Langmuir 1995,11, 1899-1904
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Surface Characterization of Conducting Polymer-Silica Nanocomposites by X-ray Photoelectron Spectroscopy Shuichi Maeda, Michael Gill, and Steven P. h e s * School of Chemistry and Molecular Sciences, University of Sussex, Falmer, Brighton, BN1 9QJ, U.K.
Ian W. Fletcher I.C.I. Materials, P.O. Box 90, Wilton, Middlesborough, Cleveland, TS90 8JE,U.K. Received October 24, 1994. In Final Form: February 10, 1995@ The surface characterizationof a range of conducting polymer-silica colloidal nanocompositesby X-ray photoelectron spectroscopy (XPS)is described. The silicon atoms in the silica component and the nitrogen atoms in the conducting polymer (polypyrrole or polyaniline) component have been utilized as unique elemental Umarkers". Thus, measurement of the silicodnitrogen atomic ratio by XPS allows a semiquantitative assessment of the silicdconductingpolymer surface composition of the nanocomposite particles. These surface atomic ratios were then compared with the bulk silicodnitrogen atomic ratios calculated for the nanocomposites from our macroscopic chemical composition data. We were able to confirmthat, for all samples investigated,the surface composition of the conductingpolymer-silica particles is silica-rich with respect to their bulk composition. These observations are consistent with the observed long-termcolloidal stabilityof these dispersions. Furthermore, the surfacecompositionof the polypyrrolesilica nanocomposites was correlated with the colloid stability of these dispersions in pH 3 and 9 buffer solutions. Finally, our XPS data confirm that, although somewhat depleted from the surface ofthe particles, the conductingpolymer component is nevertheless present. Thisobservation is consistent with the relatively high solid state electrical conductivities obtained for compressed pellets of these materials.
Introduction Both polypyrrole and polyaniline are relatively airstable organic conducting polymers. Although there have been some promising recent advances in improving the processability of polyaniline using suitable surfactanttype dopant anions,l polypyrrole is generally considered to be a cross-linked, and hence inherently intractable, material.2 Our conducting polymer research program at Sussex is focused on developing new synthetic routes for the preparation of colloidal forms of both these materials as a means of improving their processability. Together with various other research groups we have described the use-ofnumerous polymeric stabilizers and small-molecule surfactants as dispersants for submicron-sized polypyrrole and polyaniline particle^.^-^ Recently we have reported a new and convenient method of preparing colloidal polypyrrole and polyaniline in ~~
published in Advance ACS Abstracts, May 1, 1995. (1)(a)Cao, Y.; Smith, P.; Heeger, A. J. Synth. Met. 1992,48,91.(b) Kulszewicz-Bajer, I.; Pretula, J.; Pron,k J.Chem.Soc.,Chem. Commun. 1994,641. (2)Bradner, F. P.; Shapiro, J. S.;Bowley, H. J.; Gerrard, D. L.; Maddams, W. F. Polymer 1989,30,914. (3)(a)Bjorklund, R.B.; Liedberg,B. J.Chem.Soc.,Chem.Commun. 1986,1293.(b)Armes,S. P.;Vincent, B. J.Chem.Soc.,Chem. Commun. 1987,288. (c) Armes, S.P. Ph.D. Thesis, University of Bristol, U.K., 1987. (4) (a) Armes, S.P.; Aldissi, M. Polymer 1990,31,569.(b) Cawdery, N.;Obey, T. M.;Vincent, B. J.Chem.Soc., Chem. Commun. 1988,1189. (c)Epron, F.;Henry, F.; Sagnes, 0.Mukromol.Chem.,Mucromol.Symp. 1990,35/36,527. (d) Odegard, R.; Skotheim, T. A.; Lee, H. S. J. Electrochem. SOC.1991,138,2930.(e)Digar, M. L.; Bhattacharyya, S. N.; Mandal, B. M. J. Chem. SOC.,Chem. Commun. 1992,18. ( 5 ) (a)Armes, S. P.; Aldissi, M. J.Chem. Soc.,Chem.Commun. 