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Structural effect of polymeric acid dopants on the characteristics of doped polyaniline composites
sqflTH|TH[ D|TRLS ELSEVIER
Synthetic Metals 92 (1998) 223-228
Structural effect of polymeric acid dopants on the characteristics of doped polyaniline composites Hyuk-Soo Moon, Jung-Ki Park * Department of Chemical Engineering, Korea Advanced Institute of Science and Technology, 373-1. Kusung-dong, Yusung-gu, Daejon 305-701. South Korea Received 28 April 1997; revised 16 October 1997; accepted 23 October 1997
Abstract
The structure and doping behaviors of poly (p-styrene sulfonic acid) (PSSA) -doped polyaniline ( PANi ) and poly ( 2-acrylamido-2-methyll-propane sulfonic acid) (PAMPS)-doped PANi were investigated by conductivity measurement, ultraviolet-visible spectroscopy, X-ray photoelectron spectroscopy, and X-ray diffraction. The conductivities of the PSSA-doped PANi composites are higher by up to two orders of magnitude than those of the PAMPS-doped PANi composites of the same composition, given by acid-to-aniline ratio. The confinement of pendant benzene sulfonic acid groups in the PSSA and that of pendant amino methyl propane sulfonic acid groups in the PAMPS may result in non-uniform and inefficient doping, such that some of the acid groups are not able to participate in the doping. The conductivity difference between the two different polymeric acid-doped PANi composites can be explained by the difference in the degree of the steric hindrance between the two polymeric acid dopants. The PSSA-doped and PAMPS-doped PANi systems reveal the same optimum composition of 1/ 1 ( moles of sulfonic acid units in the polymeric acids/moles of aniline units in PANi) for maximum conductivity. The maximum conductivity seems to be attributed to the increase in the doping level and the decrease in the order and packing density of the PANi subchain alignment with increase of the acid-to-aniline ratio. © 1998 Elsevier Science S.A. Keywords: Polyaniline and derivatives; Polymeric acid dopant; Doping level; Chain alignment; Conductivity
I. Introduction
The discovery of the exceptional electrical properties of polyacetylene has spurred a great deal of research work in the area of electroactive polymers in recent years [ 1-4]. Of particular interest is century-old polyaniline (PANi) mainly due to its good thermal and environmental stability [ 5-7 ]. However, up to now, commercial applications of PANi have been limited. One of the obstacles to practical use of PANi is its poor processability when it is doped with conventional small molecular organic or inorganic acids. With a rigid 7rconjugated backbone molecular structure, PANi decomposes without melting when heated and few proper solvents have been found for PANi [8-11 ]. The conducting form of PANi powder synthesized in aqueous HC1, referred to as PANi hydrochloride, is insoluble in common organic solvents and even in l-methyl-2-pyrrolidone (NMP), which is the only organic solvent having been found so far that can dissolve high molecular weight PANi. Additionally, these small molecular acid dopants for PANi may evaporate at room * Corresponding author. Tel.: + 82 42 869 3925; fax: + 82 42 869 3910; e-mail: pjk @sorak.kaist.ac.kr 0379-6779/98/$19.00 © 1998 Elsevier Science S.A. All rights reserved P H S 0 3 7 9 - 6 7 7 9 ( 9 7 ) 0 4 0 9 1-5
temperature or higher, causing a depression in the conductivity of the acid-doped PANi [ 11,12]. Moreover, the small molecular acid-doped PANis are not suitable for use as cathodic material in a lithium rechargeable battery, since the movable anions of the acid dopants are capable of diffusing out to the electrolyte phase and, thus, the charge compensation for the positive electrode redox reaction will be based on the doping and undoping of anions rather than the intercalation and deintercalation of lithium cations [ 1,12-14]. These obstacles can be overcome in an appropriate way, that is, by using polymeric acid dopants. In addition, polymeric acid dopants can be expected to enhance the flexibility of the PANi composite since the polymeric acid usually has a lower glass transition temperature (Tg) than that of PANi [11,15]. However, little research has been carried out on the microstructural feature of the polymeric acid-doped PANi including how many of the acid groups are capable of doping PANi and how the dopability is affected or changed by the molecular structure of polymeric acid, such as chemical structures of pendant protonic acid groups. In addition, neither the way the dopant interacts with PANi nor how the doped PANi subchains align have been investigated yet. It has been
224
H.-S. Moon, J.-K. Park/Synthetic Metals 92 (1998)223-228
expected that not all of the acid groups would occur in polymeric acid-doped PANi and that the acid groups would be distributed non-uniformly due to the confinement of protonic acid groups on polymer chains and the conformational hindrance created by the flexible chain [ 16-21 ]. This study has been devoted to investigate systematically the structural effect of the pendant protonic acid groups of the two different polymeric acids, i.e., poly(p-styrene sulfonic acid) ( PSSA ) and poly ( 2-acrylamido-2-methyl- 1-propane sulfonic acid) (PAMPS), on the conductivity of the doped PANi composite, the doping level of PANi, and the subchain alignment of the doped PANi.
to ,.Q <
4000
2. Experimental
3000
2000
1000
Wave Number (cm1)
Fig. 1. FT-IR spectrum of the polyaniline base synthesized by oxidative polymerization.
