Langmuir 1998, 14, 6767-6771
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Polyaniline Dispersions. 9. Dynamic Light Scattering Study of Particle Formation Using Different Stabilizers Andrea Riede,* Martin Helmstedt, and Volker Riede Faculty of Physics and Geosciences, University of Leipzig, D-04103 Leipzig, Germany
Jaroslav Stejskal Institute of Macromolecular Chemistry, Academy of Sciences of the Czech Republic, 162 06 Prague 6, Czech Republic Received April 2, 1998. In Final Form: August 25, 1998 Dynamic light scattering was used to study the particle formation in the dispersion polymerization of aniline hydrochloride. The organic polymers poly(N-vinylpyrrolidone) and hydroxypropylcellulose are compared with inorganic silica colloids in their role as steric stabilizers of polyaniline dispersions. The particle size, the relative width of the radii distribution of the particles, the intensity of scattered light, and the optical absorption have all been monitored during the polymerization. The aggregation of dispersion particles stabilized with hydroxypropylcellulose has been observed at the final stages of polymerization. The effect of temperature on the polymerization rate of aniline is reported in terms of activation energies.
1. Introduction Polyaniline1,2 (PANI) is an electrically conducting polymer with many features that could be exploited in various applications.2,3 The preparation of PANI dispersions is one of the ways to improve the processibility of this polymer4,5 and to obtain electrically conducting composites. In a typical dispersion polymerization,6 the monomer (here aniline hydrochloride) is miscible with the reaction medium (water) while the resulting polymer (PANI) is insoluble under the same conditions. The macroscopic precipitation of polymer is prevented by the presence of the steric stabilizer, and colloidal particles of submicrometer size are obtained. The steric stabilizers range from simple water-soluble polymers such as poly(vinyl alcohol),7,8 poly(N-vinylpyrrolidone),7,9 poly(vinyl methyl ether),10 or cellulose ethers11,12 to sophisticated tailor-made copolymer architectures.13,14 Also inorganic colloids have been successfully used for the stabilization of PANI dispersions.15-19 * To whom correspondence should be addressed. E-mail: ariede@ physik.uni-leipzig.de. For part 8, see ref 20. (1) MacDiarmid, A. G.; Epstein, A. J. Faraday Discuss. Chem. Soc. 1989, 88, 317. (2) Trivedi, D. C. In Handbook of Organic Conductive Molecules and Polymers; Nalwa, H. S., Ed.; J. Wiley: Chichester, 1997; Vol. 2, pp 505-572. (3) Stenger-Smith, J. D. Prog. Polym. Sci. 1998, 23, 57. (4) Armes, S. P. Curr. Opin. Colloid Interface Sci. 1996, 1, 214. (5) Elyashevich, G. K.; Kozlov A. G.; Gospodinova, N.; Mokreva, P.; Terlemezyan, L. J. Appl. Polym. Sci. 1997, 64, 2665. (6) Dispersion Polymerization in Organic Media; Barrett, K. E. J., Ed.; J. Wiley: London, 1975. (7) Eisazadeh, H.; Gilmore, K. J.; Hodgson, A. J.; Spinks, G.; Wallace, G. G. Colloids Surf., A 1995, 103, 281. (8) Nagaoka, T.; Nakao, H.; Ogura, K. Anal. Sci. 1996, 12, 119. (9) Stejskal, J.; Kratochvı´l, P.; Helmstedt, M. Langmuir 1996, 12, 3389. (10) Banerjee, P.; Bhattacharyya, S. N.; Mandal, B. M. Langmuir 1995, 11, 2414. (11) Chattopadhyay, D.; Mandal, B. M. Langmuir 1996, 12, 1585. (12) Chattopadhyay, D.; Banerjee, S.; Chakravorty, I.; Mandal, B. M. Langmuir, in press. (13) DeArmitt, C.; Armes, S. P. J. Colloid Interface Sci. 1992, 150, 134. (14) Maeda, S.; Cairns, D. B.; Armes, S. P. Eur. Polym. J. 1997, 33, 245.
