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Synthesis and Characterization of Colloidal Polypyrrole Particles Using Reactive Polymeric Stabilizers M. R. Simmons, P. A. Chaloner, and S. P. Armes* School of Chemistry, Physics and Environmental Science, University of Sussex, Falmer, Brighton BN1 9QJ, U.K.
S. J. Greaves and J. F. Watts Department of Materials Science and Engineering, University of Surrey, Guildford, Surrey GU2 5XH, U.K. Received August 28, 1997. In Final Form: November 14, 1997 The use of new “tailor-made” reactive statistical copolymers for the synthesis of sterically stabilized polypyrrole colloids is described. These copolymer stabilizers are readily prepared by free-radical copolymerization of (bi)thiophene-based vinylic monomers with various hydrophilic vinyl monomers such as 2-(dimethylamino)ethyl methacrylate, 2-vinylpyridine, N-vinylpyrrolidone or oligo(ethylene oxide) methacrylate. Monitoring the oxidation of the bithiophene graft sites using visible absorption spectroscopy provided evidence for stabilizer grafting. Relatively high stabilizer efficiencies were obtained and the resulting spherical polypyrrole particles contained 14 to 48% stabilizer by mass and had reasonably narrow size distributions in the 50-100 nm range. Pressed pellet conductivities were as high as 4 S cm-1. X-ray photoelectron spectroscopy studies indicated that the polypyrrole particles were coated with an overlayer of grafted stabilizer, as expected from steric stabilization theory. This route to polypyrrole particles is believed to be completely general and is expected to allow the rational design of steric stabilizers containing a wide range of functional comonomers. This should be useful for the improved design and performance of immunodiagnostic assays based on polypyrrole “marker” particles.
Introduction Polypyrrole is a relatively air-stable organic conducting polymer which suffers from poor processability.1 Numerous research groups have reported the synthesis of sterically stabilized polypyrrole particles via dispersion polymerization, usually in aqueous media.2-6 In most cases the polymeric stabilizer is probably only physically adsorbed onto the surface of the polypyrrole particles. There is no doubt that these dispersions are significantly more processable than conventional polypyrrole “bulk powders” or films. In addition, Tarcha and co-workers at Abbott Laboratories reported that poly(vinyl alcohol)stabilized particles have considerable potential as novel “marker” particles in immunodiagnostic strip assays.7 In this context, high electrical conductivity is irrelevant: the * To whom correspondence should be addressed. (1) See, for example, the Proceedings of the 1992 International Conference on Synthetic Metals (ICSM ‘92) Synth. Met. 1993, 55-57. (2) Bjorklund, R. B.; Liedberg, B. J. Chem. Soc., Chem. Commun. 1986, 1293. (3) Armes, S. P.; Vincent, B. J. Chem. Soc., Chem. Commun. 1987, 288. Armes, S. P.; Miller, J. F.; Vincent, B. J. Colloid Interface Sci. 1987, 118, 410. (4) (a) Cawdery, N.; Obey, T. M.; Vincent, B. J. Chem. Soc., Chem. Commun. 1988, 1189. (b) Epron, F.; Henry, F.; Sagnes, O. Makromol. Chem., Macromol. Symp. 1990, 35/36, 527. (c) R. Odegard, R.; Skotheim, T. A.; Lee, H. S. J. Electrochem. Soc. 1991, 138, 2930. (d) Digar, M. L.; Bhattacharyya, S. N.; Mandal, B. M. J. Chem. Soc., Chem. Commun. 1992, 18. (5) (a) Armes, S. P.; Aldissi, M.; Agnew, S. F. Synth. Met. 1989, 28, 837. (b) Armes, S. P.; Aldissi, M.; Idzorek, G. C.; Keaton, P. W.; Rowton, L. J.; Stradling, G.; Collopy, M. T.; McColl, D. B. J. Colloid Interface Sci. 1991, 141, 119. (6) (a) Armes, S. P.; Aldissi, M. Synth. Met. 1990, 37, 137. (b) Beaman, M.; Armes, S. P. Colloid Polym. Sci. 1993, 271 (1), 70. (7) Tarcha, P. J.; Misun, D.; Wong, M.; Donovan, J. J. In Polymer Latexes: Preparation, Characterisation and Applications; Daniels, E. S., Sudol, E. D., El-Aassar, M. S., Eds.; ACS Symposium Series No. 492; American Chemical Society: Washington, DC, 1992; Vol. 22, p 347.
