Langmuir 1996, 12, 1585-1588
Methyl Cellulose Stabilized Polyaniline Dispersions Dipankar Chattopadhyay and Broja M. Mandal* Polymer Science Unit, Indian Association for the Cultivation of Science, Calcutta 700032, India Received June 28, 1995. In Final Form: December 4, 1995X Methyl cellulose has been used as a stabilizer in the oxidative dispersion polymerization of aniline using ammonium persulfate oxidant. The dispersion polymerization is unsuccessful in water but successfully effected in aqueous alcohol media. Evidence has been presented that the stabilizer incorporation in polyaniline is increasingly facilitated as the alcohol concentration of the medium is increased. Particle morphology changes from long needle-shaped to oblong (doublets of spheres) or spherical as the alcohol concentration is increased from 30% to 70%.
Introduction Among the conducting polymers, polyaniline (PANI) has the attractive property of environmental stability and dopability by protonation.1-3 In the deprotonated (undoped, nonconducting) state it can be processed from solutions in N-methylpyrrolidone (NMP).4 However, the polymer needs to be redoped by protonation following processing in order to restore its conductivity. On the other hand, the HCl-doped polymer is soluble in pyrrolidine and tripropylamine as well as concentrated acids and can be processed from such solutions.5,6 Recent developments, however, reveal that if the dopant anions have sufficiently large organic structures, PANI becomes soluble in common organic solvents.7-9 This discovery has effected a marked improvement in processability of the polymer. These developments notwithstanding, the polymer in dispersion form retains its attractiveness as far as processability is concerned. Several reports have appeared in the literature of preparing the dispersion by way of oxidative dispersion polymerization of aniline using polymeric stabilizers.10-29 Conventional water-soluble polymeric stabilizers such as methyl cellulose (MC), polyX Abstract published in Advance ACS Abstracts, February 15, 1996.
(1) Chiang, J.-C.; MacDiarmid, A. G. Synth. Met. 1986, 13, 193. (2) Ray, A.; Asturias, G. E.; Kershner, D. L.; Richter, A. F.; MacDiarmid, A. G. Synth. Met. 1989, 29, E141. (3) ICSM 1988 Conference Proceedings. Synth. Met. 1989, 27-29. (4) Angelopoulos, M.; Asturias, G. E.; Ermer, S. P.; Sherr, E. M.; MacDiarmid, A. G. Mol. Cryst. Liq. Cryst. 1988, 160, 151. (5) Han, C. C.; Shacklette, L. N.; Elsenbaumer, R. L. Meet of the Materials Research Society, Symp. Electricals, Optical and Magnetic Properties of Organic Solid State Materials, Boston, MA, Dec 2-6, 1991; 105. (6) Andreatta, A.; Cao, Y.; Chiang, J.-C.; Heeger, A. J.; Smith, P. Synth. Met. 1988, 26, 383. (7) Cao, Y.; Smith, P.; Heeger, A. J. Synth. Met. 1992, 48, 91. (8) Yang, C. Y.; Cao, Y.; Smith, P.; Heeger, A. J. Synth. Met. 1993, 53, 293. (9) Pron, A.; Laska, J.; Osterholm, J. E.; Smith, P. Polymer 1993, 34, 4235. (10) Armes, S. P.; Aldissi, M. J. Chem. Soc., Chem. Commun. 1989, 88. (11) Armes, S. P.; Aldissi, M. Polym. Prepr. (Am. Chem. Soc., Div. Polym. Chem.) 1989, 60, 751. (12) Armes, S. P.; Aldissi, M.; Gottesfeld, S.; Agnew, S. F. Mol. Cryst. Liq. Cryst. 1990, 190, 63. (13) Armes, S. P.; Aldissi, M.; Gottesfeld, S.; Agnew, S. F. Langmuir 1990, 6, 1745. (14) Armes, S. P.; Aldissi, M.; Hawley, M.; Berry, J. G.; Gottesfeld, S. Langmuir 1991, 7, 1447. (15) Tadros, P.; Armes, S. P.; Luk, S. Y. J. Mater. Chem. 1992, 2, 125. (16) Bay, R. M. C.; Armes, S. P.; Pickett, C. J.; Ryder, K. S. Polymer 1991, 32, 2456. (17) Cooper, E. C.; Vincent, B. J. Phys. D: Appl. Phys. 1989, 22, 1580. (18) Vincent, B.; Waterson, J. J. Chem. Soc., Chem. Commun. 1990, 683.
