J. Phys. Chem. B 2001, 105, 11901-11905
11901
Antiferromagnetic Pairing in Polyaniline Salt-Zeolite Nanocomposites† Harry L. Frisch,* Huihong Song, and Junqing Ma Department of Chemistry, State UniVersity of New York at Albany, Albany, New York 12222
Miriam Rafailovich and Shaoming Zhu Materials Science & Engineering, State UniVersity of New York at Stony Brook, Stony Brook, New York 11794-2275
Nan-Loh Yang and Xingzhong Yan Department of Chemistry, Staten Island College, City UniVersity of New York, Staten Island, New York 10314 ReceiVed: June 14, 2001; In Final Form: September 11, 2001
Polyaniline (PANI) zeolite composites have been prepared by oxidative polymerization of anilinein the presence of zeolite 13X, producing composite materials with PANI/zeolite weight ratios from about 0.5 to 55. The DC conductivity of the composite materials decreased exponentially with zeolite weight fraction. Pure PANI was paramagnetic while the PANI/zeolite complexes with weight ratios larger than 20 showed an appreciable antiferromagnetic component as indicated by temperature dependence of their electron spin resonance spectra. This novel antiferromagnetism is speculated to arise from π-dimer pairing of PANI chain layers stacking on the zeolite particle surface.
Introduction Zeolite 13X is a polycrystalline, microporous material with a uniform pore size distribution of 13 Å and 50% pore volume.1 Composites in which aniline was polymerized to the polyaniline (PANI) in zeolites have been previously reported,2,3 but they did not show a measurable conductivity. Similar hybrid composites of organic monomers (styrene and ethyl acrylate) polymerized in zeolite 13X have provided considerable evidence that the polymers could fill the pores in the zeolite.4-8 Pohl and Engelhardt9 using paramagnetic resonance spectroscopy (EPR) showed earlier that PANI contained high levels of paramagnetic centers. Recent EPR studies have not been exploiting the properties of PANI nanocomposites.10 Therefore it would be interesting to determine the effect on the paramagnetic properties of the PANI when it is prepared under the influence of zeolite. Samples of PANI, PANI/zeolite, and extracted PANI/zeolite were examined using CW as well as pulsed electron spin resonance. Spin density, g-value, and maximum slope line width, ∆Hmsl, were determined. As expected, the pure PANI was paramagnetic. The PANI/zeolite samples showed antiferromagnetism. This antiferromagnetism increased with the PANI/zeolite weight ratio of samples and was diminished by extraction with 1-methyl-2-pyrrolidinone. We speculate that the antiferromagnetism is associated with the formation of interchain π-dimers in PANI chain layers stacking on the surface of the zeolite. This may be the first observation of antiferromagnetic pairing of electrons in PANI composites. Experimental Section A. Materials. All starting materials were purchased from Aldrich. Zeolite 13X was dried at 832 K for 6 h before using. Hexane was refluxed with calcium hydride overnight and †
Part of the special issue “Howard Reiss Festschrift”. * To whom correspondence should be addressed.
distilled. Aniline was vacuum distilled in a stream of nitrogen. Ammonium persulfate ((NH4)2S2O8) was used without further purification. All solvents were dried over 4 Å molecular sieves. B. Preparation of the Polyaniline Salt in the Zeolite 13X. We followed the basic procedure for preparation in ref 3. All
steps were performed in a purified nitrogen atmosphere. The dry zeolite was suspended in dry hexane, and aniline was slowly added under stirring at 295 K. After stirring for 12 h, the loaded zeolites were filtered, washed with ultrapure water several times and then dried. The loaded zeolite was then suspended in 1 M hydrochloric acid solution at 273 K, and an aqueous solution of ammonium persulfate was added to the stirred suspension at a molar ratio of 1:4 oxidant to aniline. The suspension was stirred for 2 h at 273 K and then allowed to warm to 295 K for 12 h and filtered. The aniline/zeolite samples turned deep blue during this treatment. Samples with different PANI/zeolite weight ratios were prepared by this method. Pure PANI was prepared by the same method using aniline with ammonium persulfate in acidified water. Extraction of PANI11 was performed using 1-methyl-2pyrrolidinone as the solvent.12 By varying the extraction times, different percentages of of PANI were extracted to obtain PANI/ zeolite samples with different weight loss. The molecular weights of PANI were obtained by Viscotek GPC with laser light scanning and T60A dual detector with mobile phase of THF using 1-methyl-2-pyrrolidinone as the solvent and polystyrene bead packed as the column. For pure PANI we obtained Mw ) 462 000, Mn ) 391 000, and a polymerization index of n ) 1.18. Comparable values for the molecular weights were obtained for PANI/zeolite samples with ratios in excess of 13:1 weight percent. Significantly lower values with somewhat slightly higher polydispersity were
10.1021/jp012278z CCC: $20.00 © 2001 American Chemical Society Published on Web 10/23/2001
11902 J. Phys. Chem. B, Vol. 105, No. 47, 2001
Frisch et al.
