- Email: [email protected]
The Effect of Poly(methyl vinyl ether-alt-maleic acid) Stabilizer on the Stability of Polyaniline–Poly(methyl vinyl ether-alt-maleic acid) Dispersions
Journal of Colloid and Interface Science 227, 316–321 (2000) doi:10.1006/jcis.2000.6898, available online at http://www.idealibrary.com on
The Effect of Poly(methyl vinyl ether-alt-maleic acid) Stabilizer on the Stability of Polyaniline–Poly(methyl vinyl ether-alt-maleic acid) Dispersions C. M. Koo, B. H. Jeon, and I. J. Chung1 Department of Chemical Engineering, Korea Advanced Institute of Science and Technology, 373-1, Kusong-dong, Yusong-gu, Taejon 305-701, South Korea Received August 2, 1999; accepted April 10, 2000
The polyaniline (PANI) dispersions have been prepared in acidic aqueous media by oxidative dispersion polymerization in the presence of a polymeric stabilizer. The polymeric stabilizer used in this study is the poly(methyl vinyl ether-alt-maleic acid) (PMVEMA) which contains acid groups (–COOH). The PANI–PMVEMA particles have a uniform size and a spherical shape. The PANI– PMVEMA dispersions show almost no desorption of the PMVEMA, even though the sonication at 500 W for 20 min and the centrifugation at 500 rpm for 60 min are performed 10 times. The existence of the PMVEMA on the surface is confirmed by X-ray photoelectron spectroscopy. The dispersion stability of the PANI–PMVEMA dispersions is extensively influenced by ζ potential which was governed by the acid group (–COOH) of the PMVEMA on the PANI– PMVEMA particle surface. °C 2000 Academic Press Key Words: stabilization; ζ potential; polyaniline dispersion; poly(methyl vinyl ether-alt-maleic acid).
INTRODUCTION
In the last decades, many researchers have studied polyaniline (PANI) which has a lot of attractive properties such as environmental stability, low cost of raw material, and easiness of synthesis. The biggest obstacle to prevent its commercial use is the poor processibility that generally is a disadvantage for conducting polymers. To improve the processability, numerous approaches have been developed. One of the most attractive approaches is the preparation of a dispersion. Polypyrrole (PPy) and PANI dispersions have been extensively studied (1–9). Their submicronic particles are prepared by oxidative dispersion polymerization in the presence of a polymeric stabilizer. Conventional polymeric stabilizers such as poly(vinyl alcohol-co-acetate), poly(vinyl pyrrolidone), methyl cellulose, etc. had been proven not to be successful in the preparation of the PANI dispersions but to be successful in the preparation of the PPy dispersions due to a poor physical adsorption of conventional polymeric stabilizers on PANI particles during
1
To whom correspondence should be addressed. Fax: 82-42-869-3984. E-mail: [email protected]. 0021-9797/00 $35.00
C 2000 by Academic Press Copyright ° All rights of reproduction in any form reserved.
polymerization (1–3). Recently, their successful uses in suitable conditions were reported (13, 14, 18, 21). To overcome the poor physical adsorption, Armes et al. used the chemical grafting adsorption strategy (4, 5). They used tailormade polymeric stabilizers, which contained pendant aniline or in-chain N-substituted aniline moieties. These pendant aniline units underwent copolymerization with aniline and thus grafted a polymeric stabilizer to PANI. The chemical-grafting technique provided stable dispersions in which desorption of a polymeric stabilizer did not take place. In the case of PANI, however, long needle-shaped particles were still obtained because graft efficiency was not good enough to make the spherical PANI particle. Also, the fact that most of their stabilizers were tailor-made was a disadvantage. A few researchers have studied the dispersion stability of a conducting polymer particle (8–10). Markham et al. reported that the dispersions of conducting polymers were less stable than those of conventional polymers and other insulating materials, because the Hamaker constants of the former using DLVO (Derjaguin and Landau, 1941; Verwey and Overbeek, 1948) theory were higher than those of the latter (10). If the stabilizer was weakly adsorbed and was easily stripped, the dispersion stability of conducting polymers might be less and less stable. To enhance the dispersion stability of conducting polymers, we used poly(methyl vinyl ether-alt-maleic acid) (PMVEMA) copolymer with acid groups (–COOH) as a polymeric stabilizer because an electrosteric stabilization between particles charged with acid groups of PMVEMA enhances the dispersion stability (11, 12). PMVEMA is copolymer different from the poly(acrylic acid) homopolymer employed by Sun’ group and Yang’s group (15, 16). Until now, PANI dispersion has been studied mainly in the viewpoint of steric stabilizaion. But we analyze the electrosteric stabilization of PANI–PMVEMA dispersions. EXPERIMENTAL
Preparation of Polyaniline Dispersions Oxidative dispersion polymerization was carried out by using the method found in an earlier paper (17). PMVEMA (3 g) (Aldrich, Mw = 210,000) was dissolved in 25 cm3 of
316
317
EFFECT OF PMVEMA STABILIZER ON PANI–PMVEMA DISPERSIONS
