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Synthesis of crystalline polyaniline
SYNTHESIS OF CRYSTALLINE POLYANILINE
S.Tamil Selvan, A.Mani, K.Athinarayanasamy, K.L.N.Phani and S.Pitchumani Emerging Concepts and Advanced Materials Group, E.E.B. Division, Central Electrochemical Research Institute, Karaikudi 623 006, India (Received March 8,1995; Refereed)
ABSTRACT The synthesis of crystalline polyaniline usi:! a water-in-oil type microemulsion as a base medium polymerization is reported. The polymer exhibits well defined crystalline phase whose observed orthorhombic lattice parameters are a = 7.65, b m 5.75, c = 10.22A been and V = 450 polymer obtained has The . A". FTIR, W and Cyclic Voltammetry $;ajact~~=;;;mi~g XI$ crystalline phase., surfactant stabilized nature, base form and dopability. MATERIALS INDEX: Microemulsion, crystalline polyaniline
Introduction conducting Polyaniline (PANI) is an important member of applications of polymer family its because of technological importance ranging fromPZ..:electronics (1) to light weight batteries (2). The ability to tune different electronic states of PAN1 by careful choice of chemical and electrochemical preparative routes has been of great interest PAN1 have been and two classes of the emeraldine form of recently distinguished (3,4). Accordingly, PAN1 can be prepared chemically by oxidative polymerization of aniline which upon dopin /undoping yields class I emeraldine salt (ES-I) and base (EB-17 respectively while class II emeraldine materials (EB-II, ES-II) are formed when EB-I is extracted with THF/NMP yielding EB-II and subsequent conversion to ES-II upon doping. Hence new attempts of making PAN1 with defined crystalline phases 699
700
S. TAMlLSBLVANet al.
Vol.3O.No.6
has become an attractive proposition. X-ray analysis (3) shows that except EB-I other forms of PANI are partially crystalline. EB-II ranges from amorphous to a maximum of 50% crystalline depending upon sample history (3-5). Therefore, to arrive at EB-II form, one should follow multiple stages of treatments such as extraction, washing and chemical conversion etc. In an attempt to provide an alternative method to evolve crystalline forms of PAN1 we have explored the possibility of using microemulsion as a medium of polymerization in accordance with our earlier work on polyparaphenylene (PPP) (6). Hitherto microemulsion and micellar approaches reported by Gan et al (7,8) and Armes et al (9) respectively, have been successfully used to produce nanoparticles of PAN1 and other conducting polymers like polypyrrole (PPy) as colloidal dispersions. It also worthwhile noting that colloidal dispersions of zinducting polymers, particularly PAN1 and PPy can be prepared with polymeric stabilizers (10,li). All these approaches have been used to obtain ultrafine spherical particles of PANI and PPY, but to our knowledge, have never before been reported for the synthesis of crystalline PANI. In this communication, we report an inverse microemulsion route for the synthesis of highly crystalline EB-II form of PAN1 in a single step without any further processing or treatment of basic precursor polymer. Experimental Water-cyclohexane-sodium bis-2-ethylhexylsulfosuccinate (AOT) or-sodium dodecyl sulfate (SDS) were employed as components of microemulsions. Two transparent microemulsions using any one of the surfactants mentioned were first prepared separately. Although different formulations were tried, best the compositions found were to (i) 15.1 g water, 67.8 g cyclohexane, 17.1 g AOT, andbe' 5.3 g 79.9 g water, cyclohexsne, 9.8 g n-butanol, 4.9 g SDS. To two parts of microemulsion, aniline hydrochloride (0.327 g. 2.53 mmol) and ammonium peroxydisulfate ( 0.582 g. 2.55 mmol) were added separately and mixed thoroughly until they turned into perfectly the two microemulsions with clear solutions. After mixing vigorous agitation, polymerization of aniline occurred readily within 15 minutes at room temperature. The colour changed from light blue to dark blue and finally to dark green. The polymerized microemulsion system was then left standing for ca. 24 hours without stirring. The very fine precipitated PAN1 atlO,OOO particles were obtained by centrifugatlon for 10 minutes and washed with methanol/THF and finally withrzter. Results and Discussion Figure 1 shows UV-vis spectrum of PAN1 with characteristic maxima ( h ) at 425, 575 and 625 nm which are comparable to the known !&tral features of PANI (12). The absorption maxima at 575 nm confirms the formation of emeraldine base (EB) and broad maxima at 625 nm indicates lightly doped state. FTIR
