2414
Langmuir 1996,11, 2414-2418
Poly(viny1 methyl ether) Stabilized Colloidal Polyaniline Dispersions Pallab Banerjee, Sailendra N. Bhattacharyya," and Broja M. Mandal" Polymer Science Unit, Indian Association for the Cultivation of Science, Jadavpur, Calcutta 700 032, India Received September 6, 1994. I n Final Form: March 24, 1995@ A polyaniline(PANI)dispersion in aqueous or aqueous organic medium has been prepared by polymerizing aniline oxidatively using poly(viny1 methyl ether) (PVME) as the stabilizer. The particles have been characterized with respect to their PVME content, extent of doping,the particle size, and specificconductivity. The specific conductivity of the particles in the pressed pellet form ranged between 0.002 and 3.7 S cm-l, depending on synthetic conditions. Transmission electron microscopy shows that the particles are oblong. This shape appears to result from the association of two grown-up spherical particles followed by their further growth by way of filling the valley across the region of contact. The UV-visible absorption spectra of the redispersions have been discussed.
Introduction Dispersions of conducting polymers have attracted considerable research interest by virtue of their ease of processing. In a n earlier communication we briefly reported on the successful dispersion polymerization of aniline using ammonium persulfate as the oxidant, poly(vinyl methyl ether) (PVME) as the polymeric stabilizer and water or aqueous alcohols as the polymerization medium to yield stable dispersions of colloidal conducting polyaniline (PANI) partic1es.l The latter were isolated from the dispersions by centrifugation. PVME is a unique stabilizer in the sense that it allows redispersion of the isolated PANI particles in both aqueous and organic media by virtue of being soluble in water and in a number of organic solvents. Furthermore, PVME being miscible with a number of vinyl polymers such a s polystyrene, polyacrylates, and poly(viny1 ester)s2allows making of welldispersed conducting composites using these latter as matrix polymers and the PVME-coated PANI particles as the dispersed polymer. In this report we present the synthesis and characterization of the polyaniline colloids stabilized by PVME using a number of aqueous organic mixtures as reaction media. This allows us to prepare PANI having a range of conductivity from ca. 0.002 to 3.7 S cm-l, depending on synthetic conditions. The composition of the PANI particles, their morphology, and the UV-visible spectra of the dispersions have also been studied. Experimental Section Reagents and Chemicals. Aniline (E. Merck.)was distilled at a reduced pressure over zinc metal. The middle fraction was collected and stored at -10 "C under argon. PVME (Aldrich) was obtained as a 50%solution in water. It was purified through three cycles of dissolution into water and isolation by warming the solution. It was then dried under vacuum at 70 "C for 72 h. Ethyl alcohol was purified as described earlier.2 All other chemicals were G. R. grade products of E. Merck. Polymerization Procedure. Dispersion polymerization of anilinewas carriedout under magnetic stirring in a double walled glass vessel through the outer jacket of which cold water (2 f 0.5 "C) was circulated. Aniline (An) was dissolved in the appropriate solvents in which the required amount of PVME was also dissolved. The solution was acidified with concentrated @
Abstract published in Advance A C S Abstracts, June 1, 1995.
(1)Banejee, P.; Digar, M. L.; Bhattarchayya, S. N.; Mandal, B. M. Eur. Polvm. J . 1994. 30. 499.
(2) Dfgar, M. L.; Bhattacharyya, S. N.; Mandal, B. M. J.Chem. SOC., Chem. Commun. 1992, 18.