1989, 88. (b) Vincent, B.;Waterson, J. W. J. Chem. Soc., Chem. Commun. 1990, 683. (c) Liu, J.-M.; Yang, C. J. Chem. Soc., Chem. Commun. 1991,1529.(d)Gospodinova,N.;Mokreva, P.; Terlemezyan,L. J.Chem. SOC.,Chem. Commun. 1992,923. (e)Armes, S.P.; Aldissi, M.; Agnew, S. F.; Gottesfeld, S. Langmuir 1990,6,1745. (f) Armes, S.P.; Aldissi, M.; Agnew, S. F.; Gottesfeld, S. Mol. Cryst. Liq. Cryst. 1990,190,63. (g)Tadros, P.; Luk, S. Y.; Armes, S. P. J. Muter. Chem. 1992,2,125. (h) DeArmitt, C.; Armes, S. P. J,Colloid InteTfaceSci. 1992,150,134. (i) DeArmitt, C.; Armes, S. P. Langmuir 1993,9,652. IB Abstract
0743-7463/95/2411-1899$09.00/0
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Monomer Oxidant
magnetic stirring 25 "c. 16 h
Dispersedcolloidal silica particles
Conducting polymer-silica particles
Figure 1. Schematicformation of conducting polymer-silica particles. aqueous media using aparticulate dispersant.6-11 In this approach the conducting polymer is synthesized in the presence of commercially available ultrafine colloidal silica. The conducting polymer "glues" the small silica particles together to form microaggregates of conducting polymer-silica particles (see Figure 1).These nanocomposites are indefinitely stable with respect to particle aggregation in acidic media. These dispersions have already been characterized in terms of their particle size, chemical composition, and nanomorphology by a wide range of techniques including transmission electron microscopy, photon correlation spectroscopy, disk centrifuge photosedimentometry, elemental microanalyses, thermogravimetry, FTIR spectroscopy, and small-angle X-ray scattering.'+ll We are currently evaluating these new colloidal materials for both military camouflage and biomedical applications. (6)Gill, M.; Mykytiuk, J.; Armes, S.P.; Edwards, J. L.; Yeates, T.; Moreland, P. J.; Mollett, C. J. Chem. Soc., Chem. Commun. 1992,108. (7)(a)Gill, M.; Armes, S.P.; Fairhurst, D.; Emmett, S. N.; Idzorek, G.;Pigott, T.Langmuir 1992,8,2178.(b)Gill, M.; Baines, F. L.;Armes, S . P.Synth. Met. 1993,55-57, 1029. (8)Terrill, N.J.;Crowley, T.; Gill, M.; h e s , S . P. Langmuir 1993, 9, 2093. (9)Maeda, S.;Armes, S . P. J. Colloid Interface Sei. 1993,159,257. (10)Maeda, S.;Armes, S . P. J. Mater. Chem. 1994,4,935. (11)Maeda, S.;Armes, S. P. ACS PMSE Preprints 1994,70,352.
0 1995 American Chemical Society
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Table 1. Synthetic Details, Particle Size, and Chemical Composition Data for the Conducting Polymer-Silica Colloidal Nanocomposites sample no. 1
2 3 4
5 6
monomer type pyrrole pyrrole pyrrole‘ pyrrole‘ aniline aniline
oxidant type FeCly6H20 (NH4)zSzOs FeCly6HzO (NH4)zSzOs NazSzOs NazSzOs
initial silica concentrationa (w/v %) 1.1 3.4 1.1 3.4 3.0
conducting polymer contentb (wt %) 69.5 37.9 52.9
1.2
particle sizec (nm) 260 100 150
37.4
100
28.5
300
45.0
150
pellet conductivityd (S cm-l) 0.2 3 10-4 0.2 1 x 10-4 7 x 10-2 3 10-3
Sample sizes: 20 nm silica(supp1iedby Nyacol Products) for samples 1-5; 8 nm silica for sample 6. Calculated from carbon microanalysis (confirmed by thermogravimetry). Estimated by transmission electron microscopy. Four-point probe method on compressed pellets. e Oxidant added last to stirred reaction solution. a
In the present paper we describe the use of X-ray photoelectron spectroscopy ( X P S ) to characterize the surface composition of various polypyrrole-silica and polyaniline-silica colloids. Our aim in these studies was to compare the surface composition of the nanocomposite particles with their bulk composition and, if possible, to correlate this surface composition data with their observed colloidal stability and solid-state electrical conductivity.