Vol. 30. No. 6
701
I
0
1
I
300
I
700 WAVELENGTH (nm ) FIG. 1 FTIR Spectrum of PAN1 powder
1947 8
6 = .E
e te be 1541‘
I
WAVENUMBER (cm” 1 FIG. 2 UV absorption spectrum of PANI powder
702
S. TAML SELVAN et al.
Vol. 30, No. 6
spectra (Figure 2) reveals the fact that PAN1 is surfactantstabilized. The presence of AOT is evident at 3232 cm'l,The coexistence of PAN1 and surfactant is observed at 1744, 1734 and 1725 cm-1 as very weak peaks. It is assumed that the surfactant is adsorbed as an outer layer surrounding the PANI particles as was found in case of polypyrrole (9). For cyclic cast onto the Pt working voltammetric studies, PAN1 was electrode (Bio-Analytical Systems Inc., 0.16 mm diameter) by evaporating a suspension of the isolated polymer in chloroform and allowed to dry. The Pt electrode coated with PANI was placed in 1 M HCl and cyclic voltammograms were then recorded. The CV (Figure 3) possesses similar features as those of conventional of PANI which mainly consists of two redox processes at the ES 0.24 and 0.67 V vs SCE, respectively. The ES for the first redox process is ca. 100 mV higher than that of Wei et al (13) for the second redox process is 100 mV lower while the Es than that of Focke et al (14), reflecting the dependence of pH of the polymerization medium. This is quite justifiable in the present context since we have used on1 very low concentration of aniline hydrochloride (2.53 mmolY whereas in the conventional synthesis concentration of 1 M HCl (pH = -0.2) (14,15) was usually employed. As a consequence of this, the CV assumes broad features.
O-6 E/V
vs SCE
FIG. 3 Cyclic voltammetric response of PAN1 film (cast from the polymerized microemulsion), in l&lHCl; sweep rate: 100 mV s-1
Vol. 30, No. 6~
SYNTHESIS OF CRYSTALLINE
POL.YANILINE
703
Figure 4 shows the XRD patterns of PAN1 prepared with SDS and AOT respectively. As is evident, PANI is highly crystalline through a number of intense sharp peaks. We have indexed the XRD pat:terns using orthorhombic unit cell; the observed latti$e parameters are a = 7.65, b = 5.75, c = 10.22 %, and V = 450 A3 which are in good agreement with the already reported (5) EB-II form of PANI, normally obtainable after polymerization, subsequent treatment with NH40H, followed by extraction with THF/NME' solvents? while our microemulsion based synthesis yields the same EB-II directly. Sinc.e microemulsion exists as droplets of size l-10 nm, each droplet can act as a nucleus for the polymer crystallization. As the polymerization proceeds in microemulsion droplet, we believe that the size of the droplet appears to control the propagation rate leading to an ordered growth of PAN1 chains. The origin of crystalline order lies in the controlled aggregation behaviour of the oligomer/polymer chains/particles in the microenvironment provided by the microemulsions (similar to the "clusters" obtained in the case of organogels and their microst:ructures(16)).Our results may be compared with the cluster-aggregation behaviour leading to monoliths demonstrated in the latter. It is to be noted here that the microemulsion approach could lead to PAN1 with well defined crystalline phase, while a similar inverse microemulsion approach (7,8) has led to the formation of PAN1 ultrafine particles of lo-50 nm and its conductivity is found to be dependent on the concentration of HCl employed for the polymerization reaction. In our approach we have deliberately used aniline as aniline hydrochloride salt for anilinium cation generation during the just enough polymerization in microemulsion and the relatively lower environment has facilitated the concentration of acidic formation of emeraldine base exclusively with well defined crystalline phase. We believe the obtainment of that crystallinity in PAN1 is an added advantage with this microemulsion approach. Conclusion This work thus demonstrates the remarkable influence of microemulsions for the synthesis of PAN1 in crystalline form. The fact that the use of an inverse microemulsion as a medium of polymerization and obtainment of highly crystalline PAN1 without resorting to different chemical treatments of basic precursor polymer, by its own merit a promising result. The size that the polymer can grow to is controlled by the size of the droplet cores, which in turn allows the growth of polymer in a particular orientation, offers a much better control than can be achieved using more conventional oxidative polymerization of aniline. We are also currently investigating this aspect by controlling the microeniuision droplet size through different ratios of water/surfactant to understand better the morphology and crystalline growth pattern of PANI.
