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Water-dispersible conducting nanocomposites of polyaniline and poly(N-vinylcarbazole) with nanodimensional zirconium dioxide
Synthetic Metals 108 Ž2000. 231–236 www.elsevier.comrlocatersynmet
Water-dispersible conducting nanocomposites of polyaniline and poly žN-vinylcarbazole/ with nanodimensional zirconium dioxide Suprakas Sinha Ray, Mukul Biswas
)
Department of Chemistry, Presidency College, Calcutta 700 073, India Received 16 June 1999; accepted 23 September 1999
Abstract Water-dispersible composites of polyaniline ŽPANI. and polyŽ N-vinylcarbazole. ŽPNVC. with nanodimensional ZrO 2 were prepared. The incorporation of the polymers in the composites was endorsed by FTIR studies. SEM analyses revealed distinct morphological features of the composites. TEM analyses confirmed the particle sizes to be in the 300–500 nm range for PNVC–ZrO 2 and in the 250–300 nm range for the PANI–ZrO 2 composites, respectively. TG analyses revealed the enhanced stabilities of the nanocomposites relative to the base polymers. DC conductivities of the PNVC–ZrO 2 composites were in the order of Ž1–1.5. = 10y5 Srcm, which were 10 7 –10 10-fold improved relative to the base polymer. The same for the PANI–ZrO 2 composite were in the range of Ž0.03–0.35. = 10y2 Srcm values increasing with increasing polymer loading in the composites. q 2000 Elsevier Science S.A. All rights reserved. Keywords: Nanocomposite; Polyaniline; PolyŽ N-vinylcarbazole.; Nanodimensional ZrO 2 ; Conductivity
1. Introduction In course of their exhaustive work w1,2x on the preparation of inorganic oxide-based water-dispersible nanocomposites from polypyrrole ŽPPY., Maeda and Armes reported w3x that zirconia, yttria, antimony ŽV. oxide and titanium ŽIV. oxide failed to act as effective particulate dispersants being unable to prevent the macroscopic precipitation of PPY in sharp contrast to silica or tin ŽIV. oxide w1,3x. Relevantly, Bhattacharyya et al. w4x reported the preparation of a PPY–ZrO 2 nanocomposite which was claimed to exhibit a conductivity variation of 1–15 Srcm depending on the weight of ZrO 2 used. However, it was not clearly established whether the nanocomposite was dispersible stably in the aqueous medium and whether a 15-fold jump in the conductivity was a justifiably typical feature of ZrO 2-based composites in general. More recently w5x, we reported the successful preparation of conducting water-dispersible nanocomposites of PNVC and PANI with nanodimensional MnO 2 . In this background, we now wish to report the preparation of stable water-dispersible conducting nanocomposites of PANI and of PNVC using ZrO 2 as the particulate dispersant. The PNVC–ZrO 2 system was particularly in)
Corresponding author. Tel.: q91-33-241-3893; fax: q91-33-2155863; e-mail: [email protected]
teresting, since, being insoluble in water unlike PY or ANI monomers, N-vinylcarbazole ŽNVC. could not be polymerized by the conventional method as used in ANI–ZrO 2 system in this study or in other oxide-based systems w1–3x. In the procedure developed by us w6x, we used a benzene solution of NVC which was polymerized by FeCl 3 impregnated ZrO 2 powder acting as a heterogeneous catalyst and isolated the composite by methanol precipitation of the polymerization system. This article will highlight the essential details of the procedures adopted and the results of characterization of PANI–ZrO 2 and PNVC–ZrO 2 composites by scanning electron micrographic, transmission electron micrographic and thermogravimetric analyses and DC conductivity measurements.
2. Experimental 2.1. Materials Aniline ŽANI, E. Merck, Germany. was freshly distilled under reduced pressure. Nanodimensional zirconium dioxide sol was prepared by the room temperature hydrolysis of zirconium–isopropanol complex in isopropanol ŽAldrich, USA. by Dr. A. Bhattacharya of Saha Institute of Nuclear Physics, Calcutta, India w4x. The resulting sol was
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S. Sinha Ray, M. Biswasr Synthetic Metals 108 (2000) 231–236
dried by lyphilization by freezing the solution first and thereafter removing the solvent by sublimation under vacuum at y358C. Transmission electron micrographic analysis revealed that the ZrO 2 sol comprised particles of diameters in the range 20–30 nm.
forming a composite and not removable by benzene extraction as observed in other comparable systems so far reported w6,11,12x.
2.2. Preparation of PANI–ZrO2 nanocomposite
FTIR spectra of the various composites were taken on a Perkin Elmer model 883 instrument. Dispersions were diluted with water and microsprayed on a mica substrate. Then the samples were sputter-coated with gold layer, and a Hitachi S415A scanning electron micrograph was used to take the micrographs. Particle sizes of these materials were measured by transmission electron microscopic analysis ŽHitachi 600.. TEM studies were made on diluted Žin the range of 100 ppm. dispersions dried on the copper grid. TGA and DTA were performed on a Shimadzu DT-40 instrument. DC conductivity measurements were conducted on pressed pellets with silver coating by the conventional four-probe technique.
A known volume of freshly distilled ANI was syringed slowly to a dispersion of ZrO 2 in 2 M aqueous HCl under continuous sonication and kept for 1 h. To this dispersion, a requisite volume of ŽNH 4 . 2 S 2 O 8 solution Žknown strength. in aqueous 2 M HCl solution was added slowly under sonication at room temperature Ž288C.. The immediate blackening of the dispersion indicated the formation of PANI. This dispersion was kept for about 3 h under the same condition. Afterwards, the total black mass was ultracentrifuged Žat 18,500 rpm. for 1 h. The black precipitate, thus obtained, was washed thoroughly with 2 M aqueous HCl solution. This process was repeated three times to remove all adhering substances and the mass was dried under vacuum for 16 h at 908C to obtain a black green mass.