Figure 1. SEM micrographs of (A) PANI/zeolite weight ratio of 13.6; (B) pure PANI; (C) 35% PANI extracted from A; (D) 72% PANI extracted from A.
obtained for samples with 1:1 wt % ratios. (See Table 1). This indicates that the presence of the zeolite surface somehow impedes diffusion of monomer thereby impeding the synthesis. Samples for DC conductivity measurement were pressed into pellets, approximately 1.5 to 2.3 mm thick, prepared by compressing the samples in a press using an applied pressure of 15 200 lb for 10 min. The DC conductivities of these pellets were measured using the four-point probe technique at 298 K.13 The phase morphological characteristics of the samples were studied using SEM. The specimens were mounted on stubs with silver paint. All the specimens for SEM were coated with platinum in a Hummer V sputter coater. They were then observed with a digital scanning microscope (DSM 940, Zeiss, Germany) operating at a voltage of 20 kV. The electron images were recorded directly from the cathode ray tube on Polaroid 55 film. C. EPR Instrumentation and Experiment Procedures. X-band EPR spectra were recorded on a Bruker ESP380E spectrometer, equipped with a variable temperature accessory (a BVT-2000 system) and a HP5361B frequency counter. Samples, sealed in 1 mm glass capillary tubes or in 10 mm NMR glass tubes under nitrogen atmosphere, were kept in a standard cavity (ER 4102 at the desired temperatures for 20∼30 min to allow for thermal equilibrium for measurement at each temperature. The g-values were estimated based on the peak positions of DPPH solid (g ) 2.0036). Spin densities were estimated based on the intensity of the third line of a Mn2+/ CaO marker calibrated against known concentrations of aqueous solutions of TMPO/cyclodextrin, CD. The CD was added to
the aqueous system to improve the solubility of TEMPO and to prevent intermolecular interaction among TEMPO molecules. For quantitative EPR determinations over a wide temperature range, experimental conditions were selected to ensure the thermal equilibrium of the samples and the absence of microwave saturation. No power saturation was observed for sample 3B up to 72∼74 mW at 250 K and 170 K. In a series of variable temperature experiments, it was determined that at least 10 min was required for thermal equilibrium of the sample. Proper conditions, including data resolution, sweep time, and modulation amplitude, were ascertained for all EPR measurement. Relaxation, T1, determinations were made using a Bruker pulse probe (model G-N2D). The response of the sample to the π/2 (16 ns) microwave pulses was detected in quadrature with application of a CYCLOPS phase-cycling routine. The microwave pulse power was attenuated to 7 dB, and the receiver video amplifier gain was adjusted to proper levels. Results A. Surface Morphology of the PANI/Zeolite Samples. SEM micrographs of a pressed powder sample with a weight ratio of 13.6 PANI salt to zeolite is shown in Figure 1. In Figure 1A we show the micrograph obtained after synthesis. The surface morphology of the sample is very similar to that of pure PANI salt imaged in Figure 1B with the same magnification. The PANI salt observed in Figure 1A completely covers the otherwise flat platelet structure of the pure zeolite. Hence for large concentrations, most of the PANI salt resides outside the
Antiferromagnetic Pairing in PANI Zeolite Composites
J. Phys. Chem. B, Vol. 105, No. 47, 2001 11903
TABLE 1: DC Conductivities (S/cm), σ sample Pure PANI PANI/zeolite ) 19:1 (wt) PANI/zeolite ) 14.5:1 (wt) PANI/zeolite ) 1:1 (wt)
σ (S/cm)
Mw
Mn
1.20 ( 0.30 462 000 391 000 0.16 ( 0.02 432 000 360 000 0.05 ( 0.02 436 000 355 200 less than 10-8 95 000 72 900
n 1.18 1.20 1.23 1.30
Figure 2. DC conductivities (S/cm), σ vs polyaniline content of samples.
