Langmuir 1992,8, 2178-2182
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Particle Size Distributions of Polyaniline-Silica Colloidal Composites M. Gill and S. P. Armes’ School of Chemistry and Molecular Sciences, University of Sussex, Falmer, Brighton, BN1 9QJ, Sussex, U.K.
D.Fairhurst Brookhaven Instruments Corporation, Brookhaven Corporate Park, 750 Blue Point Road, Holtsville, New York 11742
S . N. Emmett Decorative Research, I.C.I. Paints, Wexham Road, Slough, Berkshire, SL2 5DS, U.K.
G. Idzorek Physics Division, Los Alamos National Laboratory, Los Alamos, New Mexico 87545
T. Pigottt Space Science and Technology Group, Los Alamos National Laboratory, Los Alamos, New Mexico 87545 Received February 21,1992. In Final Form: June 1,1992 We have characterized a new polyaniline-silica composite colloid by various particle sizing techniques.
Our transmission electron microscopy studies have confiied for the f i t time an unusual “raspberry“ morphology,with the small silica particles held together by the polyaniline ‘binder”. These particles have average diameters in the sizerange 15Ck500 nm. Chargevelocity analysis experimentsindicateda numberaverage particle diameter of 300 f 80 nm, but only poor statistics were obtained (172 particles counted). Photon correlation spectroscopy studies suggested an intensity-average particle diameter of 380 nm. Disk centrifugephotosedimentometry(DCP)turned out to be our preferred sizingtechnique for the polyaniline silica colloids, since it was both quick and reliable and, more importantly, produced the true particle size distribution (PSD) curve with excellent statistics. The DCP data indicated a weight-averageand numberaverage particle diameter of 330 70 nm and 280 f 70 nm, respectively, and moreover confirmedthe PSD to be both broad and unimodal. Finally,these colloidalcompositeswere sized using the MalvernAerosizer. Using this instrument in conjunction with a nebulizer attachment (which allowed particle sizing of the “wet”dispersion)rather than in the conventional “dry powder” mode, we obtained particle size data which were in reasonable agreement with the DCP results.
*
Introduction Polyaniline is generally recognized to be the only completely air-stable conducting polymer.’ It can be as synthesized either or ele~trochemically6-~ a bulk powder or fii. Although soluble in various solvents such as tetrahydrofuran, dimethylformamide, or N-methylpyrrolidone in the undoped state, the doped (conducting) form is insoluble in most solvents (except concentrated acidsa) and is therefore an intractable and unprocessable material. Various workers have attempted
* Author to whom correspondence should be addressed.
Permanent address: Parks College, Cahokia, IL 62206. for example, the conference proceedings of the ICSM ‘88; Aldiasi, M., Ed. Synth. Met. 1989,27-29, and references therein. (2) (a)Armes, S. P., Miller, J. F. Synth. Met. 1988,22,385. (b)Miller, J. F. B.Sc. Thesis, University of Bristol, 1987. (3) Pron, A.; Genoud, F.; Menardo, C.; Nechtechein,M. Synth. Met. f
(1) See,
1988,24,193. (4) Chiang, J.-C.; MacDiannid, A. G. Synth. Met. 1986, 13, 193. (5) McManus, P. M.;Yana. - S. C.: Cushman. R. J. J. Chem. Soc.. Chem. Commun. 1986,1556. (6) Lacroix, J. C.,Kanazawa,K. K., Dim, A. J.Electrochem. SOC.1989, 136,1308. (7) Watanabe, A.; Mori, K.; Iwaaki, Y.; Murakawi, 5.;Nakamura, Y. J . Polym. Sci., Polym. Chem. 1989,27, 4431. (8) Cao, Y.; Andreatta, A.; Heeger, A. J.; Smith, P. Polymer 1989,30, 2305.