Experimental Section ConductingPolymer- Silica Nanocomposite Syntheses. The commercial silica particles used to prepare five of the six conducting polymer-silica nanocomposite samples originated from two batches (Nyacol Products for samples 1-4 and Nalfloc for sample 51, each of approximately 20 nm particle diameter. Nanocomposite sample 6 was prepared using silica particles of 8 nm diameter (Nyacol Products). These silica particle sizes are nominal manufacturers’ values and should be treated with some caution; our small-angle X-ray scattering studies suggest that the Nalfloc “20 nm” sol actually has a weight-average particle diameter of 23 nm with a polydispersity of 24%.8 All silica samples were supplied as concentrated (20-40 wt %) aqueous dispersions; they possessed a negative surface charge with sodium cations as the counterions. The polyaniline-silica samples were synthesized as described previously.6-8 Two of the polypyrrolesilica colloids (samples 1 and 2) were prepared as described previously, with the pyrrole monomer being added last to the reaction ~ o l u t i o n . ~In - ~ the ~ synthesis of the other two polypyrrole-silica colloids (samples 3 and 4) the order of addition of the reagents was changed. Thus, in these latter samples the chemical oxidant [(NH&SzOs or FeC131 was added last to a stirred solution containing the pyrrole monomer and the silica particles. The pertinent synthetic details and characterization data of our six conducting polymer-silica samples are summarized in Table 1. All details of the instrumentation and methods for the characterization of the conducting polymer-silica samples have been described in full elsewhere.6-11 X-ray Photoelectron Spectroscopy Measurements. All samples were prepared as follows: aca. 5 x 5 mm piece of doublesided adhesive tape was used to stick a ca. 10 x 10 mm piece of “Sellotape” to the sample stub with the Sellotape being the adhesive side up. Amicrospatula tip-full ofthe sample was then deposited in the center of the Sellotape and pressed down gently. Any loose material was carefully removed using an air duster. A separate control experiment was performed to confirm that the Sellotape adhesive was purely hydrocarbon-based and contained no silicon contaminants which might otherwise affect the results. Duplicate experiments enabled u s to estimate a random error of around 1.5 atom %. For quantitative integration we selected the Sizppeak due to the silicon atoms in the silica particles. This peak has a more favorable signa1:background ratio t h a n the Sizs peak. An appropriate relative sensitivity factor was included to account for this choice in our experiments.12 XPS studies on the polypyrrole-silica nanocomposites were carried out using a JEOL JPS-80 photoelectron spectrometer. The analysis area was a circle of approximately 6.0 mm diameter for each sample. (12)Moulder, J. F.; Stickle, W. F.; Sobol, P. E.; Bomben, K. D. Handbook ofx-rayPhotoelectron Spectroscopy;Chastain, J.,Ed.;PerkinElmer Corp.: Eden Prairie, MN, 1992.
Measurements on the polyaniline-silica nanocomposites were made using a VG ESCASCOPE spectrometer. The sample analysis area in these latter experiments was a circle of approximately 1.1mm diameter. Colloid Stability of Conducting Polymer-Silica Dispersions. The polypyrrole-silica nanocomposite samples were dispersed in pH 3 and 9 buffer solutions prior to analysis using a Brookhaven disk centrifuge photosedimentometer. The scattering correction factor was taken to be the same as carbon black. Typical r u n conditions were 5 000-7 000 rpm for 10-20 min at 22-25 “C using the external gradient method. Small quantities of methanol were added to the pH-buffered aqueous spin fluid so as to obtain a suitable density gradient. Standard deviations were calculated assuming normal statistics for the particle size distributions. The disk centrifuge technique yields true weight-average particle size distributions and has become our preferred sizing technique for such conducting polymer-silica colloids.8J0 In the context of the present study, the inherent high resolution of the disk centrifuge also allows us to examine the degree of aggregation ofthe nanocomposite particles which occurs as a result of changes in the solution pH.