zeolite pores and is visible as submicron sized granular multilayer structures adsorbed to the zeolite surfaces. Figure 1C is an SEM micrograph of the sample after extraction of 35 wt % of PANI. From the figure we can now see areas where the distribution of granules is less dense and other areas where some exposed zeolite platelets are present. The areal fraction of exposed areas, approximately 20-30%, is consistent with the weight fraction of PANI extracted. In Figure 1D we show the SEM micrograph of the same sample after 72% of the PANI was extracted. Here the bare zeolite platelets are clearly seen to occupy at least 70% of the surface area. The magnification of the images is too low to see the nanometer scaled zeolite pores. Since the PANI surface coverage scales approximately with weight loss, we can conclude that even if the PANI synthesis was initiated inside the pores, most of the PANI was resident on the surface of the zeolite where it could be imaged with the SEM. B. DC Conductivity. The DC conductivity was measured as a function of zeolite weight fraction using the four point probe method. The results are tabulated in Table 1. From the table we can see that the conductivity decreases drastically with increasing zeolite content. In Figure 2 we plot the log of the conductivity, σ, versus the weight fraction of PANI salt. From the figure we see that the decrease is in fact exponential, decreasing by 8 orders of magnitude, while the volume fraction decreases by a factor of 10. Hence the similarity in morphology observed between the pure PANI sample and the zeolite/PANI sample is only superficial. The interconnectivity of the granules in the samples containing zeolite is rather poor. This may be a result of the confinement of the PANI within the pores of the non conducting zeolite matrix in the initial stages of polymerization. Hence even though the chains then emerge from the pores as the synthesis proceeds, their overlap is hindered, which results in decreased electrical conductivity. On the other hand, The possible confinement the PANI salt into pores approximately the size of a monomer can result in unusual magnetic properties, which were investigated by EPR. C. Magnetic Properties. EPR spectroscopy was applied to examine the paramagnetic properties of the PANI and PANI/
TABLE 2: Spin Density, g-Value, and ∆Hmsl of Samples with Different PANI/Zeolite Weight Ratio spin densityc (10-20 spins/ g polymer)
antiferro/ Curie ratiod
0.129
0
namea
g-valueb
∆Hmsl (G)
sample 2A (PA/Z ) 0.45:1) sample 1A (PA/Z ) 1.27:1)
2.0032 1.9997 2.0127 2.0037 1.9997 2.0032
11.00 0.857 0.348 12.00 1.318 3.573
0.079
0
1.61
∼
2.0031
2.012
0.98
∼
2.0028
4.645
1.10
∼
2.0029
2.858
0.57
∼
2.0032
3.573
2.37
∼
2.0030
2.882
1.67
∼
2.0029
3.574
2.18
0.7
2.0029
4.716
2.26
1.6
2.0028
3.430
3.11
0.8
2.0028
2.858
2.55
0.1
2.0028
2.286
1.88
0
2.0028
1.715
2.60
0
2.0029
2.001
3.60
0
2.0034
4.859
0.019
0
2.0029
4.288
1.15
0
sample 51 (PA/Z ) 13.6:1) sample 51 (-29%) sample 32 (PA/Z ) 14.5:1) sample 32 (-16%) sample 31 (PA/Z ) 19:1) sample 31 (-6%) sample 1B (PA/Z ) 28:1) sample 2B (PA/Z ) 55:1) sample 3B (PA/Z ) 20.3:1) sample 3B (-8%) sample 3B (-19.4%) sample 3B (-38.5%) sample 3B (-49%) sample 3B (dried extracted solution) polyaniline a
(-xx%) refers to xx% of PANI extracted from the sample. PA/Z is the PANI/zeolite weight ratio. b g-Value estimated by resonance frequency and field offset, using DPPH (g ) 2.0036) as a reference. c The spin densities were estimated based on the intensity of the 3rd line of the Mn2+/CaO marker as calibrated against TEMPO in β-cyclodexitrin, β-CD (TEMPO/β-CD weight ratio ) 1:10) water solution. Sample 1A exhibits three peaks, and sample 2A, two peaks. d (∼) is the low antiferro contribution.