to improve the processability of polyaniline, usually by modifyingthe polymer chain with alkyl, alkoxy, or sulfonic groups.*’l In the last few years several groups have reported the preparation of sterically stabilized colloidal polyaniline particles via dispersion polymerization.2bJ2-20This approach generally requires the use of a tailor-made polymeric stabilizer which becomes chemically grafted to the (9) Ni, S.;Wang, L.;Wang, F.; Shen, L. Polym. Commun. 1989,30,123. (IO) MacInnee, D.; Funt, B. L. Synth. Met. 1988,25, 235. (11) KallitaiB, J.; Koumanakos, E.; D a h , E.; Sakkopoulos,S.; Koub soukos, P. G. J . Chem. SOC.,Chem. Commun. 1989,1146. (12) (a) Cooper, E. C.; Vincent, B. J. Phys. D Appl. Phys. 1989,22, 1580. (b) Cooper, E. C. Ph.D. Thesis University of Bristol, 1988. (13) Arm&, 9. P.; Aldimi, M. J. Chem. Soc., Chem. Commun. 1989,89. (14) Armes,S. P.; Aldiesi, M. Mater. Res. SOC.Symp. Bot. 1989,173, 311. (15) Armea, S. P.; Aldiaei, M.; Agnew, S.F.; Gottesfeld, S. Langmuir 1990,6,1745. (16) Armes,S. P.; Aldiesi, M.; Agnew, S.F.; Gottesfeld,S. Mol. Cryst. Lio. Crvst. 1990. 190.63.
i17)Bay, R. F. C.;’Armes,S. P.; Pickett, C. J.; Ryder, K. S. Polymer
1991.32. 2466.
(18)Tadros, P.; Armes, S. P.; Luk,S. Y. J. Mater. Chem. 1992,2,125. (19) Vincent, B.; Waterson, J. J . Chem. SOC.,Chem. Commun. 1990, 683. (20) DeArmitt, C.; Armes, S.P. J . Colloid. Interface Sci. 1992, 150, 134.
0743-7463/92/2408-2178$03.00/00 1992 American Chemical Society
Langmuir, Vol. 8, No. 9, 1992 2179
Particle Size Distributions of Polyaniline-Silica polyaniline particles during the synthesis. These dispersions usually have a polydisperse morphology, which may either nonspherical12-18 or s p h e r i ~ a l ’ in ~ *nature. ~~ In contrast to our earlier work on conducting polymer composites with large (-1 pm diameter) silica particles,2l we have recently reported that small (138nm diameter) silica particles can also act as effective dispersants, leading to the formation of stable colloidal dispersions of polyaniline-silica composites.22 Our preliminary scanning electron microscopy studies suggested, but were unable to confirm, that the composite particles were submicronic aggregates of the original small silica particles.23 These dispersions were also characterized by FTIR and visible absorption spectroscopy, microanalyses, and thermogravimetric analyses. In the present paper we compare the various particle sizing techniques which we have recently utilized in order to characterize this novel colloidal form of polyaniline. These techniques are transmission electron microscopy (TEM),chargevelocity analysis (CVA), photon correlation spectroscopy (PCS),disk centrifuge photosedimentometry (DCP), and a new “Aerosizer” instrument developed by Malvern Instruments for dry powder sizing.