S.
704
SELVAN
TAME
etal.
Vol. 30, No. 6
a
1000
J 0 3
I
I
I
I7
37
BP
-
28 (degreea)
1000
3
I I?
I 37
I
57
28 (dOgfW8) FIG. 4 XRD patterns of PAN1 powders - surfactant employed in microemulsion being (a) SDS, and (b) AOT
Vol. 30, No. 6
SYNTHESIS OF CRYSTALLINE POLYANILINE
Acknowledgements S.'T thanks CSIR, Associateship.
New
Delhi
for
the
award
705
of
a
Research
References 1. S. Chao and M.S. Wrighton,
J.
Am.
Chem.
sot.
lO9,
6227
M.A. Drury, P.J. Nigrey, D.P. Nairns, 2. ~lg!l%innes Jr A:G. MacDiarmid*)andA.J. Heeger, J. Chem. Sot. Chem. Commun. 317 (1981). Epstein, X. Tang and Jozefowicz 3. :I:: M%%%&idM*EMacromolecule~ 2tJ?79 (1991) 4. Y.B. Moon, Y.
S.Tamil Selvan, A.Mani, K.Athinarayanasamy, K.L.N.Phani and S.Pitchumani Emerging Concepts and Advanced Materials Group, E.E.B. Division, Central Electrochemical Research Institute, Karaikudi 623 006, India (Received March 8,1995; Refereed)
ABSTRACT The synthesis of crystalline polyaniline usi:! a water-in-oil type microemulsion as a base medium polymerization is reported. The polymer exhibits well defined crystalline phase whose observed orthorhombic lattice parameters are a = 7.65, b m 5.75, c = 10.22A been and V = 450 polymer obtained has The . A". FTIR, W and Cyclic Voltammetry $;ajact~~=;;;mi~g XI$ crystalline phase., surfactant stabilized nature, base form and dopability. MATERIALS INDEX: Microemulsion, crystalline polyaniline
Introduction conducting Polyaniline (PANI) is an important member of applications of polymer family its because of technological importance ranging fromPZ..:electronics (1) to light weight batteries (2). The ability to tune different electronic states of PAN1 by careful choice of chemical and electrochemical preparative routes has been of great interest PAN1 have been and two classes of the emeraldine form of recently distinguished (3,4). Accordingly, PAN1 can be prepared chemically by oxidative polymerization of aniline which upon dopin /undoping yields class I emeraldine salt (ES-I) and base (EB-17 respectively while class II emeraldine materials (EB-II, ES-II) are formed when EB-I is extracted with THF/NMP yielding EB-II and subsequent conversion to ES-II upon doping. Hence new attempts of making PAN1 with defined crystalline phases 699