zeolite samples. All samples showed high levels of spin density (∼1020 spins/g PANI). The spin density of the composite depended on the PANI/zeolite weight ratio and reached a plateau of ca. 3 × 1020 spins/g-PANI at the ratio of 20 (Figure 3 ). Spin density, g-value, and maximum slope line width, ∆Hmsl, for samples of different PANI/zeolite weight ratios and extent of extraction are summarized in Table 2. Samples with a high PANI/zeolite ratio show one resonance peak. The EPR line shape of these samples approaches Lorentzian. For example, sample 3B at room temperature shows a Lorentzian/Gaussian component ratio of 7:3 based on spectral simulation. They do not have significant Dysonian components since in the present system the conductive layers are not expected to correspond to conductive systems with “thick skin”. Samples with a low PANI/ zeolite ratio (samples 1A and 2A) show spin density much lower than those with a high PANI/zeolite ratio and exhibit different spectral features (1A, three peaks; 2A, two peaks). Except for samples with a low ratio, i.e., 2A and 1A, all g-values are closer to that of free electrons compared with DPPH, indicating significant delocalization of electrons in PANI from high ratio systems. Only for samples 1A and 2A, should most of the PANI
11904 J. Phys. Chem. B, Vol. 105, No. 47, 2001
Figure 3. Spin density vs PANI/zeolite weight ratio of PANI/zeolite samples.
Frisch et al.
Figure 5. χT vs temperature plots of zeolite polyanilines with different antiferromagnetic component: (]), blank polyaniline; (b), sample 1B; (2), sample 2B; (1), sample 3B; (9), sample 3B extracted 8%. The fitting function for blank polyaniline is χT ) 4.6497 - 4.6987 × 10-3 T, and that for the other three curves is χT ) A0 + A1/{1 + exp[-(T - θ)/A3}.
Figure 4. 1/χ vs temperature plots of PANI and PANI/zeolite samples with different antiferromagnetic component: (]), blank polyaniline; (b), sample 1B; (2), sample 2B; (1), sample 3B; (9), sample 3B extracted 8%.
reside inside the zeolite pores, thus the low electronic conductivity and the lower spin density with broad line width. The delocalization of the electrons of the PANI chain in the zeolite cage would likely be hindered due to constraints imposed on the macromolecules. The temperature dependence of the spin susceptibility, χ, of the high PANI/zeolite ratio samples suggests two types of spin are present as depicted in the 1/χ vs temperature (T) plots (Figure 4). One type of spin follows Curie law; a second type exhibits a tendency for antiferromagnetic pairing. One part of the plot shows the normal decrease in 1/χ as T decreases, i.e., the Curie component; the second part in the higher temperature region shows an unusual increase in 1/χ as T decreases, i.e., the antiferromagnetic component. Polyaniline control displays the normal Curie behavior. From Figure 5, the spin ratio for antiferro/Curie can be obtained for samples with the PANI/zeolite weight ratio above 20 (1B, 2B, and 3B). At the PANI/zeolite ratio level of 20∼28, 41% of the spin is antiferromagnetic; at 55, 62% of the spin is antiferromagnetic. Solvent extraction of 3B led to a sharp decrease of the antiferro component. At 19% extraction of 3B, the antiferro component is no longer present. Their temperature behavior can be more readily depicted in χT vs T plots (Figure 6), since χT is a more direct representation of relative spin concentration over a wide range of temperature. From the temperature dependence of the antiferro components, one can obtain the enthalpy change ∆H and entropy change ∆S for the spin-paired to free-spin conversion based on the lnKT
Figure 6. Plot of lnKT vs 1/T for antiferro component for PANI/zeolite samples 1B, 2B, and 3B equilibrium: spin paring ) spin + spin, KT ) (χantiferroT)2/[(χantiferroT)0 - (χantiferroT)]. (b), sample 1B; (2), sample 2B; (1), sample 3B.
vs 1/T plot where KT is the equilibrium constant at T for the process shown in Figure 6: “Spin-paired” 2 “Free spins”. A ∆H barrier of ca. 60 kJ/per mole of spin for the process was estimated. We propose a model consistent with the EPR magnetic data obtained for the antiferromagnetic component present in the PANI/zeolite samples. The PANI macromolecules form a multilayer coating on the zeolite particle. The morphology of stacking of the layers is influenced by the zeolite surface as well as the charged nature of the doped PANI. Since each polymer chain carries high positive charge density along the backbone with large amounts of counterion, Cl-, some forms of stacking would be preferred to minimize charge and dipole repulsion.14 The PANI spin density indicates a relatively large average separation (ca. 65 repeat units of aniline). Spin pairing can originate from intrachain itinerant spin along the chain and/ or spin interchain antiferromagnetic pairing at the proper sites, i.e., formation of π-dimer. Since solvent extraction or simple immersion of samples in solvent led to a narrowing of EPR line width, the elimination of the antiferromagnetic component and the tendency of lowering spin density, the spin-pairing is more likely to involve interchain, rather than intrachain, spin antiferromagnetic pairing, facilitated by the composite morphology of chain stacking.