2.1. Preparation of materials
The PANi hydrochloride powder was synthesized by the oxidative polymerization of aniline in 1 M aqueous HC1 with (NH4) 2S208 as oxidant at 5 °C, which is similar to the method used by MacDiarmid and Epstein [6] and Ko [23]. The powder was then converted to PANi base by treatment with 0.5 M aqueous NH4OH, followed by washing with water/ methanol several times and evacuating at 40 °C for 2 days. The PANi base obtained as described above has a ratio of IR absorption intensity at 1589 c m - ~ (due to the quinoid ring) to that at 1496 c m - J (due to the benzenoid ring) nearly the same as that of the emeraldine base with a 60 mol% oxidation level (Fig. 1 ) [22,23]. Thus, the synthesized PANi base is an emeraldine base and becomes emeraldine salt after acid doping. The styrene sulfonic acid-based polymeric dopant (PSSA) was obtained by solution polymerization of styrene using 2,2'-azobis-isobutyronitrile (AIBN) as an initiator in tetrahydrofuran (THF) at 60 °C, and subsequent sulfonation of the recovered polystyrene (PS). The sulfonation was conducted at 50 °C in ethylene dichloride using acetyl sulfate as a sulfonation agent, which had been prepared by the reaction of acetic anhydride and concentrated sulfuric acid. The other type of sulfonic acid, AMPS-based polymeric dopant (PAMPS), was also synthesized using the solution method, where AIBN and water were used as the initiator and reaction medium, respectively. 2.2. Doping of PANi
The polymeric acid-doped PANi was prepared by mixing appropriate volumes of the 0.22 M PANi (based on the approximate repeat unit -C6HaNH- ) in NMP solution and the 0.22 M polymeric acid (based on its acidic repeat unit) solution. The dissolving solvent for the PSSA was NMP, whereas dimethyl formamide (DMF) was selected as a solvent for the PAMPS because NMP was a poor solvent for the PAMPS. The mixed solutions of polymeric acid-doped PANi
were then agitated vigorously and cast on an evaporating dish under vacuum at 50-60 °C for more than 48 h. 2.3. Conductivity measurement
Electrical d.c. conductivity measurements of the compressed pellets of the polymeric acid-doped PANi were made by the conventional four-point probe technique at room temperature, where the four points in the sample surface were in line at an equal spacing of I mm. During the measurements, an appropriate constant current in the range 0.01-50 IxA was applied to the two outer probes, and the voltage drop across the two inner probes was measured to determine the conductivity. The current was supplied by a Keithley model 220 constant current source, and the voltage drop was measured using a Keithley model 182 sensitive digital voltmeter. Prior to performing measurements, all pellets used in the experiments were vacuum-dried overnight to minimize the effect of moisture on conductivity. 2.4. Doping level measurement
The ultraviolet-visible (UV-Vis) absorption spectra in the wavelength range 300-1300 nm of thin films of the polymeric acid-doped PANi on glass plates were taken using a Shimadzu UV-3100S spectrophotometer. X-ray photoelectron spectroscopy (XPS) analysis for the polymeric acid-doped PANi was carried out using a monochromatic microspot X-ray beam originating from the Mg K~ source ( 1253.6 eV photons ) with a spot diameter of about 600 txm from a Surface Analysis System SPECS LHS10 spectrometer. The X-ray source was run at a reduced power of 120 W (12 kV and 10 mA). Throughout the measurements, the pressure in the analysis chamber was maintained at 10- 8 mbar or lower. To compensate for surface-charging effect, all binding energies (BEs) were calibrated with reference to the neutral carbon C ( l s ) line taken at 284.6 eV. The XPS data were analyzed with a least-squares fitting routine. Each spectrum was decomposed into individual Gaus-
H.-S. Moon,J.-K. Park/SyntheticMetals 92 (1998)223-228 sian peaks and a linear background was assumed over the energy range of the fit. Deconvolution was performed with the constraint of having a nearly equal full width at halfmaximum (FWHM) value for a particular line among different spectra. This, together with the peak position and height, was optimized f o r modeling the experimental spectra.
2.5. Chain alignment measurement X-ray diffraction (XRD) patterns for the polymeric aciddoped PANi powders were determined to examine chain ordering in the samples using a Rigaku model D / M A X -3C X-ray generator. The X-ray beam was nickel-filtered Cu Ks radiation from a sealed tube operated at 40 kV and 45 mA. Diffraction data were obtained from 2 to 40 ° (20) at a scan rate of 2°/min.
225
-3
~-4 == o
O13B -5 O .J
-6 0.0
[
[
f
[
.5
1.0
1.5
2.0
Composition (Acid / Aniline)
Fig. 2. Room-temperatured.c. conductivitiesvs. moleratiosof sulfonicacid units in (a) PSSAand (b) PAMPS to aniline units in PANi.
3. Results
3.1. Structural effects of pendant acid dopants on the electrical conductivities of the doped PANi composites Room-temperature d.c. conductivities of the pellets of the PSSA-doped and PAMPS-doped PANi with the compositions 1/4, 1/2, 1/ 1 and 2/1 are shown in Fig. 2, where the compositions are expressed in mole ratios of the sulfonic acid units of polymeric acids to the repeat units of PANi. It is noticeable that the conductivities of the PSSA~doped PANi are higher than those of the PAMPS-doped PANi for the entire range of composition examined in this study, except for the composition of 1/4 at which there seems to be an insufficient amount of polymeric acid dopants to reveal the structural effect of polymeric acid dopants on the conductivities of the polymeric acid-doped PANi. Even though, it is clear from Fig. 2 that the room-temperature d.c. conductivities of the PSSA-doped PANi are higher than those of the PAMPS-doped PANi by up to about two orders of magnitude at the same composition. It is also noted from Fig. 2 that there exists an optimum composition of polymeric acid to PANi showing maximum conductivity, and the optimum composition is the same ( 1/ 1 ) for the PSSA-doped PANi and the PAMPS-doped one.
3.2. Structural effects of the pendant acid dopants on the doping levels of the doped PANis The UV-Vis spectra for the films of PANi, PSSA-doped PANi and PAMPS-doped PANi are shown in Fig. 3, where the composition for the polymeric acid-doped PANis is 1/ 1. The UV-Vis spectrum of the PANi shows two absorption peaks at 324 and 639 nm, which are due to the xr-~r* transition of the benzonoid rings and the exciton absorption of the quinoid rings, respectively [ 24,25 ]. The exciton absorption peak of the PANi completely disappears in the absorption spectrum of the PSSA-doped PANi, which may indicate corn-
3/<, /
I --
400
!
