Langmuir 1990,6, 1745-1749
1745
Aqueous Colloidal Dispersions of Polyaniline Formed by Using Poly(vinylpyridine)-Based Steric Stabilizers S. P. Armes,? M. Aldissi,* S. Agnew, and S. Gottesfeld Los Alamos National Labortory, P.O. Box 1663, Los Alamos, New Mexico 87545 Received December 8, 1989.I n Final Form: May 23, 1990 Colloidal polyaniline has been prepared in acidic aqueous media by chemical grafting of polyaniline onto a tailor-made polymeric surfactant. The surfactant, which acts as a steric stabilizer, used in this study is the random copolymer poly(2-vinylpyridine-co-p-aminostyrene).This surfactant contains pendant aniline units which participate in the aniline polymerization,resulting in the formation of stericallystabilized polyaniline particles which have a nonspherical "rice grain" morphology. It is shown that this novel form of polyaniline is more processable than the bulk powder that is normally obtained from a conventional chemical or electrochemicalsynthesis. The solid-state conductivity of solution-cast films or compressed pellets of these dispersions is surprisingly high ( N 1 S/cm), despite the presence of the insulating stabilizing outer layer. The colloids were characterized by a wide range of techniques including electron microscopy, cyclic voltammetry, and FTIR, visible absorption, and X-ray photoelectron spectroscopies.
Introduction The polymer is typically prepared by oxidation of aniline monomer in aqueous acidic media using reagents such as ammonium persulfate or potassium A nonoxidative proton-doping process results in a highly environmentally stable material with a room temperature conductivity in the range 1-10 S/cm. Although its air stability is attractive, polyaniline suffers from poor processability,being only slightly soluble in polar solvents such as dimethylformamide (DMF) or dimethyl sulfoxide (DMS0).3 However, a better solubility is achieved in strong acidic media such as concentrated sulfuric acid.4 Intractability is typical of conjugated polymers because of the aggregated character associated with strong interchain interactions. Taking advantage of this property in the preparation of conducting polymers as colloidal dispersions seemed to be an excellent approach toward processability. Since 1986,several groups have described the preparation of spherical submicronic polypyrrole colloidal particles in aqueous media.&10 The particles are sterically stabilized by an outer layer of physically adsorbed polymeric surfactant such as methylcellulose, poly(viny1 alcohol-co-vinyl acetate), poly(vinylpyrrolidone), poly(vinylpyridine-co-butyl methacrylate), etc. Most of our initial attempts to produce colloidal polyaniline particles by analogous methods have resulted in macroscopic precipitation due to inefficient adsorption of 'To whom correspondence may be addressed at Champlain Cable Corp., P.O.Box 7, Winooski, VT 05404. + Presently at University of Sussex, School of Chemistry and Molecular Sciences, Brighton BN1 SQJ,United Kingdom. (1)Travers, J. P.; Chroboczek, J.;Devreux, F.; Genoud, F.; Nechtschein, M.; Syed, A.; Genies, E. M. Mol.Cryst. Liq. Cryst. 1985,121, 195. ( 2 ) Armes, S. F.; Miller, J. F. Synth. Met. 1988,22,385. ( 3 ) Li, 5.;Cao, Y.;Xue, 2. Synth. Met. 1987,20, 147. (4) Andereatta,A.; Cao, Y.;Chiang, J. C.; Smith, P.; Heeger, A. J. Synth.
Met. 1988, 26, 383. (5) Bjorklund, R. B.; Liedberg, B. J. Chem. Soc., Chem. Commun. 1986, 1293. (6) Armes, S. P.; Vincent, B. J.Chem. Soc., Chem. Commun. 1987,288. (7) Armes, S. P.; Miller, J. F.; Vincent, B. J. Colloid Interface Sci. 1987, 118 (2), 410.