2.4. Characterization and property eÕaluation
3. Results and discussion
2.3. Preparation of PNVC–ZrO2 nanocomposite
3.1. General features of polymerizationr composite formation
FeCl 3 impregnated ZrO 2 powder was prepared by a thorough mixing of an acetone solution containing a known weight of anhydrous FeCl 3 with powdery ZrO 2 , followed by solvent evaporation under reduced pressure at 508C w7,8x. This FeCl 3 impregnated ZrO 2 powder was used as a heterogeneous catalyst during polymerization. A typical polymerization was carried out by stirring a 5 ml benzene solution of NVC of desired concentration with a known amount of FeCl 3 impregnated ZrO 2 at 508C in a well-stoppered Pyrex tube. After a definite polymerization time, the total contents of the reaction vessel were precipitated in methanol and the precipitate was quantitatively filtered on a Gooch crucible, repeatedly washed with methanol to remove all FeCl 3 and finally washed with boiling methanol to remove all unreacted monomer w9x and dried at 608C for 7 h. The entire quantity of the precipitated PNVC–ZrO 2 mass, thus isolated was extracted with benzene through continuous stirring for 30 min. Then, the total contents of the tube were centrifuged and the centrifugate was separated from the residue. This process was expected to dissolve out all surface adsorbed PNVC. This procedure was repeated for various lengths of time, till the extracts did not give any precipitate with methanol. After subsequent drying of the benzene extracted mass, an increase in the weight of the catalyst was realised which varied under different experimental conditions. This mass after various physicochemical characterization was confirmed to contain some residual PNVC along with ZrO 2 in which PNVC was as if glued w5,10,11x on the ZrO 2 layer
Table 1 presents some results on the polymerizationrcomposite formation and conductivity measurements for the ANI–ZrO 2 – ŽNH 4 . 2 S 2 O 8 –water system and NVC– ZrO 2 – ŽFeCl 3 . system. Entries 4–1 indicate that at fixed ZrO 2 and oxidant amounts in the initial feed, the total conversion to PANI increased with increasing ANI amount up to a certain value of the latter Žentry 2. and thereafter decreased with still higher ANI amounts. For an increase in the ANI amount from 0.15 g Žentry 3. to 0.20 g, only a marginal decrease in the overall yield was experienced Žcf. entries 3 and 2. which was apparently magnified with further increase in the ANI amount Žentry 1.. Entries 2, 6 and 7 indicate that at fixed ANI and oxidant weights, the total conversion to PANI and also the PANI loading per gram of the composite showed a tendency to fall with increasing ZrO 2 amount. This could be reasonable since ZrO 2 was not involved in the initiation process and the marginal difference in the total yield would imply a substantial difference when computations were done taking into account the ZrO 2 weight in the composite. Entries 8–12 indicate the variation of the total yield and the extent of loading of PNVC in the nanocomposite with the weight of FeCl 3 in the initial feed at fixed weights of ZrO 2 and NVC. Unlike MnO 2 , in the NVC–MnO 2 nanocomposite system recently reported by us w5x, ZrO 2 failed to initiate directly the NVC polymerization and accordingly FeCl 3 impregnated ZrO 2 was used as the catalyst. A maximum loading of ca. 50% of PNVC per gram of the composite was realized under the conditions used in this experiment. Notably, as with other PNVC-
S. Sinha Ray, M. Biswasr Synthetic Metals 108 (2000) 231–236
233
Table 1 Data on polymerizaton of ANI and NVC with nanodimensional ZrO 2 and composite formation thereof Entry no. 1 2 3 4 5 6 7 8 9 10 11 12
Weight Žg. ZrO 2
Oxidant
0.10 0.10 0.10 0.10 0.10 0.10 0.10 0.10 0.10 0.10 0.15 0.15 0.20 0.20 0.10 0.10 0.10 0.10 0.10 0.10 0.10 0.15
0.60 0.60 0.60 0.60 0.60 0.60 0.60 0.60 0.50 0.50 0.60 0.60 0.60 0.60 0.04 0.04 0.06 0.06 0.08 0.08 0.10 0.10
a
Monomer 0.25 0.25 0.20 0.20 0.15 0.15 0.10 0.10 0.20 0.20 0.20 0.20 0.20 0.20 0.10 0.10 0.10 0.10 0.10 0.10 0.10 0.10
% Yield of polymer
% Polymer per gram of composite
Conductivity ŽSrcm.
70.48 70.44 82.20 82.00 83.06 83.60 40.00 45.00 59.65 60.00 81.45 83.55 80.00 87.00 50.52 49.02 87.20 87.00 100.00 100.00
63.79 63.78 62.03 62.12 55.47 55.92 28.57 31.03 54.40 54.65 52.06 52.69 44.00 46.00 33.33 33.12 46.52 46.52 50.00 50.00
0.35 = 10y2
b
0.3 = 10y2 0.2 = 10y2 0.03 = 10y2 0.14 = 10y2 0.15 = 10y2 0.2 = 10y2 – ; 10y5 1.2 = 10y5
c
c
c
c
c
c
a
The oxidants for entries Ž1–7. and Ž8–12. were ŽNH 4 . 2 S 2 O 8 and FeCl 3 , respectively. The monomers for entries Ž1–7. and Ž8–12. were ANI and NVC, respectively. c A solid mass was formed after evaporation of acetone which was intractable.
b
based nanocomposite systems w5,6,12x, repeated benzene extraction removed all the surface-adsorbed PNVC excluding the PNVC loaded in the composite. 3.2. FTIR spectral characteristics The infrared spectrum of PANI–ZrO 2 composite revealed the following characteristic absorptions: 3425 cmy1 Ž) NH stretching.; 1570–1476 cmy1 ŽN–H bending.; 1302–1243 cmy1 ŽC–N stretching. thereby endorsing the presence of PANI in the composite. Similarly, the infrared spectrum of PNVC–ZrO 2 composite revealed the following characteristic absorptions: 724 cmy1 Žring deform. of subs. aromatic structure.; 1150–1232 cmy1 ŽCH in plane deform..; 1327 cmy1 ŽCH in plane deform. of vinylidene gr..; 1406 CHy1 Ž) CH 2 deform. of vinylidene gr..; 1481–1449 cmy1 Žring vib. of NVC moiety.; 1627 cmy1 ŽC s C str. of vinylidene gr.. which endorsed the presence of PNVC in the composites.