In the classical dispersion polymerization6 of, e.g., styrene or methyl methacrylate, the corresponding polymer is soluble in its monomer. Consequently, the polymer particles produced in such polymerization, once nucleated, become swollen by the monomer, and the polymerization proceeds preferentially within the particles. In the case of polyaniline dispersions, the monomer, aniline hydrochloride, is not a solvent for polyaniline and does not swell the nuclei of polyaniline particles. The polymerization takes place in the continuous phase outside the dispersion particles. The precipitating polyaniline macromolecules aggregate together with the attached steric stabilizer to form particles of polyaniline dispersion. In our preceding paper17 we used dynamic light scattering (DLS) to investigate the particle formation during the dispersion polymerization of aniline hydrochloride stabilized by colloidal silica. In this communication we compare the use of such an inorganic colloid to the organic polymer stabilizers. Two water-soluble polymers, earlier found to be efficient steric stabilizers,9,20 have been selected for the present study: poly(N-vinylpyrrolidone) and hydroxypropylcellulose. 2. Experimental Section 2.1. Dispersion Polymerization of Aniline. For dispersion polymerization of aniline hydrochloride three different stabilizers were used: poly(N-vinylpyrrolidone) (PVP, type K90, Fluka, Switzerland), hydroxypropylcellulose (HPC, type Klucel GF, Aqualon GmbH, Germany), and colloidal silica (Ludox AS-40, 40 wt % SiO2, E. I. DuPont de Nemours product distributed by Aldrich). Aniline hydrochloride (259 mg, 2 mmol) was dissolved in 2.8 mL of water, and 5 mL of stabilizer solution (4 wt % aqueous (15) Stejskal, J.; Kratochvı´l, P.; Armes, S. P.; Lascelles, S. F.; Riede, A.; Helmstedt, M.; Prokesˇ, J.; Krˇivka, I. Macromolecules 1996, 29, 6814. (16) Stejskal, J.; Kratochvı´l, P.; Gospodinova, N.; Terlemezyan, L.; Mokreva, P. Polymer 1992, 33, 4857. (17) Riede, A.; Helmstedt, M.; Riede, V.; Stejskal, J. Colloid Polym. Sci. 1997, 275, 814. (18) Gill, M.; Baines, F. L.; Armes, S. P. Synth. Met. 1993, 55-57, 1029. (19) Gill, M.; Armes, S. P.; Fairhurst, D.; Emmett. S.; Pigott, T.; Idzorek, G. Langmuir 1992, 8, 2178. (20) Stejskal, J.; Sˇ pı´rkova´, M.; Riede, A.; Helmstedt, M.; Mokreva, P.; Prokesˇ, J. Polymer, in press.
10.1021/la980365l CCC: $15.00 © 1998 American Chemical Society Published on Web 10/16/1998
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Figure 1. Scanning electron micrograph of polyaniline dispersion particles stabilized with hydroxypropylcellulose and prepared at 0 °C. stock solution in the case of PVP and HPC or 1 mL of colloidal silica diluted with 4 mL of water) was added. The mixture was then brought to desired temperature and 2 mL of 1 M aqueous solution of ammonium peroxodisulfate ()456 mg, 2 mmol of peroxodisulfate) of the same temperature was added to start the oxidative polymerization21 of aniline hydrochloride. The mixture was briefly stirred and then kept at rest9 during the following reaction. 2.2. Dynamic Light Scattering. A direct observation of scattered light during polymerization is prevented by the high absorption of light by PANI. Thus 10 µL of reaction mixture was diluted with 10 mL of 1 M hydrochloric acid in 60 s intervals. After dilution, the polymerization was stopped, and the diluted mixture was then immediately characterized by DLS to determine the hydrodynamic radius, Rh. It has been checked that the particle size is not affected by the dilution. Measurements were carried out with a DLS-SLS-5000 laser light-scattering spectrometer/goniometer (ALV, Germany) equipped with a Nd:YAG laser DPY 315 II (Adlas, Germany) operating at λ0 ) 532 nm and a Multiple Tau Correlator ALV-5000.17 The observation angle was 90° in all cases; the duration of each measurement was 1-5 min. The CONTIN program was used to evaluate the hydrodynamic radius; the detailed description of the data treatment has been reported earlier.17 2.3. UV-vis-Near-IR Spectroscopy. The spectra of the diluted dispersions used for the DLS measurements were taken with a UV-vis-Near IR spectrometer Lambda 19 (PerkinElmer). The absorption coefficient R of the dispersions was calculated from the optical transmission, τλ ) τ0 exp(-Rd), at λ0 ) 532 nm, where d is the thickness of cell. The experimental parameter τ0 is the transmission coefficient of the blank reaction mixture containing all components with exception of aniline hydrochloride or PANI.