Experimental Section (1) Preparation of Polyaniline-Silica Composites. The general preparative procedure has been described elsewhere.n Briefly, the polyaniline-silica composites were prepared as follows: 3.0 g of a 50% (w/w) aqueous solution of colloidal silica was added to 50 mL of 1.2 M HC1 solution containing 1.57 g of ammonium persulfateat room temperaturewith constantstirring. Aniline (0.50mL)was then injected and the solutionturned green within 5 min. This reaction mixture was stirred for a further 16 h prior to centrifugation at 5000 rpm for 30 min. The resulting dark green sediment was redispersed in 1.2 M HCl using an ultrasonics bath. This centrifugation-redispersion cycle was repeated twice in order to remove excess nonadsorbed silica particles from the composites. The same batch sample was used for all of the particle sizing experimenta described below. (2) Characterization of the Polyanilins-Silica Colloid. (a)Compositional Analysis. The chemical compositionof the polyaniline-silica colloid was determined by both thermogravimetric analysis (TGA-7 instrument; scan rate 20 OC/min in air) and CHN microanalysis (Perkin-Elmer 2400 instrument). The former technique indicated a polyaniline mass content of approximately28.5% (assumingthe water content of the silica component to be ca. 10% as determinedfrom thermogravimetric analysisof the pure silicaparticleellee Figure I), while the latter technique suggested polyaniline mass content of 28.1 % . (b) Particle Size Analysis. Electron Microscopy. Transmission electron microscopy studies were made using a Philips EM 400 instrument. The diluted colloids were allowed to dry on a carbon-coated gold TEM grid (3 mm diameter Bio-Rad). Chargevelocity Analysis. Charge-velocity analysis experiments were carried out as described elsewhere using the 60kV accelerator at Los Alamos National Laboratory.% Malvern Aerosizer. The compositeswere analyzed both as dry powders (pulsed feed mode using a pulse jet disperser) and as ‘wet” dispersions using a nebulizer attachment. With this technique particle sizing is achieved by time-of-flight measurementa on particles in accelerated hypersonic air flow conditions using two laser beams and a calibration curve computed from (21)Armes, S. P.; Gottesfeld, S.; Beery,J. G.; Garzon, F.; Agnew, S. F. Polymer 1991,32,2325. (22)(a) Gill, M.; Mykytiuk, J.; Armes, S. P.; Edwards, J. L.; Yeatas, T.;Moreland, P. J.; Mollett, C. J. Chem. Soc., Chem. Commun. 1992,108. (b) Terrill, N.;Crowley, T.; Gill, M.; Armes, S. P. To be submitted to Langmuir. (c) Gill, M.; Armes, S. P. To be submitted to Synth. Met. (23)Gill, M.; Armee, S. P. Unpublished results. (24)Armea, S. P.; Aldissi, M.; Idzorek, G. C.; Keaton, P. W.; Rowton, L. J.: Stradlina, G. L.: Collouy. _ _ M. T.;McColl, D. B.J . Colloid. Interface Sci. issi,141;-119.
20
170
320
470
620
770
Temperature (C)
Figure 1. Thermograms of the polyaniline-silica colloid composite (-) and the bare silica (- -) in air at a scan rate of 20 OC/mm. the particle density. The manufacturer’s specification for this instrument claims a particle size range of 200 nm to 60 gm.26 Photon Correlation Spectroscopy. PCS measurements were made on the dust-free, ultrafiitered (1 pm PTFE fiiters, Millipore) polyaniline-silicadispersion (0.025 % (w/v) solids in 1.2 M HC1) at 25 O C using a Malvern 4700 instrument at a scattering angle of 90°. Disk Centrifuge. A BI-DCP instrument (Brookhaven Instruments)was utilized with an external densitygradient%under the following operating conditions: 0.2 mL of a 1-2% (w/v) polyaniline-silica dispersion in either 1.2 M HC1 or HzO; 15 mL of aqueous spin fluid containing 0.2 mL of methanol (this latter component is added in order to create the “density gradient” which leads to fractionation of the particles within the sample); 2000-3000 rpm centrifugationrate (correspondingto run times of approximately20 min);extinctioncoefficientcorrectionfactor = 40 (assuming the highly absorbing, dark green polyanilinesilica particles have similar scattering characteristics to carbon black);temperature = 25 O C . Dodecane was added to the surface of the spin fluid after injection of the sample as a nonvolatile ‘thermal blanket”. This procedureeliminatesevaporationeffects and significantly reduces the possibility of ‘streaming” or nonlaminar flow. Three separateDCP analyseswere carried out on the same sample and the averageparticle diameters obtained were reproducible to within 2 % . Standard deviations were calculated assuming normal statistics for the particle size distributions.
Results and Discussion In our recent communication concerning the synthesis of these novel polyaniline-silica colloids, we reported our preliminary sizing results obtained from scanning electron microscopy and photon correlation spectroscopy studies.22 Our initial PCS experiments confirmed that the polyaniline-silica particles were certainly colloidal in nature but the resolution of our scanning electron microscope was toolow to allow us to obtain unambiguous information with regard to the composite particle morphology. Bearing these results in mind, and since our thermogravimetry analyses indicated that the major component of the colloid by mass was silica,we proposed that the composite particles were probably submicronic aggregates of the original small silica particles held together by the polyaniline component which acted as a “binder”. The higher resolution of the transmission electron microscope has now enabled us to confiim this hypothesis (see Figure 2). The polyanilinesilica composite particles are, t o a first ap(25) Jones, R., Malvem Instruments, personal communication, 1991.