distilled water; 35 wt% concentrated HCl (5.7 cm3 ), and aniline (0.54 cm3 ) were successively added to the solution. Aqueous solution (18.5 cm3 ) of ammonium peroxydisulfate (APS) (0.65 g) was added in drops for 30 min. The reaction was carried out at 4◦ C for 24 h. The PANI–PMVEMA was purified three times by centrifugation at about 6000 rpm for 60 min and dried in a vacuum oven at 60◦ C for 72 h. The stable PANI–PMVEMA dispersions have been prepared at various PMVEMA concentrations (1–8 wt%) in aqueous solutions with mole ratios of oxidant/aniline from 0.25 to 2. The Characterization of the PANI–PMVEMA Dispersions To investigate morphology and size of particles, transmission electron microscopy (TEM) was performed. The sample was prepared by the casting and drying of dilute PANI–PMVEMA dispersions on carbon-coated copper grids using EM OMEGA 912 (Carl Zeiss Instrument). The electrical conductivity of pelletized PANI–PMVEMA was measured by the standard fourpoint probe technique at room temperature. Dry samples were pelletized at 3000 psi for 2 min. Chemical compositions of the PANI–PMVEMA were determined by CE EA-1110 elemental analyzer. X-ray photoelectron spectroscopy (XPS) analysis was carried out using a VG ESCALAB MK II spectrometer with a Mg K α X-ray source (1253.6-eV photons) to determine the redox states. The X-ray source was run at a reduced power of 120 W (12 kV and 10 mA) and the pressure in the analysis chamber was kept below 10−8 Torr. To compensate for surfacecharging effects, all binding energies were referenced to the C 1s neutral carbon peak at 284.6 eV. In peak analysis, the linewidth (full width at half maximum, FWHM) of the Gaussian peaks was maintained constant for all components. The ζ potentials were measured with a Zeta Plus Instrument (Brookhaven Instruments Co.). ζ potentials from 10 measurements were averaged. The solution pH was adjusted by the addition of HCl and NaOH. All measurements were performed at 20◦ C. The relative concentration of the PANI–PMVEMA particles was determined indirectly by UV-absorption spectroscopy (Duksan Mecasys Optizen II) to investigate the dispersion stability. RESULT AND DISCUSSIONS
Table 1 gives the characterization of the PANI–PMVEMA dispersions prepared using various concentrations of the PMVEMA in 1.25 M HCl aqueous solution. APS was used as an oxidant and the PMVEMA as a polymeric stabilizer. The third column in Table 1 presents the amount of the PMVEMA adsorbed on PANI–PMVEMA particles. The adsorption increases with the concentration of PMVEMA in the reaction mixture. The PANI– PMVEMA exhibits high conductivity between 0.8 and 2.0 S/cm. Even though PMVEMA acts as an insulator, its conductivity slightly decreases with an increase of the PMVEMA content. The yields of the PANI were as high as about 40%. Figure 1a shows a photograph of the PANI–PMVEMA particles (sample no. 4) (temperature = 4◦ C, [APS]/[aniline] = 0.5)
TABLE 1 Preparation and Characterization of the PANI–PMVEMA Dispersions at Various PMVEMA Concentrations (Acidic Aqueous Medium = 50 cm3 ; 1.25 M HCl; Aniline = 0.54 g; [Aniline]/[APS] = 2; Polymerization Temperature = 4◦ C)
Sample
PMVEMAa (wt%)
1 2 3 4 5
1 2 4 6 8
PMVEMAb PANI
Yieldc (%)
σ (S/cm)
Size (nm)
0.18 0.23 0.27 0.33 0.40
55.3 40.7 36.5 34.9 34.4
2.0 1.6 1.2 0.9 0.8
700 ± 50 500 ± 50 200 ± 40 130 ± 10 125 ± 10
a
Weight percent based on the total reaction volume. Weight ratio. c Yield of polyaniline hydrochloride. b
taken by TEM. The particles have almost a uniform size and a spherical shape. The average size of the particles becomes smaller as the PMVEMA concentration increases as given in Table 1. Table 2 gives the characteristics of the PANI–PMVEMA dispersions prepared at various APS concentrations. Regardless of the APS concentration, the PMVEMA content adsorbed in the PANI–PMVEMA dispersions appears almost constant with the value of about 0.35 and the conductivity has almost a constant value of about 0.8 S/cm. The yield increases linearly up to the ratio [APS]/[aniline] of 1 and levels off over this ratio. This behavior may be explained by the reason that the amount of aniline that participated in polymerization increases up to the ratio [APS]/[aniline] of 1.25 since the stoichiometry of APS to aniline is 1.25 (18). Figures 1b and 1c show the transmission electron spectroscopies of sample no. 7 and 9 in Table 2, respectively. The PANI–PMVEMA particles have almost a uniform size and a spherical shape, irrespective of the concentration of the oxidant. The average size increases up to the ratio of 1. Beyond this ratio, however, the size is almost constant. It can also be explained by TABLE 2 Preparation and Characterization of the PANI–PMVEMA Dispersions at Various APS Concentrations (Acidic Aqueous Medium = 50 cm3 ; 1.25 M HCl; Aniline = 0.54 g; PMVEMA = 3 g; Polymerization Temperature = 4◦ C) [APS]a
PMVEMAb
Sample
[aniline]
6 4 7 8 9
0.25 0.5 1 1.5 2
a
PANI
Yieldc (%)
σ (S/cm)
Size (nm)
0.42 0.33 0.29 0.34 —
13.3 26.6 54.9 56.99 —
0.8 0.9 0.6 0.8 0.4
90 ± 20 130 ± 10 170 ± 10 175 ± 10 180 ± 10
Mole ratio. Weight ratio. c Yield of polyaniline hydrochloride. b
318
FIG. 1. 4◦ C).