F
1
1
600
800
1000
1200
Wavelength (nm) Fig. 3. UV-Vis spectra of doped PANi films: (a) PANi film, (b) PSSA/
PANi ( 1/ 1) composite,(c) PAMPS/PANi ( 1/ l ) composite. plete protonation of the imine N atoms. However, the exciton absorption peak appears in the PAMPS-doped PANi, which implies that there are undoped PANi chains in the PAMPSdoped PANi composite and that phase separation exists. The XPS N ( l s ) peaks for the PSSA-doped and PAMPSdoped PANis with the composition of 1/1 are shown in Fig. 4(a) and (b), respectively. It is evident that, in all the cases, the N( I s) signal does not originate from a single transition. A standard lineshape analysis with Gaussian fitting functions reveals that the peaks can be deconvoluted using the BEs of 398.2, 399.4 and above 400 eV for the - N = (imine), - N H - (amine) and N + (positively charged nitrogen), respectively [27-30]. On the basis of the fixed FWHM approach, the high BE tail attributed to the N + species has been resolved with one (Fig. 4(b) ) or two (Fig. 4(a) ) component peaks, even though it is probable that the N + species have a continuous BE distribution as a result of charge nonuniformity. Since the AMPS has an amide N species, the XPS N ( l s ) spectrum from the PAMPS-doped PANi in Fig. 4(b) shows a line centered at about 398.8 eV with an FWHM value of 1.6 eV, which can be attributed to the amide N atom in the
226
H.-S. Moon, J.-K. Park~Synthetic Metals 92 (1998) 223-228 i
~
i
i
i
~" t~ ~t~ ~
~
(a)
(a)
r~
t
I
b
I
P
g '~
/ //~
~
~ ( d )
J I
I
I
I
I
396
398
400
402
404
5
~ I
I
10
15
Binding Energy (eV) Fig. 4. N ( ls ) core-level spectra of (a) PSSA-doped and (b) PAMPS-doped PANi with the same composition of 1/ 1. The dashed curves in each spectrum represent the well-resolved Gaussian lineshapes corresponding to nonequivalent nitrogen sites.
(f)
I
I
I
I
I
20
25
30
35
40
20 (degrees) Fig. 5. XRD patterns of (a) PANi powder, (b) HCl-doped PANi powder,
(c) PSSApowder, (d) PAMPSpowder, (e) PSSA/PANi ( 1/ 1) composite, and (f) PAMPS/PANi ( 1/ 1) composite.
Table 1 The relative contents of the various nitrogen species and the doping level determined by the integrated intensities of the XPS N(Is) lines Dopant
-N=/N
-CONH-/N
-NH-/N
N+/N
N+/N in PANi
PSSA
0
0
0.544
0.456
PAMPS
0.051
0.501
0.289
0.159
0.456 (100%) 0.319 (75.8%) " "
_
i
,
"~
i 2/1
Theoretical percent of protonation of imine N atoms, as given by - N = / (-N=+N÷).
PAMPS. The doping level N + / N in the PANi and the proportions of the various N species obtained from the XPS spectra of the three PANi composites are listed in Table 1. It is obvious from Fig. 4(b) that a considerable fraction of the imine N atoms in the PANi composites remains unprotonated even after doping at the composition of 1/ 1 in contrast to the case of the PSSA-doped PANi in Fig. 4(a). The lower doping level of the PAMPS-doped PANi as indicated by XPS is consistent with the lack of polaron band and intraband free carrier excitations in the UV-Vis spectral observations, and much poorer conductivities (~r < 10 -4 S / c m ) of these composites.
3.3. Structural effects of the pendant acid dopants on the alignment of the doped PANi subchains XRD patterns of the PANi powder, HCl-doped PANi powder, PSSA and PAMPS powders, and the PSSA-doped and PAMPS-doped PANi powders with composition of 1/1 are shown in Fig. 5. The PANi powder exhibits only a broad amorphous reflection at 2 0 = 1 9 ° ( F i g . 5 ( a ) ) , while the PSSA-doped PANi powder shows some diffraction peaks at 2 0 = 10 and 26 ° (Fig. 5 ( e ) ) , which are characteristic diffractions by the crystalline PANi salts of the HCl-doped PANi
5
10
,
I
,
,
15
20
25
30
I-2 35
TM
40
20 (degrees) Fig. 6. XRD patterns of the (a) PSSA/PANi and (b) PAMPS/PANi composites with the compositions of 2/1, 1/ l, 1/2 and 1/4.
(Fig. 5(b) ) [31 ]. On the contrary, there are no pronounced diffraction peaks in the pattern of the PAMPS-doped PANi powder (Fig. 5 (f)). The PAMPS powder exhibits reflection shoulders at 8 and 14°. However, the reflection shoulder at 14° cannot be identified in the XRD spectrum of the PAMPSdoped PANi powder, indicating a relatively strong interaction between the two polymers. Fig. 6(a) and (b) shows the composition effect upon the ordering or chain alignment of the PSSA-doped and the PAMPS-doped PANi subchains, respectively. The XRD pattern of the PSSA-doped PANi with the composition of 1/1 shows the clearest reflection peak at 26 °, indicating the bestordered PANi subchains. However, the XRD patterns of the PSSA-doped PANi with other compositions and those of the PAMPS-doped PANi show almost no diffraction peaks except for one weak peak at 9 °.