(26)Particle Size Distribution: Assessment and Characterization; Provder, T., Ed.;ACS Symp. Ser. No. 332;American Chemical Society: Washington, DC, 1987;p 191.
2180 Langmuir, Vol. 8, No. 9, 1992
Gill et al. 15
I C
.Y
0
0 0
0
0
0 0
Intensity
- Weighted Particle
Diameter (nm)
Figure 3. Intensity-weighted particle size distribution of the polyaniline-silica colloid as determined by photon correlation spectroscopy.
Figure 2. Transmission electron micrograph of the diluted polyaniline-silica colloidal composite.
proximation, polydisperse spheres lying in the size range 150-500 nm, but closer examination a t higher magnification clearly reveals an unusual “raspberry” morphology. The close proximity of these particles on the TEM grid suggests the possibility of weak flocculation (particle aggregation) of the dispersion prior to solvent evaporation. However, our solution studies by both PCS and DCP techniques indicate that the colloid consists of welldispersed, nonaggregated primary particles (see below). Thus we believe the apparent particle aggregation to be simply an artifact of the TEM sample preparation procedure. We have observed similar effects with other conducting polymer colloid systems.27 The CVA, DCP, and Aerosizer techniques each require a knowledge of the particle density for the accurate sizing of colloidal dispersions. Fortunately this parameter can be easily calculated from the chemical composition of the polyaniline-silica dispersions. Taking the density of amorphous silica to be 2.00 g cm-3 and the density of polyaniline to be 1.50 g cm-3and allowing for the water content of the silica, we calculate the colloid composite density to be approximately 1.80 g cm-3 on the basis of our thermogravimetry data. This value is in excellent agreement with that estimated from our solvent flotation experiments using iodobenzene, iodopropane, and 2-iodoanisole (1.79 f 0.02 g ~ m - and ~ ) was used to analyze the particle size data obtained from the CVA, DCP, and Aerosizer measurements. In the CVA technique24 the dried colloidal particles acquire a surface charge (q)and are then accelerated under vacuum in an applied electric field ( VO).The velocity (u) (27)Armes, S. P.Ph.D. Thesis, University of Bristol 1987.
of each individual particle is determined by time-of-flight measurements over a known distance and hence the mass (m) of the particle is calculated from the relation (1/2)mu2 = qV0. Given the particle density (d) and assuming the particles are spherical, the particle radius (R) is easily calculated from the equation m = (4/3)rR3d. Clearly, a prerequisite for this technique is that the colloidal particles must possess sufficiently high intrinsic electrical conductivity in order to be able to sustain a surface electric charge. Thus this sizing method is limited to colloidal dispersions of materials such as metals, carbon black, and organic conducting polymers. We have previously established that this technique is a useful method for the determination of the numberaverage particle diameter of sterically-stabilizedpolypyrrole colloids, provided their average particle diameter exceeded approximately 100 nm.24928 The solid-state electrical conductivity of the polyaniline-silica particles22 is similar to that of the polypyrrole colloids previously studied,28so we anticipated that the CVA technique would be appropriatefor this new system. Our CVA experiments a t Los Alamos National Laboratory indicated a numberaverage “spherical” particle diameter of 300 f 80 nm but due to time constraints we were only able to analyze 172 particles (rather than several thousand particles as in our earlier e ~ p e r i m e n t s ~ Nevertheless, ~~~~). these results are in good agreement with the particle size range suggested by our TEM studies. This observation suggests that, unlike the polyaniline-coated silica particles previously r e p ~ r t e d ,these ~ ~ , polyaniline-silica ~~ colloidal composites retain their integrity (i.e. do not undergo significant fragmentation) under the rather extreme conditions of the CVA experiment. Our photon correlation spectroscopy experiments indicate an average particle size of approximately 380 nm (see Figure 3). This result, taken together with the TEM and CVA data, suggests that the polyaniline-silica colloids consist of well-dispersed primary particles, with little or no particle aggregation. Thus the PCS experiments suggest that the particle-particle contacts observed in Figure 1 are simply an artifact of the TEM sample preparation procedure. The absolute value of the particle size obtained from the PCS experiment should be treated with some caution, since our assumptionthat the refractive index of the particles is identical to that of the dispersion medium (in the absence of any refractive index literature data for polyaniline) is almost certainly erroneous. How(28)Armes, S.P.;Aldissi, M. Synth. Met. 1990,37, 137. (29)Armes, S.P.;Idzorek, G.; Stradling,G. L. Unpublished results, 1991.