KOO, JEON, AND CHUNG
Transmission electron microscopy PANI–PMVEMA dispersion at various conditions (1.25 M HCl; aniline = 0.54 g; polymerization temperature =
the same reason as that for the yield. A conclusion can be drawn from the two results that even though the amount of aniline that participated in polymerization increases up to the ratio of 1.25, the number of particles generated is almost constant (19). The size and shape of particles such as needle, oblong (doublets of spheres), or sphere are determined by a balance between the adsorption rate of stabilizers and the growth rate of a particle (rate of polymerization) (12, 20–22). The shape departs increasingly from sphere in the order of oblong, rice grain, and needle as
the balance increasingly tilts in favor of the growth rate of a particle. Many researchers have tried to obtain the spherical shape. They have recognized that the spherical shape is obtained when the adsorption rate of stabilizers is superior to the polymerization rate. Our results suggest that the PMVEMA acts as an effective polymeric stabilizer to obtain the spherical particles. The PANI– PMVEMA dispersions also show the good desorption stability of PMVEMA. It was confirmed by elemental analysis that even though sonication at 500 W for 20 min and centrifugation at
319
EFFECT OF PMVEMA STABILIZER ON PANI–PMVEMA DISPERSIONS
FIG. 2. XPS spectra of (a) N 1s, (b) Cl 2 p, (c) S 2 p, and (d) C 1s for the PANI–PMVEMA dispersion (sample no. 4).
6000 rpm for 60 min were performed 10 times, the desorption of the PMVEMA scarcely took place. Figure 2 shows XPS spectra to characterize the surface of PANI–PMVEMA particles (sample no. 4). Figure 2a shows the N 1s spectra of PANI–PMVEMA particles. Peak i (398.4 eV) and peak ii (399.6 eV) are assigned to the imine and amine components of PANI, respectively. Peaks iii and iv (>400 eV) are assigned to positively charged nitrogen in relation to the doping level (23). The quantitative analysis of relative composition is shown in Table 3. The relative content of charged nitrogen to total nitrogen is about 42% which indicates the extent of doping. Figure 2b shows the Cl 2 p spectra of PANI–PMVEMA particles. Chlorine in the particles exists in various forms. Each form of chlorine has Cl 2 p3/2 and 2 p1/2 . Peaks i and ii (197.2 and 198.1 eV) are assigned to the chlorine complexed with an imine group and peaks iii and iv (199.1 and 200.2 eV) are assigned to the chlorine complexed with an amine group. Peaks v and vi (201.7 and 202.4 eV) could be ascribed to a covalantly bonded chlorine compound (24). The relative content of chlorine anion to total nitrogen is 25%. Figure 2c shows the S 2 p spectra of PANI–PMVEMA particles; its peak (168.7 eV) is assigned to
sulfate compounds arising from an oxidant. The relative content of sulfate compounds to total nitrogen is 9%. Figure 2d shows the C 1s spectra of PANI–PMVEMA particles. Peak i (284.6 eV) is assigned to the main neutral carbon. Peaks ii and iii (286.2 and 288.7 eV) are assigned to the –C–O– and the –O–(C==O)– component arising from the adsorbed PMVEMA (24–26). The existence of the PMVEMA on the particle surface is confirmed by peaks ii and iii. Also, the peaks ii and iii represent higher TABLE 3 Relative Composition of Charged Atoms Based on the Total Nitrogen Content for the PANI–PMVEMA Dispersions (Sample No. 4) from XPS Relative composition based Charged dopant Relative composition Nitrogen species on nitrogen anions based on nitrogen Imine-N Amine-N Charged N (N+ ) Sum
7% 51% 42% 100%
Imine-Cl− Amine-Cl− Sulfate Sum
7% 18% 9% 34%
320
KOO, JEON, AND CHUNG
FIG. 3. The ζ potential for the PANI–PMVEMA dispersions prepared at various PMVEMA concentrations.
intensities than the values which are expected by the elemental analysis in Table 1. This fact implies that the PMVEMA is highly distributed on the particle surface rather than in the particle core (27). In addition, the total anion content (34%) originated from sulfur and chlorine is not consistent with the positively charged nitrogen (42%), as given in Table 3. This inconsistency implies that –COOH in PMVEMA acts as a dopant. The participation of PMVEMA in the doping procedure may assist the successful preparation of the PANI–PMVEMA particles as well as the strong adsorption of the PMVEMA because it enhances the interaction between PANI and PMVEMA. Figure 3 shows the zeta potential of the PANI–PMVEMA dispersions prepared at various PMVEMA concentrations. The zeta potential indicates the surface potential of a particle. The zeta potentials show a negative value over a wide range of pH. This negative potential is generated by partially ionized acid groups of PMVEMA on the particle surface. Isoelectric point is observed near pH 2–3. The great deviation occurs at around pH 4, which is close to the pKa of acid goup (–COOH) (8). So the acid groups influence considerably the zeta potential of the PANI–PMVEMA dispersions. The zeta potential increases with the PMVEMA content in the PANI–PMVEMA dispersions. Figure 4 shows the relationship between the absorption of light with the wavelength of 800 nm and the PANI–PMVEMA concentration in a solution with a pH of 5. The concentration of the PANI–PMVEMA was determined from the mass of solids after drying. The absorbance shows the exact linearity with the PANI–PMVEMA concentration. The absorption coefficient is evaluated from the slope to have ε = 6457 cm2 /g. Though the absorption coefficient shows a little deviation as the pH varies, the absorbance always shows the exact linearity with the PANI–PMVEMA concentration. This implies that the relative absorbance is equivalent to the relative PANI–PMVEMA concentration in a dispersion regardless of pH. Thereafter, relative absorbance is used as a measure of the relative PANI–PMVEMA concentration to investigate the dispersion stability.
FIG. 4. Absorption spectra at various concentrations of the PANI– PMVEMA dispersions (sample no. 4) at pH 5 and the correlation of absorbance to the concentration of the PANI–PMVEMA dispersions.