HCl and cooled. To the cold admixture was added dropwise (over 30 min) the required amount of ammonium persulfate (APS)or other oxidants dissolved in water. The amount of HC1 used was 1.25 mol/dm3more than the stoichiometric amount required to neutralize the aniline. The reaction mixture was stirred for different time periods. The colloidal PANI particles were isolated from the dispersion by centrifugation at ca. about 15 000 rpm. The separated particles were washed with hot water (in which PVME is insoluble) to free them from the unreacted material. Characterizationof the PANIProducts. For conductivity measurements, the PANI particles were pressed into pellets, and the conductivity was measured at room temperature by the standard four-probemethod using a constantdc source (Keithley, model 224) and a nanovoltmeter (Keithley,model 181). Transmission electron microscopic studies were made on diluted (ca. 200 ppm) dispersion dried on carbon-coated copper grids using a JEOL JEM 100 CT electron microscope. The N content of PANI was determined using a semimicro Kjeldahl technique.3 For the estimationof C1 and S,the samples were burned in an oxygen flask (Heraeus). For C1, the gases were absorbed in water inside the oxygen flask and the solution was titrated against standard mercuric nitrate solution using diphenylcarbazone and bromophenol blue mixed indi~ator.~ For S, the gases were absorbed in water containing HzOz. The solution was titrated against the standard barium perchlorate using dimethyl sulfanazo (111) indicator.5 DSC thermograms,were recorded using a Perkin-ElmerDelta series DSC-7. The UV-visible spectra of the dispersionswere recorded using a ShimadzuUV-2100 UV-vis spectrophotometer.
Results Table 1 gives the details of the polymerization using 50%ethanol as the medium and the characterization of the polymers. From the data given in Table 1it is clear that the PVME content of the PANI particles increases upon increasing the PVME concentration in the reaction mixture. The presence of PVME in the particles is also established by differential scanning calorimetry (DSC). Figure 1 shows the DSC trace of a PANI-PVME composite which contains 19 wt % PVME as estimated from elemental analysis (Table 1,entry no. 5). The thermogram reveals the glass transition of PVME at -29 "C, proving its presence. Table 1shows also that the conductivity of the PANI particles in pressed pellet form decreases with increased incorporation of nonconducting PVME in them. The PANI as formed has both C1 and S, signifying the (3) Cole, J. 0.; Parks, C.R. Ind. Eng. Chem. 1946, 18, 61. (4) Dirscherl, A.Mikrochim. Acta. 1968, 316. ( 5 ) Budesinsky, B.; Krumlova, L. Anal. Chim. Acta 1967, 39, 375.
1995 American Chemical Society
Langmuir, Vol. 11,No. 7, 1995 2415
PVME Stabilized PANI Dispersions Table 1. Preparation of PANI Dispersions in Acidic (HC1) Aqueous (50%) Ethanola and the Characterization of PANI Particles
entry no.
WE used %
1 2 3
0.0 0.50 1.00
4 5
1.50 2.50
PANI characterization data PVME/ (C1+ u particle size PANP CVNc S)/Nc (Scm-1) (nmP 0 0.40 0.49 4.42 0.08 0.41 0.51 3.74 195 f 17 (L) 0.11 0.41 0.51 2.52 121 f 22(L) 57 f 20 (B) 0.14 0.41 0.51 2.35 0.19 0.41 0.52 1.10 110 f 28 (L) 47 f 21 (B)
[Any[APSl=2;[An] = 0.180mom, Temperature = 2 "C; time Weight ratio, calculated on the basis of reduced N content of the particles relative to pure polyaniline (entry no. 1). Atom ratio. L = length, B = maximum breadth;values are an average of 50 measurements of oblong-shaped particles. = 2 h.
I
5.001
I
Figure 2. Transmissionelectron micrographof PANI particles prepared in 50% ethanol using 0.5% PVME (entry no. 2, Table 1)and redispersed in 50% ethanol after isolation and drying.
1*251 0 -40
-20
0
20
Temperature
40 (OC
60
80
1
Figure 1. DSC trace of PANI-PVME composite containing 19% PVME (entry no. 5, Table 1).