Results and Discussion Many groups have reported the successful use of X-ray photoelectron spectroscopy for the characterization of conducting polymer^.^^-^^ Most of these studies have focused on chemically synthesized bulk powders or electrochemically synthesized thin films. Since XPS is highly surface-specific(the typical sampling depth is 2- 10 nm), this technique has proved particularly useful in assessing the surface oxidation and degradation chemistry of conducting p 0 l y m e r s . l ~ ~ JVery ~ 2 ~ ~recently we have utilized XPS (and other established surface science techniques such as surface-enhancedRaman spectroscopy and time-of-flight secondary ion mass spectroscopy) in order to examine the surface composition of both surfactant- and polymer-stabilized dispersions of polypyrrole particle^.^^,^^ In these systems we have confirmed that (13) (a) Huger, P.; Street, G. B. J . Chem. Phys. 1984,80, 544. (b) Inganas, 0.;Erlandsson, R.; Nylander,C.; Lundstrom,I. J . Phys. Chem. Solids 1984,45 (4), 427. (14)Erlandsson, R.; Inganas, 0.; Lundstrom, I.; Salaneck, W. R. Synth. Met. 1985, 10, 303. (15) Eaves, J. G.; Munro, H. S.; Parker, D. Polym. Commun. 1987, -28. _ , 3%
(16) Kang, E. T.; Tan, K. L.; Neoh, K. G.; Chan, H. S. 0.;Tan, B. T. G. Polym. Bull 1989, 21, 53. (17) Neoh, K. G.; Kang, E. T.; Tan, K. L. J . Polym. Sci.,Polym. Chem.
1991,29, 759. (18) Kang, E. T.; Neoh, K. G.; Ong, Y. K.; Tan, K. L.; Tan, B. T. G. Macromolecules 1991,24, 2822. (19) Kang, E. T.; Neoh, K. G.;Tan, K. L.Adu. Polym. Sci. 1993,106, 4 “ C
LJD.
(20) Chan, H. S. 0.; Hor, T. S. A.; Ho, P. K. H. J . Macromol. Sci: Chem. 1990, A27 (81, 1081. (21) Sabbatini, L.; Malitesta, C.; Morea, G.; Torsi, L.; Zambonin, P. G. Mikrochim. Acta 1991, II, 237. (22) Moss, B. K.; Burford, R. P. Polymer 1992, 33, 1902. (23) Luk, S. Y.; Lineton, W.; Keane, M.; DeArmitt, C.; Armes, S. P. J . Chem. SOC., Faraday Trans. 1 1995, 91, 905.
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300 nm
’ A
Figure 2. Typical transmission electron micrograph of a polypyrrole-silica colloidal nanocomposite (sample 1 in Table 1). Note the distinctive ‘‘raspberry“ particle morphology.