(vinylpyrrolidone) (PVP), and poly(vinyl alcohol)-coacetate) (PVA) have, however, proved to be ineffective although in certain circumstances low yields of colloidal PANI were obtained using some of these stabilizers.26,27 These results were interpreted to indicate that the above stabilizers adsorb poorly on PANI particles. In order to overcome this difficulty Armes et al.10-15,23 used tailormade stabilizers which are capable of being chemically grafted to PANI and help stabilize the PANI dispersions. Gospodinova et al. as well as Stejskal et al., however, found that by proper choice of the concentrations of the reagents dispersion polymerization is possible using PVA as stabilizer.19-22 Some other commercially available stabilizers that have been successfully used are poly(ethylene oxide)17,18 and poly(vinyl methyl ether).24,25 In conductive coating applications and also in blending with other polymers the chemical as well as the physical properties of the stabilizer present on the surface of the PANI particles may play important roles. Here we describe the use of a cellulose-based stabilizer, namely, MC, in the dispersion polymerization of aniline. A preliminary account of the efficacy of MC as the stabilizer has been recently reported by us for the first time.30 Experimental Section Purification of Reagents and Chemicals. Aniline (E. Merck, India) was vacuum distilled over zinc metal; the middle fraction was collected and stored under argon at -10 °C. MC was a product of British Drug Laboratories and used as received. We determined a degree of substitution of 1.7 and a viscosity (19) Gospodinova, N.; Mokreva, P.; Terlemezyan, L. J. Chem. Soc., Chem. Commun. 1992, 923. (20) Stejskal, J.; Kratochvil, P.; Gospodinova, N.; Terlemezyan, L.; Mokreva, P. Polym. Commun. 1992, 33, 4857. (21) Stejskal, J.; Kratochvil, P.; Radhakrishnan, N. Synth. Met. 1993, 61, 225. (22) Stejskal, J.; Kratochvil, P.; Gospodinova, N.; Terlemezyan, L.; Mokreva, P. Polym. Int. 1993, 32, 401. (23) DeArmitt, C.; Armes, S. P. J. Colloid Interface Sci. 1992, 150, 134. (24) Banerjee, P.; Digar, M. L.; Bhattacharyya, S. N.; Mandal, B. M. Eur. Polym. J. 1994, 30, 499. (25) Banerjee, P.; Bhattacharyya, S. N.; Mandal, B. M. Langmuir 1995, 11, 2414. (26) Miller, J. F. B.Sc. Thesis, University of Bristol, U.K., 1987 (quoted in refs 10 and 17). (27) Cooper, E. C. Ph.D. Thesis, University of Bristol, U.K., 1988 (quoted in refs 10 and 17). (28) Osterholm, J.-E.; Cao, Y.; Klavetter, F.; Smith, P. Polymer 1994, 35, 1142. (29) Luk, S. Y.; Lineton, W.; Keane, M.; DeArmitt, C.; Armes, S. P. J. Chem. Soc., Faraday Trans. 1995, 91, 905. (30) Chattopadhyay, D.; Banerjee, P.; Mandal, B. M. In Macromolecules Current trends; Venkatachalan, S., Joseph, V. C., Ramaswamy, R., Krishnamurthy, V. N., Eds.; Allied Publishers Ltd.: New Delhi, India, 1995; Vol. 1, p 227.