3. Results and Discussion 3.1. Particle Formation. Dispersion polymerization of aniline produces spherical PANI particles when HPC is used for the steric stabilization (Figure 1). These are comparable both in the size, shape, and uniformity with the particles stabilized by the colloidal silica.15 Both static16 and dynamic light scattering17 have been used for the characterization of dispersion particles, and especially the latter method proved to be a potent tool in the characterization of particle formation. A typical course of this process can be illustrated for PANI particles prepared in the presence of PVP (Figure 2). A detailed model of the particle formation was presented in a previous paper.17 At the beginning of reaction, the hydrodynamic (21) Stejskal, J.; Kratochvı´l, P.; Jenkins, A. D. Polymer 1996, 37, 367.
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Figure 2. Determination of the induction period, ti, and polymerization time, tp ) te - ti, from the time dependence of hydrodynamic radius, Rh, by sigmoidal fit of the data according to eq 1 (full line). Stabilization by poly(N-vinylpyrrolidone), 0 °C.
radius corresponds to PVP coils, Rs, which are the only macromolecular species in the system. After the induction period ti, PANI produced in the subsequent polymerization is accommodated into dispersion particles that grow as the polymerization proceeds. They reach their final size, Rd, when the polymerization is complete at time te. For the quantitative assessment of the particle formation, the sigmoidal function17
Rh(t) ) (Rs - Rd)[1 + exp(t - t0)g]-1 + Rd
(1)
fits well the time dependence of the hydrodynamic radius (Figure 2), where t0 is the position of the inflection point and g is a parameter related to the rate of polymerization. In the analysis, we have defined the onset of polymerization, ti, as the time where the sigmoidal fit deviated from the radius of the steric stabilizer, Rs, for 1 nm. On the other hand at the end of polymerization, te, the sigmoidal fit differed for less than 1 nm from the final size, Rd. The times ti and te found in this way corresponded in the best way to the onset of the blue coloration at the start of polymerization and to the blue-to-green transition typical for its end.20 Hence, the time of polymerization is defined as tp ) te - ti. 3.2. Comparison of Stabilizers. During the induction period the reaction mixture is in all cases colorless. As we reported earlier,17 in the presence of silica, the lightscattering measurements during the induction period were strongly disturbed by occasional fluctuations of scattered light. This was explained by the production of oxygen microbubbles resulting from decomposition of ammonium peroxodisulfate by organic compounds on the surface of the silica particles. In the presence of water-soluble polymers, like PVP and HPC, the formation of microbubbles is dramatically reduced. By use of PVP and HPC, PANI particles are produced much faster compared to the silica-containing reaction mixture (Figure 3a). This is especially due to the shortening of the induction period, rather than by the time of polymerization itself. The increased rate of aniline oxidation in the presence of various substrates has been reported by Tzou and Gregory.23 In the present case, the difference in the acidity of the stabilizer solutions may be responsible (in the absence of other reaction components PVP solution has pH ) 6.4, HPC has pH ) 7.0, and colloidal (22) Stejskal, J.; Sˇ pı´rkova´, M.; Kratochvı´l, P. Acta Polym. 1994, 45, 385. (23) Tzou, K.; Gregory, R. V. Synth. Met. 1992, 47, 267.