(8)Armes, S. P.; Aldieei, M.; Agnew, S. F. Synth. Met. 1989,28,C837. (9) Armes, S. P.; Aldissi, M. Polymer 1990,31, 569. (10)Cawdery, N.; Obey, T. M.; Vincent, B. J. Chem. Soc., Chem. Commun. 1989,1189.
0743-7463/90/2406-1745$02.50/0
the stabilizer, although in certain cases a low yield of colloidal polyaniline has been reported.11J2 To achieve colloidal stability of polyaniline particles and avoid the problems associated with physical adsorption/desorption, we employed a different synthetic approach which consisted of graft copolymerization of aniline onto the appropriate polymeric surfactant. In a recent preliminary c~mmunication,~~ we described the preparation of colloidal polyaniline particles using a tailor-made random copolymer poly(2-vinylpyridine-co-p-aminostyrene) surfactant. A detailed account of the synthesis and properties of the colloidal particles using poly(viny1alcohol)-based surfactant will be published elsewhere.14 Preparation of the latter two compounds and evidence of grafting were described recently.15 In this paper, we describe the preparation and characterization of polyaniline colloids using poly(2-vinylpyridineco-p-aminostyrene), abbreviated throughout the paper as P2VP, as a steric stabilizer in the graft copolymerization process. T h e analogous poly(4-vinylpyridine-co-paminostyrene), abbreviated P 4 W , was also studied, though less extensively.
Experimental Section Preparation of the Steric Stabilizer. Copolymerization of 2- or 4-vinylpyridine with p-aminostyrenewas carried out by using 2,2'-azobisisobutyronitrile and free radical polymerization at 60 O C under argon in either toluene or ethanol. The reaction was allowed to proceed for 24-48 h. The resulting copolymer was purified by repeated precipitation in methanol/water solvent/nonsolvent mixtures. The reaction scheme is shown in Figure 1. Preparation of Polyaniline Colloids. The synthesis which utilized potassium iodate to oxidize aniline was carried out as follows: 0.9 g of potassium iodate was dissolved in 20 mL of 1.25 M HCl, and 1.0 g of PPVP or 0.75 g of PIVP was dissolved in 80 mL of the same medium. The two solutions were mixed together at room temperature and stirred for 1 h to ensure some preactivation (oxidation) of the aminostyrene moieties on the (11)Miller, J. F. BSc. Thesis, University of Bristol, UK, 1987. (12) Cooper, E. C. Ph.D., Thesis, University of Bristol, UK, 1988. (13) Armes, S. P.; Aldissi, M. J. Chem. SOC.,Chem. Commun. 1989, 88. (14) Armes, S. P.; Aldissi, M. In press. (15) Armes, S. P.; Aldissi, M. Polym. Mater. Sci. Eng. 1989,60, 751.
0 1990 American Chemical Society
1746 Langmuir, Vol. 6, No. 12, 1990 CH,=CH
CH,=CH
/- CH,
X/Y
Armes et al.
-CH + ~ - c H ,-CH-
\
15-20
Figure 1. Synthesis of the poly(viny1pyridine-co-p-aminostyrene) stabilizers. stabilizer's chain. Longer aging times led to gradual precipitation of the stabilizer, probably due to cross-linking of the oxidized p-aminostyrene units. Aniline monomer (1.0 mL) was then injected into the reaction mixture, and the copolymerization reaction was allowed to proceed for 120 h at room temperature. At the end of this period, the reaction medium turned dark green. The dispersion was centrifuged at 50 000 rpm for 1-1.5 h, the yellow-orange supernatant decanted, and darkgreen sediment redispersed in 1.25 M HCl via ultrasonics and filtered to remove nonredispersed sedimenta. The reaction yield is typically about 1.0 g of colloidal dispersion. Unlike PVA-stabilizedpolyaniline colloids, the poly(viny1pyridine)-stabilized ones do not form good-quality coherent films upon solvent evaporation at room temperature. The films instead are brittle, thus making conductivity measurements difficult. We attempted to improve the film quality by co-mixing solutions of nonadsorbed commercial poly(viny1 alcohol-covinyl acetate), abbreviated PVA, with the polyaniline colloids prior to solvent removal. Because of its film-formingproperty, water-soluble PVA behaved as a plasticizer in these experiments to promote film coherence; 0.1 mL of 1.0 wt/vol So solutions of PVA was mixed with 0.84-1.20 wt/vol $1dispersions of polyaniline colloids. This resulted in much better quality films with no significant loss in conductivity. Characterization of Stabilizer and Colloids. The random copolymer steric stabilizers were characterized by gel permeation chromatography (GPC)and N M R t o determine molecular weights, distribution, and comonomer ratios, respectively. Polyaniline colloids were characterized by using a wide range of techniques including transmission, scanning, and scanning tunneling microscopies; cyclic voltammetry; and FTIR, UV-vis, absorption, and X-ray photoelectron spectroscopies. Conductivity measurements were performed on pellets of the dried-down dispersions or films cast from the colloidal dispersions.