Figure 5 gives the relative absorbance of the PANI–PMVEMA dispersions (sample no. 4) as a function of pH at 800 nm. The PANI dispersion concentration was below the critical coagulation concentration to investigate the pH effect. From Fig. 5, it is clear that a dispersion is pretty stable over the wide range of pH except around pH 2–3. The unstable pH 2–3 region exactly corresponds to the isoelectric point. Even though Maeda et al. have shown that the colloid stability of a conducting polymer (polypyrrole) is not governed wholly by the magnitude of ζ potential (28, 29), the dispersion stability of the prepared particles is considered to be extensively governed by the ζ potential. Though the PANI dispersions are stabilized by not only the electrostatic repulsion but also the steric hindrance, the PANI
FIG. 5. The relative absorbance the PANI–PMVEMA dispersions (sample no. 4) as a function of pH at 800 nm. The relative absorbance represents the absorbance after 5 h of settling divided by initial absorbance when the initial concentration of the PANI–PMVEMA was cPANI–PMVEMA = 3.3 × 10−4 g/ml.
EFFECT OF PMVEMA STABILIZER ON PANI–PMVEMA DISPERSIONS
321
and function as a dopant. A dispersion is stable over the wide range of pH except around pH 2–3. REFERENCES
FIG. 6. The dispersion stability of the PANI–PMVEMA dispersions at pH 10. cPANI–PMVEMA = 3.3 × 10−4 g/ml.
dispersion stability below the critical coagulation concentration is extensively influenced by the electric effect. Figure 6 shows the dispersion stability of the PANI– PMVEMA dispersions prepared at various PMVEMA concentrations at pH 10. Sample no. 1 and 2 are slightly unstable, but others are considerably stable, even after 5 days of settling. The dispersion stability increases with the concentration of PMVEMA. This can be explained by the fact that the steric effect as well as the ζ potential become higher with PMVEMA concentration. CONCLUSION
The PANI–PMVEMA particles of nanometer size have been successfully prepared by the oxidative dispersion polymerization using the PMVEMA in acidic aqueous media. They have a uniform size and a spherical shape. Most PMVEMAs are attached on the particle surface firmly and hardly desorbed by sonicating and washing of the PANI–PMVEMA dispersions. The existence of PMVEMA on the particle surface is confirmed by XPS. The dispersion stability of PANI–PMVEMA dispersions is influenced by the ζ potential. The acid groups (–COOH) of PMVEMA on the particle surface contribute to the ζ potential
1. Armes, S. P., and Vincent, B., J. Chem. Soc. Chem. Commun. 288 (1987). 2. Armes, S. P., Miller, J. F., and Vincent, B., J. Colloid Interface Sci. 118, 410 (1987). 3. Bjorklund, R. B., and Liedberg, B., J. Chem. Soc. Faraday Trans. 83, 1507 (1987). 4. Armes, S. P., and Aldissi, M., J. Chem. Soc. Chem. Commun. 88 (1989). 5. Bay, R. F. C., Armes, S. P., Pickett, C. J., and Rydar, K. S., Polymer 32(13), 2456 (1991). 6. DeArmitt, C., and Armes, S. P., J. Colloid Interface Sci. 150, 134 (1992). 7. Armes, S. P., Aldissi, M., Gottesfeld, S., and Agnew, S. F., Langmuir 6, 1745 (1990). 8. Butterworth, M. D., Corradi, R., Johal, J., Lascelles, S. F., Maeda, S., and Armes, S. P., J. Colloid Interface Sci. 174, 510 (1995). 9. Maeda, S., Corradi, R., and Armes, S. P., Macromolecules 28, 2905 (1995). 10. Markham, G., Obey, T. M., and Vincent, B., Colloids Surf. 51, 239 (1990). 11. Napper, D. H., “Polymeric Stabilization of Colloidal Dispersions.” Academic Press, San Diego, 1983. 12. Vincent, B., and Waterson, J., J. Chem. Soc. Chem. Commun. 683 (1990). 13. Eisazadeh, H., Spinks, G., and Wallace, G. G., Polym. Int. 37, 87 (1995). 14. Stejskal, J., Spirkova, M., Riede, A., Helmstedt, M., Mokreva, P., and Prokes, J., Polymer 40, 2487 (1999). 15. Sun, L. F., and Liu, J. M., Polym. Prepr. 33, 379 (1992). 16. Yang, S. M., Chen, W. M., and You, K. S., Synth. Met. 84, 77 (1997). 17. Banerjee, P., Figar, M. L., Bhattacharyya, S. N., and Mandal, B. M., Eur. Polym. J. 30(4), 499 (1994). 18. Stejskal, J., Helmstedt, M., and Kratochvil, P., Langmuir 12, 3389 (1996). 19. Stejskal, J., Kratochvil, P., Gospodinova, N., Terlemezyan, L., and Mokreva, P., Polym. Int. 32, 401 (1993). 20. Banerjee, P., Bhattacharyya, S. N., and Mandal, B. M., Langmuir 11, 2414 (1995). 21. Chattopadhyay, D., and Mandal, B. M., Langmuir 12, 1585 (1996). 22. Digar, M. L., Bhattacharyya, S. N., and Mandal, B. M., Polymer 35, 377 (1994). 23. Tang, J., Jing, X., Wang, B., and Wang, F., Synth. Met. 24, 231 (1988). 24. Pun, M. Y., Neoh, K. G., Kang, E. T., Loh, F. C., and Tan, K. L., J. Appl. Polym. Sci. 56, 355 (1995). 25. Neoh, K. G., Teo, H. W., and Kang, E. T., Langmuir 14, 2820 (1998). 26. Zhang, X., Kang, E. T., Neoh, K. G., Tan, K. L., Kim, D. Y., and Kim, C. Y., J. Appl. Polym. Sci. 60, 625 (1996). 27. Simmons, M. R., Chaloner, P. A., and Armes, S. P., Polymer 37, 2743 (1996). 28. Maeda, S., Gill, M., Armes, S. P., and Fletcher, I. W., Langmuir 11, 1899 (1995). 29. Maeda, S., and Armes, S. P., Chem. Mater. 7, 171 (1995).