Polyaniline Dispersions
Figure 3. (a) Hydrodynamic radius of particles, Rh, (b) polydispersity of particles characterized by relative width of the radius distribution, σ*, (c) intensity of scattered light (count rate in kHz), I, and (d) absorption coefficient, R (at λ0 ) 532 nm), during the dispersion polymerization of aniline in the presence of poly(N-vinylpyrrolidone) (3) and hydroxypropylcellulose (2) at 0 °C. The data for the stabilization by colloidal silica17 (9) are shown for comparison.
silica has pH ) 9.7) because the oxidation of aniline is faster in more acidic medium.22 By use of PVP and silica as stabilizers, the final particle size is comparable in both cases. For the HPC-stabilized system we observed an additional increase in particle size after the polymerization has been completed (Figure 3a), attributed to the aggregation of dispersion particles. Otherwise we would obtain particles of comparable sizes in all three cases (Figure 3a). This conclusion is substantiated by two findings: (1) The radius of HPCstabilized particles was found by electron microscopy to be about 200 nm (Figure 1) and corresponds to the size of particles observed by DLS before aggregation (Figure 3a). (2) After shaking of diluted dispersions the hydrodynamic size became lower. After 30 min of treatment in an ultrasonic bath, it is reduced to the values observed for the other two stabilizers (cf. also Figure 5 below). If the aggregation occurred during the polymerization, it could be explained by the simultaneous incorporation of the stabilizing chains into two separately growing particles. As it takes place at the final stages of polymerization, a limited aggregation of the final particles seems to be more likely. The polydispersity σ* (defined as the relative width of the radii distribution) during the induction period is high in all cases (Figure 3b), but this value cannot be exactly determined given the short measuring time and the low intensity of scattered light. At the beginning of the
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polymerization, the first crop of PANI macromolecules precipitates into polydisperse aggregates that gradually convert into nuclei of the future particles. Once the nuclei start to grow, the polydispersity of the system decreases and remains at a low level with both PVP and silica stabilizer. Parts c and d of Figure 3 provide some additional information on the course of dispersion polymerization. The intensity of scattered light, proportional to the product of the particle mass and the particle concentration, rapidly increases as the particles are produced and grow (Figure 3c). Soon the absorption of light by PANI starts to dominate (Figure 3d), due to the intense coloration of dispersions, and the observed light scattering is consequently reduced again (Figure 3d). At the end of polymerization, the blue color of PANI (protonated pernigraniline) changes to green (protonated emeraldine),20 and the absorption of green light used in the DLS apparatus is reduced and does not change further (Figure 3d). An additional slow postpolymerization growth of particle size reported earlier for silica-stabilized particles was not observed with polymeric stabilizers. In the case of silica it was explained by the formation of the final overlayer of colloidal silica on the dispersion particle surface.17 Dispersions stabilized by PVP and HPC are stable, and the particle size does not change over long-term storage (months to years). The free HPC steric stabilizer as well as that bound to the particles are, however, prone to the hydrolysis if kept under acidic conditions (pH ≈ 2 after polymerization).22 Although the viscosity of these dispersions markedly decreases due to the hydrolysis of HPC chains during the storage, the particle size remains unaffected. The hydrolysis can be prevented by the neutralization of the dispersions. 3.3. Temperature Dependence of Particle Formation. The rate of particle formation increases with temperature (Figure 4), because the formation of PANI is faster. In PVP-stabilized systems the sigmoidal function (1) fits well the time dependence of the hydrodynamic radius (Figure 4a). Only at temperatures over 30 °C does the procedure become imprecise, because the polymerization is too fast and the diffusion too slow to accommodate all PANI chains in well-defined particles.20 The formation of particles followed by their aggregation has been observed for HPC-stabilized dispersions prepared at low temperatures (0 and 10 °C). In this case, the sigmoidal fit was applied only to that part before the aggregation started. At higher temperatures the particle formation was too fast for reliable data analysis. The growth of the particle size with increasing temperature seems to be the result of the higher rate of polymerization. These results should be viewed with some care: Dynamic light scattering yields the diffusion coefficient of the scattering species, and this quantity is converted to hydrodynamic radius by assuming a spherical particle morphology and by using the Stokes-Einstein relation between the diffusion coefficient and the hydrodynamic radius.17 The earlier experiments indicated15 that the spherical morphology of silica-stabilized particles changes to less regular structures as the temperature and, consequently, the rate of polymerization increases. Recent studies show that this is also the case when HPC is used for steric stabilization:20 particles produced at 0 °C are spherical (Figure 1), while at 20 °C the particle morphology changes to less regular objects. Finally, at 40 °C corallike structures with no resemblance to spheres are produced.20 Thus the hydrodymic radii evaluated by assuming the spherical shape of particles have a qualitative meaning only, unless the spherical shape of particles
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Figure 4. (a) Hydrodynamic radius, Rh, during the dispersion polymerization of aniline in the presence of (a) poly(Nvinylpyrrolidone) and (b) hydroxypropylcellulose at various temperatures. Where feasible, the sigmoidal fits according to eq 1 are shown as full lines.