700
S. TAMlLSBLVANet al.
Vol.3O.No.6
has become an attractive proposition. X-ray analysis (3) shows that except EB-I other forms of PANI are partially crystalline. EB-II ranges from amorphous to a maximum of 50% crystalline depending upon sample history (3-5). Therefore, to arrive at EB-II form, one should follow multiple stages of treatments such as extraction, washing and chemical conversion etc. In an attempt to provide an alternative method to evolve crystalline forms of PAN1 we have explored the possibility of using microemulsion as a medium of polymerization in accordance with our earlier work on polyparaphenylene (PPP) (6). Hitherto microemulsion and micellar approaches reported by Gan et al (7,8) and Armes et al (9) respectively, have been successfully used to produce nanoparticles of PAN1 and other conducting polymers like polypyrrole (PPy) as colloidal dispersions. It also worthwhile noting that colloidal dispersions of zinducting polymers, particularly PAN1 and PPy can be prepared with polymeric stabilizers (10,li). All these approaches have been used to obtain ultrafine spherical particles of PANI and PPY, but to our knowledge, have never before been reported for the synthesis of crystalline PANI. In this communication, we report an inverse microemulsion route for the synthesis of highly crystalline EB-II form of PAN1 in a single step without any further processing or treatment of basic precursor polymer. Experimental Water-cyclohexane-sodium bis-2-ethylhexylsulfosuccinate (AOT) or-sodium dodecyl sulfate (SDS) were employed as components of microemulsions. Two transparent microemulsions using any one of the surfactants mentioned were first prepared separately. Although different formulations were tried, best the compositions found were to (i) 15.1 g water, 67.8 g cyclohexane, 17.1 g AOT, andbe' 5.3 g 79.9 g water, cyclohexsne, 9.8 g n-butanol, 4.9 g SDS. To two parts of microemulsion, aniline hydrochloride (0.327 g. 2.53 mmol) and ammonium peroxydisulfate ( 0.582 g. 2.55 mmol) were added separately and mixed thoroughly until they turned into perfectly the two microemulsions with clear solutions. After mixing vigorous agitation, polymerization of aniline occurred readily within 15 minutes at room temperature. The colour changed from light blue to dark blue and finally to dark green. The polymerized microemulsion system was then left standing for ca. 24 hours without stirring. The very fine precipitated PAN1 atlO,OOO particles were obtained by centrifugatlon for 10 minutes and washed with methanol/THF and finally withrzter. Results and Discussion Figure 1 shows UV-vis spectrum of PAN1 with characteristic maxima ( h ) at 425, 575 and 625 nm which are comparable to the known !&tral features of PANI (12). The absorption maxima at 575 nm confirms the formation of emeraldine base (EB) and broad maxima at 625 nm indicates lightly doped state. FTIR
Vol. 30. No. 6
701
I
0
1
I
300
I
700 WAVELENGTH (nm ) FIG. 1 FTIR Spectrum of PAN1 powder
1947 8
6 = .E
e te be 1541‘
I
WAVENUMBER (cm” 1 FIG. 2 UV absorption spectrum of PANI powder
702
S. TAML SELVAN et al.
Vol. 30, No. 6
spectra (Figure 2) reveals the fact that PAN1 is surfactantstabilized. The presence of AOT is evident at 3232 cm'l,The coexistence of PAN1 and surfactant is observed at 1744, 1734 and 1725 cm-1 as very weak peaks. It is assumed that the surfactant is adsorbed as an outer layer surrounding the PANI particles as was found in case of polypyrrole (9). For cyclic cast onto the Pt working voltammetric studies, PAN1 was electrode (Bio-Analytical Systems Inc., 0.16 mm diameter) by evaporating a suspension of the isolated polymer in chloroform and allowed to dry. The Pt electrode coated with PANI was placed in 1 M HCl and cyclic voltammograms were then recorded. The CV (Figure 3) possesses similar features as those of conventional of PANI which mainly consists of two redox processes at the ES 0.24 and 0.67 V vs SCE, respectively. The ES for the first redox process is ca. 100 mV higher than that of Wei et al (13) for the second redox process is 100 mV lower while the Es than that of Focke et al (14), reflecting the dependence of pH of the polymerization medium. This is quite justifiable in the present context since we have used on1 very low concentration of aniline hydrochloride (2.53 mmolY whereas in the conventional synthesis concentration of 1 M HCl (pH = -0.2) (14,15) was usually employed. As a consequence of this, the CV assumes broad features.