presence of both C1- and so42-/Hso4- as dopant anions. The (C1+ S)/N atom ratio is about 0.5, indicating thereby that the PANI is in the half-oxidized emeraldine state.'jv7 Redispersing the PANI particles in 1.2 mol dm-3 HC1 followed by centrifugation and washing with HC1 eliminates all the S-containing dopant anion^.^^^ Figures 2-4 show the transmission electron micrographs (TEM) of the PANI particles which were isolated from the dispersions,dried, and redispersed by sonication. The particles appear to be aggregates of sphericalprimary particles. The sizes of the oblong less aggregated particles have been measured and are given in Table 1. Figures 2 and 3 reveal that the less aggregated oblong particles evolve from the associationof two spherical particles. The average size of the particles decreases with an increase in stabilizer concentration, which is as is expected. The higher aggregates seen in the micrographs might have formed during isolation and drying of the particles. Table 2 gives the results of the PANI colloid preparation in different aqueous organic media using 1%PVME stabilizer. For the sake of comparison, the characterization data of the PANI particles prepared in each medium in the absence of PVME are also included in Table 2. As is expected, incorporation of PVME in the PANI particles decreases the conductivity of the latter, and this is true for all media. Although the medium effect on conductivity (6) Ray, A.; Asturias, G. E.; Kershner, D. L.; Richter, A. F.; MacDiarmid,A. G. Synth. Met. 1989,29, E 141. (7) Chiang, J. C.; MacDiarmid,A. G. Synth. Met. 1986,13, 193. (8)Armes, S. P.;Miller, J. F. Synth. Met. 1988,22, 385. (9)Armes, S. P.; Aldissi, M.; Agnew, S.; Gottesfeld, S. Langmuir 1990,6, 1745.
-
",+-a?
*. 7-.5?!Tp--
-
Figure 3. Transmissionelectron micrographof PANI particles prepared in 50% ethanol using 1%PVME (entry no. 3, Table 1)and redispersed in 50% ethanol after isolation and drying.
I
Figure 4. Transmissionelectronmicrographof PANI particles prepared in 50% ethanol using 2.5% PVME (entry no. 5,Table 1)and redispersed in 50% ethanol after isolation and drying.
is not very large, conductivity decreases in the following order: aqueous alcohols > aqueous acetone = aqueous CH3CN. Table 3 gives the results of using various oxidants for the preparation of PANI colloids in 50% ethanol. All the three oxidants used viz., K2Cr207, KIOs, and FeCls, yielded PANI dispersions. The mole ratio of aniline to oxidant in
Banerjee et al.
2416 Langmuir, Vol. 11, No. 7, 1995 Table 2. Preparation of PANI Dispersions in Acidic (HCl)Aqueous (60%) Organic Medium and the Characterization of PANI Particlee PANI characterization data entry reaction mixture PVME PVME/ (C1+ u no. medium" used(g) PANIb CVNc S)/Nc (Scm-') 50% methanol
1
50%ethanol
2
5
50% isopropyl alcohol 50% tert-butyl alcohol 50% acetone
6
50% acetonitrile
3 4
0.0 0.5 0.0
0.14
0.5
0.11
0.0 0.5 0.0 0.5 0.0 0.5
0.13
0.12 0.12
0.0 0.5
0.13
0.40 0.41 0.40 0.41 0.41 0.40 0.41 0.41 0.40 0.41 0.41 0.41
0.53 0.53 0.49 0.51 0.52 0.53 0.52 0.54 0.54 0.53 0.52 0.54
3.10 3.06 4.42 2.52 2.10 1.84 1.60 1.40 1.30 0.90 1.10 0.70
a Reaction volume = 50 mL, [An.]/[APS] = 2;An. = 0.82mL; temperature = 2 "C; time = 2 h. Weight ratio; calculated on the basis of reduced N content of the particles relative to pure PANI (prepared without PVME). Atom ratio.
Table 3. Preparation of PANI Dispersions Using Various Oxidants (Ox.) in 60% Ethanol and the Characterization of PANI Particles
entry no.
Ox.