reminiscent of the typical raspberry morphology proposed by Armes et al. w1,3,13x for the various oxide-based PPYrPANI composites, where the ultrafine oxide particles would be present not only on the particle surface but also distributed through the interior of the particles. The ZrO 2 powder itself appeared to comprise finer particles which however became glued together by the polymer particles acting as binders in the nanocomposites — a situation quite similar to that reported by Armes et al. w1,3,10x for
3.3. Scanning electron micrographic analysis Fig. 1a and b represent the scanning electron micrographs of PANI–ZrO 2 and PNVC–ZrO 2 composites, respectively. The formation of microaggregates of composite particles of spherical shapes could be clearly noted in either systems and certaining was most pronounced in the PANI–ZrO 2 system. The observed morphologies were
Fig. 1. Scanning electron micrographs of: Ža. PANI–ZrO 2 nanocomposite; Žb. PNVC–ZrO 2 nanocomposite.
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S. Sinha Ray, M. Biswasr Synthetic Metals 108 (2000) 231–236
the PANI–SiO 2 systems and to the PANI–MnO 2 system reported by us w5x. 3.4. Transmission electron micrographic characteristics The transmission electron micrographs ŽFig. 2a. of a PANI–ZrO 2 composite dispersion in the 100 ppm dilution range clearly confirmed the formation of spherical particles with average particle diameters in the 250–300 nm range. The TEM pattern for an undiluted PANI–ZrO 2 composite ŽFig. 2b. revealed prominently the microaggregatercluster formation involving ZrO 2 and the precipitating PANI particles. In general, the morphology of the PANI–ZrO 2 system was comparable to that reported for the well-studied PANI–SiO 2 system w10,13x and the same for the polyŽvinylmethylether. stabilized PANI dispersions w14x. Fig. 3a and b represent the transmission electron micrographs of two PNVC–ZrO 2 composites with PNVC loading of 33% Žentry 8, Table 1. and of 50% Žentry 10, Table 1., respectively. In general, these micrographs revealed a relatively non-uniform coverage of ZrO 2 particles by the precipitating PNVC. This problem would be difficult to avoid while working with a water-insoluble monomer like
Fig. 3. Transmission electron micrographs of: Ža. PNVC–ZrO 2 nanocomposite Ž11% PNVC loading.; Žb. PNVC–ZrO 2 nanocomposite Ž40% PNVC loading..
NVC, which unlike the water soluble PY or PANI monomers would not be distributed homogeneously in the polymerization medium containing the oxides and the oxidants. However, for the PNVC–ZrO 2 system, the procedure followed by us would possibly allow of a better coverage of the ZrO 2 particles by the precipitating PNVC by controlling the rate of precipitation by methanol addition to the benzene solution of the polymer. The formation of spherical particles, in general, could be clearly distinguished in the TEM pattern for the PNVC–ZrO 2 nanocomposite system. The average particle diameters of the PNVC–ZrO 2 composite were in the range 300–500 nm. 3.5. Thermal stability of the composites Fig. 2. Transmission electron micrographs of: Ža. PANI–ZrO 2 nanocomposite dispersion Ž100 ppm order dilution.; Žb. Undiluted PANI–ZrO 2 nanocomposite.
Thermogravimetric analysis of the PANI–ZrO 2 composite ŽFig. 4a. revealed a total loss of ca. 68.1% at ca. 5508C, which remained constant till 951.18C. In contrast,
S. Sinha Ray, M. Biswasr Synthetic Metals 108 (2000) 231–236
235
whereby 70% of the PANI degradation was completed. In contrast, in the DTA curve for PANI–ZrO 2 composite a broad hump at 439.98C appeared corresponding to a loss of 43% of the composite. No minor PANI peaks could be detected in this range. Thus, these data readily confirmed the enhanced thermogravimetric stability for PANI–ZrO 2 composite relative to PANI. 3.6. ConductiÕity characteristics
Fig. 4. TGA and DTA scans for: Ža. PANI–ZrO 2 nanocomposite; Žb. PANI.
PANI ŽFig. 4b. showed a complete loss at the same temperature range while ZrO 2 revealed only a partial loss of volatile impurities of about 17% in the same temperature range. In the light of the reasoning put forward by Armes et al. w2x for PANI–SiO 2 system, these data implied that the polymer content per gram of the composite was Ž68.1 y 17.25.% s 50.75% which compared reasonably well with the actual data Žentry no. 6, Table 1.. A similar consistency in the estimate of PNVC loading was also realized from the thermal analysis data for the PNVC–ZrO 2 system — which behaved in a similar fashion to the PNVC–SiO 2 nanocomposite system recently reported by us w6x. A comparison of DTA scans in the thermograms revealed the manifestation of smaller exothermic peaks at 2098C, 362.78C and a sharp peak at 532.98C for PANI
The conductivity of the PANI–ZrO 2 nanocomposites systematically increased with increasing PANI loading in the composites Žentries 4–1, Table 1.. Entries 2, 6 and 7 indicate that with an increase in the ZrO 2 amount at a fixed level of ANI and ŽNH 4 . 2 S 2 O 8 , the conductivities showed only a marginal change tending to level off. This trend was more or less parallel to the same for PANI loading on the composites. Reported conductivity values of PANI–SiO 2 w1,2x and of PANI–MnO 2 nanocomposites w5x were in the order of Ž0.4–0.6. = 10y3 and Ž1–2.5. = 10y2 Srcm, respectively. Thus, these data did not allow any correlation between the observed conductivity and the oxide chosen. The deciding factor would be the extent of PANI loading in a particular composite. As for the PNVC–ZrO 2 nanocomposites, the conductivities recorded in entries 9 and 10 in Table 1, did not reveal any change. However, these values were appreciably enhanced compared to the conductivity of the unmodified PNVC Ž; 10y1 2 to 10y16 Srcm. w15x. For an essentially nonconducting polymer like PNVC, the enhancement in the conductivity was dramatic — although increasing extent of PNVC loading in the composite would not obviously change the conductivity magnitude. The exact mechanism for the enhancement of conductivity is obscure at the moment. However, in PPY–ZrO 2 system w4x, the large increase in conductivity was attributed to an improved linking between PPY moieties due to intimate association of PPY moieties with ZrO 2 particles in the nanocomposite. The dependence of the conductivities of these conducting nanocomposites on the nature of the oxide chosen is yet an open question since the available literature is indeed
Table 2 A comparison of the conductivities of ZrO 2 - and MnO 2 -based composites of PANI and PNVC Composites
Average particle diameter Žnm. a
Polymer loading Ž%. per gram of composite
Conductivity ŽSrcm.
