, Fe, Zn, Sr, and Ba.

135



,. ,

Technique
Ultraviolet-visible (UV/Vis ; Hitachi U-,***) absorption spectra of the polymers in N-

methylpyrrolidone (NMP) were measured. Infrared (IR) absorption spectra (Jasco FT-IR/-**)of the polymers were obtained using the KBr method. Electron spin resonance (ESR) measurements of solid polymer samples were conducted using a JEOL JES TE-,** spectrometer with+** kHz modulations (X-band). The spin concentration was determined using CuSO.ῌ/H,O asa standard. ICP analysis was carried out with PerkinElmer, Optima 1-**DV. Scanning electron microscopy (SEM) observations were performed with a JEOL ED-SEM. The electricalconductivity of the polymers was measured at room temperature using the DC four-pointprobe method (Mitsubishi Lowrester-GP MCTP-T0+* with MCP-TP*0P probe).
,. -Polymerization
We performed a series of six polymerizations in the VW with concentration variation ofaniline (monomer) for obtaining PANI-+, PANI-,, PANI--, PANI-., PANI-/, and PANI-0 to
examine reactivity of aniline in the VW and dependence of physical properties (electricalconductivity and spin concentration) of resultant polyaniline on the concentration in thepolymerization reaction (Table +).
Aniline (*./ g) was added to the VW (,*ῌ,/* mL) with stirring at room temperature. After-* min, visual inspection conﬁrmed that the aniline was completely dissolved in the water.
The mixture was then continuously cooled to * and APS (+.2./ g, 2 mmol) was gradually
added to the solution (Fig. +). Here, polymerization of aniline is usually performed at *ῌ/because higher temperature causes cross link reaction for the resultant polyaniline. The
cross-linking results in two dimensionally propagated materials, which shows black color andvery low electrical conductivity. Conducting polymers require one-dimensional skeleton due
to requirement of generation of polarons and bipolarons as charge careers. Fraction of twodimensional structure for the conducting polymer depresses conductivity in the entire sysTable + Polymerizationa of aniline in volcanic hot spring water, and results of electrical conductivity andspin concentration.
SampleWaterb (g)Monomer (g)cQuantity (g)dT ()es (S/cm)f

Ns (spins/g)g

PANI-+
PANI-,
PANI-PANI-.
PANI-/
PANI-0

,*
/*
+**
+/*
,**
,/*

*4/
*4/
*4/
*4/
*4/
*4/

*4/+.
*4/**4.3/
*4./0
*4./2
*4.03

*
*
*
*
*
*

/4.+*.
/4-+*.
/42+*.
,4- +*.
,4. +*.
34- +*/+4.+*,*+4-+*,*+4++*,*+4,+*,*+4++*,*+4*+*,*

PANI-RT
PANI-/*
PANI-Ca

,**
,**
,**h

*4/
*4/
*4/

*4.02
*4-10
*4/.*

,/
/*
*

+40+*.
.4/+*1/4*+*-+4-+*,*
14-+*+3+43+*,*

Ammonium persulfate+.2./ g (2 mmol). b Volcanic hot spring water. c Aniline. d Quantity of resultant polyaniline. e Polymerization temperature. f Electrical conductivity. g Spin concentration. hSmall
amount of sulfuric acid was added to the reaction mixture to prepare the same pH value as thespring water sample (pH ,.3) employed in this study after addition of CaCl, (*.. g).
a136 0*  ,*+*

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tem. While, in the present study, we attempted polymerization of aniline at room temperature
and /* (hot spring water temperature) to examine dependence on reaction temperature inview point of natural temperature of VW (resultant polymer : PANI-RT and PANI-/*, Table+). Further, polymerization under a presence of CaCl, was carried out to examine if Ca, ion
as the main cation in the VW contributes the reaction.
All of the solutions color changed from dark brown to dark green in the polymerization
over time. After ,. h, the dark green solution with metallic reﬂection was ﬁltered. The ﬁltrate
was thoroughly washed with excess distilled water and excess methanol at room temperature. The dark green material thus obtained was dried under vacuum to yield PANI. The
constituents and their quantities used for the polymerizations are summarized in Table +.
The pristine PANIs are in the oxidized (doping) state (Fig. +). Here, the counter anion can bemainly SO., and HSO. because these are main anions in the VW. In the polyaniline chemistry, it is generally known that sulfuric acid is statically coupled with nitrogen atom ofaniline.
Quantity of the resultant polyaniline are higher than that of monomer amount employed
for the polymerization, i.e., PANI-+ (monomer*./ g, resultant polyaniline*./+. g). This isbecause the resultants possess APS and counter anion as dopants in general. Therefore,
estimation of exact synthetic yield after the polymerization is limited in the chemistry ofpolyaniline. However, reduction of the doped polyaniline (as prepared polymer) by using
hydrazine allows dedoping. The synthetic yield might be obtained by this method.
Quantity of resultant polyaniline is slightly decreased with decreasing the concentrationof the monomer in the VW in the polymerization (Table +). This result suggests that decreaseof the concentration depresses the reaction activity of the monomer. However, the small
decrease is tolerable for practical applications.
The change in pH of the PANI-, polymerization solution was monitored during the reaction as a function of time (Fig. ,). The pH increased after addition of the aniline monomer,
because the aniline monomer is coupled with ions in the VW. The pH value then decreased
after addition of APS. An inﬂection point is observed at around ++*. s. The pH value decreased with time, increases slightly at +.00+*. s (Fig. ,, inset), and then gradually decreases.
The decrease of the pH value in the reaction corresponds to the progress of the polymerization. Although the polymerization rate is somewhat low compared to the polymerization
reaction of aniline with acetic acid and sulfuric acid (Konyushenko et al., ,**3), the proﬁleobtained in this research is characteristic of the polymerization behavior of aniline monomer