H. -S. Moon. J.-K. Park / Synthetic Metals 92 (1998) 2 2 3 ~ 2 8
4. Discussion The conductivity differences between the PSSA-doped and PAMPS-doped PANis with varying acid-to-aniline ratios, as shown in Fig. 2, indicate that the conductivities of the polymeric acid-doped PANis are strongly dependent on the structural features of the polymeric acid dopants. The large differences in the conductivities between the PSSA-doped and PAMPS-doped PANis can be explained as follows. PSSA-doped PANi may align more efficiently than the PAMPS-doped PANi due to the less steric hindrance of the styrene sulfonic acid group that is also capable of aligning with the aromatic backbone of PANi, which is not for the case of PAMPS. Since the amino methyl propane sulfonic acid group is much bulkier than the benzene sulfonic acid group, it may induce larger intermolecular separation between the neighboring PANi chains than the benzene sulfonic acid group and, thus, lead to greater reduction in the coherence lengths between the chains. It is also suggested that the conductivity difference between the PSSA-doped and PAMPS-doped PANis is attributed to the dopability difference of PSSA and PAMPS which is again a function of compatibility with PANi. Another interesting result from Fig. 2 is that an optimum composition of polymeric acid to PANi exists for maximum conductivity. That the optimum composition is the same for the two polymeric acid dopants can be interpreted as follows. As the polymeric acid content increases, it is expected that the doping level of PANi increases; however, the volume fraction of PANi in the polymeric acid/PANi composite becomes inevitably reduced. Although the conductivity may be enhanced with the doping level, there is a limiting value in doping level, 50% of nitrogen atoms in PANi, and the polymeric dopant extraneously added may lower the volume fraction of the conducting PANi phase in the composite and subsequently reduce the conductivity. It is evident that the insulating polymeric acid backbone chains and the sulfonic acid units, which do not participate in doping the imine sites of PANi, will hinder the alignment of the doped PANi chains and intervene in the polaronic hopping in the conducting PANi phase. Consequently, the bulk conductivity of the PANi composite becomes lowered when the composition of the composite is greater than 1/ 1. The similar increasing tendency of conductivity of the composite with the amount of polymeric dopant was reported by Chen and Lee [ 10,11 ] for the poly(acrylic acid)-doped PANi system, and they explained the phenomenon by considering the incomplete doping of imine N atoms in PANi due to the conformational hindrance of the long chain poly(acrylic acid) (Mw=250 000). However, they examined the polymeric acid-doped PANi only up to the composition of 1/ 1, and thus just reported the increasing trend of conductivity with the content of polymeric acid in the composite within the composition range studied. In the PANi, half of the N atoms are in amine groups and the other half are in imine groups. Since only the imine groups
227
can be doped by acids, the maximum doping level of the PANi is 0.5. While the N atoms in the imine groups are protonated, N and its neighboring quinoid ring become a semiquinoid radical cation and the exciton absorption peak intensity decreases [26,27]. Therefore, when the N atoms in the imine groups are fully protonated, the exciton absorption peak of the PANi disappears, as in the case of the PSSAdoped PANi in Fig. 3(b). We can also confirm the complete protonation of the PSSA-doped PANi via the XPS N ( l s ) core-level spectrum of the composite as given in Fig. 4(a) which shows no imine N species. The remaining exciton absorption peak in the PAMPS-doped PANi in Fig. 3(c) implies that there are undoped PANi chains in the PAMPSdoped PANi composite and that phase separation exists, which can be attributed to conformational hindrance of long chains and steric hindrance of the bulkier pendant amido- lmethyl-2-propane sulfonic acid group. This result is consistent with that of conductivity measurement, and suggests the ineffective doping of the PAMPS mainly due to its steric hindrance. The diffraction peaks of the PSSA-doped PANi in Fig. 5 (e) imply that PANi chains become more rigid and ordered after the doping, since the diffraction peaks in Fig. 5 (e) are characteristic of the diffractions by the crystalline PANi salts [ 31 ]. Even though the volume fraction of the crystalline phase is small, which is obvious from the weak diffractions of the PSSA-doped PANi, it is clear that there is considerable ordering of PANi subchains in the PSSA-doped PANi composite. However, the PAMPS-doped PANi does not show any pronounced diffractions, which suggests that the PAMPS-doped PANi subchains are more randomly aligned and have no crystalline order. These results together with the results from Fig. 6 are consistent with those of the conductivity measurements and UV-Vis spectral observations, which can be interpreted as the bulkier polymeric acid reducing more the degree of chain ordering, as well as the doping level.
5. Conclusions For the PSSA-doped and PAMPS-doped PANi composite systems, the conductivities of the PSSA-doped PANi are higher by up to about two orders of magnitude than those of the PAMPS-doped PANi of the same composition, given by acid-to-aniline ratio. It is suggested that the conductivity difference is attributed to the difference in the degree of steric hindrance and, hence, the more inefficient doping of bulkier amino methyl propane sulfonic acid groups in PAMPS. However, the confinement of pendant benzene sulfonic acid groups on the flexible polymer chains of PSSA and that of pendant amino methyl propane suifonic acid groups on the PAMPS chains both lead to a non-uniform and inefficient doping, such that some of the acid groups are not able to participate in the doping. As a result, the PSSA-doped and PAMPS-doped PANi systems show maximum conductivity
228
H.-S. Moon, J.-K. Park/Synthetic Metals 92 (1998) 223-228
at the composition of 1/1 which is half the value of the theoretical composition for the stoichiometric doping. The occurrence of maximum conductivity can be explained by the increase of doping level and the decrease of order and packing in the PANi subchain alignment due to the neighboring flexible polymeric acids with increasing acid-to-aniline ratio. Therefore, we can deduce a general conclusion again that the high doping level and well-developed chain alignment could induce high conductivity.
References [1] B. Scrosati, in P.G. Bruce (ed.), Solid State Electrochemistry, Cambridge University Press, Cambridge, 1995. [ 2 ] K. Doblhofer, in J. Lipkowski and P.N. Ross (eds.), Electrochemistry of Novel Materials - - Frontiers of Electrochemistry, VCH, New York, 1994. [ 3 ] J.M. Morgolis (ed.), Conductive Polymers and Plastics, Chapman and Hall, London, 1989. [4] J.A. Chilton and M.T. Goosey (eds.), Special Polymers for Electronics and Optoelectronics, Chapman and Hall, London, 1995. [5] E.M. Geni~s, A. Boyle, M. Lapkowski and C. Tsintavis, Synth. Met., 36 (1989) 139. [6] A.G. MacDiarmid and A.J. Epstein, Faraday Discuss. Chem. Soc., 85 (1989) 317. [ 7 ] P.M. Borsenberger and W.T. Gruenbaum, J. Polym. Sci., Polym. Phys., 34 (1996) 575. [8] M. Angelopoulos, A. Ray, A.G. MacDiarmid and A.J. Epstein, Synth. Met., 21 (1987) 21. [9] K.R. Cromack, M.E. Jozefowicz, J.M. Ginder, A.J. Epstein, R.P. McCall, G. Du, J.M. Leng, K. Kim, C. Li, Z.H. Wang, M.A. Druy, P.J. Glatkowski, E.M. Scherr and A.G. MacDiarmid, Macromolecules, 24 (1991) 4157.