Results and Discussion Oxidant/Stabilizer Compatibility. The oxidation/ polymerization of aniline in the presence of PBVP- or P4VP-based stabilizers was attempted using several oxidants that are typically used for the synthesis of bulk polyaniline. It was expected that the pendant aniline moiety in the graft copolymer would participate in the polymerization of aniline via a mechanism similar to that outlined p r e v i ~ u s l y .However, ~ we encountered initially some difficulties in the choice of a suitable oxidant. Ammonium persulfate, sodium persulfate, ammonium dichromate, and iron trichloride all proved to be incompatible with the stabilizer, resulting in the formation of insoluble complexes. Recently, Pron et al.16 reported that potassium iodate could be used over a wide range of reaction conditions to produce good-quality bulk polyaniline powder. We found t h a t this oxidant did not cause precipitation of the stabilizer; as a consequence, it became the reagent of choice for the preparation of colloidal polyaniline. Molecular Weight a n d Composition of the Stabilizer. The steric stabilizers were characterized by using GPC and viscometry. Chromatographs of the various samples indicated that the use of toluene as the polym(16) Pron, A.; Genoud, F.; Menardo, C.; Nechtschein, M. Synth. Met. 1988, 24, 193.
Table I. Molecular Weights and Relative Monomer Composition of Poly(viny1pyridine)-Based Stabilizers stabilizer P2VP-1 P2VP-2 P4VP P2VP-3 P2VP-4c
solvent
aminostyrene, mol %
4.8 5.2 6.7 14.3 3.2
toluene ethanol
ethanol toluene toluene
Mn5 21500 11900
54000 77400
14500 12300
46500 36000
M,O
Myb
105 000
As measured by gel permeation chromatography in THF using P2VP standards and highly cross linked styrene/ divinylbenzene copolymer for packing the columns. As measured by differential viscometry. Resulted in precipitation of polyaniline due to the low content of p-aminostyrene.