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average molecular weight of 4.3 × 105 for it. Commercial distilled water was redistilled over alkaline permanganate and used. Ethyl alcohol (Bengal Chemical and Pharmaceutical Works India) was purified following the method of Danner and Hildebrand.31 It was then fractionally distilled using a 1 m fractionating column packed with 3 mm porcelain beads. Ammonium persulfate (APS) (E. Merck, Germany) was used as received. Polymer Synthesis. Dispersion polymerization of aniline was carried out in aqueous ethanol (30-70% by volume) media inside a double-walled glass vessel. The temperature was maintained at 2 °C by circulating cold water from a thermostat through the outer jacket of the reaction vessel. MC was dissolved in acidified (1.25 N HCl) aqueous ethanol; aniline was added to the solution. An aqueous solution of APS was added dropwise to the admixture, which was stirred magnetically. In all syntheses, a ratio of aniline to persulfate concentration of 1 was used. The PANI was obtained as finely dispersed particles, which were separated by centrifugation. PANI particles thus isolated were washed with warm water. The washed particles were used for preparing the redispersions using sonication. For characterization, the polymer was dried in vacuum for 48 h at 70 °C. Polymer Characterization. Dried particles of PANI were pelletized, and the conductivity was measured following the standard four-probe method using a constant dc source (Keithley Model 224) and a nanovoltmeter (Keithley Model 181). Transmission electron microscopic studies were made on diluted dispersions dried on carbon-coated copper grids using a JEOL JEM 100CT electron microscope. The N content of PANI was estimated by a semimicro Kjeldahl technique.32 The Cl and S were estimated by burning of the samples in an oxygen flask (Heraeus, Germany) followed by absorption of gases and titration according to standard methods.33 The amounts of MC adsorbed per unit mass of PANI in the composities were calculated on the basis of reduced nitrogen content in the PANI-MC composites relative to that of pure PANI. An FTIR infrared spectrophotometer (Perkin-Elmer Model 1600) was used to record the IR spectra of the polymers pelletized with KBr. The pellets were dried at 70 °C in a vacuum oven for 2 days before the spectra were taken.
Chattopadhyay and Mandal Table 1. Polymerization Recipe Used in Aqueous Ethanola and Characterization Data of PANI-MC Composite Particles reaction mixture (%, w/v) entry
1 2 3 4
0.0 0.5 0.5 0.5
0.25 0.25 0.20 0.15
5 6 7 8
0.0 0.5 0.5 0.5
0.50 0.50 0.37 0.25
product characterization MCb particle size (%) (nm)c
(Cl + S)/ σ Nd (S cm-1)
Medium ) 30% Ethanol 0.0 9.0 13.0 18.0
0.46 0.39 0.40 0.40
0.52 0.51 0.52 0.52
14.7 1.34 0.9 0.66
Medium ) 50% Ethanol 0.0 0.42 3.8 0.40 8.2 0.40 22.0 0.38
0.51 0.52 0.51 0.52
13.9 1.7 1.2 1.2
Medium ) 70% Ethanol 0.0 0.42 4.7 203 ( 26 (l) 0.41 137 ( 24 (b) 15.0 0.40 22.0 206 ( 13 (l) 0.40 113 ( 16 (b) 31.0 0.38 38.0 36 ( 4 (l) 0.39 32 ( 5 (b)
a [Aniline]/[APS] ) 1; temperature ) 2 °C; time ) 2 h. b Weight ratio; calculated on the basis of reduced nitrogen content of the PANI-MC composite particles. c l and b refer respectively to the length and breadth of an average of 20 particles as determined from the TEM micrographs. d Atomic ratio.