PANI PVME characterization data [An.]/ used time yield PVME/ (3 [Ox.] (g)" (h) (%) PANI* CVNc (Scm-')
Id KzCrz07 6.0
0.0 0.5
2e
No3
6.0
0.0 0.5
3f
FeC13
1.0
0.0 0.5
4 4 2 2 24 24
48.6 42.6 36.0 42.0 9.42 10.20
0 0.09 0 0.09 0 0.09
0.48 0.48 0.48 0.51 0.48 0.49
0.19 0.50
1.20 0.69 0.30 0.002
" Reaction volume = 50 mL, An. = 0.82 mL. Weight ratio; calculated on the basis of reduced N content of the particles relative to pure PANI (i.e. prepared without PVME). Atom ratio. 2 "C. e 25 "C. f Besides FeCl3, 0.5 mol/dm3 HzOz was also used in this preparation; temperature 2 "C. these experiments was set following the method of Pron et al. a t 1mol of aniline for 1equiv of oxidant.1° Also, a 1%(w/v) PVME concentration was used in every case. The yield is much lower when FeC13 is used as the oxidant. The conductivity of the particles prepared using these oxidants are lower than those prepared using (NH4)2S208 oxidant. The lowest conductivity is obtained with FeCl3 oxidant. These results conform with those reported earlier by Cao et al.ll on chemical oxidative polymerization of aniline in the absence of any stabilizer. The UV-visible spectra of the PANI colloids prepared in acidic (HC1) water using four different PVME concentrations are shown in Figure 5. For the recording of the spectra the PANI particles in the original dispersions were sedimented by centrifugation, redispersed in water, and sedimented again. The process was repeated two more times. The wet particles were then used to prepare the final redispersion for spectral examination. Each spectrum in Figure 5 shows three major transitions with peaks a t around 350 and 440-460 nm and a flat band in the 800-900 nm region. Spectrum d, which represents the dispersion ofthe lowest size particles in the series, showed some blue shift in the peaks. The other three dispersions h a v e almost the same spectra. Thus, by focusingattention to the better defined peak in the 440-460 nm region and (10) Pron, A.; Genoud, F. Menardo, C.; Nechtschein, M. Synth. Met. 1988, 24, 193. (11)Cao, Y.; Andreatta, A,; Heeger, A. J.; Smith P. Polymer 1989, 30, 2305.
0 250
COO 600 BOO W a v e l e n g t h (nm)
Figure 5. W-visible spectra of PANI colloids prepared i n acidic (HC1) water using four different PVME concentrations: (a) 0.25% PVME, (b) 0.5% PVME, (c) 1.5%PVME, and (d) 2.5% PVME. The spectra refer to t h e redispersions of wet particles i n water.
to the position of the minimum, it is found that the spectra a-c all peak a t 460 nm and the minimum in all of them appears a t 570 nm, while in spectrum d the peak appears at 440 nm and the minimum at 560 nm. Thus, a blue shift of 20 nm in the peak position has occurredin spectrum d with respect to the three other spectra a, b, and c. The literature provides a n interpretation of the spectra of the emeraldine ~ a l t . ' ~The - ~ peak ~ around 350 nm has been attributed to the x-x* transition while the other two peaks to transitions to the polaron band. The peak positions show a blue shift for solutions of PANI in 80% acetic acid with respect to the emeraldine salt films. Additionally, the 1.5 eV polaron band becomes sharpened in the solution spectrum. These results have been attributed to the localization of the polarons in the solution. Peak positions also depend on the extent of protonation of the emeraldine base. The spectra of PANI dispersions have also been r e p ~ r t e d . ~Stejskal ~ - ~ ~ et al. studied the influence of pH on the spectra of PANI dispersions in They have shown that the polaron band in the red region of the spectrum develops gradually from the 610nm band of the base with decreasing pH. This band of the PANI base has been attributed to the electronic excitation from benzenoid rings to quinoid rings.14J9,23,24 With decreasing pH, increasing protonation of the imine nitrogens of the emeraldine base occurs, resulting in polarons. Increasing protonation increases the conjugationlength of the polaron structure, resulting in the decrease of the polaron band energy. This effect continues up to about pH 4 for PANI colloids.27 As a result, the polaron band shows a gradual (12)Zuo, F.; McCall, R. P.; Ginder, J. M.; Roe, M. G.; Leng, J. M.; Epstein, A. J.; Asturias, G. E.; Ermer, S. P.; Ray, A,; MacDiarmid, A. G. Synth. Met. 1989,29, E 445. (13) Asturias, G. E.; MacDiarmid, A. G.; McCall, R. P.; Epstein, A. J. Synth. Met. 1989,29, E 157. (14) Huang, W. S.; MacDiarmid, A. G. Polymer 1993, 34, 1833. (15)Glarum, S.H.; Marshall, J. H. J. Phys. Chem. 1988,92,4210. (16) Cao, Y.; Smith, P.; Heeger, A. J. Synth. Met. 1989,32, 263. (17) Genies, E. M.; Lapkowski, M. J. Electroanal. Chem. 1987,220,