[ 10] S.A. Chert and H.T. Lee, Macromolecules, 26 (1993) 3254. [ 11 ] S.A. Chert and H.T. Lee, Macromolecules, 28 (1995) 2858. [ 12] H. Tsutsumi, S. Fukuzawa, M. Ishikawa, M. Morita and Y. Matsuda, J. Electrochem. Soc., 142 (1995) L168. [ 13 ] M. Morita, S. Miyazaki, H. Tanoue, M. Ishikawa and Y. Matsuda, J. Electrochem. Soc., 141 (1994) 1409. [ 14] D.K. Moon, A.B. Padias, H.K. Hall, Jr., T. Huntoon and P.D. Calvert, Macromotecules, 28 (1995) 6205. [15] M.F. Rubner, S.K. Tripathy, J. Georger and P. Cholewa, Macromolecules, 16 (1983) 870. [ 16] M. Lapkowski, Synth. Met., 55-57 (1993) 1558. [17] J.M. Liu and S.C. Yang, J. Chem. Soc., Chem. Commun., (1991) 1529. [ 18] J.H. Hwang and S.C. Yang, Synth. Met., 27-29 (1989) E271. [19] L. Lei, H. Li, G. Zhang, H. Zhang and Y. Li, Polym. J., 27 (1995) 867. [20] M. Angelopoulos, N. Patel and R. Saraf, Synth. Met., 55-57 (1993) 1552. [21 ] B.D. Malhotra, J. Appl. Polym. Sci., 40 (1990) 1049. [22] Y. Kang, M. Lee and S.B. Rhee, Synth. Met., 52 (1992) 319. [23] J.M. Ko, Ph.D. Thesis, KAIST, 1995. [24] F.L. Lu, F. Wudl, M. Nowak and A.J. Heeger, J. Am. Chem. Soc., 108 (1986) 8311. [ 25 ] S. Stafstr6m, J.L. Br6das, AJ. Epstein, H.S. Woo, D.B. Tanner, W.S. Huang and A.G. MacDiarmid, Phys. Rev. Lett., 59 (1987) 1464. [26] Y. Furukawa, F. Ueda, Y. Hyodo, I. Harada, T. Nakajima and T. Kawagoe, Macromolecules, 21 (1988) 1297. [27] K.G. Noeh, E.T. Kang and K.L. Tan, J. Polym. Sci., Polym. Phys., 33 ( 1995 ) 833. [28] K.L. Tan, B.T.G. Tan, E.T. Kang and K.G. Neoh, Phys. Rev. B, 39 (1989) 8070. [29] S.N. Kumar, F. Gaillard, G. Bouyssoux and A. Sartre, Synth. Met., 36 (1990) 111. [30] M. Aldissi and S.P. Armes, Macromolecules, 25 (1992) 2963. [31] J.P. Pouget, M.E. Jozefowicz, A.J. Epstein, X. Tang and A.G. MacDiarmid, Macromolecules, 24 ( 1991 ) 779.
Synthetic Metals 92 (1998) 223-228
Structural effect of polymeric acid dopants on the characteristics of doped polyaniline composites Hyuk-Soo Moon, Jung-Ki Park * Department of Chemical Engineering, Korea Advanced Institute of Science and Technology, 373-1. Kusung-dong, Yusung-gu, Daejon 305-701. South Korea Received 28 April 1997; revised 16 October 1997; accepted 23 October 1997
Abstract
The structure and doping behaviors of poly (p-styrene sulfonic acid) (PSSA) -doped polyaniline ( PANi ) and poly ( 2-acrylamido-2-methyll-propane sulfonic acid) (PAMPS)-doped PANi were investigated by conductivity measurement, ultraviolet-visible spectroscopy, X-ray photoelectron spectroscopy, and X-ray diffraction. The conductivities of the PSSA-doped PANi composites are higher by up to two orders of magnitude than those of the PAMPS-doped PANi composites of the same composition, given by acid-to-aniline ratio. The confinement of pendant benzene sulfonic acid groups in the PSSA and that of pendant amino methyl propane sulfonic acid groups in the PAMPS may result in non-uniform and inefficient doping, such that some of the acid groups are not able to participate in the doping. The conductivity difference between the two different polymeric acid-doped PANi composites can be explained by the difference in the degree of the steric hindrance between the two polymeric acid dopants. The PSSA-doped and PAMPS-doped PANi systems reveal the same optimum composition of 1/ 1 ( moles of sulfonic acid units in the polymeric acids/moles of aniline units in PANi) for maximum conductivity. The maximum conductivity seems to be attributed to the increase in the doping level and the decrease in the order and packing density of the PANi subchain alignment with increase of the acid-to-aniline ratio. © 1998 Elsevier Science S.A. Keywords: Polyaniline and derivatives; Polymeric acid dopant; Doping level; Chain alignment; Conductivity
I. Introduction
The discovery of the exceptional electrical properties of polyacetylene has spurred a great deal of research work in the area of electroactive polymers in recent years [ 1-4]. Of particular interest is century-old polyaniline (PANi) mainly due to its good thermal and environmental stability [ 5-7 ]. However, up to now, commercial applications of PANi have been limited. One of the obstacles to practical use of PANi is its poor processability when it is doped with conventional small molecular organic or inorganic acids. With a rigid 7rconjugated backbone molecular structure, PANi decomposes without melting when heated and few proper solvents have been found for PANi [8-11 ]. The conducting form of PANi powder synthesized in aqueous HC1, referred to as PANi hydrochloride, is insoluble in common organic solvents and even in l-methyl-2-pyrrolidone (NMP), which is the only organic solvent having been found so far that can dissolve high molecular weight PANi. Additionally, these small molecular acid dopants for PANi may evaporate at room * Corresponding author. Tel.: + 82 42 869 3925; fax: + 82 42 869 3910; e-mail: pjk @sorak.kaist.ac.kr 0379-6779/98/$19.00 © 1998 Elsevier Science S.A. All rights reserved P H S 0 3 7 9 - 6 7 7 9 ( 9 7 ) 0 4 0 9 1-5
temperature or higher, causing a depression in the conductivity of the acid-doped PANi [ 11,12]. Moreover, the small molecular acid-doped PANis are not suitable for use as cathodic material in a lithium rechargeable battery, since the movable anions of the acid dopants are capable of diffusing out to the electrolyte phase and, thus, the charge compensation for the positive electrode redox reaction will be based on the doping and undoping of anions rather than the intercalation and deintercalation of lithium cations [ 1,12-14]. These obstacles can be overcome in an appropriate way, that is, by using polymeric acid dopants. In addition, polymeric acid dopants can be expected to enhance the flexibility of the PANi composite since the polymeric acid usually has a lower glass transition temperature (Tg) than that of PANi [11,15]. However, little research has been carried out on the microstructural feature of the polymeric acid-doped PANi including how many of the acid groups are capable of doping PANi and how the dopability is affected or changed by the molecular structure of polymeric acid, such as chemical structures of pendant protonic acid groups. In addition, neither the way the dopant interacts with PANi nor how the doped PANi subchains align have been investigated yet. It has been
224
H.-S. Moon, J.-K. Park/Synthetic Metals 92 (1998)223-228
expected that not all of the acid groups would occur in polymeric acid-doped PANi and that the acid groups would be distributed non-uniformly due to the confinement of protonic acid groups on polymer chains and the conformational hindrance created by the flexible chain [ 16-21 ]. This study has been devoted to investigate systematically the structural effect of the pendant protonic acid groups of the two different polymeric acids, i.e., poly(p-styrene sulfonic acid) ( PSSA ) and poly ( 2-acrylamido-2-methyl- 1-propane sulfonic acid) (PAMPS), on the conductivity of the doped PANi composite, the doping level of PANi, and the subchain alignment of the doped PANi.