Ref.
PANI–MnO 2 PANI–ZrO 2 PANI–MnO 2 PANI–ZrO 2 PNVC–MnO 2 PNVC–ZrO 2 PNVC–MnO 2 PNVC–ZrO 2
100–150 250–300 100–150 250–300 200–250 300–500 200–250 300–500
63 62 51 52 23 46 36 50
1.8 = 10y2 0.3 = 10y2 1 = 10y2 0.15 = 10y2 3.5 = 10y5 1 = 10y5 4.4 = 10y5 1.2 = 10y5
w5x this study w5x this study w5x this study w5x this study
a
Particle diameters of MnO 2 and ZrO 2 were in the range of Ž6–10. and Ž20–30. nm, respectively.
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S. Sinha Ray, M. Biswasr Synthetic Metals 108 (2000) 231–236
too meager to suggest any correlation. A comparison of our recent results on the conductivity of the nanodimensional MnO 2-based nanocomposites of PANI and PNVC with those on the present ZrO 2-based systems ŽTable 2. indicate that at comparable PANIrPNVC loading in the composite, MnO 2-based composites exhibited appreciably higher conductivities compared to the ZrO 2-based systems. The particle sizes of the MnO 2 particles as also of the MnO 2-based PANIrPNVC colloids were also significantly lower than the corresponding ZrO 2-based systems. This probably allowed a more compact grain formation in the particles of MnO 2-based systems which could again enhance the conductivity. In view of the limited data presented and made available in this context, such a suggestion should be regarded as purely tentative. 3.7. Water dispersibility of the PANI–ZrO2 and PNVC– ZrO2 nanocomposites The PANI–ZrO 2 nanocomposite was obtained as a black colloidal suspension during polymerization of ANI. These suspensions retained their stability almost permanently. An exactly similar behavior was noted for the PANI–MnO 2 nanocomposite system recently reported by us w5x. Although formation of the water-dispersible nanocomposite of PANI was relatively more facile when nanodimensional MnO 2 was used, the PNVC–ZrO 2 nanocomposites required sonication in water medium for 3–4 h for producing a stable colloidal dispersion in water. Notably, in the PNVC–MnO 2 system w5x, no sonication was required for producing a stable dispersion. Thus, the behavior of ZrO 2 and MnO 2 in regard to PNVC nanocomposite formation was remarkable since for producing PNVC–SiO 2 water suspension a polymeric stabilizer polyŽN-vinylpyrrolidone. was essential w6x. As yet, we do not have any satisfactory explanation for the observed difference in behavior of the various oxides towards nanocomposite formation. Indeed, as felt by Armes et al. w1x, factors such as preadsorption characteristics of the reacting species, on the oxide surface high surface area, surface charge on the oxide might be relevant factors.
The polymers under study involved cationic chain growth w16x and accordingly, a negative oxide surface charge would presumably be relevant in the formation of the stable colloids w1x. 4. Conclusion Stable water dispersible nanocomposites of PANI and PNVC could be obtained from the ANI– ŽNH 4 . 2 S 2 O 8 –2 M HCl–water and NVC Žbenzene. –FeCl 3 –water polymerization systems in the presence of nanodimensional ZrO 2 . Acknowledgements Thanks are due to CSIR, New Delhi, India for generous funding of an Emeritus Scientist project in favour of MB and to the authorities of Presidency College Calcutta, for use of its facilities. References w1x J. Stejskel, P. Kratochvil, S.P. Armes, S.F. Lascelles, A. Riede, M. Helmstedt, J. Prokes, I. Krivka, Macromolecules 29 Ž1996. 6814. w2x S.P. Armes, S. Gottesfeld, J.G. Beery, F. Garzone, S.F. Agnew, Polymer 32 Ž1992. 2325. w3x S. Maeda, S.P. Armes, Chem. Mater. 7 Ž1995. 171. w4x A. Bhattacharya, K.M. Ganguly, A. De, S. Sarkar, Mater. Res. Bull. 31 Ž1996. 527. w5x M. Biswas, S. Sinha Ray, Y.P. Liu, Synthetic Metals 105 Ž1999. 99. w6x S. Sinha Ray, M. Biswas, Mater. Res. Bull. 33 Ž1998. 533. w7x A. Cornelis, P. Laszao, Synthesis Ž1980. 849. w8x P. Laszao, E. Polla, Tetrahedron Lett. 25 Ž1984. 3309. w9x L.P. Ellinger, Polymer 5 Ž1964. 559. w10x S. Maeda, S.P. Armes, Synthetic Metals 73 Ž1995. 151. w11x S. Sinha Ray, M. Biswas, J. Appl. Polym. Sci. 74 Ž1999. 2971. w12x M. Biswas, S. Sinha Ray, Polymer 39 Ž1998. 6423. w13x S. Maeda, S.P. Armes, J. Mater. Chem. 4 Ž1994. 935. w14x N.J. Terrill, T. Crowley, M. Gill, S.P. Armes, Langmuir 9 Ž1993. 2093. w15x M. Biswas, S.K. Das, Polymer 23 Ž1982. 1713. w16x S. Huang, B.D. Humphrey, A.G. MacDiarmid, J. Chem. Soc., Faraday Trans. 82 Ž1986. 2825.