67. (18) Cushman, R. J.; McManus, P. M.; Yang, S. C. J. Electroanal. Chem. 1987,219,335. (19) Stafstrom, S.; Sjogren, B.; Bredas, J. L. Synth. Met. 1989,29, E219 (20) Genies, E. M.; Lapkowski, M. Synth. Met. 1987,21, 117. (21) Sariciftci, N. S.; Kuzmany, H. Synth. Met. 1987,21, 157. (22) Li, Y.; Yan, B.; Yang, J.; Cao, Y.; Quian, R. Synth. Met. 1988, 25, 79. (23) Wan, M. J. Polym. Sci. Polym. Chem. 1992, 30, 543. (24) Stafstrom, S.; Bredas, J. L.; Epstein, A. J.; Woo, H. S.; Tanner, D. B.; Huang, W. S.;MacDiarmid,A. G. Phys. Reu. Lett. 1989,59,1464. (25) Gospodinova, N.; Mokreva, P.; Terlemezyan, L. J. Chem. Soc., Chem. Commun. 1992,923. (26) DeArmit, C.; Armes, S. P. J. Colloid Interface Sci. 1992, 150, 134. (27) Stejskal, J.; Kratochvil, P. Synth. Met. 1993, 61, 225.
PVME Stabilized PANI Dispersions
250
400
600
Langmuir, Vol. 11, No. 7, 1995 2417
800
Wavelength (nm)
Figure 6. Same as Figure 5 exceptthat the redispersionswere
prepared using vacuum-dried particles.
red shift with increasing pH until a pH of -4 is reached. Increasing the acidity of the medium still h r t h e r to 1 M HCl does not apparently effect further protonation; i.e. the emeraldine base is converted fully to the emeraldine salt in the pH range -0-4. In order to examine whether the spectral shift observed in spectrum d of Figure 5 arises from a pH effect, the pH of the dispersions was measured. They are much below 4,being 2.04,2.20,1.82,and 2.33 for curves a, b, c, and d, respectively. In view of the above discussion on pH effect, the PANI colloid particles should have been fully protonated to the emeraldine salt in each case. Therefore, there should not have been any difference in the spectra. However, as will be discussed later, the degree of protonation depends not only on the pH but also on the physical state of PANI. Figure 6 shows the spectra of the same PANI colloids as Figure 5 with the exception that the redispersion is now carried out after the particles are dried at room temperature for 48 h in a vacuum. The pH of the redispersions have now increased to 4.04,4.12,4.02, and 4.08 for redispersions a, b, c, and d, respectively. This increase of pH from -2 to -4 on drying the particles before redispersing them is attributable to the removal of any free HC1 in the inadequately washed particles on vacuum drying. However, the spectral shape for redispersions a-c has not altered much as a result of increasing the pH to -4. The peak position of the 3 eV polaron band now appears a t 460,450,460,and 430 nm, respectively, for spectra a, b, c, and d. The spectrum d, representing the redispersion of the smallest size particles, again showed a blue shift in the peak position of the 3 eV polaron band. Furthermore, the low-energy (1.5eV) polaron band has become much less flattened and blue shifted in spectrum d. This feature was not observed at the lower pH (pH = 2.33,see Figure 5). This fact suggests that the spectral change in the 1.5 eV band of spectrum d relative to the three other spectra, a, b, and c, arises out of a difference in the degree of protonation. In finding out an answer to the question why spectrum d is only affected on increasing the pH from -2 to -4, we found from the literature that the response of the spectrum to pH depends on the physical state of PANI. Thus, the spectrum of the emeraldine base in the film form remains unchanged on changing the pH from 6 to 4;14whereas in the colloidal state the spectra of the emeraldine salt completelydeveloped at pH -4.27 The morphology of the particles prepared with the lowest (0.25%)and the highest (2.5%)PVME concentration is shown in Figures 7 and 8, respectively. These particles are composed of primary n a n o p a r t i c l e ~ . ~The ~ . ~particles ~ shown in Figure 7can be (28) Armes, S. P.; Aldissi,M.; Hawley, M.; Beery, J. G.; Gottesfeld,
S.Langmuir 1991, 7, 1447.