The Effect of Poly(methyl vinyl ether-alt-maleic acid) Stabilizer on the Stability of Polyaniline–Poly(methyl vinyl ether-alt-maleic acid) Dispersions C. M. Koo, B. H. Jeon, and I. J. Chung1 Department of Chemical Engineering, Korea Advanced Institute of Science and Technology, 373-1, Kusong-dong, Yusong-gu, Taejon 305-701, South Korea Received August 2, 1999; accepted April 10, 2000
The polyaniline (PANI) dispersions have been prepared in acidic aqueous media by oxidative dispersion polymerization in the presence of a polymeric stabilizer. The polymeric stabilizer used in this study is the poly(methyl vinyl ether-alt-maleic acid) (PMVEMA) which contains acid groups (–COOH). The PANI–PMVEMA particles have a uniform size and a spherical shape. The PANI– PMVEMA dispersions show almost no desorption of the PMVEMA, even though the sonication at 500 W for 20 min and the centrifugation at 500 rpm for 60 min are performed 10 times. The existence of the PMVEMA on the surface is confirmed by X-ray photoelectron spectroscopy. The dispersion stability of the PANI–PMVEMA dispersions is extensively influenced by ζ potential which was governed by the acid group (–COOH) of the PMVEMA on the PANI– PMVEMA particle surface. °C 2000 Academic Press Key Words: stabilization; ζ potential; polyaniline dispersion; poly(methyl vinyl ether-alt-maleic acid).
INTRODUCTION
In the last decades, many researchers have studied polyaniline (PANI) which has a lot of attractive properties such as environmental stability, low cost of raw material, and easiness of synthesis. The biggest obstacle to prevent its commercial use is the poor processibility that generally is a disadvantage for conducting polymers. To improve the processability, numerous approaches have been developed. One of the most attractive approaches is the preparation of a dispersion. Polypyrrole (PPy) and PANI dispersions have been extensively studied (1–9). Their submicronic particles are prepared by oxidative dispersion polymerization in the presence of a polymeric stabilizer. Conventional polymeric stabilizers such as poly(vinyl alcohol-co-acetate), poly(vinyl pyrrolidone), methyl cellulose, etc. had been proven not to be successful in the preparation of the PANI dispersions but to be successful in the preparation of the PPy dispersions due to a poor physical adsorption of conventional polymeric stabilizers on PANI particles during
1
To whom correspondence should be addressed. Fax: 82-42-869-3984. E-mail: [email protected]. 0021-9797/00 $35.00
C 2000 by Academic Press Copyright ° All rights of reproduction in any form reserved.
polymerization (1–3). Recently, their successful uses in suitable conditions were reported (13, 14, 18, 21). To overcome the poor physical adsorption, Armes et al. used the chemical grafting adsorption strategy (4, 5). They used tailormade polymeric stabilizers, which contained pendant aniline or in-chain N-substituted aniline moieties. These pendant aniline units underwent copolymerization with aniline and thus grafted a polymeric stabilizer to PANI. The chemical-grafting technique provided stable dispersions in which desorption of a polymeric stabilizer did not take place. In the case of PANI, however, long needle-shaped particles were still obtained because graft efficiency was not good enough to make the spherical PANI particle. Also, the fact that most of their stabilizers were tailor-made was a disadvantage. A few researchers have studied the dispersion stability of a conducting polymer particle (8–10). Markham et al. reported that the dispersions of conducting polymers were less stable than those of conventional polymers and other insulating materials, because the Hamaker constants of the former using DLVO (Derjaguin and Landau, 1941; Verwey and Overbeek, 1948) theory were higher than those of the latter (10). If the stabilizer was weakly adsorbed and was easily stripped, the dispersion stability of conducting polymers might be less and less stable. To enhance the dispersion stability of conducting polymers, we used poly(methyl vinyl ether-alt-maleic acid) (PMVEMA) copolymer with acid groups (–COOH) as a polymeric stabilizer because an electrosteric stabilization between particles charged with acid groups of PMVEMA enhances the dispersion stability (11, 12). PMVEMA is copolymer different from the poly(acrylic acid) homopolymer employed by Sun’ group and Yang’s group (15, 16). Until now, PANI dispersion has been studied mainly in the viewpoint of steric stabilizaion. But we analyze the electrosteric stabilization of PANI–PMVEMA dispersions. EXPERIMENTAL
Preparation of Polyaniline Dispersions Oxidative dispersion polymerization was carried out by using the method found in an earlier paper (17). PMVEMA (3 g) (Aldrich, Mw = 210,000) was dissolved in 25 cm3 of
316
317
EFFECT OF PMVEMA STABILIZER ON PANI–PMVEMA DISPERSIONS
distilled water; 35 wt% concentrated HCl (5.7 cm3 ), and aniline (0.54 cm3 ) were successively added to the solution. Aqueous solution (18.5 cm3 ) of ammonium peroxydisulfate (APS) (0.65 g) was added in drops for 30 min. The reaction was carried out at 4◦ C for 24 h. The PANI–PMVEMA was purified three times by centrifugation at about 6000 rpm for 60 min and dried in a vacuum oven at 60◦ C for 72 h. The stable PANI–PMVEMA dispersions have been prepared at various PMVEMA concentrations (1–8 wt%) in aqueous solutions with mole ratios of oxidant/aniline from 0.25 to 2. The Characterization of the PANI–PMVEMA Dispersions To investigate morphology and size of particles, transmission electron microscopy (TEM) was performed. The sample was prepared by the casting and drying of dilute PANI–PMVEMA dispersions on carbon-coated copper grids using EM OMEGA 912 (Carl Zeiss Instrument). The