the surface of the conducting polymer particles has a relatively high concentration of stabilizer (either surfactant or polymer) compared to the bulk composition of the particles. These observations are consistent with the currently acceptedtheories of steric stabilizationin colloid science.25 We further note that Davies and co-workers have drawn similar conclusions after carefully examining various polymer- and charge-stabilized polymer latexes by both XPS and static SIMS.26-30 In the present work we have used XPS to determine the surface composition of conducting polymer colloids stabilized with particulate dispersants, in this case ultrafine silica particles (see Figure 2). Table 1summarizes the synthetic details and characterization data of the six conducting polymer-silica samples examined in this study. The conducting polymer content, particle size, and solid state conductivity of the nanocomposites can depend markedly on their synthesis conditions,eg. oxidant type, initial silica concentration, silica particle size, and order of addition of reagents. In our experimental strategy we utilized the silicon atoms in the silica component and the nitrogen atoms in the conducting polymer (polypyrrole or polyaniline) component as elemental “markers”. Our XPS survey spectra confirmed that these “markers” were unique for each component. For example, the pristine silica sols showed no sign of any nitrogen species (see Figures 3a and 4a), while the bulk polypyrrole and polyaniline samples contained essentiallyno silicon atoms (see Figures 3b and 4b). Thus we were confident that simply measuring the silicodnitrogenatom ratios in the nanocompositesamples by XPS (see Figures 3c and 4c) would allow a semiquantitative assessment of their silicdconducting polymer surface composition. Such XPS experiments required careful consideration to be given to the nanocomposite syntheses. For example, ultrafine silica sols with both (24) (a) Chapman, S.; Armes, S. P.; Billingham, N. C.; Fletcher, I. W. Manuscript in preparation. (b) Beadle, P.; Armes, S. P.; Greaves, S.; Watts, J. F. Unpublished results. (25) Napper, D. H. Polymeric Stabilization of Colloidal Dispersions; Academic Press: London, 1983. (26)Lynn, R. A. P.;Davis, S.S.;Short;R. D.; Davies, M. C.;Vickerman, J. C.; Humphrey, P.;Johnson,D.; Hearn, J. Polym. Commun. 1988,29, 365. (27)Brindley, A; Davies, M. C.; Lynn, R. A. P.; Davis, S. S.; H e m , J.; Watts, J. F. Polymer 1992,33,1112. (28) Davies, M. C.; Lynn, R. A. P.; Davis, S. S.; Heam, J.; Watts, J. F.; Vickerman, J. C.; Johnson, D. J. Colloid Interface Sci. 1993,156, 229. (29) Davies, M. C.; Lynn, R. A. P.; Davis, S. S.;H e m , J.; Watts, J. F.; Vickerman, J. C.; Paul, A. J. Langmuir 1993,9, 1637. (30) Davies, M. C.; Lynn, R. A P.; Davis, S. S.;Hearn, J.; Watts, J. F.; Vickerman, J. C.; Johnson, D. Langmuir 1994,10, 1399.
[-de; i ,FL I
I
I
lo00 900 800
I
I
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500 400 300 Binding Energy [ev]
700
600
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Figure 3. XPS survey spectra of (a) pristine 20 n m silica particles (upper),(b)polypyrrole bulk powder (middle),and (c) polypyrrole-silica nanocomposite sample 1 (lower).
sodium and ammonium counterions were available for the syntheses of the conducting polymer-silica colloids. However, since the presence of the nitrogen-containing ammonium counterions would be likely to interfere with our XPS measurements (Le. the nitrogen atoms of the polypyrrole component would.nolonger be “unique”),only the silica samples with sodium counterions were used. Similarly, we used the Naps208 oxidant, rather than (N’H&&O8, to synthesize the two polyaniline-silica nanocomposites to avoid possible contamination from ammonium cations derived from the oxidant salt. Unfortunately, polypyrrole-silica nanocomposite samples 2 and 4 had already been synthesized using the (NH4)2S208 oxidant (and characterized by XPS) before we realized this potential source of error. However, we wish to emphasize that the main conclusions drawn from the current study are still valid even if these two samples were contaminated with ammonium cations. Finally, despite its lower surface concentration, nitrogen was preferred to carbon as an elemental “marker“ for the conducting polymer component because our preliminary XPS measurements revealed that the surface of the “pristine”silica sols were slightly contaminated with an unidentified carbon impurity. (The XPS survey spectrum of the 8 nm silica sol used to make sample 6 also revealed an unexpected chlorine peak; this is almost certainly due to adsorbed HC1 vapor caused by sample cross-contamination during oven-drying. No such chlorine peak was found in either of the two 20 nm silica sols.)