“value-added” properties of the polypyrrole colloids are their intense intrinsic coloration (due to their highly conjugated structure) and their relatively narrow particle size distributions. Suitable steric stabilizers for polypyrrole have usually been commercially available watersoluble polymers selected on a rather ad hoc basis; there are very few papers describing the rational design and synthesis of “tailor-made” stabilizers.8,9 In contrast, a wide range of copolymer stabilizers have been synthesized and shown to be successful for the preparation of polyaniline colloids.10-14 Armes and co-workers copolymerized 4-aminostyrene with various hydrophilic comonomers such as 2-vinylpyridine or N-vinylpyrrolidone to produce reactive copolymer stabilizers containing pendent aniline units.10,12,14 Similarly, Vincent and Waterson reported that glycidyl- and methacrylate-functionalized polymers were effective steric stabilizers. However, other workers have shown that unmodified water-soluble polymers such as poly(vinyl alcohol-co-vinyl acetate) can also be used to prepare colloidally stable polyaniline dispersions. (8) Armes, S. P.; Aldissi, M. Polymer 1990, 31, 569. (9) Arca, E.; Cao, T.; Webber, S. E.; Munk, P. Polym. Prepr. (Am. Chem. Soc., Div. Polym. Chem.) 1994, 35 (1), 334. (10) Armes, S. P.; Aldissi, M. J. Chem. Soc, Chem. Commun. 1989, 88. (11) Vincent, B.; Waterson, J. J. Chem. Soc., Chem. Commun. 1990, 683. (12) Armes, S. P.; Aldissi, M.; Agnew, S. F.; Gottesfeld, S. Langmuir 1990, 6, 1745. (13) (a) Liu, J.-M.; Yang, S. C. J. Chem. Soc., Chem. Commun. 1991, 1529. (b) Gospodinova, N.; Mokreva, P.; Terlemezyan, L. J. Chem. Soc., Chem. Commun. 1992, 923. (c) Gospodinova, N.; Terlemezyan, L.; Mokreva, P.; Stejskal, J.; Kratochvil, P. Eur. Polym. J., 1993, 29, 1305. (d) Stejskal, J.; Kratochvil, P.; Gospodinova, N.; Terlemezyan, L.; Mokreva, P. Polym. Int. 1993, 32, 401. (14) (a) DeArmitt, C.; Armes, S. P. J. Colloid Interface Sci. 1992, 150, 134. (b) Maeda, S.; Cairns, D. B.; Armes, S. P. Europ. Polym. J. 1997, 33, 245.