electrical conductivity of pelletized PANI–PMVEMA was measured by the standard fourpoint probe technique at room temperature. Dry samples were pelletized at 3000 psi for 2 min. Chemical compositions of the PANI–PMVEMA were determined by CE EA-1110 elemental analyzer. X-ray photoelectron spectroscopy (XPS) analysis was carried out using a VG ESCALAB MK II spectrometer with a Mg K α X-ray source (1253.6-eV photons) to determine the redox states. The X-ray source was run at a reduced power of 120 W (12 kV and 10 mA) and the pressure in the analysis chamber was kept below 10−8 Torr. To compensate for surfacecharging effects, all binding energies were referenced to the C 1s neutral carbon peak at 284.6 eV. In peak analysis, the linewidth (full width at half maximum, FWHM) of the Gaussian peaks was maintained constant for all components. The ζ potentials were measured with a Zeta Plus Instrument (Brookhaven Instruments Co.). ζ potentials from 10 measurements were averaged. The solution pH was adjusted by the addition of HCl and NaOH. All measurements were performed at 20◦ C. The relative concentration of the PANI–PMVEMA particles was determined indirectly by UV-absorption spectroscopy (Duksan Mecasys Optizen II) to investigate the dispersion stability. RESULT AND DISCUSSIONS
Table 1 gives the characterization of the PANI–PMVEMA dispersions prepared using various concentrations of the PMVEMA in 1.25 M HCl aqueous solution. APS was used as an oxidant and the PMVEMA as a polymeric stabilizer. The third column in Table 1 presents the amount of the PMVEMA adsorbed on PANI–PMVEMA particles. The adsorption increases with the concentration of PMVEMA in the reaction mixture. The PANI– PMVEMA exhibits high conductivity between 0.8 and 2.0 S/cm. Even though PMVEMA acts as an insulator, its conductivity slightly decreases with an increase of the PMVEMA content. The yields of the PANI were as high as about 40%. Figure 1a shows a photograph of the PANI–PMVEMA particles (sample no. 4) (temperature = 4◦ C, [APS]/[aniline] = 0.5)
TABLE 1 Preparation and Characterization of the PANI–PMVEMA Dispersions at Various PMVEMA Concentrations (Acidic Aqueous Medium = 50 cm3 ; 1.25 M HCl; Aniline = 0.54 g; [Aniline]/[APS] = 2; Polymerization Temperature = 4◦ C)
Sample
PMVEMAa (wt%)
1 2 3 4 5
1 2 4 6 8
PMVEMAb PANI
Yieldc (%)
σ (S/cm)
Size (nm)
0.18 0.23 0.27 0.33 0.40
55.3 40.7 36.5 34.9 34.4
2.0 1.6 1.2 0.9 0.8
700 ± 50 500 ± 50 200 ± 40 130 ± 10 125 ± 10
a
Weight percent based on the total reaction volume. Weight ratio. c Yield of polyaniline hydrochloride. b
taken by TEM. The particles have almost a uniform size and a spherical shape. The average size of the particles becomes smaller as the PMVEMA concentration increases as given in Table 1. Table 2 gives the characteristics of the PANI–PMVEMA dispersions prepared at various APS concentrations. Regardless of the APS concentration, the PMVEMA content adsorbed in the PANI–PMVEMA dispersions appears almost constant with the value of about 0.35 and the conductivity has almost a constant value of about 0.8 S/cm. The yield increases linearly up to the ratio [APS]/[aniline] of 1 and levels off over this ratio. This behavior may be explained by the reason that the amount of aniline that participated in polymerization increases up to the ratio [APS]/[aniline] of 1.25 since the stoichiometry of APS to aniline is 1.25 (18). Figures 1b and 1c show the transmission electron spectroscopies of sample no. 7 and 9 in Table 2, respectively. The PANI–PMVEMA particles have almost a uniform size and a spherical shape, irrespective of the concentration of the oxidant. The average size increases up to the ratio of 1. Beyond this ratio, however, the size is almost constant. It can also be explained by TABLE 2 Preparation and Characterization of the PANI–PMVEMA Dispersions at Various APS Concentrations (Acidic Aqueous Medium = 50 cm3 ; 1.25 M HCl; Aniline = 0.54 g; PMVEMA = 3 g; Polymerization Temperature = 4◦ C) [APS]a
PMVEMAb
Sample
[aniline]
6 4 7 8 9
0.25 0.5 1 1.5 2
a
PANI
Yieldc (%)
σ (S/cm)
Size (nm)
0.42 0.33 0.29 0.34 —
13.3 26.6 54.9 56.99 —
0.8 0.9 0.6 0.8 0.4
90 ± 20 130 ± 10 170 ± 10 175 ± 10 180 ± 10
Mole ratio. Weight ratio. c Yield of polyaniline hydrochloride. b
318
FIG. 1. 4◦ C).
KOO, JEON, AND CHUNG
Transmission electron microscopy PANI–PMVEMA dispersion at various conditions (1.25 M HCl; aniline = 0.54 g; polymerization temperature =
the same reason as that for the yield. A conclusion can be drawn from the two results that even though the amount of aniline that participated in polymerization increases up to the ratio of 1.25, the number of particles generated is almost constant (19). The size and shape of particles such as needle, oblong (doublets of spheres), or sphere are determined by a balance between the adsorption rate of stabilizers and the growth rate of a particle (rate of polymerization) (12, 20–22). The shape departs increasingly from sphere in the order of oblong, rice grain, and needle as
the balance increasingly tilts in favor of the growth rate of a particle. Many researchers have tried to obtain the spherical shape. They have recognized that the spherical shape is obtained when the adsorption rate of stabilizers is superior to the polymerization rate. Our results suggest that the PMVEMA acts as an effective polymeric stabilizer to obtain the spherical particles. The PANI– PMVEMA dispersions also show the good desorption stability of PMVEMA. It was confirmed by elemental analysis that even though sonication at 500 W for 20 min and centrifugation at
319
EFFECT OF PMVEMA STABILIZER ON PANI–PMVEMA DISPERSIONS
FIG. 2. XPS spectra of (a) N 1s, (b) Cl 2 p, (c) S 2 p, and (d) C 1s for the PANI–PMVEMA dispersion (sample no. 4).