Water-dispersible conducting nanocomposites of polyaniline and poly žN-vinylcarbazole/ with nanodimensional zirconium dioxide Suprakas Sinha Ray, Mukul Biswas
)
Department of Chemistry, Presidency College, Calcutta 700 073, India Received 16 June 1999; accepted 23 September 1999
Abstract Water-dispersible composites of polyaniline ŽPANI. and polyŽ N-vinylcarbazole. ŽPNVC. with nanodimensional ZrO 2 were prepared. The incorporation of the polymers in the composites was endorsed by FTIR studies. SEM analyses revealed distinct morphological features of the composites. TEM analyses confirmed the particle sizes to be in the 300–500 nm range for PNVC–ZrO 2 and in the 250–300 nm range for the PANI–ZrO 2 composites, respectively. TG analyses revealed the enhanced stabilities of the nanocomposites relative to the base polymers. DC conductivities of the PNVC–ZrO 2 composites were in the order of Ž1–1.5. = 10y5 Srcm, which were 10 7 –10 10-fold improved relative to the base polymer. The same for the PANI–ZrO 2 composite were in the range of Ž0.03–0.35. = 10y2 Srcm values increasing with increasing polymer loading in the composites. q 2000 Elsevier Science S.A. All rights reserved. Keywords: Nanocomposite; Polyaniline; PolyŽ N-vinylcarbazole.; Nanodimensional ZrO 2 ; Conductivity
1. Introduction In course of their exhaustive work w1,2x on the preparation of inorganic oxide-based water-dispersible nanocomposites from polypyrrole ŽPPY., Maeda and Armes reported w3x that zirconia, yttria, antimony ŽV. oxide and titanium ŽIV. oxide failed to act as effective particulate dispersants being unable to prevent the macroscopic precipitation of PPY in sharp contrast to silica or tin ŽIV. oxide w1,3x. Relevantly, Bhattacharyya et al. w4x reported the preparation of a PPY–ZrO 2 nanocomposite which was claimed to exhibit a conductivity variation of 1–15 Srcm depending on the weight of ZrO 2 used. However, it was not clearly established whether the nanocomposite was dispersible stably in the aqueous medium and whether a 15-fold jump in the conductivity was a justifiably typical feature of ZrO 2-based composites in general. More recently w5x, we reported the successful preparation of conducting water-dispersible nanocomposites of PNVC and PANI with nanodimensional MnO 2 . In this background, we now wish to report the preparation of stable water-dispersible conducting nanocomposites of PANI and of PNVC using ZrO 2 as the particulate dispersant. The PNVC–ZrO 2 system was particularly in)
Corresponding author. Tel.: q91-33-241-3893; fax: q91-33-2155863; e-mail: [email protected]
teresting, since, being insoluble in water unlike PY or ANI monomers, N-vinylcarbazole ŽNVC. could not be polymerized by the conventional method as used in ANI–ZrO 2 system in this study or in other oxide-based systems w1–3x. In the procedure developed by us w6x, we used a benzene solution of NVC which was polymerized by FeCl 3 impregnated ZrO 2 powder acting as a heterogeneous catalyst and isolated the composite by methanol precipitation of the polymerization system. This article will highlight the essential details of the procedures adopted and the results of characterization of PANI–ZrO 2 and PNVC–ZrO 2 composites by scanning electron micrographic, transmission electron micrographic and thermogravimetric analyses and DC conductivity measurements.
2. Experimental 2.1. Materials Aniline ŽANI, E. Merck, Germany. was freshly distilled under reduced pressure. Nanodimensional zirconium dioxide sol was prepared by the room temperature hydrolysis of zirconium–isopropanol complex in isopropanol ŽAldrich, USA. by Dr. A. Bhattacharya of Saha Institute of Nuclear Physics, Calcutta, India w4x. The resulting sol was
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S. Sinha Ray, M. Biswasr Synthetic Metals 108 (2000) 231–236
dried by lyphilization by freezing the solution first and thereafter removing the solvent by sublimation under vacuum at y358C. Transmission electron micrographic analysis revealed that the ZrO 2 sol comprised particles of diameters in the range 20–30 nm.
forming a composite and not removable by benzene extraction as observed in other comparable systems so far reported w6,11,12x.
2.2. Preparation of PANI–ZrO2 nanocomposite
FTIR spectra of the various composites were taken on a Perkin Elmer model 883 instrument. Dispersions were diluted with water and microsprayed on a mica substrate. Then the samples were sputter-coated with gold layer, and a Hitachi S415A scanning electron micrograph was used to take the micrographs. Particle sizes of these materials were measured by transmission electron microscopic analysis ŽHitachi 600.. TEM studies were made on diluted Žin the range of 100 ppm. dispersions dried on the copper grid. TGA and DTA were performed on a Shimadzu DT-40 instrument. DC conductivity measurements were conducted on pressed pellets with silver coating by the conventional four-probe technique.
A known volume of freshly distilled ANI was syringed slowly to a dispersion of ZrO 2 in 2 M aqueous HCl under continuous sonication and kept for 1 h. To this dispersion, a requisite volume of ŽNH 4 . 2 S 2 O 8 solution Žknown strength. in aqueous 2 M HCl solution was added slowly under sonication at room temperature Ž288C.. The immediate blackening of the dispersion indicated the formation of PANI. This dispersion was kept for about 3 h under the same condition. Afterwards, the total black mass was ultracentrifuged Žat 18,500 rpm. for 1 h. The black precipitate, thus obtained, was washed thoroughly with 2 M aqueous HCl solution. This process was repeated three times to remove all adhering substances and the mass was dried under vacuum for 16 h at 908C to obtain a black green mass.