Figure 7. Transmission electronmicrograph of PANI particles prepared in water using 0.2% PVME and redispersed in water after isolation and drying (particle size 285 x 220 nm).
-
-
I' I
Figure 8. Transmission electron micrograph of PANI particles prepared in water using 2.5% PVME and redispersed in water after isolation and drying (particle size 145 x 65 nm).
broken down easily to nanoparticles of -20 nm diameter or less by sonication, while very little disintegration into nanoparticles could be achieved for particles shown in Figure 8.29 This proves that the nanoparticles are much more compacted in the latter case than in the former. This difference in degree of compactness might have resulted in the degree of protonation at pH -4. The small blue shift in the 3 eV polaron band for spectrum b from the positions seen in spectra a and c may be in tune with the higher pH (pH = 4.12)of this dispersion compared to those of a (pH = 4.04)and c (pH = 4.02). However, this pH effect is not reflected in the 1.5 eV band due perhaps to the flatness of the band and the smallness of the effect. The adsorption isotherms at 2 "C relating the mass of PVME ( m )adsorbed to that of PANI with the equilibrium PVME concentration in two media, viz., H20 and 50% aqueous ethanol, are shown in Figure 9. The isotherm in water is characterized by a n initial region of increased adsorption with increasing concentration of PVME and a plateau region of constant adsorption for the equilibrium PVME concentration range 5- 10g/dm3which is followed by another region of increased adsorption with further increase in PVME concentration. Adsorption curves of such shape have been reported for polypyrrole particles prepared using poly(viny1alcohol)stabilizer. In that case, (29) Banejee, P.; Mandal, B. M. Macromolecules, in press.
2418 Langmuir, Vol. 11, No. 7, 1995
Banerjee et al.
APS (as was used in the unsuccessful workz6),was also used by them. To overcomethe problem of low adsorption of conventional stabilizers Armes et al. as well a s Vincent et al. used tailor-made stabilizers which are able to undergo graft copolymerization with aniline and thus get anchored on PANI particles to help the dispersion polymerization of aniline.34-39 PVME is unique a s a stabilizer since it helps dispersion polymerization even when used a t concentrations as low as 0.25%. The oblong rice-grain shape of the PANI colloid particles appears to result in the majority of cases from the association of two precursor spherical particles. Nucleation of particles takes place by way of aggregation of PANI moleculeswhich separate from the medium because I I I I I 1 I I of their insolubility in the latter. The nuclei adsorb the 0 5000 10000 15000 20000 stabilizer on their surface in order to reduce their Equm. P V M E Conc ( p p m ) interfacial energy. However, the rate of adsorption ofthe Figure 9. Adsorption isotherm of PVME on PANI latex stabilizer may not be sufficiently high to give a stabilizer particles in water ( 0 )and in 50% ethanol ( 0 )under conditions coverage sufficient enough to prevent the aggregation of given in Table 1. nuclei. The process of nuclei aggregation and growth by capturing nascent PANI molecules continues, leading to the increased adsorption beyond the plateau region was particles with adequate stabilizer coverage. Because attributed to multilayer a d s ~ r p t i o n . ~ ~ PANI is s e m i ~ r y s t a l l i n e , 4 ~ the - ~ ~aggregation of nuclei/ The isotherm in 50%ethanol is distinctly different. The particles will not lead to their coalescence as in the case adsorption monotonicallyincreases with increasing PVME of liquid drops. Smaller particles may adhere together in concentration used, although a knee is observed in the a spherical cluster wherein the stabilizer molecules from isotherm a t the equilibrium PVME concentration of 5 g/L. the surface area of the constituent particles in contact The greater adsorption