Fig. + Polymerization of aniline in volcanic hot spring water.

137





Fig. , Change in pH as a function of polymerization time. Inset shows enlarged proﬁle.

and conﬁrms that the polymerization of aniline progressed in VW. The polymerization rate
can be improved by addition of large amount of oxidizer to the reaction mixture or metal ionssuch as Ca,.

-.
-. +

Results and discussion

UV/Vis optical absorption
Figure - displays the UV/Vis optical absorption spectrum of a PANI-, solution in NMP.

The absorption band at 0,0 nm is a doping band. The band at -,. nm is due to the pop*transition of the benzene ring in the polymer. Other polymers synthesized in the VW show
the same absorption band. This result indicates that the PANIs were successfully preparedin the VW. It should be noted that this spectral form is the typical PANI emeraldine base(intermediate doping state). PANI-RT and PANI-/*show the absorption peak at high en-ergy region compared to the polymers prepared at *. This can be due to the fact that thehigher temperatures cause a cross-link reaction, and e#ective conjugation length of the resultant polyaniline is shortened. On the other hand, the reaction under the presence of calcium chloride allows the polymer (PANI-Ca) with absorption maximum at long wavelengths.
Also, quantity of the resultant polyaniline under the presence of calcium chloride is thehighest in the series of the polymerization (Table +). This result implies that the calcium
chloride aids progress of the present polymerization. In other words, the calcium chloride
may play a role of auxiliary agent for the aniline polymerization.
-. ,Infrared (IR) optical absorption
Infrared (IR) optical absorption of the PANIs was examined using the KBr method, andthe results are shown in Fig. -. The absorption band at +,/1* cm+ is ascribed to the quinoid138 0*  ,*+*



Fig. - UV/Vis optical absorption spectra of PANIs in N-methylpyrrolidone(NMP) solution.

Fig. . IR spectra of PANIs in various concentrations of hot spring water. (a) PANI-+,(b) PANI-,, (c) PANI--, (d) PANI-., (e) PANI-/, (f) PANI-0, (g) PANI-RT. (h) PANI/*, and (i) PANI-CaCl,. Inset shows the quinoid and benzenoid structures in
the PANI main chain. (Right) IR absorption signals at 2*. and 2,- cm+.

structure (Q) in the main chain, and that at +,.3* cm+ is due to the benzenoid structure (B)(Fig. ., inset). The quinoid structure in the polymer reﬂects increase of electrical conductivity
accompanied by increase of charge career because generation of the charge career in the main
chain produces quinoid structure. The intensity ratio (+,/1* cm+/+,.3* cm+) is almost constant for PANI-+, PANI-,, PANI--, PANI-., PANI-/, PANI-0. However, PANI-RT and PANI-/*
show low absorption intensities for the quinoid structure. The band at +,-*+ cm+ is assignable to the CN stretching vibration of the quinoid-benzenoid-quinoid (QBQ) sequence. The
139

ῑ῔῕ῒ

῎ΐ῏ῐ

absorption band at +,,.+ cmῌ+ is due to C-N stretching in the benzene-benzene-benzene (BBB)
sequence (tang et al., +322). Absorption bands at 2*. and 2,- cmῌ+ are ascribed to C-H outof-plane vibrations on the aromatic main chain. The intensity of the band at 2*. cmῌ+ decreased with the decrease in the concentration of monomer in the polymerization (Fig. . (right),(a)῍*.,2 M, (f)῍*.*, M). This result may be related to the length of the main chain or content
of Q in the main chain. PANI-Ca shows relatively strong absorption band at 2*. cmῌ+. Characteristic absorption band at +,+.* cmῌ+ was assigned as an electronic-like band (delocalization of electrons on the PANI main chain) related to the electrical conductivity (Yamauch,+33,). Basically, all of the PANI samples had the same characteristic absorption IR bands ofstandard PANI (Quillard et al., +33.).
-. -Electron spin resonance (ESR)
The pristine PANI thus synthesized is partly in the oxidized (doping) state. It has charge

carriers (polarons (cation radical) and bipolarons (dication)), and exhibits electrical conductivity. ESR measurements can evaluate the presence of charge carriers in PANI. A representative ESR spectrum for the PANI in this study is shown in Fig. /. The spectrum is a symmetrical Lorentzian absorption with DH+/,/DHpp῍,.11. The inset of Fig. / indicates DH+/, (linewidth at half maximum) and DHpp (peak-to-peak line width) in the ESR signal (Konyushenko
et al., ,**0). The spin concentration (Ns) is increased with the concentration of monomer in
VW water during the synthesis of PANI. This result may correspond to the increase in themolecular weight of PANI with increasing monomer concentration during synthesis, becauseof an increase in reactivity. The g-values are almost constant, which indicates that the charge
carriers are polarons and the species are not changed if the concentration of monomer isFig. / ESR spectrum of PANI-+. Inset shows DH+/, and DHpp in the spectrum.