2.4. Characterization and property eÕaluation
3. Results and discussion
2.3. Preparation of PNVC–ZrO2 nanocomposite
3.1. General features of polymerizationr composite formation
FeCl 3 impregnated ZrO 2 powder was prepared by a thorough mixing of an acetone solution containing a known weight of anhydrous FeCl 3 with powdery ZrO 2 , followed by solvent evaporation under reduced pressure at 508C w7,8x. This FeCl 3 impregnated ZrO 2 powder was used as a heterogeneous catalyst during polymerization. A typical polymerization was carried out by stirring a 5 ml benzene solution of NVC of desired concentration with a known amount of FeCl 3 impregnated ZrO 2 at 508C in a well-stoppered Pyrex tube. After a definite polymerization time, the total contents of the reaction vessel were precipitated in methanol and the precipitate was quantitatively filtered on a Gooch crucible, repeatedly washed with methanol to remove all FeCl 3 and finally washed with boiling methanol to remove all unreacted monomer w9x and dried at 608C for 7 h. The entire quantity of the precipitated PNVC–ZrO 2 mass, thus isolated was extracted with benzene through continuous stirring for 30 min. Then, the total contents of the tube were centrifuged and the centrifugate was separated from the residue. This process was expected to dissolve out all surface adsorbed PNVC. This procedure was repeated for various lengths of time, till the extracts did not give any precipitate with methanol. After subsequent drying of the benzene extracted mass, an increase in the weight of the catalyst was realised which varied under different experimental conditions. This mass after various physicochemical characterization was confirmed to contain some residual PNVC along with ZrO 2 in which PNVC was as if glued w5,10,11x on the ZrO 2 layer
Table 1 presents some results on the polymerizationrcomposite formation and conductivity measurements for the ANI–ZrO 2 – ŽNH 4 . 2 S 2 O 8 –water system and NVC– ZrO 2 – ŽFeCl 3 . system. Entries 4–1 indicate that at fixed ZrO 2 and oxidant amounts in the initial feed, the total conversion to PANI increased with increasing ANI amount up to a certain value of the latter Žentry 2. and thereafter decreased with still higher ANI amounts. For an increase in the ANI amount from 0.15 g Žentry 3. to 0.20 g, only a marginal decrease in the overall yield was experienced Žcf. entries 3 and 2. which was apparently magnified with further increase in the ANI amount Žentry 1.. Entries 2, 6 and 7 indicate that at fixed ANI and oxidant weights, the total conversion to PANI and also the PANI loading per gram of the composite showed a tendency to fall with increasing ZrO 2 amount. This could be reasonable since ZrO 2 was not involved in the initiation process and the marginal difference in the total yield would imply a substantial difference when computations were done taking into account the ZrO 2 weight in the composite. Entries 8–12 indicate the variation of the total yield and the extent of loading of PNVC in the nanocomposite with the weight of FeCl 3 in the initial feed at fixed weights of ZrO 2 and NVC. Unlike MnO 2 , in the NVC–MnO 2 nanocomposite system recently reported by us w5x, ZrO 2 failed to initiate directly the NVC polymerization and accordingly FeCl 3 impregnated ZrO 2 was used as the catalyst. A maximum loading of ca. 50% of PNVC per gram of the composite was realized under the conditions used in this experiment. Notably, as with other PNVC-
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Table 1 Data on polymerizaton of ANI and NVC with nanodimensional ZrO 2 and composite formation thereof Entry no. 1 2 3 4 5 6 7 8 9 10 11 12
Weight Žg. ZrO 2
Oxidant
0.10 0.10 0.10 0.10 0.10 0.10 0.10 0.10 0.10 0.10 0.15 0.15 0.20 0.20 0.10 0.10 0.10 0.10 0.10 0.10 0.10 0.15
0.60 0.60 0.60 0.60 0.60 0.60 0.60 0.60 0.50 0.50 0.60 0.60 0.60 0.60 0.04 0.04 0.06 0.06 0.08 0.08 0.10 0.10
a
Monomer 0.25 0.25 0.20 0.20 0.15 0.15 0.10 0.10 0.20 0.20 0.20 0.20 0.20 0.20 0.10 0.10 0.10 0.10 0.10 0.10 0.10 0.10
% Yield of polymer
% Polymer per gram of composite
Conductivity ŽSrcm.
70.48 70.44 82.20 82.00 83.06 83.60 40.00 45.00 59.65 60.00 81.45 83.55 80.00 87.00 50.52 49.02 87.20 87.00 100.00 100.00
63.79 63.78 62.03 62.12 55.47 55.92 28.57 31.03 54.40 54.65 52.06 52.69 44.00 46.00 33.33 33.12 46.52 46.52 50.00 50.00
0.35 = 10y2
b
0.3 = 10y2 0.2 = 10y2 0.03 = 10y2 0.14 = 10y2 0.15 = 10y2 0.2 = 10y2 – ; 10y5 1.2 = 10y5
c
c
c
c
c
c
a
The oxidants for entries Ž1–7. and Ž8–12. were ŽNH 4 . 2 S 2 O 8 and FeCl 3 , respectively. The monomers for entries Ž1–7. and Ž8–12. were ANI and NVC, respectively. c A solid mass was formed after evaporation of acetone which was intractable.
b
based nanocomposite systems w5,6,12x, repeated benzene extraction removed all the surface-adsorbed PNVC excluding the PNVC loaded in the composite. 3.2. FTIR spectral characteristics The infrared spectrum of PANI–ZrO 2 composite revealed the following characteristic absorptions: 3425 cmy1 Ž) NH stretching.; 1570–1476 cmy1 ŽN–H bending.; 1302–1243 cmy1 ŽC–N stretching. thereby endorsing the presence of PANI in the composite. Similarly, the infrared spectrum of PNVC–ZrO 2 composite revealed the following characteristic absorptions: 724 cmy1 Žring deform. of subs. aromatic structure.; 1150–1232 cmy1 ŽCH in plane deform..; 1327 cmy1 ŽCH in plane deform. of vinylidene gr..; 1406 CHy1 Ž) CH 2 deform. of vinylidene gr..; 1481–1449 cmy1 Žring vib. of NVC moiety.; 1627 cmy1 ŽC s C str. of vinylidene gr.. which endorsed the presence of PNVC in the composites.