6000 rpm for 60 min were performed 10 times, the desorption of the PMVEMA scarcely took place. Figure 2 shows XPS spectra to characterize the surface of PANI–PMVEMA particles (sample no. 4). Figure 2a shows the N 1s spectra of PANI–PMVEMA particles. Peak i (398.4 eV) and peak ii (399.6 eV) are assigned to the imine and amine components of PANI, respectively. Peaks iii and iv (>400 eV) are assigned to positively charged nitrogen in relation to the doping level (23). The quantitative analysis of relative composition is shown in Table 3. The relative content of charged nitrogen to total nitrogen is about 42% which indicates the extent of doping. Figure 2b shows the Cl 2 p spectra of PANI–PMVEMA particles. Chlorine in the particles exists in various forms. Each form of chlorine has Cl 2 p3/2 and 2 p1/2 . Peaks i and ii (197.2 and 198.1 eV) are assigned to the chlorine complexed with an imine group and peaks iii and iv (199.1 and 200.2 eV) are assigned to the chlorine complexed with an amine group. Peaks v and vi (201.7 and 202.4 eV) could be ascribed to a covalantly bonded chlorine compound (24). The relative content of chlorine anion to total nitrogen is 25%. Figure 2c shows the S 2 p spectra of PANI–PMVEMA particles; its peak (168.7 eV) is assigned to
sulfate compounds arising from an oxidant. The relative content of sulfate compounds to total nitrogen is 9%. Figure 2d shows the C 1s spectra of PANI–PMVEMA particles. Peak i (284.6 eV) is assigned to the main neutral carbon. Peaks ii and iii (286.2 and 288.7 eV) are assigned to the –C–O– and the –O–(C==O)– component arising from the adsorbed PMVEMA (24–26). The existence of the PMVEMA on the particle surface is confirmed by peaks ii and iii. Also, the peaks ii and iii represent higher TABLE 3 Relative Composition of Charged Atoms Based on the Total Nitrogen Content for the PANI–PMVEMA Dispersions (Sample No. 4) from XPS Relative composition based Charged dopant Relative composition Nitrogen species on nitrogen anions based on nitrogen Imine-N Amine-N Charged N (N+ ) Sum
7% 51% 42% 100%
Imine-Cl− Amine-Cl− Sulfate Sum
7% 18% 9% 34%
320
KOO, JEON, AND CHUNG
FIG. 3. The ζ potential for the PANI–PMVEMA dispersions prepared at various PMVEMA concentrations.
intensities than the values which are expected by the elemental analysis in Table 1. This fact implies that the PMVEMA is highly distributed on the particle surface rather than in the particle core (27). In addition, the total anion content (34%) originated from sulfur and chlorine is not consistent with the positively charged nitrogen (42%), as given in Table 3. This inconsistency implies that –COOH in PMVEMA acts as a dopant. The participation of PMVEMA in the doping procedure may assist the successful preparation of the PANI–PMVEMA particles as well as the strong adsorption of the PMVEMA because it enhances the interaction between PANI and PMVEMA. Figure 3 shows the zeta potential of the PANI–PMVEMA dispersions prepared at various PMVEMA concentrations. The zeta potential indicates the surface potential of a particle. The zeta potentials show a negative value over a wide range of pH. This negative potential is generated by partially ionized acid groups of PMVEMA on the particle surface. Isoelectric point is observed near pH 2–3. The great deviation occurs at around pH 4, which is close to the pKa of acid goup (–COOH) (8). So the acid groups influence considerably the zeta potential of the PANI–PMVEMA dispersions. The zeta potential increases with the PMVEMA content in the PANI–PMVEMA dispersions. Figure 4 shows the relationship between the absorption of light with the wavelength of 800 nm and the PANI–PMVEMA concentration in a solution with a pH of 5. The concentration of the PANI–PMVEMA was determined from the mass of solids after drying. The absorbance shows the exact linearity with the PANI–PMVEMA concentration. The absorption coefficient is evaluated from the slope to have ε = 6457 cm2 /g. Though the absorption coefficient shows a little deviation as the pH varies, the absorbance always shows the exact linearity with the PANI–PMVEMA concentration. This implies that the relative absorbance is equivalent to the relative PANI–PMVEMA concentration in a dispersion regardless of pH. Thereafter, relative absorbance is used as a measure of the relative PANI–PMVEMA concentration to investigate the dispersion stability.
FIG. 4. Absorption spectra at various concentrations of the PANI– PMVEMA dispersions (sample no. 4) at pH 5 and the correlation of absorbance to the concentration of the PANI–PMVEMA dispersions.