to ,.Q <
4000
2. Experimental
3000
2000
1000
Wave Number (cm1)
Fig. 1. FT-IR spectrum of the polyaniline base synthesized by oxidative polymerization.
2.1. Preparation of materials
The PANi hydrochloride powder was synthesized by the oxidative polymerization of aniline in 1 M aqueous HC1 with (NH4) 2S208 as oxidant at 5 °C, which is similar to the method used by MacDiarmid and Epstein [6] and Ko [23]. The powder was then converted to PANi base by treatment with 0.5 M aqueous NH4OH, followed by washing with water/ methanol several times and evacuating at 40 °C for 2 days. The PANi base obtained as described above has a ratio of IR absorption intensity at 1589 c m - ~ (due to the quinoid ring) to that at 1496 c m - J (due to the benzenoid ring) nearly the same as that of the emeraldine base with a 60 mol% oxidation level (Fig. 1 ) [22,23]. Thus, the synthesized PANi base is an emeraldine base and becomes emeraldine salt after acid doping. The styrene sulfonic acid-based polymeric dopant (PSSA) was obtained by solution polymerization of styrene using 2,2'-azobis-isobutyronitrile (AIBN) as an initiator in tetrahydrofuran (THF) at 60 °C, and subsequent sulfonation of the recovered polystyrene (PS). The sulfonation was conducted at 50 °C in ethylene dichloride using acetyl sulfate as a sulfonation agent, which had been prepared by the reaction of acetic anhydride and concentrated sulfuric acid. The other type of sulfonic acid, AMPS-based polymeric dopant (PAMPS), was also synthesized using the solution method, where AIBN and water were used as the initiator and reaction medium, respectively. 2.2. Doping of PANi
The polymeric acid-doped PANi was prepared by mixing appropriate volumes of the 0.22 M PANi (based on the approximate repeat unit -C6HaNH- ) in NMP solution and the 0.22 M polymeric acid (based on its acidic repeat unit) solution. The dissolving solvent for the PSSA was NMP, whereas dimethyl formamide (DMF) was selected as a solvent for the PAMPS because NMP was a poor solvent for the PAMPS. The mixed solutions of polymeric acid-doped PANi
were then agitated vigorously and cast on an evaporating dish under vacuum at 50-60 °C for more than 48 h. 2.3. Conductivity measurement
Electrical d.c. conductivity measurements of the compressed pellets of the polymeric acid-doped PANi were made by the conventional four-point probe technique at room temperature, where the four points in the sample surface were in line at an equal spacing of I mm. During the measurements, an appropriate constant current in the range 0.01-50 IxA was applied to the two outer probes, and the voltage drop across the two inner probes was measured to determine the conductivity. The current was supplied by a Keithley model 220 constant current source, and the voltage drop was measured using a Keithley model 182 sensitive digital voltmeter. Prior to performing measurements, all pellets used in the experiments were vacuum-dried overnight to minimize the effect of moisture on conductivity. 2.4. Doping level measurement
The ultraviolet-visible (UV-Vis) absorption spectra in the wavelength range 300-1300 nm of thin films of the polymeric acid-doped PANi on glass plates were taken using a Shimadzu UV-3100S spectrophotometer. X-ray photoelectron spectroscopy (XPS) analysis for the polymeric acid-doped PANi was carried out using a monochromatic microspot X-ray beam originating from the Mg K~ source ( 1253.6 eV photons ) with a spot diameter of about 600 txm from a Surface Analysis System SPECS LHS10 spectrometer. The X-ray source was run at a reduced power of 120 W (12 kV and 10 mA). Throughout the measurements, the pressure in the analysis chamber was maintained at 10- 8 mbar or lower. To compensate for surface-charging effect, all binding energies (BEs) were calibrated with reference to the neutral carbon C ( l s ) line taken at 284.6 eV. The XPS data were analyzed with a least-squares fitting routine. Each spectrum was decomposed into individual Gaus-
H.-S. Moon,J.-K. Park/SyntheticMetals 92 (1998)223-228 sian peaks and a linear background was assumed over the energy range of the fit. Deconvolution was performed with the constraint of having a nearly equal full width at halfmaximum (FWHM) value for a particular line among different spectra. This, together with the peak position and height, was optimized f o r modeling the experimental spectra.
2.5. Chain alignment measurement X-ray diffraction (XRD) patterns for the polymeric aciddoped PANi powders were determined to examine chain ordering in the samples using a Rigaku model D / M A X -3C X-ray generator. The X-ray beam was nickel-filtered Cu Ks radiation from a sealed tube operated at 40 kV and 45 mA. Diffraction data were obtained from 2 to 40 ° (20) at a scan rate of 2°/min.
225
-3
~-4 == o
O13B -5 O .J
-6 0.0
[
[
f
[
.5
1.0
1.5
2.0
Composition (Acid / Aniline)
Fig. 2. Room-temperatured.c. conductivitiesvs. moleratiosof sulfonicacid units in (a) PSSAand (b) PAMPS to aniline units in PANi.