Results Dispersion polymerization using MC as steric stabilizer in water resulted in macroscopic precipitation of PANI particles. On the other hand, in aqueous ethanol media (30-70% ethanol) colloidal dispersions of PANI can be synthesized.30 Table 1 gives the recipe of the dispersion polymerization of aniline in aqueous ethanol and the characterization data of the PANI-MC composite particles. It would be evident from the data that the incorporation of MC in the colloid particles increases with decreasing aniline and consequently also APS concentration in the reaction mixture when the MC concentration was kept constant for a given medium. The specific conductivity of the PANI-MC composite particles is lower than that of the pure PANI particles. In general, conductivity decreases with the increase of the percentage of nonconducting MC in the PANI-MC composite particles in a given series of experiments. The PANI as formed has both Cl and S, signifying the presence of both Cl- and HSO4-/SO42- as dopant anions, the latter being derived from S2O82- following its reaction with aniline. The (Cl (31) Danner, P. S.; Hildebrand, J. H. J. Am. Chem. Soc. 1922, 44, 2824. (32) Cole, J. O.; Parks, C. R. Ind. Eng. Chem. 1946, 18, 61. (33) Dirscherl, A. Mikrochim. Acta 1968, 316. (34) O’Connor, R. T. In Cellulose and Cellulose derivatives; Bikales, M., Seagal, L., Eds.; Wiley-Interscience: New York, 1971; Vol. V, Part II, p 51. (35) Blackwell, J.; Marchessault, R. H. In Cellulose and Cellulose derivatives; Bikales, M., Seagal, L., Eds.; Wiley-Interscience: New York, 1971; Vol. V, Part II, p 1. (36) Chen, S.-A.; Lee, H.-T. Macromolecules 1993, 26, 3254. (37) Furukawa, Y.; Ueda, F.; Hyodo, Y.; Harada, I.; Nakalima, T.; Kawagoe, T. Macromolecules 1988, 21, 1297. (38) Chen, S.-A.; Fang, W.-G. Macromolecules 1991, 24, 1242.
Figure 1. Incorporated MC in PANI particles plotted against alcohol concentration in the polymerization medium. MC ) 0.5% and AN ) 0.25% were used in the polymerization recipe in every case.
+ S)/N atom ratio is ∼0.5, which points out that the PANI is in the half-oxidized emeraldine state. A comparison of the results (Table 1) obtained using three media differing in ethanol content reveals that under otherwise identical experimental conditions the extent of MC incorporation in the composite particles increases with increasing alcohol content of the medium. This is shown in Figure 1, which plots the weight fraction of MC in the composites vs alcohol concentration of the medium when the initial reaction mixture contained 0.5% MC and 0.25% aniline in every case (results of entries 2, 8, and 14 in Table 1). Since the MC content of the composites is determined by the relative rate of polymerization vs rate of MC adsorption, the above results also mean that incorporation of MC occurs at increasingly faster rates as the alcohol concentration is increased. The curve may be extrapolated to 0% MC for pure water as the medium.
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Figure 2. Transmission electron micrograph of PANI-MC composite particles: (a) prepared in 30% ethanol (entry 2, Table 1); (b) prepared in 50% ethanol (entry 6, Table 1).
Figure 3. Transmission electron micrograph of PANI-MC composite particles prepared in 70% ethanol using different amounts of MC and aniline: (a) 0.5% MC and 0.75% aniline (entry 10, Table 1); (b) 0.5% MC and 0.25% aniline (entry 14, Table 1).
This quantitative data thus substantiates the result that in pure water medium MC fails to act as a stabilizer for the dispersion polymerization of aniline. One consequence of faster rate of incorporation of MC in the composites is that a higher PANI loading is tolerated in the dispersion with increasing alcohol concentration of the medium in the range of 30-70% alcohol. Thus, using 0.5% MC a maximum of 0.75% aniline is tolerated in 70% ethanol in contrast to 0.5% in 50% ethanol and 0.25% in 30% ethanol (see entries 2, 6, and 10 in Table 1). The morphology of the particles prepared in 30% and 50% ethanol is shown in parts a and b, respectively of Figure 2. The morphology was determined using transmission electron microscopy. The particles are long needle-shaped in the former case and aggregated in a complex way with evidence of chaining in the latter case. When the alcohol concentration is increased to 70%, an oblong-shape (doublet of spheres) morphology results for particles prepared using higher aniline and APS concentrations (Figure 3a). Spherical particles are obtained when the aniline and APS concentrations are reduced, the MC concentration being kept the same as in the former system (Figure 3b). FTIR spectra of MC, PANI, and PANI-MC composite particles containing 38% MC are shown in curves a, b, and c, respectively, of Figure 4. The presence of MC in the composite particles is evident from the presence of the C-H stretching bands in the 2850-2975 cm-1 region of the spectrum of the latter (curve c). Discussion For a dispersion polymerization to take place, the polymeric stabilizer should adsorb on the emerging particles and the polymerization medium should be a reasonably good solvent for the stabilizer. The latter condition ensures that the excluded volume effect of the adsorbed stabilizer becomes large enough to bring about adequate repulsion between particles to overcome the van der Waals attraction between them. As far as the MC stabilizer is concerned, aqueous (30-70%) ethanol solutions are better solvents than water. This is borne out by two facts. First, it is well-known that the cloud temperature of a solution of MC in water increases with the
Figure 4. FTIR spectra of MC, PANI, and PANI-MC composite particles containing 38% MC: (a) MC; (b) PANI; (c) PANI-MC.