Figure 5 gives the relative absorbance of the PANI–PMVEMA dispersions (sample no. 4) as a function of pH at 800 nm. The PANI dispersion concentration was below the critical coagulation concentration to investigate the pH effect. From Fig. 5, it is clear that a dispersion is pretty stable over the wide range of pH except around pH 2–3. The unstable pH 2–3 region exactly corresponds to the isoelectric point. Even though Maeda et al. have shown that the colloid stability of a conducting polymer (polypyrrole) is not governed wholly by the magnitude of ζ potential (28, 29), the dispersion stability of the prepared particles is considered to be extensively governed by the ζ potential. Though the PANI dispersions are stabilized by not only the electrostatic repulsion but also the steric hindrance, the PANI
FIG. 5. The relative absorbance the PANI–PMVEMA dispersions (sample no. 4) as a function of pH at 800 nm. The relative absorbance represents the absorbance after 5 h of settling divided by initial absorbance when the initial concentration of the PANI–PMVEMA was cPANI–PMVEMA = 3.3 × 10−4 g/ml.
EFFECT OF PMVEMA STABILIZER ON PANI–PMVEMA DISPERSIONS
321
and function as a dopant. A dispersion is stable over the wide range of pH except around pH 2–3. REFERENCES
FIG. 6. The dispersion stability of the PANI–PMVEMA dispersions at pH 10. cPANI–PMVEMA = 3.3 × 10−4 g/ml.
dispersion stability below the critical coagulation concentration is extensively influenced by the electric effect. Figure 6 shows the dispersion stability of the PANI– PMVEMA dispersions prepared at various PMVEMA concentrations at pH 10. Sample no. 1 and 2 are slightly unstable, but others are considerably stable, even after 5 days of settling. The dispersion stability increases with the concentration of PMVEMA. This can be explained by the fact that the steric effect as well as the ζ potential become higher with PMVEMA concentration. CONCLUSION
The PANI–PMVEMA particles of nanometer size have been successfully prepared by the oxidative dispersion polymerization using the PMVEMA in acidic aqueous media. They have a uniform size and a spherical shape. Most PMVEMAs are attached on the particle surface firmly and hardly desorbed by sonicating and washing of the PANI–PMVEMA dispersions. The existence of PMVEMA on the particle surface is confirmed by XPS. The dispersion stability of PANI–PMVEMA dispersions is influenced by the ζ potential. The acid groups (–COOH) of PMVEMA on the particle surface contribute to the ζ potential
1. Armes, S. P., and Vincent, B., J. Chem. Soc. Chem. Commun. 288 (1987). 2. Armes, S. P., Miller, J. F., and Vincent, B., J. Colloid Interface Sci. 118, 410 (1987). 3. Bjorklund, R. B., and Liedberg, B., J. Chem. Soc. Faraday Trans. 83, 1507 (1987). 4. Armes, S. P., and Aldissi, M., J. Chem. Soc. Chem. Commun. 88 (1989). 5. Bay, R. F. C., Armes, S. P., Pickett, C. J., and Rydar, K. S., Polymer 32(13), 2456 (1991). 6. DeArmitt, C., and Armes, S. P., J. Colloid Interface Sci. 150, 134 (1992). 7. Armes, S. P., Aldissi, M., Gottesfeld, S., and Agnew, S. F., Langmuir 6, 1745 (1990). 8. Butterworth, M. D., Corradi, R., Johal, J., Lascelles, S. F., Maeda, S., and Armes, S. P., J. Colloid Interface Sci. 174, 510 (1995). 9. Maeda, S., Corradi, R., and Armes, S. P., Macromolecules 28, 2905 (1995). 10. Markham, G., Obey, T. M., and Vincent, B., Colloids Surf. 51, 239 (1990). 11. Napper, D. H., “Polymeric Stabilization of Colloidal Dispersions.” Academic Press, San Diego, 1983. 12. Vincent, B., and Waterson, J., J. Chem. Soc. Chem. Commun. 683 (1990). 13. Eisazadeh, H., Spinks, G., and Wallace, G. G., Polym. Int. 37, 87 (1995). 14. Stejskal, J., Spirkova, M., Riede, A., Helmstedt, M., Mokreva, P., and Prokes, J., Polymer 40, 2487 (1999). 15. Sun, L. F., and Liu, J. M., Polym. Prepr. 33, 379 (1992). 16. Yang, S. M., Chen, W. M., and You, K. S., Synth. Met. 84, 77 (1997). 17. Banerjee, P., Figar, M. L., Bhattacharyya, S. N., and Mandal, B. M., Eur. Polym. J. 30(4), 499 (1994). 18. Stejskal, J., Helmstedt, M., and Kratochvil, P., Langmuir 12, 3389 (1996). 19. Stejskal, J., Kratochvil, P., Gospodinova, N., Terlemezyan, L., and Mokreva, P., Polym. Int. 32, 401 (1993). 20. Banerjee, P., Bhattacharyya, S. N., and Mandal, B. M., Langmuir 11, 2414 (1995). 21. Chattopadhyay, D., and Mandal, B. M., Langmuir 12, 1585 (1996). 22. Digar, M. L., Bhattacharyya, S. N., and Mandal, B. M., Polymer 35, 377 (1994). 23. Tang, J., Jing, X., Wang, B., and Wang, F., Synth. Met. 24, 231 (1988). 24. Pun, M. Y., Neoh, K. G., Kang, E. T., Loh, F. C., and Tan, K. L., J. Appl. Polym. Sci. 56, 355 (1995). 25. Neoh, K. G., Teo, H. W., and Kang, E. T., Langmuir 14, 2820 (1998). 26. Zhang, X., Kang, E. T., Neoh, K. G., Tan, K. L., Kim, D. Y., and Kim, C. Y., J. Appl. Polym. Sci. 60, 625 (1996). 27. Simmons, M. R., Chaloner, P. A., and Armes, S. P., Polymer 37, 2743 (1996). 28. Maeda, S., Gill, M., Armes, S. P., and Fletcher, I. W., Langmuir 11, 1899 (1995). 29. Maeda, S., and Armes, S. P., Chem. Mater. 7, 171 (1995).