O-6 E/V
vs SCE
FIG. 3 Cyclic voltammetric response of PAN1 film (cast from the polymerized microemulsion), in l&lHCl; sweep rate: 100 mV s-1
Vol. 30, No. 6~
SYNTHESIS OF CRYSTALLINE
POL.YANILINE
703
Figure 4 shows the XRD patterns of PAN1 prepared with SDS and AOT respectively. As is evident, PANI is highly crystalline through a number of intense sharp peaks. We have indexed the XRD pat:terns using orthorhombic unit cell; the observed latti$e parameters are a = 7.65, b = 5.75, c = 10.22 %, and V = 450 A3 which are in good agreement with the already reported (5) EB-II form of PANI, normally obtainable after polymerization, subsequent treatment with NH40H, followed by extraction with THF/NME' solvents? while our microemulsion based synthesis yields the same EB-II directly. Sinc.e microemulsion exists as droplets of size l-10 nm, each droplet can act as a nucleus for the polymer crystallization. As the polymerization proceeds in microemulsion droplet, we believe that the size of the droplet appears to control the propagation rate leading to an ordered growth of PAN1 chains. The origin of crystalline order lies in the controlled aggregation behaviour of the oligomer/polymer chains/particles in the microenvironment provided by the microemulsions (similar to the "clusters" obtained in the case of organogels and their microst:ructures(16)).Our results may be compared with the cluster-aggregation behaviour leading to monoliths demonstrated in the latter. It is to be noted here that the microemulsion approach could lead to PAN1 with well defined crystalline phase, while a similar inverse microemulsion approach (7,8) has led to the formation of PAN1 ultrafine particles of lo-50 nm and its conductivity is found to be dependent on the concentration of HCl employed for the polymerization reaction. In our approach we have deliberately used aniline as aniline hydrochloride salt for anilinium cation generation during the just enough polymerization in microemulsion and the relatively lower environment has facilitated the concentration of acidic formation of emeraldine base exclusively with well defined crystalline phase. We believe the obtainment of that crystallinity in PAN1 is an added advantage with this microemulsion approach. Conclusion This work thus demonstrates the remarkable influence of microemulsions for the synthesis of PAN1 in crystalline form. The fact that the use of an inverse microemulsion as a medium of polymerization and obtainment of highly crystalline PAN1 without resorting to different chemical treatments of basic precursor polymer, by its own merit a promising result. The size that the polymer can grow to is controlled by the size of the droplet cores, which in turn allows the growth of polymer in a particular orientation, offers a much better control than can be achieved using more conventional oxidative polymerization of aniline. We are also currently investigating this aspect by controlling the microeniuision droplet size through different ratios of water/surfactant to understand better the morphology and crystalline growth pattern of PANI.
S.
704
SELVAN
TAME
etal.
Vol. 30, No. 6
a
1000
J 0 3
I
I
I
I7
37
BP
-
28 (degreea)
1000
3
I I?
I 37
I
57
28 (dOgfW8) FIG. 4 XRD patterns of PAN1 powders - surfactant employed in microemulsion being (a) SDS, and (b) AOT
Vol. 30, No. 6
SYNTHESIS OF CRYSTALLINE POLYANILINE
Acknowledgements S.'T thanks CSIR, Associateship.
New
Delhi
for
the
award
705
of
a
Research
References 1. S. Chao and M.S. Wrighton,
J.
Am.
Chem.
sot.
lO9,
6227
M.A. Drury, P.J. Nigrey, D.P. Nairns, 2. ~lg!l%innes Jr A:G. MacDiarmid*)andA.J. Heeger, J. Chem. Sot. Chem. Commun. 317 (1981). Epstein, X. Tang and Jozefowicz 3. :I:: M%%%&idM*EMacromolecule~ 2tJ?79 (1991) 4. Y.B. Moon, Y.