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Several years ago we decided to explore the feasibility of preparing polypyrrole colloids using “tailor-made” reactive statistical copolymers which were capable of grafting onto the surface of the polypyrrole particles. It is well-known that alkylation of the thiophene ring system lowers its oxidation potential with respect to electrochemical polymerization.15 Furthermore, the oxidation potentials of 2,2′-bithiophene and pyrrole are quite similar; recently it has been shown that copolymerizing these two comonomers can give a “hybrid” conducting copolymer.16 Thus there appeared to be good literature evidence that pendent alkyl-substituted (bi)thiophene groups should be effective graft sites for the in situ polymerization of pyrrole. Work by both Fernandez and co-workers and Stanke et al. suggested that pendent pyrrolic groups would also be suitable.17 However, in our experience such pyrrolic groups tend to be rather prone to aerial oxidation, leading to somewhat discolored copolymers.18 In a recent communication19 we reported the synthesis of colloidal polypyrrole particles using a tailor-made “reactive” copolymer stabilizer based on 2-(dimethylamino)ethyl methacrylate (DMAEMA). This water-soluble copolymer contained a small percentage of polymerized vinyl (bi)thiophene groups which leads to the in situ chemical grafting of the stabilizer onto the surface of the polypyrrole particles during dispersion polymerization (see Figure 1). Four different (bi)thiophene-based comonomers were examined as potential graft sites: 2-vinylthiophene (2VT), 3-vinylthiophene (3VT), 5-vinyl-2,2′-bithiophene (2VBT) or 2,2′-bithienylmethyl methacrylate (BTMA) (see Figure 2). In the present paper a more detailed account of this work is provided, together with our latest results. In particular, various hydrophilic vinyl monomers have been used to provide the water-soluble component of the “reactive stabilizer”, including both DMAEMA and a sulfopropylbetaine derivative (SBT), oligo(ethylene oxide) methacrylate (OEGMA), N-vinylpyrrolidone (NVP), and 2-vinylpyridine (2VP) (see Figure 2). These examples serve to illustrate the generality of our synthetic route. Thus the rational design of copolymer stabilizers, perhaps with the incorporation of appropriate functional comonomers, can be contemplated for the first time.
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Figure 1. Schematic representation of the synthesis of polypyrrole colloids via aqueous dispersion polymerization using statistical copolymer stabilizers which contain reactive (bi)thiophene groups.
Experimental Section Copolymer Stabilizers. The copolymer stabilizers were synthesized via free-radical copolymerization of DMAEMA with 2VT, 3VT, VBT, or BTMA using AIBN in toluene at 70 °C. Each of these (bi)thiophene-based monomers was synthesized according to known literature procedures.20,21 The comonomer feed ratio was typically 10:1 in favor of the DMAEMA comonomer. The resulting copolymers were purified by precipitation into n-hexane. The dried copolymers contained 7-15 mol % (bi)thiophene-based comonomer as judged by 1H NMR spectroscopy (e.g., in the case of the 3VT-DMAEMA copolymers, the peak integrals at δ 6.7 to 7.3 due to the aromatic thiophene protons (15) Waltman, R. J.; Diaz, A. F.; Bargon, J. J. Electrochem. Soc. 1984, 131 (6), 1452. (16) (a) Peters, E. M.; Van Dyke, J. D. J. Polym. Sci., Polym. Chem. 1991, 29, 1379. (b) Liang, Q. Y.; Neoh, K. G.; Kang, E. T.; Tan, K. L.; Wong, H. K. Eur. Polym. J. 1992, 28 (7), 755. (17) (a) Finzi, C.; Fernandez, J. E.; Randazzo, M.; Toppare, L. Macromolecules 1992, 25, 245. (b) Stanke, D.; Hallensleben, M. L.; Toppare, L. Synth. Met. 1995, 72 (1), 89. (18) Simmons, M. R. DPhil Thesis, University of Sussex, UK, 1996. (19) Simmons, M. R.; Chaloner, P. A.; Armes, S. P. Langmuir 1995, 11, 4222. (20) (a) Trumbo, D. L. J. Polym. Sci., Polym. Chem. 1988, 26, 3127. (b) Trumbo, D. L. J. Polym. Sci., Polym. Chem. 1991, 29, 603. (21) (a) Trumbo, D. L. Polym. Bull. 1988, 19, 217. (b) Wei, Y.; Hariharan, R.; Bakthavatchalam, R. J. Chem. Soc., Chem. Commun. 1993, 1160.