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1902 Langmuir, Vol. 11, No. 6, 1995 01,
(a)
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4600
u" 4200 3800
408
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n
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406
404 402 400 Binding Energy (eV)
398
110 ioe 106 Binding Energy (eV)
104
396
102
Figure 5. Typical N1, and SizpXPS core-line signals used for determiningthe surfacecompositionof the conductingpolymer-
silica nanocomposites. The Si2fl1, atomic ratios cited in Table
2 were calculated from the ratio of respective integrated peak
areas after including appropriate sensitivity factors.
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Figure 4. XPS survey spectra of (a)pristine 8 nm silica particles (upper), (b) polyaniline bulk powder (middle), and (c) polyaniline-silica nanocomposite sample 6 (lower).
With the silicon elemental marker we chose to utilize only the SizP signal (see Experimental Section) for comparison with the N1,signal. Typical XPS core-line signals are shown in Figure 5. The integrated peak areas of such spectra were adjusted using established sensitivity factors12to determine the surface SUN atomic ratios to within an estimated experimental uncertainty of 10- 12% (see Table 2). We utilized the chemical composition data presented in Table 1to calculate the corresponding bulk SUN atomic ratios for the nanocomposites. Clearly the observed differences between the surface and bulk SUN atomic ratios for each of the six samples listed in Table 2 are appreciably greater than the respective experimental errors; i.e. they are real differences. We conclude that the particle surface of all six of the conducting polymer-silica nanocomposites is distinctly silica-rich. This observation is entirely consistent with the excellent long-term colloid stability exhibited by these dispersions and suggests that these systems are charge-stabilized just like the original ultrafine silica particles. We further note that this silicarich surface should, in principle, allow the surfacefunctionalization of these conducting polymer-silica particles with functional silane compounds such as (trimetho~ysily1)propylamine.~~ Such surface derivatization could be very beneficial for certain biomedical application^.^^,^^ The relatively high solid-state electrical conductivities exhibited by the nanocomposites indicate that there must (31)Giesche, H.; Matijevic, E. Dyes Pigments 1991,17, 323. (32) Kawaguchi, H. In Microspheres forDiagnosis and Bioseparation; Tsuruta, T., et al., Eds.; CRC Press: London, 1993; p 294.
Table 2. Surface and Bulk SUN Atomic Ratios of Conducting Polymer-Silica Colloids as Calculated from X-ray Photoelectron Spectroscopy and Elemental Microanalysesmermogravimetry Data, Respectively SUN atomic ratio sample no.
bulka
surfaceb
1
0.5 f 0.1 1.7 f 0.2 0.9 f 0.1
4.4 f 0.4 6.4 f 0.6 6.7 f 0.7 9.5 f 0.9 14.3 f 1.5 11.1 f 1.2
2 3 4 5 6
1.8f 0.3 4.6 f 0.5 1.3 f 0.2
a Calculated from combined elemental microanalysis and thermogravimetric analysis data. Calculated from XPS data.
be a reasonably efficient interparticle charge-transfer mechanism operating at the microscopic level. Our X P S observations are consistent with this hypothesis. Since the N1,signal is visible in the XPS spectra of each of the six nanocomposites, we conclude that the conducting polymer component is always present at the particle surface of these samples (although somewhat depleted relative to its bulk concentration). We have already reportedlo that the use of the FeCl3 oxidant invariably leads to the formation of polypyrrolesilica nanocomposites of significantly higher conductivity than those synthesized with the (NH&SzOs oxidant. (This observation is also apparent from Table 1; samples 1and 3 have conductivities almost 3 orders of magnitude higher than those of samples 2 and 4.) We attributed these conductivity differences to overoxidationof the conducting polymer by the latter oxidant, as evidenced by the (33)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 Symp. Ser. No. 492, American Chemical Society: Washington, DC, 1992; p 347.