from 50% ethanol is in accord with would migrate to the solid-liquid interface of the cluster the lower particle size obtained in this medium compared in order to help minimize the interfacial free energy. to that in water under otherwise identical experimental Indeed, using scanning tunneling microscopy, Armes et conditions (compare Figure 2 with Figure 7, and Figure al. did find 5-10 nm diameter spherical particles inside 4 with Figure 8). the bigger (-100 nm) spherical polypyrrole (PPY) or the rice-grain-shaped PANI colloid particles.28 Discussion As the particles grow, the grown-up spherical particles In our previous papers on the dispersion polymerization might further need to aggregate. Once again, the staof polypyrrole using PVME stabilizer we established from bilizer from the surface area in contact in the aggregates a n intrinsic viscosity study that 50% ethanol is a better would be released and available for adsorption on the solvent for PVME than water and the solvent power solid-liquid interface. If the aggregated particle is a increases with a decrease in the temperature, so dispersion doublet, an oblong shape would eventually result because polymerization in water is only feasible a t lower temwhen the doublet grows still further, the middle region peratures (ca. 0 "C) but not at ambient t e m p e r a t ~ r e . ~ ~ which ~ ~ has a dip is likely to be preferentially filled since On the other hand, in aqueous alcohol media this that would lead to a decrease in the interfacial area. sensitivity to temperature does not exist. The greater effectiveness of aqueous alcohols as dispersion media was Acknowledgment. The authors thank the Regional attributed to their better solvent behavior toward PVME, Sophisticated Instrumentation Centre located a t Bose which helps to exert greater excluded volume effect for Institute, Calcutta, India, for the transmission electron the adsorbed PVME stabilizer so that the steric stabilizamicroscopic work. They also thank one of the referee for tion of the PANI particles is facilitated. drawing attention to the earlier work on the spectra of Previous studies on the synthesis of polyaniline disperPANI dispersions. sions reported a lack of success with conventional polymeric stabilizers like poly(viny1alcohol-co-acetate) (PVA), LA9407121 methyl cellulose, and poly(vinylpyrrolidone).32 It was therefore concluded that these stabilizers adsorb poorly (34) h e s , S. P.; Aldissi, M.; Gottesfeld, S.; Agnew, S. F. Langmuir 1990.6. on PANI particles. On the contrary, Gospodinova et al. -, - , 1745. (35) h e s , S. P.;Aldissi, M.; Gottesfeld, S.; Agnew, S. F.Mol. Cryst. claimed successful dispersion polymerization using PVA Liq. Cryst. 1990, 190, 63. stabili~er.~ , ~ ~the reason for their success they As~ for (36) Bay, R. F.C.; Armes, S. P.; Pickett, C. J.; Ryder, K.S. Polymer suggested that under their experimental conditions gr&1991.32.2456. (37) Tadros, P.;Armes, S. P.; Luk, S. Y. J . Matter. Chem. 1992,2, ing of PVA onto PANI particles occurred, although no 125. proof of grafting was given and the same oxidant, viz, (38) Cooper,E. C.;Vincent, B. J . Phys. D.App1. Phys. 1989,22,1580.
I/
ov
~~
~
~
(30) Armes, S. P.;Miller, J. F.; Vincent, B. J . Colloid Interface Sci. 1987, 118,410. (31) Digar, M. L.;Bhattacharyya, S.N.;Mandal,B. M.Polymer 1994, 35, 377. (32) Armes, S. P.; Aldissi, M. J . Chem. SOC.,Chem. Commun. 1989, 88. (33) Stejskal, J.; Kratochvil, P.; Gospodinova, N.; Terlemezyan, L.; Mokreva, P.Polymer Commun. 1992, 33, 4857.
~~
(39) Waterson, J.;Vincent, B. J . Chem. Soc., Chem. Commun 1990, 683. (40) Andreatta, A.; Cao, Y.; Chiang, J. C.; Heeger, A. J.; Smith, P. Synth. Met. 1988,26, 383. (41)Pouget, J. P.; J6zefowicz, M. E.; Epstein, A. J.; Tang, X.; MacDiarmid, A. G. Macromolecules 1991,24, 779. (42) J6zefowicz, M. E.; Laversanne, R.; Javedi, H. H. S.; Epstein, A. J.; Pouget, J. P.; Tang, X.; MacDiarmid, A. G. Phys. Rev. B 1989, 39, 12958.