Figure 2. Chemical structures of the various comonomers used to synthesize the reactive statistical copolymers required for the preparation of sterically stabilized polypyrrole particles. were compared to the peak integrals at δ 4.0 due to the -OCH2protons of the DMAEMA residues. These NMR copolymer compositions were generally in good agreement with those
Colloidal Polypyrrole Particles
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Table 1. Summary of the Molecular Weights, Polydispersities, and Copolymer Compositions of the Various Reactive Copolymer Stabilizers Used in This Study copolymer stabilizer
Mna (g mol-1)
Mw/Mna
copolymer compositionb by NMR
copolymer compositionb by microanalyses
P(2VT-DMAEMA) P(3VT-DMAEMA) P(VBT-DMAEMA) P(BTMA-DMAEMA) P(2VT-2VP) P(3VT-2VP) P(VBT-2VP) P(3VT-OEGMA) P(BTMA-OEGMA) P(3VT-NVP)
29 200 25 400 27 900 17 800 28 200 31 600 10 600 19 200 27 600 22 000
1.88 2.13 1.92 1.92 1.91 1.78 1.92 2.15 1.84 2.34
9:91 10:90 11:89 7:93 c c c 9:91 9:91 6:94
10:90 10:90 8:92 9:91 8:92 9:91 18:82 13:87 7:93 5:95
a
As determined from GPC (PMMA standards). b Molar ratio of comonomers. c Not determined due to overlapping peaks.
calculated from CHNS microanalytical data. Gel permeation chromatography (GPC) analyses indicated polydispersities of 1.5 to 2.5 and Mws in the 10 000 to 32 000 range (see Figure 2 and Table 1). Synthesis of Betaine-Based Stabilizers. Polysulfobetaine (SBT) stabilizers were prepared by derivatizing the DMAEMAbased stabilizers using a 10% excess of 1,3-propanesultone (based on DMAEMA residues) in THF at room temperature as described by Lowe et al.22 Purification was achieved by Soxhlet extraction using refluxing THF to remove unreacted 1,3-propanesultone. Both elemental microanalyses and 1H NMR spectroscopy analysis confirmed that the degree of betainization was near quantitative (>95%) and the (bi)thiophene graft sites remained intact.18 Polysulfobetaine molecular weights were calculated, assuming complete betainization, from the precursor copolymer molecular weight. Polypyrrole Colloids. A typical preparation procedure was as follows: Pyrrole (1.00 mL) was added to an aged (0-30 min at room temperature), stirred aqueous solution (100 mL) containing the reactive copolymer stabilizer (0.15-1.00 g) and FeCl3 oxidant (5.80 g). The solution turned black within a few seconds and was stirred at room temperature for at least 16 h. The resulting black dispersion was centrifuged at 10 000 to 40 000 rpm for 1-2 h, the colored supernatant was carefully decanted, and the black sediment was redispersed in water using an ultrasonics bath. This centrifugation-redispersion cycle was repeated twice to ensure the complete removal of inorganic byproducts and excess, nongrafted copolymer stabilizer. Other chemical oxidants used were [NH4]2S2O8 and the H2O2/HBr/Fe3+ catalytic system described by Yamamoto and co-workers.23 The polymerization temperature was also varied in the 0 to 50 °C range. Stabilizer and Colloid Characterization. (a) Gel Permeation Chromatography. Copolymer stabilizer molecular weights were determined using our in-house GPC instrument (THF eluent, refractive index detector, and poly(methyl methacrylate) calibration standards. (b) Particle Size and Morphology. Polypyrrole colloids were sized using both transmission electron microscopy (Hitachi 7100 instrument, carbon-coated copper grids) and dynamic light scattering (Malvern 4700 PCS instrument). The latter measurements were carried out on dilute dispersions at 25 °C using a fixed 90° scattering angle. (c) NMR, FTIR, and Visible Absorption Spectroscopy. NMR spectra of the copolymer stabilizers were recorded in CDCl3 or D2O using a 250 MHz Bruker instrument. FTIR spectra (KBr disk) were recorded using a Nicolet Magna 550 Series II spectrometer at a resolution of 4 cm-1. Visible absorption spectra of aging stabilizer-oxidant solutions were recorded at various time intervals using a Perkin-Elmer Lambda 2S spectrophotometer. (d) Conductivity Measurements. Electrical conductivities of the dried colloids were determined on pressed pellets at room temperature using the conventional four-point probe technique. (e) X-ray Photoelectron Spectroscopy (XPS). Samples were mounted on double-sided adhesive tape: excess loose powder (22) Lowe, A. B.; Billingham, N. C.; Armes, S. P. Chem. Commun. 1996, 1555. (23) Liu, C.-F.; Maruyama, T.; Yamamoto, T. Polym. J. 1993, 4, 363.