addition of lower alcohols.39 Second, the intrinsic viscosity [η] of the MC used in this work increases from 0.396 to 0.443 to 0.476 dm3 g-1 at 25 °C as the solvent is changed from water to 30% ethanol to 50% ethanol. Increasing the ethanol concentration still further to ca. 70% brings about a decrease in [η] to 0.447. Thus, among the above solvents, water is the poorest for MC and hence is an inferior medium for dispersion polymerization. However, MC has been used successfully as the stabilizer in the aqueous dispersion polymerization of other monomers, e.g., pyrrole.40 Thus, water may be an inferior but not an impossible medium to support dispersion polymerization using MC. As a corollary, the failure in the case of dispersion polymerization of aniline in water must be due to the poor adsorption of MC on the PANI surface from water. This is supported by results shown in Figure 1 and discussed earlier in this paper. Again, the success achieved on replacing water by aqueous ethanols suggests that the adsorption of MC on polyaniline per se is not weak. In the adsorption process segments of the polymeric stabilizer must replace some of the solvent molecules adsorbed on the particle surface. Obviously, MC fails to replace water molecules adsorbed on PANI (emeraldine salt) surface but succeeds when the adsorbed molecules are ethanol. The facile incorporation of MC in the composites with increasing alcohol content of the medium (Figure 1) tends to support this inference. However, the situation may be more complex than discussed above since other mechanisms of stabilization, e.g., through stabilizer grafted with PANI in situ, may be operative.19,20 Regarding morphology, the formation of needle-shaped, oblong (doublets of spheres), or spherical particles has been discussed already in the literature.17,18,23,25 In brief, the particle size and shape are determined by a balance between the rate of stabilizer adsorption and the rate of particle growth (rate of polymerization).18,23,25 The spherical shape is produced when the balance is in favor of the former.18 The increasing departure from spherical shape, i.e., oblong, rice-grain, needle-shape (in that order), arises (39) Heymen, E.; Trans. Faraday Soc. 1935, 31, 846. (40) Bjorklund, R. B.; Liedberg, B. J. Chem. Soc., Chem. Commun. 1986, 1243.
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as the balance increasingly tilts in favor of the latter. The precursors of these particles are spherical41 and are formed at early stages of the particle growth process. The shape is determined by the size of the precursor particles25 and extent of their aggregation, for which a chain-like structure is favored.18 In view of the discussion on the adsorption of MC on a PANI surface presented earlier in this work (Figure 1), it follows that only a medium rich in ethanol provides an
environment for efficient adsorption of MC. The needleshaped particles are therefore obtained in 30% ethanol, where the adsorption of MC on PANI is the most inefficient of the three media used in this work. In 70% ethanol, adsorption of MC is most efficient and spherical or oblong (doublets of spheres) particles are produced, the former being obtained under conditions of slower polymerization rate (lower aniline and APS concentrations) than the latter.
(41) Armes, S. P.; Aldissi, M.; Hawley, M.; Beery, J. G.; Gottesfeld, S. Langmuir 1991, 7, 1447.