Antiferromagnetic Pairing in PANI Zeolite Composites The presence of the antiferromagnetic region allowing for spin pairing can be facilitated by high local dielectric constant consisting of high positive charge of the chain backbone together with its counterion. As thermal energy is increased, the spins of the pair can be separated to form two free spins, i.e., high spin susceptibility at higher temperature. The process of this separation is estimated to involve a ∆H of ca. 60 kJ per mole of spin and a ∆S of ca. 200 J/molK. These values of ∆H and ∆S are in line with those for the formation and dissociation of π-dimers of π-conjugated cation radical segments based on pyrrole and thiophene.15 The ∆S value for the present system is about twice that for the case of π-dimers (200 J/molK vs 110 J/molK). This higher value is expected, since spin separation in a conductive matrix should involve a greater increase in disorder than for simple dimer separation. The antiferromagnetic region is most likely to be more densely packed than the Curie region, thus more conducive for interchain processes as evidenced by data of spin relaxation time, T1, from pulsed FTEPR investigation. The T1 values of sample 3B series are in the following order: sample 3B (102ns) < sample 3B (-8%) < sample 3B (-19.4%) < sample 3B (-38.5%) < sample 3B (-49%) < 210 ns. Although a precise T1 value for each case is not available due to the fast signal decays contributed by more than one component, their ranking does indicate a better spin/ lattice interaction in the antiferromagnetic region. Conclusion The presence of zeolite alters the processes for oxidative polymerization of aniline. The resulting zeolite/polyaniline salt composites show properties distinctively different from pure PANI salt. Low levels of zeolite in the composites decrease
J. Phys. Chem. B, Vol. 105, No. 47, 2001 11905 drastically the DC conductivity and lead to the unusual magnetic property of the system. The influence of less than 5% of zeolite in the polymerization system gives PANI with a large fraction of antiferromagnetic component instead of the usual all paramagnetic PANI. Acknowledgment. H.L.F. was supported by NSF grant DMR9628224 and the donors of the Petroleum Fund of the American Chemical Society, M.R. was supported by NSF grant NSF-MRSEC (DMR9632525), and N.Y. was supported by New York State Higher Education Applied Technology Program (HEAT) and NSF-MRSEC (DMR9632525). References and Notes (1) Al-ghamdi, A. M. S.; Mark, J. E. Polym. Bull. 1988, 20, 537. (2) Bein, T.; Enzel, P. Synth. Met. 1989, 29, E163. (3) Enzel, P.; Bein, T. J. Phys. Chem. 1989, 93, 6270. (4) Frisch, H. L.; Xue, Y. J. Polym. Sci. A 1995, 33, 1979. (5) Frisch, H. L.; Xue, Y.; Maaref, S.; Beaucage, G.; Pu, Z.; Mark, J. E. Macromol. Symp. 1996, 106, 147. (6) Frisch, H. L.; Maaref, S.; Xue, Y.; Beaucage, G.; Pu, Z.; Mark, J. E. J. Polym. Sci. A 1996, 34, 673. (7) Frisch, H. L.; Mark, J. E. Chem. Mater. 1996, 8, 1735. (8) Frisch, H. L.; West, J. M.; Go¨ltner, C. G.; Attard, G. S. J. Polym. Sci. A 1996, 34, 1823. (9) Pohl, A.; Engelhardt, E. H. J. Phys. Chem. 1962, 66, 2085. (10) Genoud, F.; Kulszewicz-Bajer, I.; Bedel, A.; Oddou, J. L.; Jeandey, C.; Pron, A. Chem. Mater. 2000, 12(3), 744, and references therein. (11) David, S.; Nicolau, Y. F.; Melis, F.; Revillon, A. Synth. Met. 1995, 69, 125. (12) Adams, P. N.; Apperley, D. C.; Monkman, A. P. Polymer 1993, 34, 331. (13) Smits, M. Bell Syst. Technol. J. 1958, 37, 711. (14) Mattes, R. Macromolecules 1998, 31(23), 8183. (15) van Haare, A. E. H.; van Boxtel, M.; Janssen, R. A. J. Chem. Mater. 1998, 10(4), 116.