3. Results
3.1. Structural effects of pendant acid dopants on the electrical conductivities of the doped PANi composites Room-temperature d.c. conductivities of the pellets of the PSSA-doped and PAMPS-doped PANi with the compositions 1/4, 1/2, 1/ 1 and 2/1 are shown in Fig. 2, where the compositions are expressed in mole ratios of the sulfonic acid units of polymeric acids to the repeat units of PANi. It is noticeable that the conductivities of the PSSA~doped PANi are higher than those of the PAMPS-doped PANi for the entire range of composition examined in this study, except for the composition of 1/4 at which there seems to be an insufficient amount of polymeric acid dopants to reveal the structural effect of polymeric acid dopants on the conductivities of the polymeric acid-doped PANi. Even though, it is clear from Fig. 2 that the room-temperature d.c. conductivities of the PSSA-doped PANi are higher than those of the PAMPS-doped PANi by up to about two orders of magnitude at the same composition. It is also noted from Fig. 2 that there exists an optimum composition of polymeric acid to PANi showing maximum conductivity, and the optimum composition is the same ( 1/ 1 ) for the PSSA-doped PANi and the PAMPS-doped one.
3.2. Structural effects of the pendant acid dopants on the doping levels of the doped PANis The UV-Vis spectra for the films of PANi, PSSA-doped PANi and PAMPS-doped PANi are shown in Fig. 3, where the composition for the polymeric acid-doped PANis is 1/ 1. The UV-Vis spectrum of the PANi shows two absorption peaks at 324 and 639 nm, which are due to the xr-~r* transition of the benzonoid rings and the exciton absorption of the quinoid rings, respectively [ 24,25 ]. The exciton absorption peak of the PANi completely disappears in the absorption spectrum of the PSSA-doped PANi, which may indicate corn-
3/<, /
I --
400
!
F
1
1
600
800
1000
1200
Wavelength (nm) Fig. 3. UV-Vis spectra of doped PANi films: (a) PANi film, (b) PSSA/
PANi ( 1/ 1) composite,(c) PAMPS/PANi ( 1/ l ) composite. plete protonation of the imine N atoms. However, the exciton absorption peak appears in the PAMPS-doped PANi, which implies that there are undoped PANi chains in the PAMPSdoped PANi composite and that phase separation exists. The XPS N ( l s ) peaks for the PSSA-doped and PAMPSdoped PANis with the composition of 1/1 are shown in Fig. 4(a) and (b), respectively. It is evident that, in all the cases, the N( I s) signal does not originate from a single transition. A standard lineshape analysis with Gaussian fitting functions reveals that the peaks can be deconvoluted using the BEs of 398.2, 399.4 and above 400 eV for the - N = (imine), - N H - (amine) and N + (positively charged nitrogen), respectively [27-30]. On the basis of the fixed FWHM approach, the high BE tail attributed to the N + species has been resolved with one (Fig. 4(b) ) or two (Fig. 4(a) ) component peaks, even though it is probable that the N + species have a continuous BE distribution as a result of charge nonuniformity. Since the AMPS has an amide N species, the XPS N ( l s ) spectrum from the PAMPS-doped PANi in Fig. 4(b) shows a line centered at about 398.8 eV with an FWHM value of 1.6 eV, which can be attributed to the amide N atom in the
226
H.-S. Moon, J.-K. Park~Synthetic Metals 92 (1998) 223-228 i
~
i
i
i
~" t~ ~t~ ~
~
(a)
(a)
r~
t
I
b
I
P
g '~
/ //~
~
~ ( d )
J I
I
I
I
I
396
398
400
402
404
5
~ I
I
10
15
Binding Energy (eV) Fig. 4. N ( ls ) core-level spectra of (a) PSSA-doped and (b) PAMPS-doped PANi with the same composition of 1/ 1. The dashed curves in each spectrum represent the well-resolved Gaussian lineshapes corresponding to nonequivalent nitrogen sites.
(f)
I
I
I
I
I
20
25
30
35
40
20 (degrees) Fig. 5. XRD patterns of (a) PANi powder, (b) HCl-doped PANi powder,
(c) PSSApowder, (d) PAMPSpowder, (e) PSSA/PANi ( 1/ 1) composite, and (f) PAMPS/PANi ( 1/ 1) composite.
Table 1 The relative contents of the various nitrogen species and the doping level determined by the integrated intensities of the XPS N(Is) lines Dopant
-N=/N
-CONH-/N
-NH-/N
N+/N
N+/N in PANi
PSSA
0
0
0.544
0.456
PAMPS
0.051
0.501
0.289
0.159
0.456 (100%) 0.319 (75.8%) " "
_
i
,
"~
i 2/1
Theoretical percent of protonation of imine N atoms, as given by - N = / (-N=+N÷).
PAMPS. The doping level N + / N in the PANi and the proportions of the various N species obtained from the XPS spectra of the three PANi composites are listed in Table 1. It is obvious from Fig. 4(b) that a considerable fraction of the imine N atoms in the PANi composites remains unprotonated even after doping at the composition of 1/ 1 in contrast to the case of the PSSA-doped PANi in Fig. 4(a). The lower doping level of the PAMPS-doped PANi as indicated by XPS is consistent with the lack of polaron band and intraband free carrier excitations in the UV-Vis spectral observations, and much poorer conductivities (~r < 10 -4 S / c m ) of these composites.
3.3. Structural effects of the pendant acid dopants on the alignment of the doped PANi subchains XRD patterns of the PANi powder, HCl-doped PANi powder, PSSA and PAMPS powders, and the PSSA-doped and PAMPS-doped PANi powders with composition of 1/1 are shown in Fig. 5. The PANi powder exhibits only a broad amorphous reflection at 2 0 = 1 9 ° ( F i g . 5 ( a ) ) , while the PSSA-doped PANi powder shows some diffraction peaks at 2 0 = 10 and 26 ° (Fig. 5 ( e ) ) , which are characteristic diffractions by the crystalline PANi salts of the HCl-doped PANi
5
10
,
I
,
,
15
20
25
30
I-2 35
TM
40
20 (degrees) Fig. 6. XRD patterns of the (a) PSSA/PANi and (b) PAMPS/PANi composites with the compositions of 2/1, 1/ l, 1/2 and 1/4.