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Figure 6. Dependence of (a) the induction period, ti, and (b) the polymerization time, tp, on reciprocal temperature, 1/T, for dispersion polymerization of aniline made with three different stabilizers: poly(N-vinylpyrrolidone) (3), hydroxypropylcellulose (2), and colloidal silica (9). Table 1. Activation Energy of the Processes Occurring during the Induction Period, EAi, and of the Polymerization of Aniline, EAp, in the Presence of Various Steric Stabilizers steric stabilizer PVP HPC colloidal silica
Figure 5. Dependence of the final hydrodynamic radius, Rh, for PANI dispersions prepared at temperature T with three stabilizers: poly(N-vinylpyrrolidone) (3), hydroxypropylcellulose (2), and colloidal silica (9). The effect of sonication is indicated by arrows.
has been established. Some apparent variations of the particle size thus may be due to changes in the morphology of scattering objects. The aggregation of HPC-stabilized particles and the reduction of hydrodynamic radius after ultrasonic treatment has been always observed in more than 20 experiments at all polymerization temperatures (Figure 5). With silica-stabilized dispersions no such aggregation was found. 3.4. Rate of Polymerization. The oxidation of aniline to PANI can be divided into two basic parts: (1) the induction period during which reactions of low molecular weight components only take place and (2) the polymer-
activation energy/kJ mol-1 EAi
EAp
42 42 55
42 42 43
ization, in which PANI macromoleculues are produced and dispersion particles formed. With a plot of the logarithm of the induction period, ti, and polymerization time, tp, vs reciprocal temperature, linear dependences are obtained (Figure 6). Both times can be taken as inversely proportional to the reaction rates. From this analogy of the Arrhenius plot, the slope of such dependences is EA/2.303R, where EA is activation energy of the corresponding process and R ) 8.314 J K-1 mol-1 is the gas constant. The results indicate that the activation energy for the processes occurring during the induction period is lower in more acidic media (Figure 6a, PVP- and HPC-stabilized dispersions compared with silica-stabilized polymerizations) (Table 1). The activation energy of the polymerization is the same within the experimental error and is thus independent of the type of steric stabilizer (Figure 6b), even though the rates of polymerization differ: they are highest for PVP-stabilized dispersions, followed by the HPC- and silica-containing system. 4. Conclusions Polyaniline dispersion particles can be obtained when water-soluble polymers, poly(N-vinylpyrrolidone) and
Polyaniline Dispersions
hydroxypropylcellulose, or inorganic colloids (silica) are used for the steric stabilization. The nature of the particle formation, as observed by dynamic light scattering, is similar in all cases. The formation of particles is faster in the presence of polymeric stabilizers, because of the higher acidity of reaction mixtures compared with silicacontaining systems. Particles stabilized with hydroxypropylcellulose form aggregates in the final stages of polymerization. These can be reduced to the individual particles by sonication. The rate of particle formation gets higher as the temperature of polymerization is
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increased. The particle size moderately increases at the same time, but the potential simultaneous changes in particle morphology may complicate the interpretation of the data. The activation energy of the aniline polymerization is not affected by the nature of the steric stabilizer. Acknowledgment. Thanks are due to the Deutsche Forschungsgemeinschaft (He 2123/2-3 and 436 TSE 113/ 6) and the Grant Agency of the Czech Republic (104/97/ 0759) for the financial support. LA980365L