140 0*  ,*+*



altered during the polymerization reaction, as shown in Fig. 0. Especially, PANI-Ca
shows the highest spin concentration. This
result is consistent with the IR (the strong
absorption band at 2*. cm+) and optical absorption measurements results (absorption
peak located at the longest wavelengths inthe samples).
-. . Electrical conductivity
The electrical conductivity of compressed pellet samples of the as-prepared
PANI was measured. The conductivity ofcompressed PANI pellets vs. concentration

Fig. 0 ESR results for the PANIs. Spin concentration(Ns) vs. aniline monomer concentration in the polymerization reaction () and g values ().

of monomer during PANI synthesis isshown in Fig. 1. The conductivity of the resultant PANI was increased with the concentrationof monomer and the value is saturated at around/+*. S/cm.

PANI-/* shows low electricalconductivity may be due to occurrence of crosslink reaction in the polymerization.

PANI-Cadisplays good electrical conductivity (/.*+*S/cm). This is consistent with the results of the
IR and the optical absorption.
-. / Surface structure
A polarized di#erential interference contrast
microscopy (DIM) image of PANI-, (compressedpellet) is shown in Fig. 2 (a). The low magniﬁcation image suggests that the surface consists ofFig. 1 Conductivity (s) of compressed PANI pellets as a function of the aniline monomer
concentration in the polymerization reaction.

ﬁne particles. An SEM image of the PANI-, isshown in Fig. 2 (b). The high magniﬁcation reveals that PANI-, forms a ﬁber structure ; theﬁber diameter is approximately +/* nm and the longest ﬁber in the micrograph is longer than..1 mm. Fiber formations can be due to aggregation during the PANI synthesis. PANI nanostructures have been prepared in the presence of weak organic acid (Tran et al., ,**2) or inpolymeric acid solution (Zhang et al., ,**1). Fundamentally, the ﬁber shape of the polyaniline
thus synthesized in the present study is the same as the previously reported method by using
camphor sulfonic acid, or naphthalene sulfonic acid (instead of use of sulfuric acid). Here, thestandard polymerization method generally produces non nano-ﬁber structure (non order
structure). While, the polyaniline thus prepared in the VW displays long nano-ﬁber, as shownin Fig. 2 (b) (dashed line : trace of the longest ﬁber in the image). The nano-ﬁber is relatively
longer in size than that of the polyaniline nano-ﬁber produced by the combination of APS and
141

ῑ῔῕ῒ

῎ΐ῏ῐ

Fig. 2 (a) Polarizing di#erential interference contrast microscopic image (DIM) of acompressed pellet sample of PANI-, (ῌ/**). (b) SEM micrograph of the PANI-,
bulk sample (ῌ,1,***). Dashed line : trace of the longest ﬁber in the image.

camphor sulfuric acid at appropriate condition. The results in this research demonstrate that
the VW provides satisfactory condition for formation of PANI nano-ﬁber having moderate
electrical conductivity, without use of any complicated preparation for the reaction medium(i.e., operation of adjustment of pH).

..

Conclusion

PANI was synthesized in natural volcanic hot spring water. Infrared absorption spectraof the synthesized product conﬁrmed the chemical structure of PANI. The moderate acidityof the water provides a good environment for the polymerization of aniline monomers. Furthermore, a set of the results suggests that change in amount of the VW as a reaction solvent
for the polymerization allows tuning of spin concentration and electrical conductivity of theresultant polyaniline. The polymerization at ,/῍ a#orded the polyaniline with the moderate142 0*  ,*+*



 electrical conductivity. Quasi-model experiment implies that calcium in the natural volcanic
hot spring water behaves auxiliary agent to promote the polymerization. Although we needto perform further research to evaluate a role of metal ions, employment of the natural
volcanic hot spring water in this study gave us the new clue for development of conducting
nanoﬁber with high performance in electro-magnetic properties.
Natural volcanic hot spring waters from other places may give another polymerization
results because the constituent depends on location. This method may provide a new approach toward the synthesis of organic electronic materials by employing natural resources.

Acknowledgments
The author thanks Hakone Hot Spring Spa Supply Co. Ltd. (Kanagawa, Japan) for generously providing the hot spring water sample. The author is grateful to the Engineering
Workshop of University of Tsukuba for glasswork and the Analysis Center for the ICP
measurements.
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