Langmuir, Vol. 8,No. 9, 1992 2181
Particle Size Distributions of Polyaniline-Silica
Table I. Comparison of the Particle Size Data Obtained for the Polyaniline-Silica Colloidal Composite Using the Various Sizing Techniques particle sizing technique
TEM
0.00 0.00
CVA PCS DCP Aerosizer 0.20
0.40
0.60
0.80
1 .oo
Diameter (um)
Figure 4. Number-average particle size distribution of the polyaniline-ailica colloid as measured by disk centrifuge photosedimentometry.
0.00 0.00
0.20
0.40
0.60
0.80
1 .oo
h) Figure 5. Weight-average particle size distribution of the polyaniline-ailica colloid as measured by disk centrifuge photosedimentometry. Diameter
ever, considering the sample polydispersity (asevidenced from the TEM and CVA data and confirmed by our DCP results-see below) and recalling that the PCS experiment leads to an intensity-weighted (rather than weightaverage) particle size,3O we believe our TEM, CVA, and PCS sizing data to be reasonably self-consistent. Disk centrifuge photosedimentometry distinguishes between different particle masses according to their sedimentation rates (via the Stokes-Einstein equation) rather than their scattered light intensities as in PCS experiments. Thus, the former technique yields the true weight-average particle diameter distribution directly without any calibration procedure^.^^ The relatively high density difference between the composite particles and the solvent is particularly advantageous for the DCP measurements since it ensures shorter run times and a lower limit cutoff in particle size close to the theoretical resolution of the instrument (8 nm). Our experiments yielded the number- and weight-average particle diameter distributions shown in Figures 4 and 5. Close inspection of these data enables us to draw the following conclusions: (1) The observed particle size distribution is both broad and unimodal and in good agreement with the TEM and CVA data. (2) The number- and weight-average particle diameters are 280 f 70 nm and 330 f 70 nm, respectively. (3) The polyaniline-silica particles exist as well(30)Particle Size Distribution: Assessment and Characterization;
Provder, T., Ed.;ACS Symp. Ser. No. 332;American Chemical Society: Washington, DC, 1987; pp 48-160. (31)High Resolution Particle Size Analysis of Coating Materials; Thomae, J. C.; Fairhurst, D. In Surface Phenomena and Fine Particles in Watepbased Coatings andPrinting Technology;Sharma, M. K., Micale, F. J., E&.; Plenum Press: New York, 1991; p 213.