reminiscent of the typical raspberry morphology proposed by Armes et al. w1,3,13x for the various oxide-based PPYrPANI composites, where the ultrafine oxide particles would be present not only on the particle surface but also distributed through the interior of the particles. The ZrO 2 powder itself appeared to comprise finer particles which however became glued together by the polymer particles acting as binders in the nanocomposites — a situation quite similar to that reported by Armes et al. w1,3,10x for
3.3. Scanning electron micrographic analysis Fig. 1a and b represent the scanning electron micrographs of PANI–ZrO 2 and PNVC–ZrO 2 composites, respectively. The formation of microaggregates of composite particles of spherical shapes could be clearly noted in either systems and certaining was most pronounced in the PANI–ZrO 2 system. The observed morphologies were
Fig. 1. Scanning electron micrographs of: Ža. PANI–ZrO 2 nanocomposite; Žb. PNVC–ZrO 2 nanocomposite.
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the PANI–SiO 2 systems and to the PANI–MnO 2 system reported by us w5x. 3.4. Transmission electron micrographic characteristics The transmission electron micrographs ŽFig. 2a. of a PANI–ZrO 2 composite dispersion in the 100 ppm dilution range clearly confirmed the formation of spherical particles with average particle diameters in the 250–300 nm range. The TEM pattern for an undiluted PANI–ZrO 2 composite ŽFig. 2b. revealed prominently the microaggregatercluster formation involving ZrO 2 and the precipitating PANI particles. In general, the morphology of the PANI–ZrO 2 system was comparable to that reported for the well-studied PANI–SiO 2 system w10,13x and the same for the polyŽvinylmethylether. stabilized PANI dispersions w14x. Fig. 3a and b represent the transmission electron micrographs of two PNVC–ZrO 2 composites with PNVC loading of 33% Žentry 8, Table 1. and of 50% Žentry 10, Table 1., respectively. In general, these micrographs revealed a relatively non-uniform coverage of ZrO 2 particles by the precipitating PNVC. This problem would be difficult to avoid while working with a water-insoluble monomer like
Fig. 3. Transmission electron micrographs of: Ža. PNVC–ZrO 2 nanocomposite Ž11% PNVC loading.; Žb. PNVC–ZrO 2 nanocomposite Ž40% PNVC loading..
NVC, which unlike the water soluble PY or PANI monomers would not be distributed homogeneously in the polymerization medium containing the oxides and the oxidants. However, for the PNVC–ZrO 2 system, the procedure followed by us would possibly allow of a better coverage of the ZrO 2 particles by the precipitating PNVC by controlling the rate of precipitation by methanol addition to the benzene solution of the polymer. The formation of spherical particles, in general, could be clearly distinguished in the TEM pattern for the PNVC–ZrO 2 nanocomposite system. The average particle diameters of the PNVC–ZrO 2 composite were in the range 300–500 nm. 3.5. Thermal stability of the composites Fig. 2. Transmission electron micrographs of: Ža. PANI–ZrO 2 nanocomposite dispersion Ž100 ppm order dilution.; Žb. Undiluted PANI–ZrO 2 nanocomposite.
Thermogravimetric analysis of the PANI–ZrO 2 composite ŽFig. 4a. revealed a total loss of ca. 68.1% at ca. 5508C, which remained constant till 951.18C. In contrast,
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whereby 70% of the PANI degradation was completed. In contrast, in the DTA curve for PANI–ZrO 2 composite a broad hump at 439.98C appeared corresponding to a loss of 43% of the composite. No minor PANI peaks could be detected in this range. Thus, these data readily confirmed the enhanced thermogravimetric stability for PANI–ZrO 2 composite relative to PANI. 3.6. ConductiÕity characteristics
Fig. 4. TGA and DTA scans for: Ža. PANI–ZrO 2 nanocomposite; Žb. PANI.
PANI ŽFig. 4b. showed a complete loss at the same temperature range while ZrO 2 revealed only a partial loss of volatile impurities of about 17% in the same temperature range. In the light of the reasoning put forward by Armes et al. w2x for PANI–SiO 2 system, these data implied that the polymer content per gram of the composite was Ž68.1 y 17.25.% s 50.75% which compared reasonably well with the actual data Žentry no. 6, Table 1.. A similar consistency in the estimate of PNVC loading was also realized from the thermal analysis data for the PNVC–ZrO 2 system — which behaved in a similar fashion to the PNVC–SiO 2 nanocomposite system recently reported by us w6x. A comparison of DTA scans in the thermograms revealed the manifestation of smaller exothermic peaks at 2098C, 362.78C and a sharp peak at 532.98C for PANI
The conductivity of the PANI–ZrO 2 nanocomposites systematically increased with increasing PANI loading in the composites Žentries 4–1, Table 1.. Entries 2, 6 and 7 indicate that with an increase in the ZrO 2 amount at a fixed level of ANI and ŽNH 4 . 2 S 2 O 8 , the conductivities showed only a marginal change tending to level off. This trend was more or less parallel to the same for PANI loading on the composites. Reported conductivity values of PANI–SiO 2 w1,2x and of PANI–MnO 2 nanocomposites w5x were in the order of Ž0.4–0.6. = 10y3 and Ž1–2.5. = 10y2 Srcm, respectively. Thus, these data did not allow any correlation between the observed conductivity and the oxide chosen. The deciding factor would be the extent of PANI loading in a particular composite. As for the PNVC–ZrO 2 nanocomposites, the conductivities recorded in entries 9 and 10 in Table 1, did not reveal any change. However, these values were appreciably enhanced compared to the conductivity of the unmodified PNVC Ž; 10y1 2 to 10y16 Srcm. w15x. For an essentially nonconducting polymer like PNVC, the enhancement in the conductivity was dramatic — although increasing extent of PNVC loading in the composite would not obviously change the conductivity magnitude. The exact mechanism for the enhancement of conductivity is obscure at the moment. However, in PPY–ZrO 2 system w4x, the large increase in conductivity was attributed to an improved linking between PPY moieties due to intimate association of PPY moieties with ZrO 2 particles in the nanocomposite. The dependence of the conductivities of these conducting nanocomposites on the nature of the oxide chosen is yet an open question since the available literature is indeed
Table 2 A comparison of the conductivities of ZrO 2 - and MnO 2 -based composites of PANI and PNVC Composites
Average particle diameter Žnm. a
Polymer loading Ž%. per gram of composite
Conductivity ŽSrcm.