*
IIIZ P4VP + K103
400
460
520
580
640
IO
A (nm)
Figure 2. Spectroscopic evidence for grafting of polyaniline. erization solvent rather than ethanol produced a product with lower polydispersity. This was presumably due to the lower rate of chain transfer to solvent in the former medium. However, P4VP copolymer was not soluble in toluene, so ethanol was the preferred solvent in this case. The presence of p-aminostyrene moiety in the copolymers was confirmed by FTIR using 4-ethylaniline as a reference compound. The exact amount of p-aminostyrene was determined by using 1H NMR. The results are summarized in Table I. Spectroscopic Evidence f o r G r a f t i n g of Polyaniline. The polymerization of aniline by KI03 (and other oxidants such as ammonium persulfate) in acid solution proceeds via a short-lived anilinium radical cation intermediate which has an absorption peak A, at 525 nm (Figure 2). If KI03 is added to an acidic solution containing the polymeric stabilizer, a similar absorption peak is observed. In a control experiment, a P4VP/KI03/HCl solution showed no absorption peak under identical conditions. Thus, we conclude that the observed absorption peak is due to the oxidation of the pendant aniline groups. In our preparative procedure for the synthesis of colloidal polyaniline, the aniline monomer is added last to a stirred, aged solution containing KI03 and the stabilizer. Since it is clear that the pendant aniline groups grafted onto PVA are activated under these conditions, we believe that these moieties inevitably participate in the aniline polymerization and constitute sites for the grafting
Aqueous Colloidal Dispersions of Polyaniline
Langrnuir, Vol. 6, No.12, 1990 1747 Colloids Morphology. Examination of the drieddown dispersions by scanning and transmission electron microscopies revealed a morphology which consisted of "rice grain" like particles. The length of the particles was 150 50 nm, and their width was 60 10 nm. Figure 3 shows transmissionelectron micrographs of the dispersions in which individual particles can be seen clearly. Because steric stabilization by the polymericsurfactant takes place during polymerization of aniline, aggregation is limited to aggregates or particles in the submicronicrange. However, the overall fibrillar shape observed in bulk p~lyaniline'~ is maintained. Although the shape of the particles is nonspherical (unlike polypyrrole), scanning tunneling microscopy revealed that any given polyaniline colloidal particle is composed of much smaller ones whose shape is undetermined. The morphology using this latter technique will be discussed elsewhere." Conductivity. Four-probe conductivity measurements were performed on films cast from the dark green dispersions mixed with 7-11 wt % of poly(viny1alcohol) for better film formation. Room temperature conductivity varied from sample to sample and was in the range 0.5-1.0 S/cm. The conductivity of pellets of the powder with no PVA is in the range 1-2 S/cm. This conductivity is comparable to t h a t of bulk polyaniline prepared chemically or electrochemically despite the presence of the nonconducting component, which is the polymeric surfactant. Scanning tunneling microscopy" indicated the absence of surfactant chains where there is contact between the conducting components once the solvent is removed. This allows for the hopping of charge carriers between the colloidal particles. Conductivity measurements as a function of pressure and temperature" indicated that conduction is pressuredependent and occurs via a variable. range hopping mechanism. Cyclic Voltammetry. Films for this study were prepared by spin coating onto a platinum electrode of a P4VP-stabilized polyaniline colloid mixed with PVA plasticizer whose nominal M , was 25 OOO. The mixture consisted of 5 mL of 1.2 wt/vol colloid and 1 mL of 1.0 wt/vol PVA. The electrolyte used was 2 M HCI, and the reference electrode was a dynamic hydrogen electrode. Cyclic voltammograms shown in Figure 4 are for both a limited range of cycling of 0 . 1 4 9 V and for cycling with higher anodic end potentials. The features of the voltammograms are very similar to those recorded for electrochemically grown polyaniline films on inert metal electrodes. The similarities include (i) the quasireversibility of the first voltammetric peak and the large capacitive current observed in the voltammograms recorded with limited anodic potentials and (ii) the loss of charge capacity in the first peak region and the apparent lowered conductivity when cycling with higher anodic end potentials. The response of the film was fully maintained a t 500 mV/s when cycling with a limited anodic end potential. Some lagging of the charging process could be seen at 500 mV/s when a much thicker film of the same material was employed as seen in Figure 5. The latter film was formed by placing a drop of the colloidal dispersion on the electrode surface and allowing the solvent to evaporate. Most of the lagging seen at 500 mV/s could be simply due to the RC time constant determined by the electrolyte resistance and the overall capacity of the film. These preliminary results show charging characteristics very similar to t h w of anodically formed polyaniline films. More detailed impedance measurements are required to
*
Figure 3. Transmiwion elertron microFaph of P2VP-based polyaniline colloids. Mugnification 10 900. prepolgmerization aging time I h.
0
0.1
02
0.3 0.4 0.5 0.6 0.1 0.8 POTEN~ALYSDHE(VO~)
1.0
1.2
Figure 4. Cyclic voltammograms of a polyaniline colloid thin film (