size range 150-500 140-560 190-760 140-550 360-800
average particle diameter (nm) number weight PCS average average average 300f80 380 280h70 ~510
330f70
360
dispersed primary particles in the colloidal state with little or no particle aggregation. (4) There is no contamination of the composite particles by the original small silica particles (of diameter 15-40 nm) used in the synthesis of the composite colloid. Thus three centrifugation cycles were clearly sufficient to “cleanup” the dispersion (remove the small silica contaminant) and also these composite particles are stable (do not undergo fragmentation) under the DCP experimental conditions (centrifugation at up to 3000 rpm). (5) The same results were obtained for the composites dispersed in both 1.2 M HC1 and HzO; therefore this polyaniline-silica colloid is not aggregated in either medium. (6) The DCP analysis also produces an equivalent “PCSaverage”particle diameter of approximately 360 nm which compares reasonably well with the particle diameter obtained in our actual PCS experiments described above. The particle size data initially obtained using the Malvern Aerosizer in the conventional “dry powder” mode were rather disappointing. The statistics were excellent (>lo6particles counted) but the modal number-average particle diameter was approximately 750 nm and the size range was 300-2000 nm. Clearly the particulate deagglomeration mechanism was inefficient under these conditions and a substantial fraction of particle aggregates or clusters were detected in addition to the primary particles, leading to an overestimate of the average particle diameter. In contrast, we noted that the Aerosizer was able to quickly and accurately size other dry powder dispersions such as silica (clpM diameter) and alumina (=450 nm diameter) under the same operating conditions. Our later experiments utilized a nebulizer attachment to size a “wet” dispersion of the polyaniline-silica colloid directly. This procedure was much more satisfactory, although poorer statistics were obtained (only -103 particle&. The numberaverage particle diameter was approximately 510 nm and the particle size range was 360-800 nm. These results are in broad agreement with the results obtained from the other techniques; we believe the observed overestimate of the true particle size in these latter experiments is simply due to the instrument’s artificial cutoff at a lower limit particle diameter of approximately 200 nm. We have observed similar effects with CVA measurements on polypyrrole colloidswith particle diameters less than 100 nm.24 A summary of the particle size data for the polyanilinesilica colloid composite obtained from the various particle sizing techniques is presented in Table I. Allowing for the obvious fundamental differences between these techniques, we believe that our results are in reasonable agreement and are generally self-consistent. The composites are submicrometer-sized and they are neither appreciably aggregated in the dispersed state nor contaminated with any of the original small silica particles. Further work is currently in progress to examine the effect of varying the composite colloid synthesis conditions (e.g. type of oxidant and monomer, acid concentration, temperature, size of the original small silica particles, etc.) on
2182 Longmuir, Vol. 8, No. 9,1992 the particle size distributions of these polyaniline-ailica dispersions. These results will be cormnunicated elsewhere.
Conclusions We have characterized the particle size distribution of a new polyaniline-silica colloidal composite by trammiasion electron microscopy,charge-velocity analysis,photon correlation spectroscopy, disk centrifuge photosedimentometry, and the Malvem Aerosizer instrument. The resulta obtained from these sizing techniques are generally in reasonable agreement. Our transmission electron microscopy studies confii for the fmt time that these composite particlea conekt of submimnic aggregatesof the original small silica particles held together by the polyaniline ‘binder”, resulting in an unusual ‘raspberry” morphology. Our preferred sizing technique is the disk centrifuge photodimentometer, which is quick and reliable and, more importantly, yields the true overallparticle diameter distribution curve in addition to the weighbaverage particle diameter. This technique c o n f i i the broad, unimodal size distribution of the polyaniline-silica composite and gives number- and weightcaverage particles diameters of 280 f 70 nm and 330 i 70 nm,respectively.
Gill et al. There is no evidence for the presence of the original small silica particles in the polyaniline-ailica dispersions
in either our transmission electron micrographs or our DCP data.
Acknowledgment. We thank the following people for their generous assistance in this work J. Smith and C. Mombourquette of Los Alamos National Laboratory (transmiasion and scanning electron microscopy, respectively); L. Rowton of Los Alamos National Laboratory (charge-velocity analysis); R. Jones of Malvern Instrumenta (Malvern Aerosizer);L. Hughes of I.C.I. Paints (disk centrifuge photosedimentometer) and J. L. Edwards of I.C.I. Corporate Colloid Science Group (donation of the silica sample used for the polyaniline-silica composite syntheeis). The SERC is thanked for capital equipment funds for the purchase of both the photon correlation spectrometer and the thermogravimetric analyzer (GR/ F/73977 and GR/G/10654). S.P.A. wishes to thank both the Nuffield Foundation and the Society for Chemical Industry for generous travel/subsistence funds which enabled this collaborative project to be carried out. M.G. gratefully acknowledges a SERC PbD. CASE studentship with I.C.I. Resins. Registry No. Polyaniline, 26233-30-1; silica, 7631-86-9.