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Figure 6. Particle size distributioncurves obtained using the disk centrifuge for polypyrrole-silica nanocomposite colloids dispersed in pH 3 and 9 buffer solutions: (a) sample 1and (b) sample 2.
appearance of a small carbonyl peak in the IR spectrum of (NH4)&0s-synthesized nanocomposites.1° The disk centrifuge is a useful instrument for assessing the degree of dispersion of the nanocomposite particles at a particular pH. In these experiments the particles are dispersed in the appropriate pH buffer prior to analysis; a shift in the weight-average particle size distribution to higher particle diameter indicates an increase in the degree of flocculation of the dispersion, rather than an actual increase in particle diameter. Our results are depicted in Figure 6. Nanocomposite samples 1and 2 are both colloidally stable (high degree of dispersion) at pH 3. However, at pH 9 sample 1 becomes appreciably flocculated (Figure 6a), whereas sample 2 remains almost unchanged (Figure 6b). These observations correlate rather well with our XPS data: the particle surface of sample 2 is more silica-rich than sample 1(see Table 2) so we would expect higher colloid stability for the former dispersion. On the other hand, we were not able to correlate our disk centrifuge experiments with the XPS data for our polyaniline-silica nanocomposites. For example, sample 5 has the most silica-rich surface of all the samples investigated in this study, yet this dispersion was only stable under relatively acidic conditions and became quantitatively flocculated at pH I3! In general we find that all the polypyrrole-silica dispersions become flocculated at ca. pH 9-1034 and the polyaniline-silica dispersions become flocculated at ca. pH 3.35 Since these values correlate quite closely with the pH regimes required (34) Maeda, S.;Armes, S. P. Unpublished results. (35) Gill, M. Ph.D. Thesis, University o f Sussex, 1994.
for deprotonation (dedoping) of p o l y p y r r ~ l eand ~ ~ polyaniline:' it is interesting to speculate whether the doped cationic conducting polymer chain actually contribute to the colloid stability mechanism of the nanocomposites. However, this seems unlikely in view of the high Hamaker constants for conducting polymers reported by Vincent's Furthermore, our recent studies39on the analogous polypyrrole-tin(IV) oxide nanocomposites have confirmed that these latter dispersions are quantitatively flocculated at pH 5 ; if flocculation were primarily due to dedoping of the conducting polymer component, such dispersions would be expected to remain stable until pH 9-10. This observation strongly suggests that the inorganic oxide component is of considerable importance in determining the colloidalstability of the nanocomposite particles. We have previously reported that, in our polypyrrolesilica nanocomposite syntheses, either the pyrrole monomer or the oxidant are partially preadsorbed onto the surface of the colloidal silica particles prior to polymerization.1° Thus, in syntheses involving the (NH4)&Os oxidant, the initial solution pH is around 7, the pyrrole monomer is adsorbed onto the silica and the Sz0s2-oxidant remains in solution. On the other hand, in the FeC13 syntheses the reaction solution is rather more acidic (pH 21, an iron species, probably [Fe(H2O)5(0H)l2+,is preadsorbed onto the silica and the pyrrole monomer remains in solution. Given these differencesin adsorption behavior and solution pH, we speculated whether the order of addition of reagents during synthesis might affect the bulk and/or surface properties of the resulting colloidal nanocomposites. For example, adding the FeCl3 oxidant last would mean that the initial solution pH of the pyrrole/ silica reaction mixture prior to the addition of oxidant would be approximately pH 7 rather than pH 2. At this pH the pyrrole monomer is partially adsorbed onto the silica particles.1° However, upon addition of the FeC13 the solution pH would rapidly decrease to pH 2 (due to acid hydrolysis of the iron salt). Thus, we might expect that the pyrrole monomer would be displaced from the silica surface and the iron species subsequently adsorbed in its place. On a similar time scale the oxidant would begin to polymerize the pyrrole monomer. The true locus of polymerization under these circumstances is somewhat uncertain to say the least! In contrast, with the (NH4)2S208 oxidant, the order of addition of reagents does not affect the solution pH. In addition, if the pyrrole monomer is added last, it can adsorb onto the silica particles without competition from the nonadsorbed oxidant, whereas if the oxidant is added last, it will not displace the preadsorbed monomer. Thus we might expect that changing the order of addition of reagents in (NH4)2S~Os-basedsyntheses would be less likely to cause dramatic changes in the bulk and/or surface properties of the resulting nanocomposite particles. Recently we have obtained some direct evidence for the effect of order of addition of reagents: in the case of closely related polypyrrole copolymer-silica colloid systems, we have observed that adding the oxidant last appears to be critical for the successful formation of a stable colloidal d i s p e r ~ i o n .Thus, ~~ as part of the present study we undertook to examine the effect of order of addition of reagents on both the bulk and surface properties of the (36) Pei, Q.; Qian, R. Synth. Met. 1991,45,35. (37) Chiang, J.-C.; MacDiannid, A. G. Synth. Met. 1986,13,193. (38) Markham, G.;Obey, T. M.;Vincent, B. Colloids Surf. 1990,51, 229
(39) Maeda, S.;Armes, S. P. Chem. Mater. 1995,7 , 171. (40) (a) Armes, S.P.; Maeda, 5. A.C.S. Polymer Preprints 1994,35 (l),217. (b) Corradi, R. BSc. thesis, University of Sussex, 1994. (c) Maeda, S.; Corradi, R.; Armes, S. P. Macromolecules, in press.