was shaken off, leaving enough sample to cover the analysis area (ca. 10 mm2) in the center of the specimen stub. XPS measurements were made using a VG Scientific ESCALAB Mk. II spectrometer interfaced to a VGS 5000S data system based on a DEC PDP 11/73 computer. The operating conditions were as follows: the X-ray source [Al KR (1486.6 eV) or Mg KR (1253.6 eV) radiation] was operated at a power of 450 W (i.e., 13 kV potential and 34 mA emission current). The spectrometer was operated in the fixed analyzer transmission mode at a pass energy of 50 eV (survey spectra) or 20 eV (high-resolution spectra). The base pressure in the sample chamber during analysis was approximately 3 × 10-8 mbar. Good quality survey spectra were obtained with a single scan; core-line high-resolution spectra were integrated over 5-10 scans depending on the intensity of the spectral region of interest. Total spectral acquisition times per sample were typically 10-20 min. Spectral analysis was carried out using the standard VGS 5000S software for quantification, which was based on peak areas calculated from the high-resolution spectra. Minor sample charging effects of the order of 1 eV were observed for some of the samples. The errors incurred by ignoring the low hydrogen contents of the samples were negligible.
Results and Discussion Preliminary Studies. DMAEMA was successfully copolymerized in turn with each of the four (bi)thiophene comonomers depicted in Figure 2. The molecular weights, polydispersities, and copolymer compositions of the resulting copolymers are summarized in Table 1. Each of the three DMAEMA-based reactive copolymers containing 3VT, VBT, or BTMA pendent groups proved to be effective steric stabilizers for polypyrrole particles. On the other hand, only macroscopic precipitation was observed for the 2VT-DMAEMA copolymer under the same reaction conditions. Similar differences were found for 2VP-based stabilizers which contained 3VT and 2VT pendent groups, respectively (see later).18 These observations are particularly perplexing in the light of a recent paper by Stanke and co-workers,24 who reported that thiophene could be successfully grafted onto methacrylate copolymers which contained pendent 2-alkylthiophene groups. The reason(s) for this anomaly are unknown at present. Evidence for Stabilizer Grafting. Some indirect evidence was obtained for the postulated stabilizer grafting mechanism. Aqueous solutions containing the BTMADMAEMA stabilizer and the FeCl3 oxidant changed color from orange-yellow to green on aging at room temperature. The concomitant evolution of an absorption band (λmax at 628 nm) in the visible absorption spectrum was observed, which was attributed to oxidative activation of the pendent bithiophene groups (the reaction conditions were identical with those used in the colloid syntheses except that the pyrrole monomer was omitted). Essentially the same (24) Hallensleben, M. L.; Hollwedel, F.; Stanke, D. Macromol. Chem. Phys. 1995, 196, 3535.
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Table 2. Summary of the Particle Size, Conductivity, and Stabilizer Contents of the Polypyrrole Colloids Synthesized Using the 2-(Dimethylamino)ethyl Methacrylate-Based Copolymer Stabilizers copolymer stabilizer
colloid formation
stabilizer concn (g/L)
oxidant type
σ (S cm-1)
TEM diameter (nm)a
PCS diameter (nm)
stabilizer content (%)b
PDMAEMA P(2VT-DMAEMA) P(3VT-DMAEMA) P(3VT-DMAEMA) P(3VT-DMAEMA) P(VBT-DMAEMA) P(BTMA-DMAEMA) P(3VT-DMAEMA) P(BTMA-DMAEMA)
ppt ppt colloid colloid colloid colloid colloid colloid colloid
10 10 10 5 1.5 10 10 10 10
FeCl3 FeCl3 FeCl3 FeCl3 FeCl3 FeCl3 FeCl3 H2O2/HBr/FeCl3 H2O2/HBr/FeCl3
4 2 1 1 2