Ref.
PANI–MnO 2 PANI–ZrO 2 PANI–MnO 2 PANI–ZrO 2 PNVC–MnO 2 PNVC–ZrO 2 PNVC–MnO 2 PNVC–ZrO 2
100–150 250–300 100–150 250–300 200–250 300–500 200–250 300–500
63 62 51 52 23 46 36 50
1.8 = 10y2 0.3 = 10y2 1 = 10y2 0.15 = 10y2 3.5 = 10y5 1 = 10y5 4.4 = 10y5 1.2 = 10y5
w5x this study w5x this study w5x this study w5x this study
a
Particle diameters of MnO 2 and ZrO 2 were in the range of Ž6–10. and Ž20–30. nm, respectively.
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too meager to suggest any correlation. A comparison of our recent results on the conductivity of the nanodimensional MnO 2-based nanocomposites of PANI and PNVC with those on the present ZrO 2-based systems ŽTable 2. indicate that at comparable PANIrPNVC loading in the composite, MnO 2-based composites exhibited appreciably higher conductivities compared to the ZrO 2-based systems. The particle sizes of the MnO 2 particles as also of the MnO 2-based PANIrPNVC colloids were also significantly lower than the corresponding ZrO 2-based systems. This probably allowed a more compact grain formation in the particles of MnO 2-based systems which could again enhance the conductivity. In view of the limited data presented and made available in this context, such a suggestion should be regarded as purely tentative. 3.7. Water dispersibility of the PANI–ZrO2 and PNVC– ZrO2 nanocomposites The PANI–ZrO 2 nanocomposite was obtained as a black colloidal suspension during polymerization of ANI. These suspensions retained their stability almost permanently. An exactly similar behavior was noted for the PANI–MnO 2 nanocomposite system recently reported by us w5x. Although formation of the water-dispersible nanocomposite of PANI was relatively more facile when nanodimensional MnO 2 was used, the PNVC–ZrO 2 nanocomposites required sonication in water medium for 3–4 h for producing a stable colloidal dispersion in water. Notably, in the PNVC–MnO 2 system w5x, no sonication was required for producing a stable dispersion. Thus, the behavior of ZrO 2 and MnO 2 in regard to PNVC nanocomposite formation was remarkable since for producing PNVC–SiO 2 water suspension a polymeric stabilizer polyŽN-vinylpyrrolidone. was essential w6x. As yet, we do not have any satisfactory explanation for the observed difference in behavior of the various oxides towards nanocomposite formation. Indeed, as felt by Armes et al. w1x, factors such as preadsorption characteristics of the reacting species, on the oxide surface high surface area, surface charge on the oxide might be relevant factors.
The polymers under study involved cationic chain growth w16x and accordingly, a negative oxide surface charge would presumably be relevant in the formation of the stable colloids w1x. 4. Conclusion Stable water dispersible nanocomposites of PANI and PNVC could be obtained from the ANI– ŽNH 4 . 2 S 2 O 8 –2 M HCl–water and NVC Žbenzene. –FeCl 3 –water polymerization systems in the presence of nanodimensional ZrO 2 . Acknowledgements Thanks are due to CSIR, New Delhi, India for generous funding of an Emeritus Scientist project in favour of MB and to the authorities of Presidency College Calcutta, for use of its facilities. References w1x J. Stejskel, P. Kratochvil, S.P. Armes, S.F. Lascelles, A. Riede, M. Helmstedt, J. Prokes, I. Krivka, Macromolecules 29 Ž1996. 6814. w2x S.P. Armes, S. Gottesfeld, J.G. Beery, F. Garzone, S.F. Agnew, Polymer 32 Ž1992. 2325. w3x S. Maeda, S.P. Armes, Chem. Mater. 7 Ž1995. 171. w4x A. Bhattacharya, K.M. Ganguly, A. De, S. Sarkar, Mater. Res. Bull. 31 Ž1996. 527. w5x M. Biswas, S. Sinha Ray, Y.P. Liu, Synthetic Metals 105 Ž1999. 99. w6x S. Sinha Ray, M. Biswas, Mater. Res. Bull. 33 Ž1998. 533. w7x A. Cornelis, P. Laszao, Synthesis Ž1980. 849. w8x P. Laszao, E. Polla, Tetrahedron Lett. 25 Ž1984. 3309. w9x L.P. Ellinger, Polymer 5 Ž1964. 559. w10x S. Maeda, S.P. Armes, Synthetic Metals 73 Ž1995. 151. w11x S. Sinha Ray, M. Biswas, J. Appl. Polym. Sci. 74 Ž1999. 2971. w12x M. Biswas, S. Sinha Ray, Polymer 39 Ž1998. 6423. w13x S. Maeda, S.P. Armes, J. Mater. Chem. 4 Ž1994. 935. w14x N.J. Terrill, T. Crowley, M. Gill, S.P. Armes, Langmuir 9 Ž1993. 2093. w15x M. Biswas, S.K. Das, Polymer 23 Ž1982. 1713. w16x S. Huang, B.D. Humphrey, A.G. MacDiarmid, J. Chem. Soc., Faraday Trans. 82 Ž1986. 2825.