(Fig. 5(b) ) [31 ]. On the contrary, there are no pronounced diffraction peaks in the pattern of the PAMPS-doped PANi powder (Fig. 5 (f)). The PAMPS powder exhibits reflection shoulders at 8 and 14°. However, the reflection shoulder at 14° cannot be identified in the XRD spectrum of the PAMPSdoped PANi powder, indicating a relatively strong interaction between the two polymers. Fig. 6(a) and (b) shows the composition effect upon the ordering or chain alignment of the PSSA-doped and the PAMPS-doped PANi subchains, respectively. The XRD pattern of the PSSA-doped PANi with the composition of 1/1 shows the clearest reflection peak at 26 °, indicating the bestordered PANi subchains. However, the XRD patterns of the PSSA-doped PANi with other compositions and those of the PAMPS-doped PANi show almost no diffraction peaks except for one weak peak at 9 °.
H. -S. Moon. J.-K. Park / Synthetic Metals 92 (1998) 2 2 3 ~ 2 8
4. Discussion The conductivity differences between the PSSA-doped and PAMPS-doped PANis with varying acid-to-aniline ratios, as shown in Fig. 2, indicate that the conductivities of the polymeric acid-doped PANis are strongly dependent on the structural features of the polymeric acid dopants. The large differences in the conductivities between the PSSA-doped and PAMPS-doped PANis can be explained as follows. PSSA-doped PANi may align more efficiently than the PAMPS-doped PANi due to the less steric hindrance of the styrene sulfonic acid group that is also capable of aligning with the aromatic backbone of PANi, which is not for the case of PAMPS. Since the amino methyl propane sulfonic acid group is much bulkier than the benzene sulfonic acid group, it may induce larger intermolecular separation between the neighboring PANi chains than the benzene sulfonic acid group and, thus, lead to greater reduction in the coherence lengths between the chains. It is also suggested that the conductivity difference between the PSSA-doped and PAMPS-doped PANis is attributed to the dopability difference of PSSA and PAMPS which is again a function of compatibility with PANi. Another interesting result from Fig. 2 is that an optimum composition of polymeric acid to PANi exists for maximum conductivity. That the optimum composition is the same for the two polymeric acid dopants can be interpreted as follows. As the polymeric acid content increases, it is expected that the doping level of PANi increases; however, the volume fraction of PANi in the polymeric acid/PANi composite becomes inevitably reduced. Although the conductivity may be enhanced with the doping level, there is a limiting value in doping level, 50% of nitrogen atoms in PANi, and the polymeric dopant extraneously added may lower the volume fraction of the conducting PANi phase in the composite and subsequently reduce the conductivity. It is evident that the insulating polymeric acid backbone chains and the sulfonic acid units, which do not participate in doping the imine sites of PANi, will hinder the alignment of the doped PANi chains and intervene in the polaronic hopping in the conducting PANi phase. Consequently, the bulk conductivity of the PANi composite becomes lowered when the composition of the composite is greater than 1/ 1. The similar increasing tendency of conductivity of the composite with the amount of polymeric dopant was reported by Chen and Lee [ 10,11 ] for the poly(acrylic acid)-doped PANi system, and they explained the phenomenon by considering the incomplete doping of imine N atoms in PANi due to the conformational hindrance of the long chain poly(acrylic acid) (Mw=250 000). However, they examined the polymeric acid-doped PANi only up to the composition of 1/ 1, and thus just reported the increasing trend of conductivity with the content of polymeric acid in the composite within the composition range studied. In the PANi, half of the N atoms are in amine groups and the other half are in imine groups. Since only the imine groups
227
can be doped by acids, the maximum doping level of the PANi is 0.5. While the N atoms in the imine groups are protonated, N and its neighboring quinoid ring become a semiquinoid radical cation and the exciton absorption peak intensity decreases [26,27]. Therefore, when the N atoms in the imine groups are fully protonated, the exciton absorption peak of the PANi disappears, as in the case of the PSSAdoped PANi in Fig. 3(b). We can also confirm the complete protonation of the PSSA-doped PANi via the XPS N ( l s ) core-level spectrum of the composite as given in Fig. 4(a) which shows no imine N species. The remaining exciton absorption peak in the PAMPS-doped PANi in Fig. 3(c) implies that there are undoped PANi chains in the PAMPSdoped PANi composite and that phase separation exists, which can be attributed to conformational hindrance of long chains and steric hindrance of the bulkier pendant amido- lmethyl-2-propane sulfonic acid group. This result is consistent with that of conductivity measurement, and suggests the ineffective doping of the PAMPS mainly due to its steric hindrance. The diffraction peaks of the PSSA-doped PANi in Fig. 5 (e) imply that PANi chains become more rigid and ordered after the doping, since the diffraction peaks in Fig. 5 (e) are characteristic of the diffractions by the crystalline PANi salts [ 31 ]. Even though the volume fraction of the crystalline phase is small, which is obvious from the weak diffractions of the PSSA-doped PANi, it is clear that there is considerable ordering of PANi subchains in the PSSA-doped PANi composite. However, the PAMPS-doped PANi does not show any pronounced diffractions, which suggests that the PAMPS-doped PANi subchains are more randomly aligned and have no crystalline order. These results together with the results from Fig. 6 are consistent with those of the conductivity measurements and UV-Vis spectral observations, which can be interpreted as the bulkier polymeric acid reducing more the degree of chain ordering, as well as the doping level.
5. Conclusions For the PSSA-doped and PAMPS-doped PANi composite systems, the conductivities of the PSSA-doped PANi are higher by up to about two orders of magnitude than those of the PAMPS-doped PANi of the same composition, given by acid-to-aniline ratio. It is suggested that the conductivity difference is attributed to the difference in the degree of steric hindrance and, hence, the more inefficient doping of bulkier amino methyl propane sulfonic acid groups in PAMPS. However, the confinement of pendant benzene sulfonic acid groups on the flexible polymer chains of PSSA and that of pendant amino methyl propane suifonic acid groups on the PAMPS chains both lead to a non-uniform and inefficient doping, such that some of the acid groups are not able to participate in the doping. As a result, the PSSA-doped and PAMPS-doped PANi systems show maximum conductivity
228
H.-S. Moon, J.-K. Park/Synthetic Metals 92 (1998) 223-228
at the composition of 1/1 which is half the value of the theoretical composition for the stoichiometric doping. The occurrence of maximum conductivity can be explained by the increase of doping level and the decrease of order and packing in the PANi subchain alignment due to the neighboring flexible polymeric acids with increasing acid-to-aniline ratio. Therefore, we can deduce a general conclusion again that the high doping level and well-developed chain alignment could induce high conductivity.
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