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1904 Langmuir, Vol. 1 2 , No. 6, 1995
homopolypyrrole-silica colloids. In Table 1 both samples 1 and 2 were prepared by the conventional method (Le. pyrrole monomer added last), whereas samples 3 and 4 were prepared by adding the oxidant last to a solution containing the pyrrole monomer and the silica particles. Comparing samples 1 and 3 we find significant differences in both the particle size and conducting polymer content of the nanocomposites. Thus, for the FeC4 oxidant, the order of addition of reagents affects the bulk properties of the nanocomposites. In view of this observation, it is difficult to make a meaningful comparison of the XPS data for samples 1and 3. On the other hand, ifwe compare samples 2 and 4 it is clear that, for nanocomposites synthesized with the (NH4)&08 oxidant, the particle size and conducting polymer content are essentially independent of the order of addition of reagents. Turning our attention to Table 2, the XPS data suggest that the particle surface of nanocomposite sample 4 [(NH4)&08 oxidant added last] is distinctly more silica-rich than sample 2 (pyrrole monomer added last). The reason(s) for these “order of addition” effects on the bulk and surface properties ofthe nanocomposites are unclear at the present time. We tentatively suggest that the preadsorption (or otherwise) of the reagents may be important in determining both the bulk and surface properties of the polypyrrole-silica nanocomposites.
Conclusions Our XPS studies have confirmed unequivocally that the surface composition of each of our six conducting polymer-silica nanocomposite samples is distinctly silicarich compared to their respective bulk compositions. This observation is consistent with the observed long-term
colloid stability of these dispersions. Moreover, for the polypyrrole-silica particles, our disk centrifuge data suggest that the more silica-rich dispersions have better colloid stability at pH 9. However, the XPS experiments also confirmed the presence of the conducting polymer component within the top 2- 10 nm of the particle surface. This observation is consistent with the relatively high solid state conductivities observed for compressed pellets of these materials. The “order of addition” effects observed in the polypyrrole-silica syntheses are complex: with the FeC13 oxidant there are significant changes in the bulk properties (particle size and chemical composition) of the nanocomposites, whereas with the (N&)&O8 oxidant the order of addition of the reagents apparently only affects the surface properties of the nanocomposites.
Acknowledgment. We wish to thank Mr. Hiroe and Mr. Miyagawa of the New Oji Paper Co., Ltd. for their kind assistance with the XPS studies on the polypyrrolesilica nanocomposites. S.M. thanks the New Oji Paper Co. for its financial support in the form of a Ph.D. studentship. M.G. thanks Zeneca Resins and the SERC for an SERC CASE Ph.D. studentship. S.P.A. gratefully acknowledges both the Royal Society (for travel funds to support a study visit to the New Oji Paper Co’s Central Research Laboratory in Tokyo) and the SERC (for capital equipment funds to purchase the disk centrifuge photosedimentometer; G W 9 3 6 0 6 ) . Finally we thank Zeneca Resins and IC1 Materials for permission to publish this work. LA940831G
